Because of the human ear's function of regulating the pressure within the head region, catarrh blockage may also cause discomfort during changes in atmospheric pressure.


The word "catarrh" comes from 15th-century French  catarrhe , Latin catarrhus, and Greek  Ancient Greek :  καταρρεῖν [ 5 ]  ( katarrhein ):  kata-  meaning "down" and  rhein  meaning "to flow." The  Oxford English Dictionary  quotes Thomas Bowes' translation of Pierre de la Primaudaye's  The [second part of the] French academic  (1594): "Sodainely choked by catarrhs, which like to floods of waters, runner downwards." [ 6 ]


Causes of  hearing loss  include  ageing ,  genetics ,  perinatal problems ,  loud sounds , and  diseases . For some kinds of hearing loss the cause may be classified as of  unknown cause .


There is a progressive loss of ability to hear high frequencies with ageing known as  presbycusis . For men, this can start as early as 25 and for women at 30. Although genetically variable, it is a normal concomitant of ageing and is distinct from hearing loss caused by noise exposure, toxins, or disease agents. [ 1 ]  Common conditions that can increase the risk of hearing loss in elderly people are high blood pressure, diabetes, or the use of certain medications harmful to the ear. [ 2 ] [ 3 ]  While everyone loses hearing with age, the amount and type of hearing loss is variable. [ 4 ]


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. [ 5 ]  The  National Institute for Occupational Safety and Health  (NIOSH) recognizes that the majority of hearing loss is not due to age, but due to noise exposure. By correcting for age in assessing hearing, one tends to overestimate the hearing loss due to noise for some and underestimate it for others. [ 6 ]


Hearing loss due to noise may be temporary, called a 'temporary threshold shift', a reduced sensitivity to sound over a wide frequency range resulting from exposure to a brief but very loud noise like a gunshot, firecracker, jet engine, jackhammer, etc. or exposure to loud sound over a few hours such as during a pop concert or nightclub session. [ 7 ]  Recovery of hearing is usually within 24 hours but may take up to a week. [ 8 ]  Both constant exposure to loud sounds (85 dB(A) or above) and one-time exposure to extremely loud sounds (120 dB(A) or above) may cause permanent hearing loss. [ 9 ]


Noise-induced hearing loss  (NIHL) typically manifests as elevated hearing thresholds (i.e. less sensitivity or muting) between 3000 and 6000  Hz, centred at 4000  Hz. As noise damage progresses, damage spreads to affect lower and higher frequencies. On an  audiogram , the resulting configuration has a distinctive notch, called a 'noise' notch. As ageing and other effects contribute to higher frequency loss (6–8 kHz on an audiogram), this notch may be obscured and entirely disappear.


Various governmental, industry, and standards organizations set noise standards. [ 10 ]


The  U.S. Environmental Protection Agency  has identified the level of 70 dB(A) (40% louder to twice as loud as normal conversation; typical level of TV, radio, stereo; city street noise) for 24‑hour exposure as the level necessary to protect the public from hearing loss and other disruptive effects from noise, such as sleep disturbance, stress-related problems, learning detriment, etc. [ 11 ]  Noise levels are typically in the 65 to 75 dB (A) range for those living near airports or freeways and may result in hearing damage if sufficient time is spent outdoors. [ 12 ]


Louder sounds cause damage in a shorter period of time. Estimation of a "safe" duration of exposure is possible using an  exchange rate  of 3 dB. As 3 dB represents a doubling of the intensity of sound, the duration of exposure must be cut in half to maintain the same energy dose. For workplace noise regulation, the "safe" daily exposure amount at 85 dB A, known as an  exposure action value , is 8 hours, while the "safe" exposure at 91 dB(A) is only 2 hours. [ 13 ]  Different standards use exposure action values between 80 dBA and 90 dBA. Note that for some people, sound may be damaging at even lower levels than 85 dB A. Exposures to other ototoxicants (such as pesticides, some medications including chemotherapy agents, solvents, etc.) can lead to greater susceptibility to noise damage, as well as causing its own damage. This is called a  synergistic  interaction. Since noise damage is cumulative over long periods, persons who are exposed to non-workplace noise, like recreational activities or environmental noise, may have compounding damage from all sources.


Some national and international organizations and agencies use an exchange rate of 4 dB or 5 dB. [ 14 ]  While these exchange rates may indicate a wider zone of comfort or safety, they can significantly underestimate the damage caused by loud noise. For example, at 100 dB (nightclub music level), a 3 dB exchange rate would limit exposure to 15 minutes; the 5 dB exchange rate allows an hour.


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. If one is exposed to loud sound (including music) at high levels or for extended durations (85 dB A or greater), then hearing loss will occur. Sound intensity (sound energy or propensity to cause damage to the ears) increases dramatically with proximity according to an inverse square law: halving the distance to the sound quadruples the sound intensity.


In the US, 12.5% of children aged 6–19 years have permanent hearing damage from excessive noise exposure. [ 15 ]  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. [ 16 ]


Hearing loss due to noise has been described as primarily a condition of modern society. [ 17 ]  In preindustrial times, humans had far less exposure to loud sounds. Studies of primitive peoples indicate that much of what has been attributed to age-related hearing loss may be long-term cumulative damage from all sources, especially noise. People living in preindustrial societies have considerably less hearing loss than similar populations living in modern society. Among primitive people who have migrated into modern society, hearing loss is proportional to the number of years spent in modern society. [ 18 ] [ 19 ] [ 20 ]  Military service in  World War II , the  Korean War , and the  Vietnam War , has likely also caused hearing loss in large numbers of men from those generations, though proving that hearing loss was a direct result of military service is problematic without entry and exit audiograms. [ 21 ]


Hearing loss in adolescents may be caused by loud noises from toys, music by headphones, and concerts or events. [ 22 ]  In 2017, the  Centers for Disease Control and Prevention  brought their researchers together with experts from the  World Health Organization  and academia to examine the risk of hearing loss from excessive noise exposure in and outside the workplace in different age groups, as well as actions being taken to reduce the burden of the condition. A summary report was published in 2018. [ 23 ]


In the United States, hearing loss may be more likely among members of the  Republican Party  due to greater firearm ownership, according to  The Washington Post  in February 2024. [ 24 ]


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 . [ 25 ]


When looking at the genetics of deafness, there are 2 different forms, syndromic and  nonsyndromic . Syndromic deafness occurs when there are other signs or medical problems aside from deafness in an individual. This accounts for around 30% of deaf individuals who are deaf from a genetic standpoint. [ 25 ]  Nonsyndromic deafness occurs when there are no other signs or medical problems associated with an individual other than deafness. From a genetic standpoint, this accounts for the other 70% of cases and represents the majority of hereditary hearing loss. [ 25 ]  Syndromic cases occur with disorders such as  Usher syndrome ,  Stickler syndrome ,  Waardenburg syndrome ,  Chudley-Mccullough syndrome ,  Alport's syndrome , and  neurofibromatosis type 2 . These are diseases that have deafness as one of the symptoms or as a common feature associated with it. Many of the genetic mutations giving rise to syndromic deafness have been identified. In nonsyndromic cases, where deafness is the only finding, it is more difficult to identify the genetic mutation although some have been discovered.


