It is known that prior exposure to  radiation  of the head and neck is associated with an increased risk of cancer, particularly thyroid, salivary gland and squamous cell carcinomas, although there is a time-delay of many years and the overall risk is still low. [ 56 ]


Some head and neck cancers, and in particular oropharyngeal cancer, are caused by the  human papillomavirus  (HPV), [ 1 ] [ 62 ]  and 70% of all head and neck cancer cases are related to HPV. [ 62 ]  Risk factors for HPV-positive oropharyngeal cancer include multiple sexual partners, anal and oral sex and a weak immune system. [ 56 ]


The incidence of HPV-related head and neck cancer is increasing, especially in the Western world. In the United States, the incidence of  HPV-positive oropharyngeal cancer  has overtaken HPV-positive  cervical cancer  as the leading HPV related cancer type. [ 63 ]  An increased incidence has particularly affected males. As a result, recent changes have resulted in the  HPV vaccine  being offered to adolescent boys between 12-13 (previously only offered to girls between this age due to cervical cancer risks) and men under 45  who have sex with men  in the UK. [ 64 ] [ 65 ]


Over 20 different high-risk HPV subtypes have been implicated in causing head and neck cancer. In particular, HPV-16 is responsible for up to 90% of oropharyngeal cancer in North America. [ 56 ]   Approximately 15–25% of head and neck cancers contain genomic DNA from HPV, [ 66 ]  and the association varies based on the site of the tumor. [ 67 ]  In the case of  HPV-positive oropharyngeal cancer , the highest distribution is in the  tonsils , where HPV DNA is found in 45–67% of the cases, [ 68 ]  and it is less often in the hypopharynx (13–25%), and least often in the oral cavity (12–18%) and larynx (3–7%). [ 69 ] [ 70 ]


Positive HPV16 status is associated with a improved prognosis over HPV-negative oropharyngeal cancer due to better response to  radiotherapy  and  chemotherapy . [ 71 ]


HPV can induce tumors by several mechanisms: [ 71 ] [ 72 ]


There are observed biological differences between HPV-positive and HPV-negative head and neck cancer, for example in terms of  mutation  patterns. In HPV-negative disease, genes frequently mutated include  TP53 ,  CDKN2A  and  PIK3CA . [ 73 ]  In HPV-positive disease, these genes are less frequently mutated, and the tumour suppressor gene p53 and  pRb  (protein retinoblastoma) are commonly inactivated by HPV oncoproteins E6 and E7 respectively. [ 74 ]  In addition, viral infections such as HPV can cause aberrant  DNA methylation  during cancer development. HPV-positive head and neck cancers demonstrate higher levels of such DNA methylation compared to HPV-negative disease. [ 75 ]


E6 sequesters p53 to promote p53 degradation, while E7 inhibits pRb. Degradation of p53 results in cells being unable to respond to checkpoint signals that are normally present to activate  apoptosis  when DNA damage is signalled. Loss of pRb leads to deregulation of  cell proliferation  and apoptosis. Both mechanisms therefore leave cell proliferation unchecked and increase the chance of  carcinogenesis . [ 76 ]


Epstein–Barr virus  (EBV) infection is associated with  nasopharyngeal cancer . Nasopharyngeal cancer caused by EBV commonly occurs in some countries of the Mediterranean and Asia, where EBV  antibody titers  can be measured to screen high-risk populations. [ 77 ] [ 78 ]


The presence of  gastroesophageal reflux disease  (GERD) or laryngeal reflux disease can also be a major factor.  Stomach acids  that flow up through the  esophagus  can damage its lining and raise susceptibility to throat cancer. [ citation needed ]


People after  hematopoietic stem cell transplantation  (HSCT) are at a higher risk for oral cancer. Post-HSCT oral cancer may have more aggressive behavior and a poorer prognosis when compared to oral cancer in non-HSCT patients. [ 79 ]  This effect is supposed to be due to continuous, lifelong  immune  suppression and chronic oral  graft-versus-host disease . [ 79 ]


Several other risk factors have been identified in the development of head and neck cancer. These include occupational environmental  carcinogen  exposure such as  asbestos ,  wood dust ,  mineral acid ,  sulfuric acid mists  and metal dusts. In addition, weakened immune systems, age greater than 55 years, poor  socioeconomic factors  such as lower incomes and occupational status, and low  body mass index  (<18.5 kg/m2) are also risk factors. [ 56 ] [ 80 ] [ 23 ]  Poor  oral hygiene  and chronic  oral cavity inflammation  (for example secondary to  chronic gum inflammation ) are also linked to an increased head and neck cancer risk. [ 81 ] [ 82 ]  The presence of  leukoplakia , which is the appearance of white patches or spots in the mouth, can develop into cancer in about 1⁄3 of cases. [ 22 ]


A significant proportion of people with head and neck cancer will present to their physicians with an already advanced stage disease. [ 11 ]  This can either be down to patient factors (delays in seeking medical attention), or physician factors (such as delays in referral from primary care, or non-diagnostic investigation results). [ 83 ]


A person usually presents to the physician complaining of one or more of the typical symptoms. These symptoms may be site specific (such as a laryngeal cancer causing  hoarse voice ), or not site specific (earache can be caused by multiple types of head and neck cancers). [ 6 ]


The physician will undertake a thorough history to determine the nature of the symptoms and the presence or absence of any risk factors. The physician will also ask about other illnesses such as heart or lung diseases as they may impact their fitness for potentially curative treatment. Clinical examination will involve examination of the neck for any masses, examining inside the mouth for any abnormalities and assessing the rest of the pharynx and larynx with a  nasendoscope . [ 84 ]


Further investigations will be directed by the symptoms discussed and any abnormalities identified during the exam. [ citation needed ]


Neck masses  typically undergo assessment with  ultrasound  and a  fine-needle aspiration  (FNA, a type of needle biopsy). Concerning  lesions  that are readily accessible (such as in the mouth) can be biopsied with a  local anaesthetic . Lesions less readily available can be biopsied either with the patient awake or under a  general anaesthetic  depending on local expertise and availability of specialist equipment. [ 85 ]


The cancer will also need to be  staged  (accurately determine its size, association with nearby structures, and spread to distant sites). This is typically done by scanning the patient with a combination of  magnetic resonance imaging  (MRI),  computed tomography  (CT) and/or  positron emission tomography  (PET). Exactly which investigations are required will depend on a variety of factors such as the site of concern and the size of the tumour. [ 86 ]


Some people will present with a neck lump containing cancer cells (identified by  FNA ) that have spread from elsewhere, but with no identifiable primary site on initial assessment. In such cases people will undergo additional testing to attempt to find the initial site of cancer, as this has significant implications for their treatment. These patients undergo MRI scanning,  PET-CT  and then  panendoscopy  and  biopsies  of any abnormal areas. If the scans and panendoscopy still do not identify a primary site for the cancer, affected people will undergo a bilateral  tonsillectomy  and tongue base  mucosectomy  (as these are the most common subsites of cancer that spread to the neck). This procedure can be done with or without  robotic assistance . [ 87 ]


Once a diagnosis is confirmed, a multidisciplinary discussion of the optimal treatment strategy will be undertaken between the  radiation oncologist ,  surgical oncologist , and  medical oncologist . A  histopathologist  and a  radiologist  will also be present to discuss the biopsy and imaging findings. [ 86 ]  Most (90%) cancers of the head and neck are  squamous cell -derived, termed "head-and-neck squamous-cell carcinomas". [ 9 ]


Throat cancers are classified according to their  histology  or cell structure and are commonly referred to by their location in the oral cavity and neck. This is because where the cancer appears in the throat affects the prognosis; some throat cancers are more aggressive than others, depending on their location. The stage at which the cancer is diagnosed is also a critical factor in the prognosis of throat cancer. Treatment guidelines recommend routine testing for the presence of HPV for all oropharyngeal squamous cell carcinoma tumors. [ 88 ]


Squamous-cell carcinoma  is a  cancer  of the  squamous cell , a kind of  epithelial  cell found in both the  skin  and  mucous membranes . It accounts for over 90% of all head and neck cancers, [ 89 ]  including more than 90% of throat cancer. [ 22 ]  Squamous cell carcinoma is most likely to appear in males over 40 years of age with a history of heavy alcohol use coupled with smoking. [ citation needed ]


All squamous cell carcinomas arising from the oropharynx, and all neck node  metastases  of unknown primary should undergo testing for HPV status. This is essential to adequately stage the tumour and adequately plan treatment. Due to the different biology of HPV positive and negative cancers, differentiating HPV status is also important for ongoing research to determine the best treatments. [ 90 ]


Nasopharyngeal carcinomas , or neck node metastases possibly arising from the nasopharynx will also be tested for Ebstein Barr virus. [ 91 ]


The tumor marker Cyfra 21-1 may be useful in diagnosing squamous cell carcinoma of the head and neck (SCCHN). [ 92 ]


Adenocarcinoma  is a cancer of the epithelial tissue that has  glandular  characteristics. Several head and neck cancers are adenocarcinomas (either of intestinal or non-intestinal cell types). [ 89 ]


Avoidance of risk factors (such as smoking and alcohol) is the single most effective form of prevention. [ 56 ]


Regular dental examinations may identify pre-cancerous lesions in the oral cavity. [ 1 ]  While  screening  in the general population does not appear to be useful, screening high-risk groups by examination of the throat might be useful. [ 2 ]  Head and neck cancer is often curable if it is diagnosed early; however, outcomes are typically poor if it is diagnosed late. [ 2 ]


When diagnosed early, oral, head, and neck cancers can be treated more easily, and the chances of survival increase tremendously. [ 1 ]  The  HPV vaccine  helps to prevent the development of HPV-related oropharyngeal cancer. [ 10 ]


Improvements in diagnosis and local management, as well as  targeted therapy , have led to improvements in quality of life and survival for people with head and neck cancer. [ 93 ]


After a histologic diagnosis has been established and tumor extent determined, such as with the use of PET-CT, [ 94 ]  the selection of appropriate treatment for a specific cancer depends on a complex array of variables, including tumor site, relative morbidity of various treatment options, concomitant health problems, social and logistic factors, previous primary tumors, and the person's preference. Treatment planning generally requires a multidisciplinary approach involving specialist surgeons, medical oncologists, and radiation oncologists.  [ citation needed ]


Surgical resection and radiation therapy are the mainstays of treatment for most head and neck cancers and remain the standard of care in most cases. For small primary cancers without regional metastases (stage I or II), wide surgical excision alone or curative radiation therapy alone is used. For more extensive primary tumors or those with regional metastases (stage III or IV), planned combinations of pre- or postoperative radiation and complete surgical excision are generally used. More recently, as historical survival and control rates have been recognized as less than satisfactory, there has been an emphasis on the use of various induction or concomitant chemotherapy regimens. [ citation needed ]


Surgery  as a treatment is frequently used for most types of head and neck cancer. Usually, the goal is to remove the cancerous cells entirely. This can be particularly tricky if the cancer is near the  larynx  and can result in the person being unable to speak. Surgery is also commonly used to resect (remove) some or all of the cervical lymph nodes to prevent further spread of the disease.  Transoral robotic surgery  (TORS) is gaining popularity worldwide as the technology and training become more accessible. It now has an established role in the treatment of early stage oropharyngeal cancer. [ 95 ]  There is also a growing trend worldwide towards TORS for the surgical treatment of laryngeal and hypopharyngeal tumours. [ 96 ] [ 97 ]


CO 2  laser surgery  is also another form of treatment.  Transoral laser microsurgery  allows surgeons to remove tumors from the voice box with no external incisions. It also allows access to tumors that are not reachable with robotic surgery. During the surgery, the surgeon and pathologist work together to assess the adequacy of excision ("margin status"), minimizing the amount of normal tissue removed or damaged. [ 98 ]  This technique helps give the person as much speech and swallowing function as possible after surgery. [ 99 ]


Radiation therapy  is the most common form of treatment. There are different forms of radiation therapy, including 3D conformal radiation therapy, intensity-modulated radiation therapy,  particle beam therapy , and brachytherapy, which are commonly used in the treatment of cancers of the head and neck. Most people with head and neck cancer who are treated in the United States and Europe are treated with intensity-modulated radiation therapy using high-energy photons. At higher doses, head and neck radiation is associated with  thyroid  dysfunction and  pituitary  axis dysfunction. [ 100 ]  Radiation therapy for head and neck cancers can also cause acute skin reactions of varying severity, which can be treated and managed with topically applied creams or specialist films. [ 101 ]


Chemotherapy  for throat cancer is not generally used to  cure  the cancer as such. Instead, it is used to provide an inhospitable environment for metastases so that they will not establish themselves in other parts of the body. Typical chemotherapy agents are a combination of  paclitaxel  and  carboplatin .  Cetuximab  is also used in the treatment of throat cancer. [ citation needed ]


Docetaxel -based chemotherapy has shown a very good response in locally advanced head and neck cancer. Docetaxel is the only  taxane  approved by the FDA for head and neck cancer, in combination with cisplatin and fluorouracil for the induction treatment of inoperable, locally advanced head and neck cancer. [ 102 ]


While not specifically a chemotherapy,  amifostine  is often administered  intravenously  by a chemotherapy clinic prior to  IMRT  radiotherapy sessions. Amifostine protects the gums and  salivary glands  from the effects of radiation. [ citation needed ]


There is no evidence that  erythropoietin  should be routinely given with radiotherapy. [ 103 ]


Photodynamic therapy  may have promise for treating mucosal dysplasia and small head and neck tumors. [ 22 ]   Amphinex  is showing good results in early clinical trials for the treatment of advanced head and neck cancer. [ 104 ]


Targeted therapy , according to the  National Cancer Institute , is "a type of treatment that uses drugs or other substances, such as monoclonal antibodies, to identify and attack specific cancer cells without harming normal cells." Some  targeted therapies  used in head and neck cancers include  cetuximab ,  bevacizumab , and  erlotinib . [ citation needed ]


Cetuximab  is used for treating people with advanced-stage cancer who cannot be treated with conventional chemotherapy ( cisplatin ). [ 105 ] [ 106 ]  However, cetuximab's efficacy is still under investigation by researchers. [ 107 ]


Gendicine  is a  gene therapy  that employs an  adenovirus  to deliver the  tumor suppressor gene   p53  to cells. It was approved in China in 2003 for the treatment of head and neck cancer. [ 108 ]


The mutational profiles of  HPV + and HPV- head and neck cancer have been reported, further demonstrating that they are fundamentally distinct diseases.
 [ 109 ] [ non-primary source needed ]


Immunotherapy  is a type of treatment that activates the immune system to fight cancer. One type of immunotherapy, immune checkpoint blockade, binds to and blocks inhibitory signals on immune cells to release their anti-cancer activities. [ citation needed ]


In 2016, the FDA granted accelerated approval to  pembrolizumab  for the treatment of people with recurrent or metastatic head and neck cancer with disease progression on or after platinum-containing chemotherapy. [ 110 ]  Later that year, the FDA approved  nivolumab  for the treatment of recurrent or metastatic head and neck cancer with disease progression on or after platinum-based chemotherapy. [ 111 ]  In 2019, the FDA approved  pembrolizumab  for the first-line treatment of metastatic or unresectable recurrent head and neck cancer. [ 112 ]


Depending on the treatment used, people with head and neck cancer may experience the following symptoms and treatment side effects: [ 22 ] [ 101 ]


Programs to support the emotional and social well-being of people who have been diagnosed with head and neck cancer may be offered. [ 113 ]  There is no clear evidence on the effectiveness of these interventions or any particular type of psychosocial program or length of time that is most helpful for those with head and neck cancer. [ 113 ]


Although early-stage head and neck cancers (especially laryngeal and oral cavity) have high cure rates, up to 50% of people with head and neck cancer present with advanced disease. [ 114 ]
Cure rates decrease in locally advanced cases, whose probability of cure is inversely related to tumor size and even more so to the extent of regional node involvement.  [ citation needed ]  HPV-associated oropharyngeal cancer has been shown to respond better to chemoradiation and, subsequently, have a better prognosis compared to non-associated HPV head and neck cancer. [ 12 ]


Consensus panels in America ( AJCC ) and Europe ( UICC ) have established staging systems for head and neck cancers. These staging systems attempt to standardize clinical trial criteria for research studies and define prognostic categories of disease. Head and neck cancers are staged according to the  TNM  classification system, where T is the size and configuration of the tumor, N is the presence or absence of lymph node metastases, and M is the presence or absence of distant metastases. The T, N, and M characteristics are combined to produce a "stage" of the cancer, from I to IVB. [ 115 ]


Survival advantages provided by new treatment modalities have been undermined by the significant percentage of people cured of head and neck cancer who subsequently develop second  primary tumors . The incidence of second primary tumors ranges in studies from 9% [ 116 ]
to 23% [ 117 ]
at 20 years. Second primary tumors are the major threat to long-term survival after successful therapy of early-stage head and neck cancer. [ 118 ]  Their high incidence results from the same carcinogenic exposure responsible for the initial primary process, called  field cancerization . [ citation needed ]


Many people with head and neck cancer are also not able to eat sufficiently. A tumor may impair a person's ability to swallow and eat, and throat cancer may affect the  digestive system . The difficulty in swallowing can cause a person to  choke  on their food in the early stages of digestion and interfere with the food's smooth travel down into the  esophagus  and beyond. [ citation needed ]


The treatments for throat cancer can also be harmful to the digestive system as well as other body systems. Radiation therapy can lead to  nausea  and  vomiting , which can deprive the body of vital fluids (although these may be obtained through intravenous fluids if necessary). Frequent vomiting can lead to an electrolyte imbalance, which has serious consequences for the proper functioning of the heart. Frequent vomiting can also upset the balance of stomach acids, which has a negative impact on the digestive system, especially the lining of the stomach and esophagus. [ citation needed ]


Enteral feeding , a method that adds nutrients directly into a person's stomach using a  nasogastric  feeding tube or a  gastrostomy tube , may be necessary for some people. [ 119 ]  Further research is required to determine the most effective method of enteral feeding to ensure that people undergoing radiotherapy or chemoradiation treatment are able to stay nourished during their treatment. [ 119 ]


Cancer in the head or neck may impact a person's mental well-being and can sometimes lead to social isolation. [ 113 ]  This largely results from a decreased ability or inability to eat, speak, or effectively communicate. Physical appearance is often altered by the cancer itself and/or as a consequence of treatment side effects. Psychological distress may occur, and feelings such as uncertainty and fear may arise. [ 113 ]  Some people may also have a changed physical appearance, differences in swallowing or breathing, and residual pain to manage. [ 113 ]


Caregiver stress


Caregivers for people with head and neck cancer show higher rates of  caregiver stress  and poorer mental health compared to both the general population and those caring for non-head and neck cancer patients. [ 120 ]  The high symptom burden patients' experience necessitates complex caregiver roles, often requiring hospital staff training, which caregivers can find distressing when asked to do so for the first time. It is becoming increasingly apparent that caregivers (most often spouses, children, or close family members) might not be adequately informed about, prepared for, or trained for the tasks and roles they will encounter during the treatment and recovery phases of this unique patient population, which span both technical and emotional support. [ 121 ]  Of note, caregivers of patients who report lower quality of life demonstrate increased burden and fatigue that extend beyond the treatment phase.


Examples of technically difficult caregiver duties include tube feeding, oral suctioning, wound maintenance, medication delivery safe for tube feeding, and troubleshooting home medical equipment. If the cancer affects the mouth or larynx, caregivers must also find a way to effectively communicate among themselves and with their healthcare team. This is in addition to providing emotional support for the person undergoing cancer therapy. [ 121 ]


Like any  cancer ,  metastasis  affects many areas of the body as the cancer spreads from cell to cell and  organ  to organ. For example, if it spreads to the  bone marrow , it will prevent the body from producing enough  red blood cells  and affect the proper functioning of the  white blood cells  and the body's  immune system ; spreading to the  circulatory system  will prevent oxygen from being transported to all the cells of the body; and throat cancer can throw the  nervous system  into chaos, making it unable to properly regulate and control the body. [ citation needed ]


Globally, head and neck cancer accounts for 650,000 new cases of cancer and 330,000 deaths annually on average. In 2018, it was the seventh most common cancer worldwide, with 890,000 new cases documented and 450,000 people dying from the disease. [ 12 ]   The risk of developing head and neck cancer increases with age, especially after 50 years. Most people who do so are between 50 and 70 years old. [ 22 ]


In North America and Europe, the tumors usually arise from the oral cavity, oropharynx, or larynx, whereas nasopharyngeal cancer is more common in the Mediterranean countries and in the Far East. In Southeast China and Taiwan, head and neck cancer, specifically  nasopharyngeal cancer , is the most common cause of death in young men. [ 123 ]


In the United States, head and neck cancer makes up 3% of all cancer cases (averaging 53,000 new diagnoses per year) and 1.5% of cancer deaths. [ 124 ]  The 2017 worldwide figure cites head and neck cancers as representing 5.3% of all cancers (not including non-melanoma skin cancers). [ 125 ] [ 5 ]   Notably, head and neck cancer secondary to chronic alcohol or tobacco use has been steadily declining as less of the population chronically smokes tobacco. [ 12 ]  However, HPV-associated oropharyngeal cancer is rising, particularly in younger people in westernized nations, which is thought to be reflective of changes in oral sexual practices, specifically with regard to the number of oral sexual partners. [ 5 ] [ 12 ]  This increase since the 1970s has mostly affected wealthier nations and male populations. [ 126 ] [ 127 ] [ 5 ]  This is due to evidence suggesting that transmission rates of HPV from women to men are higher than from men to women, as women often have a higher immune response to infection. [ 5 ] [ 128 ]


Immunotherapy with immune checkpoint inhibitors is being investigated in head and neck cancers. [ 131 ]


A  head mirror  is a simple diagnostic device,  stereotypically  worn by  physicians , but less so in recent decades as they have become somewhat obsolete. [ 1 ]


A head mirror is mostly used for examination of the  ear, nose and throat  (ENT). It comprises a circular  concave mirror , with a small hole in the middle, and is attached to a headband. The mirror is worn over the physician's eye of choice, with the concave mirror surface facing outwards and the hole directly over the physician's eye, providing illumination like a  ring light .