A 2023 systematic review and meta-analysis found that alcohol consumption is associated with an increased risk of hearing loss. [ 35 ]


Some medications may reversibly 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 . The link between NSAIDs and hearing loss tends to be greater in women, especially those who take ibuprofen six or more times a week. [ 36 ]  Others may cause permanent hearing loss. [ 37 ]  The most important group is the  aminoglycosides  (main member  gentamicin ) and platinum based chemotherapeutics such as  cisplatin  and  carboplatin . [ 38 ] [ 39 ]


In 2007, the  U.S. Food and Drug Administration  (FDA) warned about possible sudden hearing loss from  PDE5 inhibitors , which are used for erectile dysfunction. [ 40 ]


Audiologic monitoring for ototoxicity allows for the early detection of changes to hearing status presumably attributed to a drug/treatment regime so that changes in the drug regimen may be considered, and audiologic intervention when handicapping hearing impairment has occurred. [ 41 ]   Co-administration of anti-oxidants and ototoxic medications may limit the extent of the ototoxic damage. [ 42 ] [ 43 ]


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 , [ 44 ]  and  automobile exhaust , [ 44 ]  for example); and  asphyxiants . [ 45 ]  Combined with noise, these  ototoxic  chemicals have an additive effect on a person's hearing loss. [ 45 ]


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 . [ 45 ]  For some ototoxic chemical exposures, particularly styrene, [ 46 ]  the risk of hearing loss can be higher than being exposed to  noise  alone. The effects are greatest when the combined exposure includes  impulse noise . [ 47 ] [ 48 ]


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. [ 50 ]


Audiologic monitoring for ototoxicity allows for the early detection of changes to hearing status presumably attributed to a drug/treatment regime so that changes in the drug regimen may be considered, and audiologic intervention when handicapping hearing impairment has occurred. [ 41 ]


Co-administration of anti-oxidants and ototoxic medications may limit the extent of the ototoxic damage. [ 42 ] [ 43 ]


There can be damage either to the ear, whether the external or middle ear, to the cochlea, or to the brain centres that process the aural information conveyed by the ears. Damage to the middle ear may include fracture and discontinuity of the ossicular chain. 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. [ 51 ] [ 52 ]


The  cervical branch of the facial nerve  is a nerve in the  neck . It is a branch of the  facial nerve  (VII). It supplies the  platysma muscle , among other functions.


The cervical branch of the facial nerve is a branch of the  facial nerve  (VII). It runs forward beneath the  platysma muscle , and forms a series of arches across the side of the neck over the  suprahyoid  region. One branch descends to join the  cervical cutaneous nerve  from the  cervical plexus .


The lateral part of the cervical branch of the facial nerve supplies the  platysma muscle . [ 1 ] [ 2 ]


The  choanae  ( sg. :  choana ),  posterior nasal apertures  or  internal nostrils  are two openings found at the back of the nasal passage between the  nasal cavity  and the  pharynx , in humans and other mammals (as well as  crocodilians  and most  skinks ). They are considered one of the most important  synapomorphies  of  tetrapodomorphs , that allowed the passage from water to land. [ 1 ]


In animals with secondary palates, they allow breathing when the mouth is closed. [ 2 ]  In tetrapods without secondary palates their function relates primarily to olfaction (sense of smell).


The choanae are separated in two by the  vomer .


A choana is the opening between the  nasal cavity  and the  nasopharynx .


It is therefore not a structure but a space bounded as follows:


The term is a latinization from the Greek χοάνη, "choanē" meaning  funnel .


Early bony fishes (~420 mya) had two pairs of  nostrils , one pair for incoming water (known as the anterior or incurrent nostrils), and a second pair for outgoing water (the posterior or excurrent nostrils), with the olfactory apparatus (for sense of smell) in between. In the first  tetrapodomorphs  (~415 mya) the excurrent nostrils migrated to the edge of the mouth, occupying a position between the maxillary and premaxillary bones, directly below the lateral rostral (a bone that vanished in early tetrapods). [ 1 ]


In all but the most basal (primitive) tetrapodomorphs (or "choanates"), the excurrent nostrils have migrated from the edge of the mouth to the interior of the mouth. In tetrapods that lack a secondary palate (basal tetrapods and amphibians), the choanae are located forward in the roof of the mouth, just inside the upper jaw. These internal nasal passages evolved while the vertebrates still lived in water. [ 3 ]  In animals with complete secondary palates (mammals, crocodilians, most skinks) the space between the primary and secondary palates contain the nasal passages, with the choanae located above the posterior end of the secondary palate.


In animals with partial secondary palates (most birds and reptiles), the median  choanal slit  separates the two halves of the posterior half of the palate, connecting the nasal cavity with the buccal cavity (mouth) and the pharynx (throat). [ 4 ]


Most  fish  do not have choanae, instead they have two pairs of external nostrils: each with two tubes whose frontal openings lie close to the upper jaw, and the posterior openings further behind near the eyes. A 395-million-year-old fossil lobe-finned fish called  Kenichthys   campbelli  has something between a choana and the external nostrils seen on other fish. The posterior opening of the external nostrils has migrated into the mouth. [ 2 ]


In lungfish, the inner nostrils are regarded as an example of  parallel evolution . The fossil lungfish  Diabolepis  shows an intermediate stage between posterior and interior nostril and supports the independent origin of internal nostrils in the lungfish. [ 2 ]


Hagfishes  have a single internal nostril that opens inside the mouth cavity, while  chimaeras  have open canals that leads water from their external nostrils into their mouths and through their gills.


Cholesteatoma  is a destructive and expanding growth consisting of keratinizing  squamous epithelium  in the middle ear and/or  mastoid process . [ 1 ] [ 2 ]  Cholesteatomas are not cancerous as the name may suggest, but can cause significant problems because of their erosive and expansile properties. This can result in the destruction of the bones of the middle ear ( ossicles ), as well as growth through the  base of the skull  into the brain. They often become infected and can result in chronically draining ears. Treatment almost always consists of surgical removal. [ 2 ] [ 3 ]


Other more common conditions (e.g.  otitis externa ) may also present with these symptoms, but cholesteatoma is much more serious and should not be overlooked. If a patient presents to a doctor with ear discharge and hearing loss, the doctor should consider cholesteatoma until the disease is definitely excluded. [ 4 ]  Other less common symptoms (all less than 15%) of cholesteatoma may include pain,  balance disruption ,  tinnitus ,  earache ,  headaches  and bleeding from the ear. [ 2 ]  There can also be  facial nerve  weakness. Balance symptoms in the presence of a cholesteatoma raise the possibility that the cholesteatoma is eroding the balance organs in the  inner ear . [ 1 ]


Doctors' initial inspections may only reveal an ear canal full of discharge. Until the doctor has cleaned the ear and inspected the entire  tympanic membrane , cholesteatoma cannot be diagnosed. [ 2 ]  Once the debris is cleared, cholesteatoma can give rise to a number of appearances. If there is significant inflammation, the tympanic membrane may be partially obscured by an aural  polyp . If there is less inflammation, the cholesteatoma may present the appearance of 'semolina' discharging from a defect in the tympanic membrane. The posterior and superior parts of the tympanic membrane are most commonly affected. If the cholesteatoma has been dry, the cholesteatoma may present the appearance of ' wax  over the attic'. The attic is just above the  eardrum .


If untreated, a cholesteatoma can eat or cause erosion of the three small bones located in the middle ear (the  malleus ,  incus  and  stapes , collectively called  ossicles ). [ 5 ]  This can result in nerve deterioration, imbalance,  vertigo , and  deafness  early in the disease. [ 6 ]  It can also affect and erode, through the  enzymes  it produces, the thin bone structure that isolates the top of the ear from the brain, as well as lay the covering of the brain open to infection with serious complications (rarely even death due to  brain abscess  and  sepsis ).


Both the acquired as well as the congenital types of the disease can affect the  facial nerve  that extends from the brain to the face and passes through the inner and middle ear and leaves at the anterior tip of the  mastoid bone , and then rises to the front of the ear and extends into the upper and lower face.