In use, the patient sits and faces the physician. A bright lamp is positioned adjacent to the patient's head, pointing toward the physician's face and hence towards the head mirror. The light from the lamp reflects off the mirror, along the line of sight of the user, with the light being somewhat concentrated by the curvature of the mirror. When used properly, the head mirror thus provides excellent shadow-free illumination.


Because they were  once in common use , notably by  general practitioners  and  otorhinolaryngologists , head mirrors are often used as a stereotypical part of a physician's uniform by costumers and as props in comic routines. The main drawback to head mirrors was that they required some skill to use well. [ 1 ]  They are rarely seen outside of the ENT setting, having been largely replaced by pen lights among  general practitioners . They are still used by some otolaryngologists, particularly for examinations and procedures involving the oral cavity, although many are switching to fiber optic headlights. [ 1 ]


Hearing loss  is a partial or total inability to  hear . [ 5 ]  Hearing loss may be present at birth or acquired at any time afterwards. [ 6 ] [ 7 ]   Hearing loss may occur in one or both ears. [ 2 ]  In children, hearing problems can affect the ability to  acquire  spoken  language , and in adults it can create difficulties with social interaction and at work. [ 8 ]  Hearing loss can be temporary or permanent.  Hearing loss related to age  usually affects both ears and is due to  cochlear hair cell  loss. [ 9 ]  In some people, particularly older people, hearing loss can result in loneliness. [ 2 ]


Hearing loss may be caused by a number of factors, including:  genetics ,  ageing ,  exposure to noise , some  infections , birth complications, trauma to the ear, and certain medications or toxins. [ 2 ]  A common condition that results in hearing loss is  chronic ear infections . [ 2 ]  Certain infections during pregnancy, such as  cytomegalovirus ,  syphilis  and  rubella , may also cause hearing loss in the child. [ 2 ] [ 10 ]  Hearing loss is diagnosed when  hearing testing  finds that a person is unable to hear 25  decibels  in at least one ear. [ 2 ]  Testing for poor hearing is recommended for all newborns. [ 8 ]  Hearing loss can be categorized as mild (25 to 40  dB ), moderate (41 to 55 dB), moderate-severe (56 to 70 dB), severe (71 to 90 dB), or profound (greater than 90 dB). [ 2 ]  There are three main types of hearing loss:  conductive hearing loss ,  sensorineural hearing loss , and mixed hearing loss. [ 3 ]


About half of hearing loss globally is preventable through public health measures. [ 2 ]  Such practices include  immunization , proper care around  pregnancy , avoiding loud noise, and avoiding certain medications. [ 2 ]  The  World Health Organization  recommends that young people limit exposure to loud sounds and the use of  personal audio players  to an hour a day in an effort to limit exposure to noise. [ 11 ]  Early identification and support are particularly important in children. [ 2 ]  For many,  hearing aids ,  sign language ,  cochlear implants  and  subtitles  are useful. [ 2 ]   Lip reading  is another useful skill some develop. [ 2 ]  Access to hearing aids, however, is limited in many areas of the world. [ 2 ]


As of 2013 hearing loss affects about 1.1 billion people to some degree. [ 12 ]  It causes disability in about 466 million people (5% of the global population), and moderate to severe disability in 124 million people. [ 2 ] [ 13 ] [ 14 ]  Of those with moderate to severe disability 108 million live in low and middle income countries. [ 13 ]  Of those with hearing loss, it began during childhood for 65 million. [ 15 ]  Those who use sign language and are members of  Deaf culture  may see themselves as having a difference rather than a  disability . [ 16 ]  Many members of Deaf culture oppose attempts to cure deafness [ 17 ] [ 18 ] [ 19 ]  and some within this community view cochlear implants with concern as they have the potential to eliminate their culture. [ 20 ]


Use of the terms "hearing impaired", "deaf-mute", or "deaf and dumb" to describe deaf and hard of hearing people is discouraged by many in the deaf community as well as advocacy organizations, as they are offensive to many deaf and hard of hearing people. [ 23 ] [ 24 ]


Human hearing extends in frequency from 20 to 20,000 Hz, and in intensity from 0 dB to 120 dB HL or more. 0 dB does not represent absence of sound, but rather the softest sound an average unimpaired human ear can hear; some people can hear down to −5 or even −10 dB.  Sound is generally uncomfortably loud above 90 dB and 115 dB represents the  threshold of pain . The ear does not hear all frequencies equally well: hearing sensitivity peaks around 3,000 Hz.  There are many qualities of human hearing besides frequency range and intensity that cannot easily be measured quantitatively. However, for many practical purposes, normal hearing is defined by a frequency versus intensity graph, or audiogram, charting sensitivity thresholds of hearing at defined frequencies.  Because of the cumulative impact of age and exposure to noise and other acoustic insults, 'typical' hearing may not be normal. [ 25 ] [ 26 ]


The presentation is as follows: [ citation needed ]


Hearing loss is sensory, but may have accompanying symptoms: [ citation needed ]


There may also be accompanying secondary symptoms: [ citation needed ]


Hearing loss is associated with  Alzheimer's disease  and  dementia . [ 27 ]  The risk increases with the hearing loss degree. There are several hypotheses including cognitive resources being redistributed to hearing and social isolation from hearing loss having a negative effect. [ 28 ]  According to preliminary data,  hearing aid  usage can slow down the decline in  cognitive functions . [ 29 ]


Hearing loss is responsible for causing  thalamocortical dysrthymia  in the brain which is a cause for several neurological disorders including  tinnitus  and  visual snow syndrome . [ citation needed ]


Hearing loss is an increasing concern especially in aging populations. The prevalence of hearing loss increases about two-fold for each decade increase in age after age 40. [ 30 ]  While the secular trend might decrease individual level risk of developing hearing loss, the prevalence of hearing loss is expected to rise due to the aging population in the US. Another concern about aging process is cognitive decline, which may progress to  mild cognitive impairment  and eventually dementia. [ 31 ]  The association between hearing loss and cognitive decline has been studied in various research settings. Despite the variability in study design and protocols, the majority of these studies have found consistent association between age-related hearing loss and cognitive decline, cognitive impairment, and dementia. [ 32 ]  The association between age-related hearing loss and Alzheimer's disease was found to be nonsignificant, and this finding supports the hypothesis that hearing loss is associated with dementia independent of Alzheimer pathology. [ 32 ]  There are several hypotheses about the underlying causal mechanism for age-related hearing loss and cognitive decline. One hypothesis is that this association can be explained by common etiology or shared neurobiological pathology with decline in other physiological system. [ 33 ]  Another possible cognitive mechanism emphasize on individual's  cognitive load . As people developing hearing loss in the process of aging, the cognitive load demanded by auditory perception increases, which may lead to change in brain structure and eventually to dementia. [ 34 ]  One other hypothesis suggests that the association between hearing loss and cognitive decline is mediated through various psychosocial factors, such as decrease in  social contact  and increase in  social isolation . [ 33 ]  Findings on the association between hearing loss and dementia have significant public health implication, since about 9% of dementia cases are associated with hearing loss. [ 35 ]


People with hearing loss are at a higher risk of falling. There is also a potential  dose–response relationship  between hearing loss and falls—greater severity of hearing loss is associated with increased difficulties in postural control and increased prevalence of falls. [ 36 ]  The underlying causal link between the association of hearing loss and falls is yet to be elucidated. There are several hypotheses that indicate that there may be a common process between decline in  auditory system  and increase in incident falls, driven by physiological, cognitive, and behavioral factors. [ 36 ]  This evidence suggests that treating hearing loss has potential to increase  health-related quality of life  in older adults. [ 36 ] Falls  have important health implications, especially for an aging population where they can lead to significant morbidity and mortality. Elderly people are particularly vulnerable to the consequences of injuries caused by falls, since older individuals typically have greater bone fragility and poorer protective reflexes. [ 37 ]  Fall-related injury can also lead to burdens on the financial and health care systems. [ 37 ]  In literature, age-related hearing loss is found to be significantly associated with incident falls. [ 38 ]


Hearing loss can contribute to decrease in health-related quality of life, increase in social isolation and decline in social engagement, which are all risk factors for increased risk of developing depression symptoms. [ 39 ]   Depression  is one of the leading causes of morbidity and mortality worldwide. In older adults, the suicide rate is higher than it is for younger adults, and more suicide cases are attributable to depression. [ 40 ]  Some chronic diseases are found to be significantly associated with risk of developing depression, such as  coronary heart disease ,  pulmonary disease ,  vision loss  and hearing loss. [ 41 ]


Prelingual deafness  is profound hearing loss that is sustained before the acquisition of language, which can occur due to a  congenital  condition or through hearing loss before birth or in early infancy. Prelingual deafness impairs an individual's ability to acquire a  spoken  language in children, but deaf children can acquire spoken language through support from cochlear implants (sometimes combined with hearing aids). [ 42 ] [ 43 ]  Non-signing (hearing) parents of deaf babies (90–95% of cases) usually go with oral approach without the support of sign language, as these families lack previous experience with  sign language  and cannot competently provide it to their children without learning it themselves. This may in some cases (late implantation or not sufficient benefit from cochlear implants) bring the risk of  language deprivation  for the deaf baby [ 44 ]  because the deaf baby would not have a sign language if the child is unable to acquire spoken language successfully. The 5–10% of cases of deaf babies born into signing families have the potential of age-appropriate development of language due to early exposure to a  sign language  by sign-competent parents, thus they have the potential to meet language milestones, in sign language in lieu of spoken language. [ 45 ]


Post-lingual deafness  is hearing loss that is sustained after the  acquisition of language , which can occur due to  disease ,  trauma , or as a side-effect of a medicine. Typically, hearing loss is gradual and often detected by family and friends of affected individuals long before the patients themselves will acknowledge the disability. [ 46 ]  Post-lingual deafness is far more common than pre-lingual deafness. Those who lose their hearing later in life, such as in late adolescence or adulthood, face their own challenges, living with the adaptations that allow them to live independently. [ citation needed ]


Hearing loss has multiple causes, including ageing, genetics, perinatal problems and acquired causes like noise and disease.  For some kinds of hearing loss the cause may be classified as  of unknown cause . [ citation needed ]


There is a progressive loss of ability to hear high frequencies with aging known as  presbycusis . For men, this can start as early as 25 and women at 30. Although genetically variable, it is a normal concomitant of ageing and is distinct from hearing losses caused by noise exposure, toxins or disease agents. [ 47 ]  Common conditions that can increase the risk of hearing loss in elderly people are  high blood pressure ,  diabetes  ( hearing loss in diabetes ), [ 48 ]  or the use of certain medications harmful to the ear. [ 49 ] [ 50 ]  While everyone loses hearing with age, the amount and type of hearing loss is variable. [ 51 ]


Noise-induced hearing loss  (NIHL), also known as  acoustic trauma , typically manifests as elevated hearing thresholds (i.e. less sensitivity or muting). Noise exposure is the cause of approximately half of all cases of hearing loss, causing some degree of problems in 5% of the population globally. [ 52 ]  The majority of hearing loss is not due to age, but due to noise exposure. [ 53 ]  Various governmental, industry and standards organizations set noise standards. [ 54 ]  Many people are unaware of the presence of environmental sound at damaging levels, or of the level at which sound becomes harmful. Common sources of damaging noise levels include car stereos, children's toys, motor vehicles, crowds, lawn and maintenance equipment, power tools, gun use, musical instruments, and even hair dryers. Noise damage is cumulative; all sources of damage must be considered to assess risk. In the US, 12.5% of children aged 6–19 years have permanent hearing damage from excessive noise exposure. [ 55 ]   The World Health Organization estimates that half of those between 12 and 35 are at risk from using  personal audio devices  that are too loud. [ 11 ]  Hearing loss in adolescents may be caused by loud noise from toys, music by headphones, and concerts or events. [ 56 ] [ 57 ]


Hearing loss can be inherited. Around 75–80% of all these cases are inherited by  recessive genes , 20–25% are inherited by  dominant genes , 1–2% are inherited by  X-linked  patterns, and fewer than 1% are inherited by  mitochondrial inheritance . [ 58 ]   Syndromic deafness occurs when there are other signs or medical problems aside from deafness in an individual, [ 58 ]  such as  Usher syndrome ,  Stickler syndrome ,  Waardenburg syndrome ,  Alport's syndrome , and  neurofibromatosis type 2 .  Nonsyndromic deafness  occurs when there are no other signs or medical problems associated with the deafness in an individual. [ 58 ]


Fetal alcohol spectrum disorders  are reported to cause hearing loss in up to 64% of infants born to  alcoholic  mothers, from the ototoxic effect on the developing fetus plus malnutrition during pregnancy from the excess  alcohol  intake.  Premature birth  can be associated with sensorineural hearing loss because of an increased risk of  hypoxia ,  hyperbilirubinaemia , ototoxic medication and infection as well as noise exposure in the neonatal units. Also, hearing loss in premature babies is often discovered far later than a similar hearing loss would be in a full-term baby because normally babies are given a hearing test within 48 hours of birth, but doctors must wait until the premature baby is medically stable before testing hearing, which can be months after birth. [ 59 ]  The risk of hearing loss is greatest for those weighing less than 1500 g at birth.


Disorders responsible for hearing loss include  auditory neuropathy , [ 60 ] [ 61 ]   Down syndrome , [ 62 ]   Charcot–Marie–Tooth disease  variant 1E, [ 63 ]   autoimmune disease ,  multiple sclerosis ,  meningitis ,  cholesteatoma ,  otosclerosis ,  perilymph fistula ,  Ménière's disease , recurring ear infections, strokes,  superior semicircular canal dehiscence ,  Pierre Robin ,  Treacher-Collins ,  Usher Syndrome ,  Pendred Syndrome , and  Turner  syndrome,  syphilis ,  vestibular schwannoma , and  viral infections  such as  measles ,  mumps , congenital  rubella  (also called German measles) syndrome, several varieties of  herpes viruses , [ 64 ] [ 65 ]   HIV/AIDS , [ 66 ]  and  West Nile virus .


Some medications may reversibly or irreversibly affect hearing. These medications are considered  ototoxic . This includes  loop diuretics  such as furosemide and bumetanide,  non-steroidal anti-inflammatory drugs  (NSAIDs) both over-the-counter (aspirin, ibuprofen, naproxen) as well as prescription (celecoxib, diclofenac, etc.), paracetamol,  quinine , and  macrolide antibiotics . [ 67 ]  Others may cause permanent hearing loss. [ 68 ]  The most important group is the  aminoglycosides  (main member  gentamicin ) and platinum based chemotherapeutics such as  cisplatin  and  carboplatin . [ 69 ] [ 70 ]


In addition to medications, hearing loss can also result from specific chemicals in the environment: metals, such as  lead ;  solvents , such as  toluene  (found in  crude oil ,  gasoline [ 71 ]  and  automobile exhaust , [ 71 ]  for example); and  asphyxiants . [ 72 ]  Combined with noise, these ototoxic chemicals have an additive effect on a person's hearing loss. [ 72 ]  Hearing loss due to chemicals starts in the high frequency range and is irreversible. It damages the  cochlea  with lesions and degrades central portions of the  auditory system . [ 72 ]  For some ototoxic chemical exposures, particularly styrene, [ 73 ]  the risk of hearing loss can be higher than being exposed to  noise  alone. The effects is greatest when the combined exposure include  impulse noise . [ 74 ] [ 75 ]  A 2018 informational bulletin by the US  Occupational Safety and Health Administration  (OSHA) and the  National Institute for Occupational Safety and Health  (NIOSH) introduces the issue, provides examples of ototoxic chemicals, lists the industries and occupations at risk and provides prevention information. [ 76 ]


There can be damage either to the ear, whether the external or middle ear, to the cochlea, or to the brain centers that process the aural information conveyed by the ears.  Damage to the middle ear may include fracture and discontinuity of the ossicular chain. [ 77 ] [ 78 ]   Damage to the inner ear (cochlea) may be caused by  temporal bone fracture .  People who sustain head injury are especially vulnerable to hearing loss or tinnitus, either temporary or permanent. [ 79 ] [ 80 ]


Sound waves reach the outer ear and are conducted down the ear canal to the  eardrum , causing it to vibrate. The vibrations are transferred by the 3 tiny ear bones of the  middle ear  to the fluid in the inner ear. The fluid moves hair cells ( stereocilia ), and their movement generates nerve impulses which are then taken to the brain by the  cochlear nerve . [ 81 ] [ 82 ]   The auditory nerve takes the impulses to the brainstem, which sends the impulses to the midbrain. Finally, the signal goes to the auditory cortex of the temporal lobe to be interpreted as sound. [ 83 ]


Hearing loss is most commonly caused by long-term exposure to loud noises, from recreation or from work, that damage the hair cells, which do not grow back on their own. [ 84 ] [ 85 ] [ 9 ]


Older people may lose their hearing from long exposure to noise, changes in the inner ear, changes in the middle ear, or from changes along the nerves from the ear to the brain. [ 86 ]


Identification of a hearing loss is usually conducted by a general practitioner  medical doctor ,  otolaryngologist , certified and licensed  audiologist , school or industrial  audiometrist , or other audiometric technician. Diagnosis of the cause of a hearing loss is carried out by a specialist physician (audiovestibular physician) or  otorhinolaryngologist .


Hearing loss is generally measured by playing generated or recorded sounds, and determining whether the person can hear them. Hearing sensitivity varies according to the  frequency  of sounds. To take this into account, hearing sensitivity can be measured for a range of frequencies and plotted on an  audiogram . Other method for quantifying hearing loss is a  hearing test  using a mobile application or hearing aid application, which includes a hearing test. [ 87 ] [ 88 ]  Hearing diagnosis using mobile application is similar to the  audiometry  procedure. [ 87 ]  Audiograms, obtained using mobile applications, can be used to adjust hearing aid applications. [ 88 ]  Another method for quantifying hearing loss is a speech-in-noise test. which gives an indication of how well one can understand speech in a noisy environment. [ 89 ]  Otoacoustic emissions test is an objective hearing test that may be administered to toddlers and children too young to cooperate in a conventional hearing test. Auditory brainstem response testing is an electrophysiological test used to test for hearing deficits caused by pathology within the ear, the cochlear nerve and also within the brainstem.


A case history (usually a written form, with questionnaire) can provide valuable information about the context of the hearing loss, and indicate what kind of diagnostic procedures to employ. Examinations include  otoscopy ,  tympanometry , and differential testing with the  Weber ,  Rinne , Bing and Schwabach tests. In case of infection or inflammation, blood or other body fluids may be submitted for laboratory analysis.  MRI and CT scans can be useful to identify the pathology of many causes of hearing loss.


Hearing loss is categorized by severity, type, and configuration. Furthermore, a hearing loss may exist in only one ear (unilateral) or in both ears (bilateral).  Hearing loss can be temporary or permanent, sudden or progressive. The  severity  of a hearing loss is ranked according to ranges of nominal thresholds in which a sound must be so it can be detected by an individual. It is measured in  decibels  of hearing loss, or dB HL. There are three main  types  of hearing loss:  conductive hearing loss ,  sensorineural hearing loss , and mixed hearing loss. [ 15 ]  An additional problem which is increasingly recognised is  auditory processing disorder  which is not a hearing loss as such but a difficulty perceiving sound. The shape of an audiogram shows the relative  configuration  of the hearing loss, such as a  Carhart notch  for otosclerosis, 'noise' notch for noise-induced damage, high frequency rolloff for presbycusis, or a flat audiogram for conductive hearing loss. In conjunction with speech audiometry, it may indicate central auditory processing disorder, or the presence of a  schwannoma  or other tumor.


People with  unilateral hearing loss  or single-sided deafness (SSD) have difficulty in hearing conversation on their impaired side, localizing sound, and understanding speech in the presence of background noise.  One reason for the hearing problems these patients often experience is due to the  head shadow effect . [ 90 ]


Idiopathic sudden hearing loss is a condition where a person as an immediate decrease in the sensitivity of their  sensorineural hearing  that does not have a known cause. [ 91 ]  This type of loss is usually only on one side (unilateral) and the severity of the loss varies. A common threshold of a "loss of at least 30 dB in three connected frequencies within 72 hours" is sometimes used, however there is no universal definition or international consensus for diagnosing idiopathic sudden hearing loss. [ 91 ]


It is estimated that half of cases of hearing loss are preventable. [ 92 ]  About 60% of hearing loss in children under the age of 15 can be avoided. [ 93 ] [ 2 ]  There are a number of effective preventative strategies, including: immunization against  rubella  to prevent  congenital rubella syndrome , immunization against  H. influenza  and  S. pneumoniae  to reduce cases of  meningitis , and avoiding or protecting against excessive noise exposure. [ 15 ]  The  World Health Organization  also recommends immunization against  measles ,  mumps , and  meningitis , efforts to prevent  premature birth , and avoidance of certain medication as prevention. [ 94 ]   World Hearing Day  is a yearly event to promote actions to prevent hearing damage.