Cholesteatomas occur in two basic classifications: Acquired cholesteatomas, which are more common, are usually caused by pathological alteration of the  ear drum  leading to accumulation of keratin within the  middle ear . [ 7 ]  Congenital cholesteatomas are usually middle ear epidermal cysts that are identified deep within an intact ear drum often in the superior anterior portion. [ 8 ]


Cholesteatomas do not contain cholesterol or fat and should not be confused with  cholesterol granulomas . [ 8 ]


Keratin-filled cysts that grow medial to the tympanic membrane are considered to be congenital if they fulfill the following criteria (Levenson's criteria): [ 3 ]


Congenital cholesteatomas occur at three important sites: the middle ear, the petrous apex, and the cerebropontinio angle. They are most often found deep to the anterior aspect of the ear drum, and a vestigial structure, the  epidermoid  formation, from which congenital cholesteatoma may originate, has been identified in this area. [ 4 ]


Not all middle ear epidermal cysts are congenital, as they can be acquired either by metaplasia of the middle ear mucosa or by traumatic implantation of ear canal or tympanic membrane skin. In addition, cholesteatoma inadvertently left by a surgeon usually regrows as an epidermal cyst. Some authors have also suggested hereditary factors. [ 9 ] [ 10 ]


More commonly, keratin accumulates in a pouch of tympanic membrane which extends into the middle ear space. This abnormal folding or 'retraction' of the tympanic membrane arises in one of the following ways:


Cholesteatoma may also arise as a result of  metaplasia  of the middle ear mucosa  [ 15 ]  or implantation following trauma.


Cholesteatoma is diagnosed by a medical doctor by physical examination of the ear. A  CT scan  may help to rule out other, often more serious causes for the patient's clinical presentation. Non-ionizing radiation imaging techniques ( MRI ) may be suitable to replace a CT scan, if determined necessary by a physician. [ 16 ] [ 17 ]


Cholesteatoma is a persistent disease. Once the diagnosis of cholesteatoma is made in a patient who can tolerate a general anesthetic, the standard treatment is to surgically remove the growth.


The challenge of cholesteatoma surgery is to permanently remove the cholesteatoma whilst retaining or reconstructing the normal functions of the structures housed within the  temporal bone .


The general objective of cholesteatoma surgery has two parts. It is both directed against the underlying  pathology  and directed towards maintaining the normal functions of the temporal bone. These aims are conflicting and this makes cholesteatoma surgery extremely challenging.


Sometimes, the situation results in a clash of surgical aims. The need to fully remove a progressive disease like cholesteatoma is the surgeon's first priority. Preservation of hearing is secondary to this primary aim. If the disease can be removed easily so that there is no increased risk of residual disease, then the ossicles may be preserved. If the disease is difficult to remove, so that there is an increased risk of residual disease, then removal of involved ossicles in order to fully clear cholesteatoma has generally been regarded as necessary and reasonable.


In other words, the aims of cholesteatoma treatment form a hierarchy. The paramount objective is the complete removal of cholesteatoma. The remaining objectives, such as hearing preservation, are subordinate to the need for complete removal of cholesteatoma. This hierarchy of aims has led to the development of a wide range of strategies for the treatment of cholesteatoma.


The variation in technique in cholesteatoma surgery results from each surgeon's judgment whether to retain or remove certain structures housed within the temporal bone in order to facilitate the removal of cholesteatoma. This typically involves some form of  mastoidectomy  which may or may not involve removing the posterior ear canal wall and the ossicles.


Removal of the canal wall facilitates the complete clearance of cholesteatoma from the temporal bone in three ways:


Thus removal of the canal wall provides one of the most effective strategies for achieving the primary aim of cholesteatoma surgery, the complete removal of cholesteatoma. However, there is a trade-off, since the functional impact of canal wall removal is also important.


The removal of the ear canal wall results in:


The formation of a mastoid cavity by removal of the canal wall is the simplest and most effective procedure for facilitating the removal of cholesteatoma, but may bestow the most lasting infirmity due to loss of ear function upon the patient treated in this way.


The following strategies are employed to mitigate the effects of canal wall removal:


Clearly, preservation and restoration of ear function at the same time as total removal of cholesteatoma requires a high level of surgical expertise.


Traditionally, ear surgery has been performed using the surgical microscope. The direct line of view dictated by that approach necessitates using the mastoid as the access port to the middle ear. It has long been recognized that failure in cholesteatoma surgery occurs in some of the out of view spaces of the tympanic cavity like the  sinus tympani  and facial recess that are out of view using the traditional microscopic technique. [ 23 ]  More recently, the endoscope has been increasingly utilized in the surgical management of cholesteatoma in one of two ways:


There are multiple advantages for the use of the endoscope in cholesteatoma surgery:


It is important that the patient attend periodic follow-up checks, because even after careful microscopic surgical removal, cholesteatomas may recur. Such recurrence may arise many years, or even decades, after treatment.


A 'residual cholesteatoma' may develop if the initial surgery failed to completely remove the original; residual cholesteatomas typically become evident within the first few years after the initial surgery.


A 'recurrent cholesteatoma' is a new cholesteatoma that develops when the underlying causes of the initial cholesteatoma are still present. Such causes can include, for example, poor  Eustachian tube  function, which results in retraction of the ear drum, and failure of the normal outward migration of skin. [ 27 ]


In a retrospective study of 345 patients with middle ear cholesteatoma operated on by the same surgeon, the overall 5-year recurrence rate was 11.8%. [ 28 ]  In a different study with a mean follow-up period of 7.3 years, the recurrence rate was 12.3%, with the recurrence rate being higher in children than in adults. [ 29 ]  The use of the endoscope as an ancillary instrument has been shown to reduce the incidence of residual cholesteatoma. [ 30 ]  Although more studies are needed, so far, new techniques addressing underlying Eustachian tube dysfunction such as transtympanic dilatation of the Eustachian tube has not been shown to change outcomes of chronic ear surgery. [ 31 ]


Recent findings indicate that the keratinizing  squamous epithelium  of the middle ear could be subjected to  human papillomavirus  infection. [ 32 ]  Indeed, DNA belonging to oncogenic HPV16 has been detected in cholesteatoma tissues, thereby underlining that keratinizing squamous epithelia could potentially be a target tissue for HPV infection. [ 32 ]


In one study, the number of new cases of cholesteatoma in  Iowa  was estimated in 1975–76 to be just under one new case per 10,000 citizens per year. [ 33 ]  Cholesteatoma affects all age groups, from infants through to the elderly. The peak incidence occurs in the second decade. [ 33 ]


Submandibular gland


Chorda tympani  is a branch of the  facial nerve  that carries  gustatory  ( taste ) sensory innervation from the front of the tongue and  parasympathetic  ( secretomotor ) innervation to the  submandibular  and  sublingual   salivary glands . [ 1 ]


Chorda tympani has a complex course from the  brainstem , through the  temporal bone  and  middle ear , into the  infratemporal fossa , and ending in the  oral cavity . [ 2 ]


Chorda tympani  fibers emerge from the  pons  of the  brainstem  as part of the  intermediate nerve  of the  facial nerve . The facial nerve exits the  cranial cavity  through the  internal acoustic meatus  and enters the  facial canal . Within the facial canal, chorda tympani branches off the  facial nerve  and enters the lateral wall of the  tympanic cavity  within the  middle ear , where it runs across the  tympanic membrane  (from posterior to anterior) and medial to the  neck of the malleus . [ 3 ]


Chorda tympani then exits the skull by descending through the  petrotympanic fissure  into the  infratemporal fossa . Here it joins the  lingual nerve , a branch of the  mandibular nerve  (CN V 3 ). Traveling with the  lingual nerve , the fibers of chorda tympani enter the  sublingual space  to reach the  anterior 2/3 of the tongue  and  submandibular ganglion . [ 4 ]


The chorda tympani carries two types of nerve fibers from their origin from the facial nerve to the  lingual nerve  that carries them to their destinations:


The chorda tympani is one of three cranial nerves that are involved in  taste . The taste system involves a complicated  feedback  loop, with each nerve acting to inhibit the signals of other nerves.