Avoiding exposure to loud noise can help prevent noise-induced hearing loss. [ 95 ]  18% of adults exposed to loud noise at work for five years or more report hearing loss in both ears as compared to 5.5% of adults who were not exposed to loud noise at work. [ 96 ]  Different programs exist for specific populations such as school-age children, adolescents and workers. [ 97 ]   But the HPD (without individual selection, training and  fit testing ) does not significantly reduce the risk of hearing loss. [ 98 ] [ 99 ]  The use of  antioxidants  is being studied for the prevention of noise-induced hearing loss, particularly for scenarios in which noise exposure cannot be reduced, such as during military operations. [ 100 ]


Noise is widely recognized as an  occupational hazard . In the United States, the  National Institute for Occupational Safety and Health  (NIOSH) and the  Occupational Safety and Health Administration  (OSHA) work together to provide standards and enforcement on workplace noise levels. [ 101 ] [ 102 ]  The  hierarchy of hazard controls  demonstrates the different levels of controls to reduce or eliminate exposure to noise and prevent hearing loss, including  engineering controls  and  personal protective equipment  (PPE). [ 103 ]  Other programs and initiative have been created to prevent hearing loss in the workplace. For example, the  Safe-in-Sound Award  was created to recognize organizations that can demonstrate results of successful noise control and other interventions. [ 104 ]  Additionally, the  Buy Quiet  program was created to encourage employers to purchase quieter machinery and tools. [ 105 ]  By purchasing less noisy power tools like those found on the  NIOSH Power Tools Database  and limiting exposure to ototoxic chemicals, great strides can be made in preventing hearing loss. [ 106 ]


Companies can also provide personal hearing protector devices tailored to both the worker and type of employment. Some hearing protectors universally block out all noise, and some allow for certain noises to be heard. Workers are more likely to wear hearing protector devices when they are properly fitted. [ 107 ]


Often interventions to prevent noise-induced hearing loss have many components. A 2017 Cochrane review found that stricter legislation might reduce noise levels. [ 108 ]   Providing workers with information on their  sound exposure levels  was not shown to decrease exposure to noise. Ear protection, if used correctly, can reduce noise to safer levels, but often, providing them is not sufficient to prevent hearing loss. Engineering noise out and other solutions such as proper maintenance of equipment can lead to noise reduction, but further field studies on resulting noise exposures following such interventions are needed. Other possible solutions include improved enforcement of existing legislation and better implementation of well-designed prevention programmes, which have not yet been proven conclusively to be effective. The conclusion of the Cochrane Review was that further research could modify what is now regarding the effectiveness of the evaluated interventions. [ 108 ]


The  Institute for Occupational Safety and Health of the German Social Accident Insurance  has created a hearing impairment calculator based on the ISO 1999 model for studying  threshold shift  in relatively homogeneous groups of people, such as workers with the same type of job.  The ISO 1999 model estimates how much hearing impairment in a group can be ascribed to age and  noise exposure .  The result is calculated via an  algebraic equation  that uses the A-weighted sound exposure level, how many years the people were exposed to this noise, how old the people are, and their sex. The model's estimations are only useful for people without hearing loss due to non-job related exposure and can be used for prevention activities. [ 109 ]


The  United States Preventive Services Task Force  recommends  neonatal hearing screening  for all newborns, as the first three years of life are believed to be the most important for language development. [ 8 ] [ 110 ]  Universal neonatal hearing screenings have now been widely implemented across the U.S., with rates of newborn screening increasing from less than 3% in the early 1990s to 98% in 2009. [ 111 ] [ 112 ]  Newborns whose screening reveals a high index of suspicion of hearing loss are referred for additional diagnostic testing with the goal of providing early intervention and access to language. [ 113 ]


The  American Academy of Pediatrics  advises that children should have their hearing tested several times throughout their schooling: [ 55 ]


While the American College of Physicians indicated that there is not enough evidence to determine the utility of screening in adults over 50 years old who do not have any symptoms, [ 114 ]  the  American Language, Speech Pathology and Hearing Association  recommends that adults should be screened at least every decade through age 50 and at three-year intervals thereafter, to minimize the detrimental effects of the untreated condition on quality of life. [ 115 ]  For the same reason, the US Office of Disease Prevention and Health Promotion included as one of  Healthy People 2020  objectives: to increase the proportion of persons who have had a hearing examination. [ 116 ]


Management depends on the specific cause if known as well as the extent, type and configuration of the hearing loss. Sudden hearing loss due to an underlying nerve problem may be treated with  corticosteroids . [ 117 ]


Most hearing loss, that result from age and noise, is progressive and irreversible, and there are currently no approved or recommended treatments.  A few specific kinds of hearing loss are amenable to surgical treatment.  In other cases, treatment is addressed to underlying pathologies, but any hearing loss incurred may be permanent. Some management options include  hearing aids ,  cochlear implants ,  middle ear implants ,  assistive technology , and  closed captioning ; [ 9 ]  in  movie theaters , a Hearing Impaired (HI) audio track may be available via headphones to better hear dialog. [ 118 ]


This choice depends on the level of hearing loss, type of hearing loss, and personal preference. Hearing aid applications are one of the options for hearing loss management. [ 88 ] [ 119 ]  For people with bilateral hearing loss, it is not clear if bilateral hearing aids (hearing aids in both ears) are better than a unilateral hearing aid (hearing aid in one ear). [ 9 ]


For people with idiopathic sudden hearing loss, different treatment approaches have been suggested that are usually based on the suspected cause of the sudden hearing loss. Treatment approaches may include corticosteroid medications, rheological drugs, vasodilators, anesthetics, and other medications chosen based on the suspected underlying pathology that caused the sudden hearing loss. [ 91 ]  The evidence supporting most treatment options for idiopathic sudden hearing loss is very weak and adverse effects of these different medications is a consideration when deciding on a treatment approach. [ 91 ]


Globally, hearing loss affects about 10% of the population to some degree. [ 52 ]  It caused moderate to severe disability in 124.2 million people as of 2004 (107.9 million of whom are in low and middle income countries). [ 13 ]  Of these 65 million acquired the condition during childhood. [ 15 ]  At birth ~3 per 1000 in  developed countries  and more than 6 per 1000 in  developing countries  have hearing problems. [ 15 ]


Hearing loss increases with age. In those between 20 and 35 rates of hearing loss are 3% while in those 44 to 55 it is 11% and in those 65 to 85 it is 43%. [ 8 ]


A 2017 report by the World Health Organization estimated the costs of unaddressed hearing loss and the cost-effectiveness of interventions, for the health-care sector, for the education sector and as broad societal costs. [ 120 ]   Globally, the annual cost of unaddressed hearing loss was estimated to be in the range of $750–790 billion  international dollars .


The  International Organization for Standardization  (ISO) developed the ISO 1999 standards for the estimation of hearing thresholds and noise-induced hearing impairment. [ 121 ]  They used data from two noise and hearing study databases, one presented by Burns and Robinson ( Hearing and Noise in Industry,  Her Majesty's Stationery Office,  London, 1970) and by Passchier-Vermeer (1968). [ 122 ]  As race are some of the factors that can affect the expected distribution of pure-tone hearing thresholds several other national or regional datasets exist, from Sweden, [ 123 ]   Norway, [ 124 ]  South Korea, [ 125 ]  the United States [ 126 ]  and Spain. [ 127 ]


In the United States hearing is one of the health outcomes measure by the  National Health and Nutrition Examination Survey  (NHANES), a  survey   research  program conducted by the  National Center for Health Statistics . It examines  health  and  nutritional  status of adults and children in the  United States .  Data from the United States in 2011–2012 found that rates of hearing loss has declined among adults aged 20 to 69 years, when compared with the results from an earlier time period (1999–2004). It also found that adult hearing loss is associated with increasing age, sex, ethnicity, educational level, and noise exposure. [ 128 ]   Nearly one in four adults had audiometric results suggesting noise-induced hearing loss. Almost one in four adults who reported excellent or good hearing had a similar pattern (5.5% on both sides and 18% on one side). Among people who reported exposure to loud noise at work, almost one third had such changes. [ 129 ]


People with extreme hearing loss may communicate through  sign languages . Sign languages convey meaning through manual communication and body language instead of acoustically conveyed sound patterns. This involves the simultaneous combination of hand shapes, orientation and movement of the hands, arms or body, and facial expressions to express a speaker's thoughts. "Sign languages are based on the idea that vision is the most useful tool a deaf person has to communicate and receive information". [ 130 ]


Deaf culture  refers to a tight-knit  cultural group  of people whose  primary language  is signed, and who practice social and cultural norms which are distinct from those of the surrounding hearing community. This community does not automatically include all those who are clinically or legally deaf, nor does it exclude every hearing person. According to Baker and Padden, it includes any person or persons who "identifies him/herself as a member of the Deaf community, and other members accept that person as a part of the community," [ 131 ]  an example being  children of deaf adults  with normal hearing ability.  It includes the set of social beliefs, behaviors, art, literary traditions, history, values, and shared institutions of communities that are influenced by deafness and which use sign languages as the main means of communication. [ 132 ] [ 133 ]   Members of the Deaf community tend to view deafness as a difference in human experience rather than a  disability  or  disease . [ 134 ] [ 135 ]   When used as a cultural label especially within the culture, the word  deaf  is often written with a capital  D  and referred to as "big D Deaf" in speech and sign. When used as a label for the  audiological  condition, it is written with a lower case  d . [ 132 ] [ 133 ]


There also multiple educational institutions for both deaf and Deaf people, that usually use sign language as the main language of instruction. Famous institutions include  Gallaudet University  and the  National Technical Institute for the Deaf  in the US, [ 136 ]  and the  National University Corporation of Tsukuba University of Technology  in Japan. [ 137 ]


A 2005 study achieved successful  regrowth of cochlea cells  in guinea pigs. [ 138 ]  However, the regrowth of cochlear hair cells does not imply the restoration of hearing sensitivity, as the sensory cells may or may not make connections with neurons that carry the signals from hair cells to the brain. A 2008 study has shown that gene therapy targeting  Atoh1  can cause hair cell growth and attract neuronal processes in embryonic mice. Some hope that a similar treatment will one day ameliorate hearing loss in humans. [ 139 ]


Recent research, reported in 2012 achieved growth of cochlear nerve cells resulting in hearing improvements in gerbils, [ 140 ]  using stem cells. Also reported in 2013 was regrowth of hair cells in deaf adult mice using a drug intervention resulting in hearing improvement. [ 141 ]  The  Hearing Health Foundation  in the US has embarked on a project called the Hearing Restoration Project. [ 142 ]  Also Action on Hearing Loss in the UK is also aiming to restore hearing. [ 143 ]