There are similarities between the tastes the chorda tympani picks up in sweeteners between mice and  primates , but not rats. Relating research results to humans is therefore not always consistent. [ 5 ]
 Sodium chloride  is detected and recognized most by the chorda tympani nerve. [ 5 ]  The recognition and responses to sodium chloride in the chorda tympani is mediated by  amiloride -sensitive sodium channels. [ 6 ]
The chorda tympani has a relatively low response to  quinine  and varied responses to  hydrochloride . The chorda tympani is less responsive to  sucrose  than is the  greater petrosal nerve . [ 7 ]


The chorda tympani nerve carries its information to the  nucleus of solitary tract , and shares this area with the  greater petrosal ,  glossopharyngeal , and  vagus  nerves. [ 8 ]  When the greater petrosal and glossopharyngeal nerves are cut, regardless of age, the chorda tympani nerve takes over the space in the terminal field. This takeover of space by the chorda tympani is believed to be the nerve reverting to its original state before competition and pruning. [ 9 ]
The chorda tympani, as part of the  peripheral nervous system , is not as plastic in early ages. In a study done by Hosley et al. and a study done by Sollars, it has been shown that when the nerve is cut at a young age, the related  taste buds  are not likely to grow back to full strength. [ 10 ] [ 11 ]
In a bilateral transection of the chorda tympani in mice, the preference for sodium chloride increases compared to before the transection. Also avoidance of higher concentrations of sodium chloride is eliminated. [ 5 ]  The amiloride-sensitive channels responsible for salt recognition and response is functional in adult rats but not neonatal rats. This explains part of the change in preference of sodium chloride after a chorda tympani transection. [ 6 ]
The chorda tympani innervates the  fungiform papillae  on the tongue. [ 11 ]  According to a study done by Sollars et al. in 2002, when the chorda tympani has been transected early in postnatal development some of the fungiform papillae undergo a structural change to become more “filiform-like”. [ 12 ]   When some of the other papillae grow back, they do so without a pore. [ 11 ]


Injury to the chorda tympani nerve leads to loss or distortion of taste from anterior 2/3 of  tongue . [ 13 ]   However, taste from the posterior 1/3 of tongue (supplied by the glossopharyngeal nerve) remains intact.


The chorda tympani appears to exert a particularly strong inhibitory influence on other taste nerves, as well as on pain fibers in the tongue. When the chorda tympani is damaged, its inhibitory function is disrupted, leading to less inhibited activity in the other  nerves . [ citation needed ]


A  cleft lip  contains an opening in the  upper lip  that may extend into the nose. [ 1 ]  The opening may be on one side, both sides, or in the middle. [ 1 ]  A  cleft palate  occurs when the  palate  (the roof of the mouth) contains an opening into the  nose . [ 1 ]  The term  orofacial cleft  refers to either condition or to both occurring together. These disorders can result in feeding problems, speech problems, hearing problems, and frequent  ear infections . [ 1 ]  Less than half the time the condition is associated with other disorders. [ 1 ]


Cleft lip and palate are the result of tissues of the face not joining properly during  development . [ 1 ]  As such, they are a type of  birth defect . [ 1 ]  The cause is unknown in most cases. [ 1 ]   Risk factors  include  smoking during pregnancy ,  diabetes ,  obesity ,  an older mother , and certain  medications  (such as some used to treat  seizures ). [ 1 ] [ 2 ]  Cleft lip and cleft palate can often be diagnosed during pregnancy with an  ultrasound exam . [ 1 ]


A cleft lip or palate can be successfully treated with surgery. [ 1 ]  This is often done in the first few months of life for cleft lip and before eighteen months for cleft palate. [ 1 ]   Speech therapy  and dental care may also be needed. [ 1 ]  With appropriate treatment, outcomes are good. [ 1 ]


Cleft lip and palate occurs in about 1 to 2 per 1000 births in the  developed world . [ 2 ]  Cleft lip is about twice as common in males as females, while cleft palate without cleft lip is more common in females. [ 2 ]  In 2017, it resulted in about 3,800 deaths globally, down from 14,600 deaths in 1990. [ 3 ] [ 4 ]  Cleft lips are commonly known as  hare-lips  because of their resemblance to the lips of  hares  or rabbits. [ 5 ]


If the cleft does not affect the palate structure of the mouth, it is referred to as cleft lip.  Cleft lip is formed in the top of the lip as either a small gap or an indentation in the lip (partial or incomplete cleft), or it continues into the nose (complete cleft). Lip cleft can occur as a one-sided (unilateral) or two-sided (bilateral) condition. It is due to the failure of fusion of the  maxillary prominence  and  medial nasal processes  (formation of the primary palate).


A mild form of a cleft lip is a microform cleft. [ 6 ]  A microform cleft can appear as small as a little dent in the red part of the lip or look like a scar from the lip up to the nostril. [ 7 ]  In some cases  muscle tissue in the lip  underneath the scar is affected and might require reconstructive surgery. [ 8 ]  It is advised to have newborn infants with a microform cleft checked with a  craniofacial team  as soon as possible to determine the severity of the cleft. [ 9 ]


Cleft palate is a condition in which the two plates of the  skull  that form the  hard palate  (roof of the mouth) are not completely joined. The  soft palate  is in these cases cleft as well. In most cases, cleft lip is also present.


Palate cleft can occur as complete (soft and hard palate, possibly including a gap in the jaw) or incomplete (a 'hole' in the roof of the mouth, usually as a cleft soft palate). When cleft palate occurs, the  uvula  is usually split. It occurs due to the failure of fusion of the lateral palatine processes, the nasal septum, or the median palatine processes (formation of the  secondary palate ).


The hole in the roof of the mouth caused by a cleft connects the mouth directly to the  inside of the nose .


Note: the next images show the roof of the mouth. The top shows the nose, the lips are colored pink. For clarity the images depict a toothless infant.


A result of an open connection between the  mouth  and inside the nose is called  velopharyngeal insufficiency  (VPI). Because of the gap, air leaks into the nasal cavity resulting in a  hypernasal  voice  resonance  and nasal emissions while talking. [ 10 ]  Secondary effects of VPI include speech  articulation  errors (e.g.,  distortions , substitutions, and omissions) and compensatory misarticulations and mispronunciations (e.g.,  glottal stops  and posterior nasal  fricatives ). [ 11 ]  Possible treatment options include  speech therapy , prosthetics, augmentation of the posterior pharyngeal wall, lengthening of the palate, and  surgical procedures . [ 10 ]


Submucous cleft palate can also occur, which is a cleft of the soft palate with a split  uvula , a furrow along the midline of the soft palate, and a notch in the back margin of the hard palate. [ 12 ]  The diagnosis of submucous cleft palate often occurs late in children as a result of the nature of the cleft. [ 13 ]  While the muscles of the soft palate are not joined, the mucosal membranes covering the roof of the mouth appear relatively normal and intact. [ 14 ]


Tooth development can be delayed with increasing severity of CLP. Some of the dental problems affect the  primary teeth , but most of the problems arise after the permanent teeth erupt. Problems may include fused teeth, missing teeth, and extra teeth erupting behind normal teeth. Missing teeth or extra teeth are both normal occurrences. Typically, the lateral incisors are missing. The  enamel  (outermost layer of the tooth) is commonly found to be hypomineralized and hypoplastic, making the teeth more likely to decay. As CLP can make oral hygiene more difficult, there is an increased rate of cavities. [ 15 ]  In addition, abnormal positioning of individual teeth may affect occlusion, which can create an open bite or cross bite. This in turn can then affect the patient's speech. [ 16 ] [ 17 ]


Cleft may cause problems with feeding, ear disease, speech, socialization, and cognition.


Due to lack of suction, an infant with a cleft may have trouble feeding. An infant with a cleft palate will have greater success feeding in a more upright position. Gravity will help prevent milk from coming through the baby's nose if he/she has cleft palate. Gravity feeding can be accomplished by using specialized equipment, such as the  Haberman Feeder . Another equipment commonly used for gravity feeding is a customized bottle with a combination of nipples and bottle inserts. A large hole, crosscut, or slit in the nipple, a protruding nipple and rhythmically squeezing the bottle insert can result in controllable flow to the infant without the stigma caused by specialized equipment.


Individuals with cleft also face many middle ear infections which may eventually lead to hearing loss. The  Eustachian tubes  and external ear canals may be angled or tortuous, leading to food or other contamination of a part of the body that is normally self-cleaning. Hearing is related to learning to speak.  Babies with palatal clefts may have compromised hearing and therefore, if the baby cannot hear, it cannot try to mimic the sounds of speech. Thus, even before expressive language acquisition, the baby with the cleft palate is at risk for receptive language acquisition. Because the lips and palate are both used in pronunciation, individuals with cleft usually need the aid of a speech therapist.


Tentative evidence has found that those with clefts perform less well at language. [ 18 ]


There is research dedicated to the  psychosocial development  of individuals with cleft palate. A cleft palate/lip may impact an individual's  self-esteem ,  social skills  and  behavior . Self-concept may be adversely affected by the presence of a cleft lip or cleft palate, particularly among girls. [ 19 ]  Negative outcomes can also be associated with the long durations of hospitalization. Psychological issues could extend not just to the individual with CLP but also to their families, particularly their mothers, that experience varying levels of depression and anxiety. [ 20 ] [ 21 ]


Research has shown that during the early preschool years (ages 3–5), children with cleft lip or cleft palate tend to have a self-concept that is similar to their peers without a cleft. However, as they grow older and their social interactions increase, children with clefts tend to report more dissatisfaction with peer relationships and higher levels of  social anxiety . Experts conclude that this is probably due to the associated stigma of visible deformities and possible  speech impediments . Children who are judged as attractive tend to be perceived as more intelligent, exhibit more positive social behaviors, and are treated more positively than children with cleft lip or cleft palate. [ 22 ]  Children with clefts tend to report feelings of anger, sadness, fear, and alienation from their peers, but these children were similar to their peers in regard to "how well they liked themselves."


The relationship between parental attitudes and a child's self-concept is crucial during the preschool years. It has been reported that elevated stress levels in mothers correlated with reduced social skills in their children. [ 23 ]  Strong parent support networks may help to prevent the development of negative self-concept in children with cleft palate. [ 24 ]  In the later preschool and early elementary years, the development of social skills is no longer only impacted by parental attitudes but is beginning to be shaped by their peers. A cleft lip or cleft palate may affect the behavior of preschoolers. Experts suggest that parents discuss with their children ways to handle negative social situations related to their cleft lip or cleft palate. A child who is entering school should learn the proper (and age-appropriate) terms related to the cleft. The ability to confidently explain the condition to others may limit feelings of awkwardness and embarrassment and reduce negative social experiences. [ 25 ]


As children reach adolescence, the period of time between age 13 and 19, the dynamics of the parent-child relationship change as peer groups are now the focus of attention. An adolescent with cleft lip or cleft palate will deal with the typical challenges faced by most of their peers including issues related to self-esteem, dating and social acceptance. [ 26 ] [ 27 ] [ 28 ]  Adolescents, however, view appearance as the most important characteristic, above intelligence and humor. [ 29 ]  This being the case, adolescents are susceptible to additional problems because they cannot hide their facial differences from their peers. Adolescent boys typically deal with issues relating to withdrawal, attention, thought, and  internalizing  problems, and may possibly develop anxiousness-depression and aggressive behaviors. [ 28 ]  Adolescent girls are more likely to develop problems relating to self-concept and appearance. Individuals with cleft lip or cleft palate often deal with threats to their  quality of life  for multiple reasons including unsuccessful social relationships, deviance in social appearance, and multiple surgeries.


Most clefts are polygenic and multifactorial in origin with many genetic and environmental factors contributing. Genetic factors cause clefts in 20% to 50% of the cases and the remaining clefts are attributable to either environmental factors (such as  teratogens ) or gene-environment interactions. The polygenic/multifactorial inheritance model predicts that most individuals will be born without clefts; however with a number of genetic or environmental factors, it can result in cleft formation. [ 17 ]


The development of the face is coordinated by complex  morphogenetic events  and rapid proliferative expansion, and is thus highly susceptible to environmental and genetic factors, rationalising the high incidence of facial malformations. During the first six to eight weeks of pregnancy, the shape of the embryo's head is formed.  Five primitive tissue lobes grow:


If these tissues fail to meet, a gap appears where the tissues should have joined (fused). This may happen in any single joining site, or simultaneously in several or all of them. The resulting birth defect reflects the locations and severity of individual fusion failures (e.g., from a small lip or palate fissure up to a completely malformed face).


The upper lip is formed earlier than the palate, from the first three lobes named a to c above. Formation of the palate is the last step in joining the five embryonic facial lobes, and involves the back portions of the lobes b and c. These back portions are called palatal shelves, which grow towards each other until they fuse in the middle. [ 30 ]  This process is very vulnerable to multiple toxic substances, environmental pollutants, and nutritional imbalance. The biologic mechanisms of mutual recognition of the two cabinets, and the way they are glued together, are quite complex and obscure despite intensive scientific research. [ 30 ]


Orofacial clefts may be associated with a syndrome (syndromic) or may not be associated with a syndrome (nonsyndromic). Syndromic clefts are part of syndromes that are caused by a variety of factors such as environment and genetics or an unknown cause. Nonsyndromic clefts, which are not as common as syndromic clefts, also have a genetic cause. [ 31 ]


Genetic factors contributing to cleft lip and cleft palate formation have been identified for some syndromic cases. Many clefts run in families, even though in some cases there does not seem to be an identifiable syndrome present. [ 32 ]  A number of genes are involved including  cleft lip and palate transmembrane protein 1  and  GAD1 , [ 33 ]  One study found an association between mutations in the  HYAL2  gene and cleft lip and cleft palate formation. [ 34 ]


In some cases, cleft palate is caused by syndromes that also cause other problems:


Many genes associated with syndromic cases of cleft lip/palate (see above) have been identified to contribute to the incidence of isolated cases of cleft lip/palate. This includes in particular sequence variants in the genes  IRF6 ,  PVRL1  and  MSX1 . [ 45 ]  The understanding of the genetic complexities involved in the  morphogenesis  of the midface, including molecular and cellular processes, has been greatly aided by research on animal models, including of the genes  BMP4 ,  SHH ,  SHOX2 ,  FGF10  and  MSX1 . [ 45 ]


Environmental influences may also cause, or interact with genetics to produce, orofacial clefts. An example of the link between environmental factors and genetics comes from a research on mutations in the gene  PHF8 . The research found that  PHF8  encodes for a  histone  lysine demethylase, [ 46 ]  and is involved in  epigenetic regulation . The catalytic activity of  PHF8  depends on molecular  oxygen , [ 46 ]  a factor considered important from reports on increased incidence of cleft lip/palate in mice that have been exposed to  hypoxia  early during pregnancy. [ 47 ]


Cleft lip and other  congenital abnormalities  have also been linked to maternal hypoxia caused by  maternal smoking , [ 48 ]  with the estimated attributable fraction of orofacial clefts due to smoking in early pregnancy being 6.1%. Orofacial clefts occur very early in pregnancy and so smoking cessation right after recognition of pregnancy is unlikely to reduce the exposure during the critical time period. [ 49 ]


Maternal  alcohol use  has also been linked to cleft lip and palate due to the effects on the cranial  neural crest  cells. The degree of the effect, however, is unknown and requires further research. [ 50 ]  Some forms of maternal  hypertension  treatment have been linked to cleft lip and palate. [ 51 ]  Other environmental factors that have been studied include seasonal causes (such as pesticide exposure); maternal diet and vitamin intake;  retinoids  (members of the vitamin A family);  anticonvulsant  drugs; nitrate compounds; organic solvents; parental exposure to lead; alcohol; cigarette use; and a number of other psychoactive drugs (e.g.  cocaine ,  crack cocaine ,  heroin ).