Researchers reported in 2015 that genetically deaf mice which were treated with  TMC1  gene therapy recovered some of their hearing. [ 144 ] [ 145 ]  In 2017, additional studies were performed to treat  Usher syndrome [ 146 ]  and here, a recombinant adeno-associated virus seemed to outperform the older vectors. [ 147 ] [ 148 ]


Besides research studies seeking to improve hearing, such as the ones listed above, research studies on the deaf have also been carried out in order to understand more about audition. Pijil and Shwarz (2005) conducted their study on the deaf who lost their hearing later in life and, hence, used cochlear implants to hear. They discovered further evidence for rate coding of pitch, a system that codes for information for frequencies by the rate that neurons fire in the auditory system, especially for lower frequencies as they are coded by the frequencies that neurons fire from the basilar membrane in a synchronous manner. Their results showed that the subjects could identify different pitches that were proportional to the frequency stimulated by a single electrode. The lower frequencies were detected when the basilar membrane was stimulated, providing even further evidence for rate coding. [ 149 ]


Tracheal intubation  (usually simply referred to as  intubation ), an  invasive medical procedure , is the placement of a flexible plastic  catheter  into the  trachea . For millennia,  tracheotomy  was considered the most reliable (and most risky) method of tracheal intubation. By the late 19th century, advances in the sciences of  anatomy  and  physiology , as well as the beginnings of an appreciation of the  germ theory of disease , had reduced the  morbidity  and  mortality  of this operation to a more acceptable rate. Also in the late 19th century, advances in  endoscopic  instrumentation had improved to such a degree that direct  laryngoscopy  had finally become a viable means to secure the  airway  by the non-surgical orotracheal route. Nasotracheal intubation was not widely practiced until the early 20th century. The 20th century saw the transformation of the practices of tracheotomy, endoscopy and non-surgical tracheal intubation from rarely employed procedures to essential components of the practices of  anesthesia ,  critical care medicine ,  emergency medicine ,  gastroenterology ,  pulmonology  and  surgery .


The earliest known depiction of a tracheotomy is found on two  Egyptian   tablets  dating back to circa 3600 BC. [ 1 ]  The 110-page  Ebers Papyrus , an  Egyptian medical papyrus  that dates to around 1550 BC, also refers to the tracheotomy. [ 1 ] [ 2 ]  Tracheotomy was described in an ancient Indian scripture, the  Rigveda : the text mentions "the bountiful one who, without a ligature, can cause the windpipe to re-unite when the cervical cartilages are cut across, provided they are not entirely severed." [ 2 ] [ 3 ] [ 4 ]  The  Sushruta Samhita  ( c.  400 BC ) is another text from the  Indian subcontinent  on ayurvedic medicine and surgery that mentions tracheotomy. [ 5 ]


The  Greek  physician  Hippocrates  ( c.  460 – c.  370 BC ) condemned the practice of tracheotomy. Warning against the unacceptable risk of death from inadvertent laceration of the  carotid artery  during tracheotomy, Hippocrates also cautioned that "The most difficult  fistulas  are those that occur in the cartilaginous areas." [ 6 ]   Homerus of Byzantium  is said to have written of  Alexander the Great  (356–323 BC) saving a soldier from  asphyxiation  by making an incision with the tip of his sword in the man's trachea. [ 7 ]


Despite the concerns of Hippocrates,  Galen  of  Pergamon  (129–199) and  Aretaeus  of  Cappadocia  (both of whom lived in  Rome  in the 2nd century AD) credit  Asclepiades of Bithynia  ( c.  124 –40 BC) as being the first physician to perform a non-emergency tracheotomy. [ 8 ] [ 9 ]  However, Aretaeus warned against the performance of tracheotomy because he believed that incisions made into the tracheal  cartilage  were prone to secondary  wound infections  and therefore would not heal. He wrote that "The lips of the wound do not coalesce, for they are both cartilaginous and not of a nature to unite". [ 10 ] [ 11 ]   Antyllus , another Greek surgeon who lived in Rome in the 2nd century AD, was reported to have performed tracheotomy when treating oral diseases. He refined the technique to be more similar to that used in modern times, recommending that a  transverse  incision be made between the third and fourth tracheal rings for the treatment of life-threatening airway obstruction. [ 10 ]  Antyllus wrote that tracheotomy was not effective however in cases of severe  laryngotracheobronchitis  because the  pathology  was distal to the operative site. Antyllus' original writings were lost, but they were preserved by  Oribasius  ( c.  320 –400) and  Paul of Aegina  ( c.  625 –690), both of whom were Greek physicians as well as historians. [ 10 ]  Galen clarified the anatomy of the trachea and was the first to demonstrate that the larynx generates the voice. [ 12 ] [ 13 ]  Galen may have understood the importance of artificial ventilation, because in one of his experiments he used bellows to inflate the lungs of a dead animal. [ 14 ] [ 15 ]


Circa 1020,  Ibn Sīnā  (980–1037) described the use of tracheal intubation in  The Canon of Medicine  to facilitate  breathing . [ 16 ]  In the 12th century medical textbook  Al-Taisir ,  Ibn Zuhr  (1091–1161) of Al-Andalus (also known as Avenzoar) provided an anatomically correct description of the tracheotomy operation. [ 17 ] [ 18 ]


The  Renaissance  saw significant advances in anatomy and surgery, and surgeons became increasingly open to surgery on the trachea. Despite this, the mortality rate failed to improve. [ 10 ]  From 1500 through 1832 there are only 28 known descriptions of successful tracheotomy in the literature. [ 10 ]  The first detailed descriptions on tracheal intubation and subsequent  artificial respiration  of animals were from  Andreas Vesalius  (1514–1564) of Brussels. In his landmark book published in 1543,  De humani corporis fabrica , he described an experiment in which he passed a  reed  into the trachea of a dying animal whose  thorax  had been opened and maintained ventilation by blowing into the reed intermittently. [ 15 ] [ 19 ]  Vesalius wrote that the technique could be life-saving.  Antonio Musa Brassavola  (1490–1554) of  Ferrara  treated a patient with  peritonsillar abscess  by tracheotomy after the patient had been refused by  barber surgeons . The patient apparently made a complete recovery and Brassavola published his account in 1546. This operation has been identified as the first recorded successful tracheostomy, despite many ancient references to the trachea and possibly to its opening. [ 10 ]


Towards the end of the 16th century, anatomist and surgeon  Hieronymus Fabricius  (1533–1619) described a useful technique for tracheotomy in his writings, although he had never actually performed the operation himself. He advised using a vertical incision and was the first to introduce the idea of a tracheostomy tube. This was a straight, short  cannula  that incorporated wings to prevent the tube from advancing too far into the trachea. Fabricius' description of the tracheotomy procedure is similar to that used today.  Julius Casserius  (1561–1616) succeeded Fabricius as professor of anatomy at the University of Padua and published his own writings regarding technique and equipment for tracheotomy, recommending a curved silver tube with several holes in it.  Marco Aurelio Severino  (1580–1656), a skillful surgeon and anatomist, performed multiple successful tracheotomies during a  diphtheria   epidemic  in  Naples  in 1610, using the vertical incision technique recommended by Fabricius. He also developed his own version of a trocar. [ 20 ]


In 1620 the French surgeon  Nicholas Habicot  (1550–1624), surgeon of the  Duke of Nemours  and anatomist, published a report of four successful "bronchotomies" he had performed. [ 21 ]  One of these is the first recorded case of a tracheotomy for the removal of a foreign body, in this instance a blood clot in the larynx of a stabbing victim. He also described the first known tracheotomy performed on a  pediatric  patient. A 14-year-old boy swallowed a bag containing 9 gold coins in an attempt to prevent its theft by a  highwayman . The object became lodged in his  esophagus , obstructing his trachea. Habicot suggested that the operation might also be effective for patients with inflammation of the larynx. He developed equipment for this surgical procedure that are similar in many ways to modern designs.


Sanctorius  (1561–1636) is believed to be the first to use a trocar in the operation. He recommended leaving the cannula in place for a few days following the operation. [ 22 ]  Early tracheostomy devices are illustrated in Habicot's  Question Chirurgicale [ 21 ]  and Julius Casserius' posthumous  Tabulae anatomicae  in 1627. [ 23 ]  Thomas Fienus (1567–1631), Professor of Medicine at the  University of Louvain , was the first to use the word "tracheotomy" in 1649, but this term was not commonly used until a century later. [ 24 ]  Georg Detharding (1671–1747), professor of anatomy at the  University of Rostock , treated a drowning victim with tracheostomy in 1714. [ 25 ] [ 26 ] [ 27 ]


Fearful of complications, most surgeons delayed the potentially life-saving tracheotomy until a patient was moribund, despite the knowledge that irreversible organ damage would have already occurred by that time. This began to change in the early 19th century, when the tracheotomy finally began to be recognized as a legitimate means of treating severe airway obstruction. In 1832, French physician  Pierre Bretonneau  (1778–1862) employed tracheotomy as a last resort to treat a case of  diphtheria . [ 28 ]  In 1852, Bretonneau's student  Armand Trousseau  (1801–1867) presented a series of 169 tracheotomies (158 of which were for  croup  and 11 for "chronic maladies of the larynx"). [ 29 ]  In 1871, the German surgeon  Friedrich Trendelenburg  (1844–1924) published a paper describing the first successful elective human tracheotomy performed to administer general anesthesia. [ 30 ] [ 31 ] [ 32 ] [ 33 ]  After the death of German Emperor  Frederick III  from  laryngeal cancer  in 1888, Sir  Morell Mackenzie  (1837–1892) and the other treating physicians collectively wrote a book discussing the then-current  indications  for tracheotomy and when the operation is absolutely necessary. [ 34 ]


In the early 20th century, physicians began to use the tracheotomy in the treatment of patients affected by paralytic  poliomyelitis  who required mechanical ventilation. The currently used surgical tracheotomy technique was described in 1909 by  Chevalier Jackson  (1865–1958), a professor of laryngology at  Jefferson Medical College  in Philadelphia. [ 35 ]  However, surgeons continued to debate various aspects of the tracheotomy well into the 20th century. Many techniques were employed, along with many different surgical instruments and tracheal tubes. Surgeons could not seem to reach a consensus on where or how the tracheal incision should be made, arguing whether the "high tracheotomy" or the "low tracheotomy" was more beneficial. Ironically, the newly developed  inhalational anesthetic agents  and techniques of general anesthesia actually seemed to increase the risks, with many patients with fatal postoperative complications. Jackson emphasised the importance of postoperative care, which dramatically reduced the mortality rate. By 1965, the surgical anatomy was thoroughly and widely understood,  antibiotics  were widely available and useful for treating postoperative infections and other major complications of tracheotomy had also become more manageable.