Current research continues to investigate the extent to which  folic acid  can reduce the incidence of clefting. [ 52 ]  Folic acid alone or in combination with vitamins and minerals prevents neural tube defects but does not have a clear effect on cleft lip palate incidence. [ 53 ]  The mechanism behind beneficial folate supplementation is due to folate playing a pivotal role in DNA synthesis and methylation and contributes to both development and gene expression. [ 54 ]


Traditionally, the diagnosis is made at the time of birth by physical examination. Recent advances in  prenatal diagnosis  have allowed  obstetricians  to diagnose facial clefts  in utero  with  ultrasonography . [ 55 ]


Clefts can also affect other parts of the face, such as the eyes, ears, nose, cheeks, and forehead. In 1976,  Paul Tessier  described fifteen lines of cleft.  Most of these  craniofacial clefts  are even rarer and are frequently described as Tessier clefts using the numerical locator devised by Tessier. [ 56 ]


Cleft lip and cleft palate is an "umbrella term" for a collection of orofacial clefts. It includes clefting of the upper lip, the  maxillary alveolus  (dental arch), and the  hard  or  soft palate , in various combinations. Proposed anatomic combinations include: [ 57 ]


Cleft lip with or without palate is classified as the most common congenital birth defect. It has been noted that the prevalence of orofacial clefts varies by race. The highest number of cases have been recorded among Asians and Native Americans, followed by Europeans, Hispanics and African-Americans. The critical period for cleft development ranges from the 4th to the 12th week of intrauterine life. Clefts of the primary palate develop between the 4th and 7th weeks of intrauterine life, while clefts of the secondary palate develop between the 8th and 12th embryonic weeks.


Accurate evaluation of craniofacial malformations is usually possible with the ultrasound scan performed during pregnancy. This is however not a routine procedure according to the American Institute of Ultrasound in Medicine. The accuracy of ultrasonography for prenatal diagnosis of cleft lip +/- palate is dependent on the experience of the sonologist, maternal body type, foetal position, the amount of amniotic fluid and the type of cleft.


Prenatal diagnosis enables appropriate and timely education and discussion with parents by the cleft team. This helps improve the quality of treatment received by the child and improves quality of life.


An accurate prenatal diagnosis of the CLP anomaly is critical for establishing long-term treatment planning, prediction of treatment outcome, and discussion and education of the parent. Although there is no intrauterine treatment for CLP, both mother and child benefit from early diagnosis and education. A multidisciplinary team approach is now accepted as the standard of care in dealing with CLP patients.


The time period immediately after the diagnosis and the first year after the birth is most challenging for parents. A systematically planned treatment plan and support system will help assist parents. The ultimate aim is to help educate parents and create awareness so as to improve care provided for the child. [ 58 ]


Cleft lip and palate is very treatable; however, the kind of treatment depends on the type and severity of the cleft.


Most children with a form of clefting are monitored by a  cleft palate team  or  craniofacial team  through young adulthood. [ 59 ]  Care can be lifelong and are looked after by craniofacial cleft teams often consist of: cleft surgeons,  orthodontists , speech and language therapists,  restorative dentists ,  psychologists , ENT surgeons and audio-logical physicians. [ 60 ] : 255   Treatment procedures can vary between craniofacial teams. For example, some teams wait on jaw correction until the child is aged 10 to 12 (argument: growth is less influential as  deciduous teeth  are replaced by  permanent teeth , thus saving the child from repeated corrective surgeries), while other teams correct the jaw earlier (argument: less speech therapy is needed than at a later age when speech therapy becomes harder). Within teams, treatment can differ between individual cases depending on the type and severity of the cleft.


Within the first 2–3 months after birth, surgery is performed to close the cleft lip. While surgery to repair a cleft lip can be performed soon after birth, often the preferred age is at approximately 10 weeks of age, following the " rule of 10s " coined by surgeons Wilhelmmesen and Musgrave in 1969 (the child is at least 10 weeks of age; weighs at least 10 pounds, and has at least 10g/dL hemoglobin). [ 61 ] [ 62 ]  If the cleft is bilateral and extensive, two surgeries may be required to close the cleft, one side first, and the second side a few weeks later. The most common procedure to repair a cleft lip is the  Millard procedure  pioneered by  Ralph Millard . Millard performed the first procedure at a  Mobile Army Surgical Hospital  (MASH) unit in Korea. [ 63 ]


Often an incomplete cleft lip requires the same surgery as complete cleft. This is done for two reasons. Firstly the group of  muscles  required to purse the lips run through the upper lip. To restore the complete group a full incision must be made. Secondly, to create a less obvious scar the surgeon tries to line up the scar with the natural lines in the upper lip (such as the edges of the  philtrum ) and tuck away stitches as far up the nose as possible. Incomplete cleft gives the surgeon more tissue to work with, creating a more supple and natural-looking upper lip.


In some cases of a severe bilateral complete cleft, the premaxillary segment will be protruded far outside the mouth.


Nasoalveolar molding  prior to surgery can improve long-term nasal symmetry where there is complete unilateral cleft lip–cleft palate, compared to correction by surgery alone, according to a retrospective  cohort study . [ 64 ]  In this study, significant improvements in nasal symmetry were observed in multiple areas including measurements of the projected length of the nasal ala (lateral surface of the external nose), position of the superoinferior alar groove, position of the mediolateral nasal dome, and nasal bridge deviation. "The nasal ala projection length demonstrated an average ratio of 93.0 percent in the surgery-alone group and 96.5 percent in the nasoalveolar molding group," this study concluded. A  systematic review  found in conclusion that nasoalveolar molding had a positive effect on the primary surgery of cleft lip/or palate treatment and aesthetics. [ 65 ]


Often a cleft palate is temporarily covered by a  palatal obturator  (a prosthetic device made to fit the roof of the mouth covering the gap). This device re-positions displaced alveolar segments and helps reduce the cleft lip separation. The obturator will improve speech as there's now proper airflow and improve feeding and breathing as the gap in the hard and soft palate is closed over so cannot affect it. [ 60 ] : 257


Cleft palate can also be corrected by  surgery , usually performed between 6 and 12 months. Approximately 20–25% only require one palatal surgery to achieve a competent velopharyngeal valve capable of producing normal, non- hypernasal speech . However, combinations of surgical methods and repeated surgeries are often necessary as the child grows. One of the new innovations of cleft lip and cleft palate repair is the  Latham appliance . [ 66 ]  The Latham is surgically inserted by use of pins during the child's fourth or fifth month. After it is in place, the doctor, or parents, turn a screw daily to bring the cleft together to assist with future lip or palate repair.


If the cleft extends into the maxillary alveolar ridge, the gap is usually corrected by filling the gap with bone tissue. The bone tissue can be acquired from the individual's own chin, rib or hip.


At age 1–7 years the child is regularly reviewed by the cleft team. [ 60 ] : 257


Age 7–12 years, for the children born with alveolar clefts, they may need to have a secondary alveolar bone graft. This is where autogenous  cancellous bone  from a donor site (often the pelvic bone) is transplanted into the alveolar cleft region. This transplant of bone will close the osseous cleft of the alveolus, close any oro-nasal  fistulae  and will become integrated with the  maxillary bone . It provides bone for teeth to erupt into and to allow implants to be placed as a possible future treatment option. The procedure should be carried out before the upper canine has erupted. Ideally the root of the canine should be one to two-thirds formed and that there is a space available to place the bone graft.  Radio-graphs  are taken to determine the quantity of missing bone in the cleft area. [ 60 ] : 258


Orthognathic surgery  – surgical cutting of bone to realign the upper jaw ( osteotomy ). The bone is cut then re-positioned and held together by wires or rigid fixation plates to ensure there's no anterior-posterior discrepancy, also to reduce scarring as it reduces growth. Single piece or multi-piece osteotomy exist. Single piece osteotomy is carried out where there is sufficient alveolar continuity achieved from a successful bone graft. Multi piece osteotomy is performed when there is a notable residual alveolar defect with a dental gap and oronasal fistula (communication between the oral and nasal cavities). The goal of both single and multi piece osteotomy is to displace the maxilla forward to obtain adequate occlusion as well to provide better support for upper lip and the nose and to close any  fistulae . [ 67 ]


Distraction osteogenesis  – bone lengthening by gradual distraction. This involves cutting bone and moving ends apart incrementally to allow new bone to form in the gap. This consists of several phases. After attachment of the distracting device and the bone cuts, there is a latency phase of 3–7 days when a  callus  forms. In the activation phase distraction of the callus induces bony ingrowth which can last up to 15 days depending on the required distraction. Once the required bone length is reached, the distraction device is left to remain in situ as it acts as a rigid skeletal fixation device until the new bone has matured (known as the consolidation period). [ 67 ]


Velopharyngeal insufficiency  (VPI) can occur as a result of an unrepaired or repaired cleft lip and palate. VPI is the inability of the soft palate to close tightly against the back of the throat during speech, resulting in incomplete velopharyngeal closure. In turn, this results in speech abnormalities. Velopharyngeal closure is necessary during speech because it forms a seal between the nose and mouth, allowing the production of normal speech sounds. VPI can cause  hypernasality  (excessive nasal resonance),  hyponasality  (reduced nasal resonance), or a mixed nasal resonance, which is when hypernasality and hyponasality occur simultaneously. [ 68 ]  In addition, CLP may cause abnormal positioning of individual teeth, which can in turn affect the patient's ability to make certain sounds when speaking such as the "f" or "v" sound and can also result in a lisp. The changes in speech may also be a manifestation on CLP's effects on the patient's occlusion. [ 16 ] [ 17 ]


Children with cleft palate have a very high risk of developing a middle ear infection, specifically  otitis media . This is due to the immature development of the different bones and muscles in the ear. Otitis media is caused by the obstruction of the  Eustachian tube , negative middle ear pressure and fluid build-up in the normally air-filled space of the middle ear. [ 69 ]  This is associated with hearing impairment or loss. The insertion of a ventilation tube into the eardrum is a surgical treatment option commonly used to improve hearing in children with otitis media. [ 70 ]  In addition, breast milk has been proven to decrease the incidence of otitis media in infants with clefts. [ 16 ]


There are different options on how to feed a baby with cleft lip or cleft palate which include:  breast-feeding , bottle feeding, spoon feeding and syringe feeding. Although breast-feeding is challenging, it improves weight-gain compared to spoon-feeding. [ 71 ]  Nasal regurgitation is common due to the open space between the oral cavity and the nasal cavity. Bottle feeding can help (with squeezable bottles being easier to use than rigid bottles). In addition, maxillary plates can be added to aid in feeding. Whatever feeding method is established, it is important to keep the baby's weight gain and hydration monitored. Infants with cleft lip or palate may require supplemental feeds for adequate growth and nutrition. Breast feeding position as suggested by specialists can also improve success rate. [ 72 ]


Babies with cleft lip are more likely to breastfeed successfully than those with cleft palate and cleft lip and palate. Larger clefts of the soft or hard palate may not be able to generate suction as the oral cavity cannot be separated from the nasal cavity when feeding which leads to fatigue, prolonged feeding time, impaired growth and nutrition. Changes in swallowing mechanics may result in coughing, choking, gagging and nasal regurgitation. Even after cleft repair, the problem may still persist as significant motor learning of swallowing and sucking was absent for many months before repair. [ 73 ]  These difficulties in feeding may result in secondary problems such as poor weight gain, excessive energy expenditure during feeding, lengthy feeding times, discomfort during feeding, and stressful feeding interactions between the infant and the mother. A potential source of discomfort for the baby during or after feeding is bloating or frequent "spit up" which is due to the excessive air intake through the nose and mouth in the open cleft. [ 16 ]  Babies with cleft lip and or palate should be evaluated individually taking into account the size and location of the cleft and the mother's previous experience with breastfeeding. [ 71 ]


Another option is feeding breast milk via bottle or syringe. Since babies with clip lip and cleft palate generate less section when breastfeeding, their nutrition, hydration and weight gain may be affected. This may result in the need for supplemental feeds. Modifying the position of holding the baby may increase the effectiveness and efficiency of breastfeeding.


Preoperative feeding – using a squeezable bottle instead of a rigid bottle can allow a higher volume of food intake and less effort to extract food. Using a syringe is practical, easy to perform and allows greater administered volume of food. It also means there will be weight gain and less time spent feeding. [ 74 ]


Post-operative feeding (isolated lip repair, or lip repair associated or not with  palatoplasty ) – post  palatoplasty , some studies believe that inappropriate negative pressure on the suture line may affect results. Babies can be fed by a  nasogastric tube  instead. Studies suggest babies required less analgesics and shorter hospital stay with nasogastric feeding post-surgery. With bottle-feeding, there was higher feeding rejection and pain and required more frequent and prolonged feeding times. [ 74 ]


Each person's treatment schedule is individualized. The table below shows a common sample treatment schedule. The colored squares indicate the average timeframe in which the indicated procedure occurs. In some cases, this is usually one procedure, for example lip repair. In other cases, it is an ongoing therapy, for example speech therapy. In most cases of cleft lip and palate that involve the alveolar bone, patients will need a treatment plan including the prevention of cavities, orthodontics, alveolar bone grafting, and possibly jaw surgery. [ 75 ]


People with CLP present with a multiplicity of problems. Therefore, effective management of CLP involves a wide range of specialists. The current model for delivery of this care is the multidisciplinary cleft team. This is a group of individuals from different specialist backgrounds who work closely together to provide patients with comprehensive care from birth through adolescence. This system of delivery of care enables the individuals within the team to function in an interdisciplinary way, so that all aspects of care for CLP patients can be provided in the best way possible. [ 76 ]


Measuring the outcomes of CLP treatment has been laden with difficulty due to the complexity and longitudinal nature of cleft care, which spans birth through young adulthood. Prior attempts to study the effectiveness of specific interventions or overall treatment protocols have been hindered by a lack of data standards for outcomes assessment in cleft care. [ citation needed ]


The International Consortium for Health Outcome Measurement (ICHOM) has proposed the Standard Set of Outcome Measures for Cleft Lip and Palate. [ 77 ] [ 78 ]  The ICHOM Standard Set includes measures for many of the important outcome domains in cleft care (hearing, breathing, eating/drinking, speech, oral health, appearance and psychosocial well-being). It includes clinician-reported, patient-reported, and family-reported outcome measures.


Cleft lip and palate occurs in about 1 to 2 per 1000 births in the  developed world . [ 2 ]


Rates for cleft lip with or without cleft palate and cleft palate alone varies within different  ethnic  groups.


According to CDC, the prevalence of cleft palate in the United States is 6.35/10000 births and the prevalence of cleft lip with or without cleft palate is 10.63/10000 births. [ 79 ]  The highest prevalence  rates  for cleft lip, either with or without cleft palate are reported for Native Americans and  Asians .  Africans  have the lowest prevalence rates. [ 80 ]


Cleft lip and cleft palate caused about 3,800 deaths globally in 2017, down from 14,600 deaths in 1990. [ 4 ]


Prevalence of "cleft  uvula " has varied from 0.02% to 18.8% with the highest numbers found among  Chippewa  and  Navajo  and the lowest generally in Africans. [ 81 ] [ 82 ]


In some countries, cleft lip or palate deformities are considered reasons (either generally tolerated or officially sanctioned) to perform an abortion beyond the legal  fetal  age limit, even though the  fetus  is not in jeopardy of life or limb. [ 83 ] [ 84 ]  Some political opponents contend this practice amounts to  eugenics  based on cosmetic defects rather than practical definitions of a  disability . [ 85 ]


The eponymous hero of  J. M. Coetzee 's 1983 novel  Life & Times of Michael K  has a cleft lip that is never corrected. In the 1920 novel  Growth of the Soil ,  by Norwegian writer  Knut Hamsun , Inger (wife of the main character) has an uncorrected cleft lip which puts heavy limitations on her life, even causing her to kill her own child, who is also born with a cleft lip. The protagonist of the 1924 novel  Precious Bane , by English writer  Mary Webb , is a young woman living in 19th-century rural  Shropshire  who eventually comes to feel that her deformity is the source of her spiritual strength. The book was later adapted for television by both the  BBC  and  ORTF  in France. Similarly, the main character in  Graham Greene 's 1936 crime noir novel  A Gun for Sale , Raven, has a cleft lip which he is sensitive about, and is described as "an ugly man dedicated to ugly deeds". In the 1976  Patricia A. McKillip  novel  The Night Gift , one of the high-school aged protagonists is shy because she has a cleft lip, but learns to have more confidence in herself.