While all these surgical advances were taking place, many important developments were also taking place in the science of  optics . Many new  optical instruments  with medical applications were invented during the 19th century. In 1805, a German army surgeon named  Philipp von Bozzini  (1773–1809) invented a device he called the  lichtleiter  (or light-guiding instrument). This instrument, the ancestor of the modern endoscope, was used to examine the  urethra , the human  urinary bladder ,  rectum , oropharynx and nasopharynx. [ 36 ] [ 37 ] [ 38 ] [ 39 ]  The instrument consisted of a candle within a metal chimney; a mirror on the inside reflected light from the candle through attachments into the relevant body cavity. [ 40 ]  The practice of gastric endoscopy in humans was pioneered by United States Army surgeon  William Beaumont  (1785–1853) in 1822 with the cooperation of his patient  Alexis St. Martin  (1794–1880), a victim of an accidental gunshot wound to the stomach. [ 41 ]  In 1853,  Antonin Jean Desormeaux  (1815–1882) of Paris modified Bozzini's lichtleiter such that a mirror would reflect light from a  kerosene lamp  through a long metal channel. [ 40 ]  Referring to this instrument as an  endoscope  (he is credited with coining this term), Desormeaux employed it to examine the urinary bladder. However, like Bozzini's lichtleiter, Desormeaux's endoscope was of limited utility due to its propensity to become very hot during use. [ 40 ]  In 1868,  Adolph Kussmaul  (1822–1902) of Germany performed the first  esophagogastroduodenoscopy  (a  diagnostic  procedure in which an endoscope is used to visualize the esophagus, stomach and  duodenum ) on a living human. The subject was a  sword-swallower , who swallowed a metal tube with a length of 47 centimeters and a diameter of 13 millimeters. [ 42 ] [ 43 ] [ 44 ] [ 45 ]  On 2 October 1877,  Berlin   urologist   Maximilian Carl-Friedrich Nitze  (1848–1906) and  Viennese  instrument maker Josef Leiter (1830–1892) introduced the first practical  cystourethroscope  with an electric light source. [ 46 ]  The instrument's biggest drawback was the  tungsten  filament  incandescent light bulb  (invented by  Alexander Lodygin , 1847–1923), which became very hot and required a complicated  water cooling  system. [ 40 ]  In 1881, Polish physician  Jan Mikulicz-Radecki  (1850–1905) created the first rigid gastroscope for practical applications. [ 47 ] [ 48 ] [ 49 ]


In 1932,  Rudolph Schindler  (1888–1968) of Germany introduced the first semi-flexible gastroscope. [ 50 ]  This device had numerous  lenses  positioned throughout the tube and a miniature light bulb at the distal tip. The tube of this device was 75 centimeters long and 11 millimeters in diameter, and the distal portion was capable of a certain degree of flexion. Between 1945 and 1952,  optical engineers  (particularly Karl Storz (1911–1996) of the  Karl Storz GmbH  company of Germany,  Harold Hopkins  (1918–1995) of England and  Mutsuo Sugiura  of the Japanese  Olympus Corporation ) built upon this early work, leading to the development of the first "gastrocamera". [ 51 ] [ 52 ]  In 1964, Fernando Alves Martins (born 17 June 1927) of Portugal applied  optical fiber  technology to one of these early gastrocameras to produce the first gastrocamera with a flexible fiberscope. [ 53 ] [ 54 ]  Initially used in esophagogastroduodenoscopy, newer devices were developed in the late 1960s for use in  bronchoscopy ,  rhinoscopy  and laryngoscopy. The concept of using a fiberoptic endoscope for tracheal intubation was introduced by Peter Murphy, an English anesthetist, in 1967. [ 55 ]  By the mid-1980s, the flexible fiberoptic bronchoscope had become an indispensable instrument within the  pulmonology  and anesthesia communities. [ 56 ]


In 1854, a Spanish  vocal pedagogist  named  Manuel García  (1805–1906) became the first man to view the functioning glottis in a living human. García developed a tool that used two mirrors for which the Sun served as an external  light source . [ 57 ]  Using this device, he was able to observe the function of his own glottic apparatus and the uppermost portion of his trachea. He presented his observations at the  Royal Society of London  in 1855. [ 57 ] [ 58 ]


In 1858,  Eugène Bouchut  (1818–1891), a  pediatrician  from Paris, developed a new technique for non-surgical orotracheal intubation to bypass laryngeal obstruction resulting from a diphtheria-related pseudomembrane. His method involved introducing a small straight metal tube into the larynx, securing it by means of a silk thread and leaving it there for a few days until the pseudomembrane and airway obstruction had resolved sufficiently. [ 59 ]  Bouchut presented this experimental technique along with the results he had achieved in the first seven cases at the  French Academy of Sciences  conference on 18 September 1858. [ 60 ]  The members of the academy rejected Bouchut's ideas, largely as a result of highly critical and negative remarks made by the influential Armand Trousseau. [ 61 ]  Undaunted, Bouchut later introduced a set of tubes ("Bouchut's tubes") for intubation of the trachea, as an alternative to tracheotomy in cases of diphtheria.


In March 1878, Wilhelm Hack of  Freiburg  published a paper describing the use of non-surgical orotracheal intubation in the removal of  vocal cord polyps . [ 62 ]  In November of that year, he published another study, this time on the use of orotracheal intubation to secure the airway of a patient with acute  glottic edema , progressively introducing sizes 3 through 11 of "Schrotter's graduated triangular  vulcanite  bougies" into the larynx. [ 63 ] [ 64 ]  In 1880, the  Scottish  surgeon  William Macewen  (1848–1924) reported on his use of orotracheal intubation as an alternative to tracheotomy to allow a patient with glottic edema to breathe, as well as in the setting of general anesthesia with  chloroform . [ 64 ] [ 65 ] [ 66 ]  All previous observations of the glottis and larynx (including those of García, Hack and Macewen) had been performed under indirect vision (using mirrors) until 23 April 1895, when  Alfred Kirstein  (1863–1922) of Germany first described direct visualization of the vocal cords. Kirstein performed the first direct laryngoscopy in Berlin, using an esophagoscope he had modified for this purpose; he called this device an  autoscope . [ 67 ]  The death in 1888 of  Emperor Frederick III [ 34 ]  may have motivated Kirstein to develop the autoscope. [ 68 ]


Until 1913,  oral and maxillofacial surgery  was performed by mask  inhalation anesthesia , topical application of  local anesthetics  to the  mucosa , rectal anesthesia, or  intravenous  anesthesia. While otherwise effective, these techniques did not protect the airway from obstruction and also exposed patients to the risk of pulmonary aspiration of blood and  mucus  into the tracheobronchial tree. In 1913,  Chevalier Jackson  was the first to report a high rate of success for the use of direct laryngoscopy as a means to intubate the trachea. [ 69 ]  Jackson introduced a new laryngoscope blade that had a light source at the distal tip, rather than the proximal light source used by Kirstein. [ 70 ]  This new blade incorporated a component that the operator could slide out to allow room for passage of an endotracheal tube or bronchoscope. [ 71 ]


That same year,  Henry H. Janeway  (1873–1921) published results he had achieved using a laryngoscope he had recently developed. [ 72 ]  While practicing at  Bellevue Hospital  in  New York City , Janeway was of the opinion that direct intratracheal  insufflation  of  volatile anesthetics  would provide improved conditions for  otolaryngologic  surgery. With this in mind, he developed a laryngoscope designed for the sole purpose of tracheal intubation. Similar to Jackson's device, Janeway's instrument incorporated a distal light source. Unique however was the inclusion of  batteries  within the handle, a central notch in the blade for maintaining the tracheal tube in the midline of the oropharynx during intubation and a slight curve to the distal tip of the blade to help guide the tube through the glottis. The success of this design led to its subsequent use in other types of surgery. Janeway was thus instrumental in popularizing the widespread use of direct laryngoscopy and tracheal intubation in the practice of anesthesiology. [ 68 ]


After  World War I , further advances were made in the field of intratracheal anesthesia. Among these were those made by Sir  Ivan Whiteside Magill  (1888–1986). Working at the  Queen's Hospital for Facial and Jaw Injuries  in  Sidcup  with plastic surgeon Sir  Harold Gillies  (1882–1960) and anesthetist E. Stanley Rowbotham (1890–1979), Magill developed the technique of awake blind nasotracheal intubation. [ 73 ] [ 74 ] [ 75 ] [ 76 ] [ 77 ] [ 78 ]  Magill devised a new type of angulated forceps (the Magill forceps) that are still used today to facilitate nasotracheal intubation in a manner that is little changed from Magill's original technique. [ 79 ]  Other devices invented by Magill include the Magill laryngoscope blade, [ 80 ]  as well as several apparati for the administration of volatile anesthetic agents. [ 81 ] [ 82 ] [ 83 ]  The Magill curve of an endotracheal tube is also named for Magill.


Sir  Robert Macintosh  (1897–1989) also achieved significant advances in techniques for tracheal intubation when he introduced his new curved laryngoscope blade in 1943. [ 84 ]  The Macintosh blade remains to this day the most widely used laryngoscope blade for orotracheal intubation. [ 85 ]  In 1949, Macintosh published a case report describing the novel use of a  gum elastic   urinary catheter  as an endotracheal tube introducer to facilitate difficult tracheal intubation. [ 86 ]  Inspired by Macintosh's report, P. Hex Venn (who was at that time the anesthetic advisor to the British firm Eschmann Brothers & Walsh, Ltd.) set about developing an endotracheal tube introducer based on this concept. Venn's design was accepted in March 1973, and what became known as the Eschmann endotracheal tube introducer went into production later that year. [ 87 ]  The material of Venn's design was different from that of a gum elastic bougie in that it had two layers: a core of tube woven from  polyester  threads and an outer  resin  layer. This provided more stiffness but maintained the flexibility and the slippery surface. Other differences were the length (the new introducer was 60 cm (24 in), which is much longer than the gum elastic bougie) and the presence of a 35° curved tip that let it be steered around obstacles. [ 88 ] [ 89 ]  The concept of using a stylet for replacing or exchanging orotracheal tubes was introduced by Finucane and Kupshik in 1978, using a  central venous catheter . [ 90 ]


The 20th century saw the transformation of the practices of tracheotomy, endoscopy and non-surgical tracheal intubation from rarely employed procedures to essential components of the practices of anesthesia,  critical care medicine ,  emergency medicine ,  gastroenterology , pulmonology and surgery. The  "digital revolution"  of the 21st century has brought newer technology to the art and science of tracheal intubation. Several manufacturers have developed  video laryngoscopes  that use  digital technology  such as the  CMOS   active pixel sensor  (CMOS APS) to generate a view of the glottis so that the trachea may be intubated. The Glidescope video laryngoscope is one example of such a device. [ 91 ] [ 92 ]


A  hoarse voice , also known as  dysphonia  or  hoarseness , [ 1 ]  is when the voice involuntarily sounds breathy, raspy, or strained, or is softer in volume or lower in pitch. [ 2 ] [ 3 ] [ clarification needed ]  A hoarse voice can be associated with a feeling of unease or scratchiness in the throat. [ 2 ]  Hoarseness is often a symptom of problems in the vocal folds of the larynx. [ 2 ]  It may be caused by  laryngitis , which in turn may be caused by an  upper respiratory infection , a  cold , or  allergies . [ 2 ]  Cheering at sporting events, speaking loudly in noisy environments, talking for too long without resting one's voice, singing loudly, or speaking with a voice that is too high or too low can also cause temporary hoarseness. [ 2 ]  A number of other causes for losing one's voice exist, and treatment is generally by resting the voice and treating the underlying cause. [ 2 ]  If the cause is misuse or overuse of the voice, drinking plenty of water may alleviate the problems. [ 2 ]


It appears to occur more commonly in females and the elderly. [ 4 ]  Furthermore, certain occupational groups, such as teachers and singers, are at an increased risk. [ 5 ] [ 6 ]


Long-term hoarseness, or hoarseness that persists over three weeks, especially when not associated with a cold or  flu  should be assessed by a medical doctor. [ 2 ]  It is also recommended to see a doctor if hoarseness is associated with coughing up blood,  difficulties swallowing , a lump in the neck, pain when speaking or swallowing,  difficulty breathing , or  complete loss of voice  for more than a few days. [ 2 ]  For voice to be classified as "dysphonic", abnormalities must be present in one or more vocal parameters: pitch, loudness, quality, or variability. [ 7 ]  Perceptually, dysphonia can be characterised by hoarse, breathy, harsh, or rough vocal qualities, but some kind of  phonation  remains. [ 7 ]


Dysphonia can be categorized into two broad main types: organic and functional, and classification is based on the underlying pathology. While the causes of dysphonia can be divided into five basic categories, all of them result in an interruption of the ability of the  vocal folds  to vibrate normally during exhalation, which affects the voice. The assessment and diagnosis of dysphonia is done by a multidisciplinary team, and involves the use of a variety of subjective and objective measures, which look at both the quality of the voice as well as the physical state of the  larynx . [ citation needed ]  Multiple treatments have been developed to address organic and functional causes of dysphonia. Dysphonia can be targeted through direct therapy, indirect therapy, medical treatments, and surgery. Functional dysphonias may be treated through direct and indirect voice therapies, whereas surgeries are recommended for chronic, organic dysphonias. [ 8 ]


Voice disorders can be divided into two broad categories: organic and functional. [ 9 ]  The distinction between these broad classes stems from their cause, whereby organic dysphonia results from some sort of physiological change in one of the subsystems of speech (for voice, usually respiration, laryngeal anatomy, and/or other parts of the vocal tract are affected). Conversely, functional dysphonia refers to hoarseness resulting from vocal use (i.e. overuse/abuse). [ 10 ]  Furthermore, according to  ASHA , organic dysphonia can be subdivided into structural and neurogenic; neurogenic dysphonia is defined as impaired functioning of the vocal structure due to a neurological problem (in the  central nervous system  or  peripheral nervous system ); in contrast, structural dysphonia is defined as impaired functioning of the vocal mechanism that is caused by some sort of physical change (e.g. a lesion on the vocal folds). [ 10 ]  Notably, an additional subcategory of functional dysphonia recognized by professionals is psychogenic dysphonia, which can be defined as a type of voice disorder that has no known cause and can be presumed to be a product of some sort of psychological stressors in one's environment. [ 10 ] [ 11 ]  It is important to note that these types are not mutually exclusive and much overlap occurs. For example,  Muscle Tension Dysphonia  (MTD) has been found to be a result of many different causes including the following: MTD in the presence of an organic pathology (i.e. organic type), MTD stemming from vocal use (i.e. functional type), and MTD as a result of personality and/or psychological factors (i.e. psychogenic type). [ 10 ] [ 12 ]


The most common causes of hoarseness is  laryngitis  (acute 42.1%; chronic 9.7%) and functional dysphonia (30%). [ 13 ]  Hoarseness can also be caused by laryngeal  tumours  (benign 10.7 - 31%; malignant 2.2 - 3.0%). [ 13 ]  Causes that are overall less common include  neurogenic  conditions (2.8 - 8.0%), psychogenic conditions (2.0 - 2.2%), and aging (2%). [ 13 ]


A variety of different causes, which result in abnormal vibrations of the vocal folds, can cause dysphonia. These causes can range from vocal abuse and misuse to  systemic diseases . Causes of dysphonia can be divided into five basic categories, although overlap may occur between categories. [ 14 ] [ 15 ] [ 16 ] [ 17 ]  (Note that this list is not exhaustive):


It has been suggested that certain occupational groups may be at increased risk of developing dysphonia [ 5 ] [ 6 ]  due to the excessive or intense vocal demands of their work. [ 19 ]  Research on this topic has primarily focused on teachers and singers, although some studies have examined other groups of heavy voice users (e.g. actors, cheerleaders, aerobic instructors, etc.). [ 5 ] [ 20 ]  At present, it is known that teachers and singers are likely to report dysphonia. [ 19 ] [ 21 ]  Moreover, physical education teachers, teachers in noisy environments, and those who habitually use a loud speaking voice are at increased risk. [ 19 ]  The term  clergyman's throat  or  dysphonia clericorum  was previously used for painful dysphonia associated with public speaking, particularly among preachers. [ 22 ]  However, the exact prevalence rates for occupational voice users are unclear, as individual studies have varied widely in the methodologies used to obtain data (e.g. employing different  operational definitions  for "singer"). [ 19 ] [ 21 ]


Located in the anterior portion of the neck is the  larynx  (also known as the voice box), a structure made up of several supporting cartilages and ligaments, which houses the  vocal folds . [ 23 ]  In normal voice production, exhaled air moves out of the lungs and passes upward through the  vocal tract . [ 23 ]  At the level of the larynx, the exhaled air causes the vocal folds to move toward the midline of the tract (a process called adduction). The adducted vocal folds do not close completely but instead remain partially open. The narrow opening between the folds is referred to as the  glottis . [ 23 ] [ 7 ]  As air moves through the glottis, it causes a distortion of the air particles which sets the vocal folds into vibratory motion. It is this vibratory motion that produces phonation or voice. [ 7 ]  In dysphonia, there is an impairment in the ability to produce an appropriate level of phonation. More specifically, it results from an impairment in vocal fold vibration or the nerve supply of the larynx. [ 7 ]


The assessment and diagnosis of a dysphonic voice is completed by a multidisciplinary team, such as an otolaryngologist (ear, nose and throat doctor) and Speech-Language Pathologist, involving the use of both objective and subjective measures to evaluate the quality of the voice as well as the condition of the vocal fold  tissue  and  vibration  patterns. [ 24 ]


Dysphonia is a broad clinical term which refers to abnormal functioning of the voice. [ 23 ] [ 7 ]  More specifically, a voice can be classified as "dysphonic" when there are abnormalities or impairments in one or more of the following parameters of voice: pitch, loudness, quality, and variability. [ 7 ]  For example, abnormal pitch can be characterized by a voice that is too high or low whereas abnormal loudness can be characterized by a voice that is too quiet or loud. [ 7 ]  Similarly, a voice that has frequent, inappropriate breaks characterizes abnormal quality while a voice that is monotone (i.e., very flat) or inappropriately fluctuates characterizes abnormal variability. [ 7 ]  While hoarseness is used interchangeably with the term dysphonia, it is important to note that the two are not synonymous. Hoarseness is merely a subjective term to explain the perceptual quality (or sound) of a dysphonic voice. [ 25 ]  While hoarseness is a common symptom (or complaint) of dysphonia, [ 23 ]  there are several other signs and symptoms that can be present such as: breathiness, roughness, and dryness. Furthermore, a voice can be classified as dysphonic when it poses problems in the functional or occupational needs of the individual or is inappropriate for their age or sex. [ 7 ]


Auditory-perceptual measures are the most commonly used tool by clinicians to evaluate the voice quality due to its quick and non-invasive nature. [ 26 ]  Additionally, these measures have been proven to be reliable in a clinical setting. [ 27 ]  Ratings are used to evaluate the quality of a patient's voice for a variety of voice features, including overall severity, roughness, breathiness, strain, loudness and pitch. These evaluations are done during spontaneous speech, sentence or passage reading or sustained vowel productions. [ 17 ]  The GRBAS (Grade, Roughness, Breathiness, Asthenia, Strain) and the CAPE-V (Consensus Auditory Perceptual Evaluation—Voice) are two formal voice rating scales commonly used for this purpose. [ 26 ]


Vocal fold imaging techniques are used by clinicians to examine the vocal folds and allows them to detect vocal  pathology  and assess the quality of the vocal fold vibrations. Laryngeal  stroboscopy  is the primary clinical tool used for this purpose. Laryngeal stroboscopy uses a synchronized flashing light passed through either a rigid or flexible  laryngoscope  to provide an image of the vocal fold motion; the image is created by averaging over several vibratory cycles and is thus not provided in real-time. [ 28 ]  As this technique relies on  periodic  vocal fold vibration, it cannot be used in patients with moderate to severe dysphonia. [ 17 ]  High speed digital imaging of the vocal folds ( videokymography ), another imaging technique, is not subject to the same limitations as laryngeal stroboscopy. A rigid endoscope is used to take images at a rate of 8000 frames per second, and the image is displayed in real time. As well, this technique allows imaging of aperiodic vibrations [ 17 ]  and can thus be used with patients presenting with all severities of dysphonia.


Acoustic measures can be used to provide objective measures of vocal function. Signal processing  algorithms  are applied to voice recordings made during sustained  phonation  or during spontaneous speech. [ 29 ]  The acoustic parameters which can then be examined include  fundamental frequency , signal  amplitude , jitter, shimmer, and noise-to-harmonic ratios. [ 17 ]  However, due to limitations imposed by the algorithms employed, these measures cannot be used with patients who exhibit severe dysphonia. [ 29 ]


Aerodynamic  measures of voice include measures of  air volume , air flow and sub glottal air pressure. The normal aerodynamic parameters of voice vary considerably among individuals, which leads to a large overlapping range of values between dysphonic and non-dysphonic patients. This limits the use of these measures as a diagnostic tool. [ 17 ]  Nonetheless, they are useful when used in adjunct with other voice assessment measures, or as a tool for monitoring therapeutic effects over time. [ 27 ]


Given that certain occupations are more at risk for developing dysphonia (e.g., teachers) research into prevention studies have been conducted. [ 30 ]  Research into the effectiveness of prevention strategies for dysphonia have yet to produce definitive results, however, research is still ongoing. [ 9 ] [ 30 ]  Primarily, there are two types of vocal training recognized by professionals to help with prevention: direct and indirect. Direct prevention describes efforts to reduce conditions that may serve to increase vocal strain (such as patient education, relaxation strategies, etc.), while indirect prevention strategies refer to changes in the underlying physiological mechanism for voice production (e.g., adjustments to the manner in which vocal fold adduction occurs, respiratory training, shifting postural habits, etc.). [ 9 ] [ 30 ]