In the first edition of  Harry Potter and the Chamber of Secrets , one of the people Gilderoy Lockhart stole credit from was a witch with a harelip who banished the Bandon Banshee. In later editions, this was changed to a witch with a hairy chin. [ 86 ]


In chapter 26 of  Mark Twain 's  The Adventures of Huckleberry Finn , Huck Finn meets the three Wilks sisters, Mary Jane, Susan, and Joanna. Joanna is described as, "the one who gives herself to good works and has a hare-lip." As a form of offensive  synecdoche , Huck Finn refers to Joanna as "the hare-lip" rather than by her name.


Cleft lip and cleft palate are often portrayed negatively in popular culture. Examples include  Oddjob , the secondary villain of the  James Bond  novel  Goldfinger  by  Ian Fleming  ( the film adaptation  does not mention this but leaves it implied) and serial killer  Francis Dolarhyde  in the novel  Red Dragon  and its screen adaptations,  Manhunter ,  Red Dragon , and  Hannibal . [ 87 ]  The portrayal of enemy characters with cleft lips and cleft palates, dubbed mutants, in the 2019 video game  Rage 2  left Chris Plante of  Polygon  wondering if the condition would ever be portrayed positively. [ 88 ] [ 89 ]


Cleft lips and palates are occasionally seen in cattle and dogs, and rarely in goats, sheep, cats, horses,  pandas  and  ferrets . Most commonly, the defect involves the lip,  rhinarium , and  premaxilla . Clefts of the hard and soft palate are sometimes seen with a cleft lip. The cause is usually hereditary.   Brachycephalic  dogs such as  Boxers  and  Boston Terriers  are most commonly affected. [ 107 ]  An inherited disorder with incomplete  penetrance  has also been suggested in  Shih tzus ,  Swiss Sheepdogs ,  Bulldogs , and  Pointers . [ 108 ]  In horses, it is a rare condition usually involving the caudal soft palate. [ 109 ]  In  Charolais cattle , clefts are seen in combination with  arthrogryposis , which is inherited as an  autosomal recessive  trait.  It is also inherited as an autosomal recessive trait in  Texel sheep .  Other contributing factors may include maternal nutritional deficiencies, exposure  in utero  to viral infections, trauma, drugs, or chemicals, or ingestion of toxins by the mother, such as certain  lupines  by cattle during the second or third month of  gestation . [ 110 ]  The use of  corticosteroids  during pregnancy in dogs and the ingestion of  Veratrum californicum  by pregnant sheep have also been associated with cleft formation. [ 111 ]


Difficulty with nursing is the most common problem associated with clefts, but  aspiration pneumonia ,  regurgitation , and  malnutrition  are often seen with cleft palate and is a common cause of death.  Providing nutrition through a  feeding tube  is often necessary, but corrective surgery in dogs can be done by the age of twelve weeks. [ 107 ]  For cleft palate, there is a high rate of surgical failure resulting in repeated surgeries. [ 112 ]  Surgical techniques for cleft palate in dogs include  prosthesis , mucosal flaps, and microvascular  free flaps . [ 113 ]  Affected animals should [ opinion ]  not be bred due to the hereditary nature of this condition. [ citation needed ]


The  cochlea  is the part of the  inner ear  involved in  hearing . It is a spiral-shaped cavity in the  bony labyrinth , in humans making 2.75 turns around its axis, the  modiolus . [ 2 ] [ 3 ]  A core component of the cochlea is the  organ of Corti , the sensory organ of hearing, which is distributed along the partition separating the fluid chambers in the coiled tapered tube of the cochlea.


The name 'cochlea' is derived from the Latin word for  snail shell , which in turn is from the  Ancient Greek  κοχλίας  kokhlias  ("snail, screw"), and from κόχλος  kokhlos  ("spiral shell") [ 4 ]  in reference to its coiled shape; the cochlea is coiled in mammals with the exception of  monotremes .


The cochlea ( pl. : cochleae) is a spiraled, hollow, conical chamber of bone, in which waves propagate from the base (near the middle ear and the  oval window ) to the apex (the top or center of the spiral). The spiral canal of the cochlea is a section of the bony labyrinth of the inner ear that is approximately 30 mm long and makes 2 3 ⁄ 4  turns about the modiolus. The cochlear structures include:


The cochlea is a portion of the inner ear that looks like a snail shell ( cochlea  is Greek for snail). [ 5 ]  The cochlea receives sound in the form of vibrations, which cause the stereocilia to move. The stereocilia then convert these vibrations into nerve impulses which are taken up to the brain to be interpreted. Two of the three fluid sections are canals and the third is the 'organ of Corti' which detects pressure impulses that travel along the auditory nerve to the brain. The two canals are called the vestibular canal and the tympanic canal.


The walls of the hollow cochlea are made of bone, with a thin, delicate lining of  epithelial tissue . This coiled tube is divided through most of its length by an inner membranous partition. Two fluid-filled outer spaces (ducts or  scalae ) are formed by this dividing membrane. At the top of the snailshell-like coiling tubes, there is a reversal of the direction of the fluid, thus changing the vestibular duct to the tympanic duct. This area is called the helicotrema. This continuation at the helicotrema allows fluid being pushed into the vestibular duct by the oval window to move back out via movement in the tympanic duct and deflection of the round window; since the fluid is nearly incompressible and the bony walls are rigid, it is essential for the conserved fluid volume to exit somewhere.


The lengthwise partition that divides most of the cochlea is itself a fluid-filled tube, the third 'duct'. This central column is called the cochlear duct. Its fluid, endolymph, also contains electrolytes and proteins, but is chemically quite different from perilymph. Whereas the perilymph is rich in sodium ions, the endolymph is rich in potassium ions, which produces an  ionic , electrical potential.


The hair cells are arranged in four rows in the organ of Corti along the entire length of the cochlear coil. Three rows consist of outer hair cells (OHCs) and one row consists of inner hair cells (IHCs). The inner hair cells provide the main neural output of the cochlea. The outer hair cells, instead, mainly 'receive' neural input from the brain, which influences their  motility  as part of the cochlea's mechanical "pre-amplifier". The input to the OHC is from the  olivary body  via the medial olivocochlear bundle.


The cochlear duct is almost as complex on its own as the ear itself.  The cochlear duct is bounded on three sides by the  basilar membrane , the  stria vascularis , and Reissner's membrane.  The  stria vascularis  is a rich bed of capillaries and secretory cells; Reissner's membrane is a thin membrane that separates endolymph from perilymph; and the basilar membrane is a mechanically somewhat stiff membrane, supporting the receptor organ for hearing, the organ of Corti, and determines the mechanical wave propagation properties of the cochlear system.


Between males and females, there are differences in the shape of the human cochlea. The variation is in the twist at the end of the spiral. Because of this difference, and because the cochlea is one of the more durable bones in the skull, it is used in ascertaining the sexes of human remains found at archaeological sites. [ 6 ] [ 7 ]


The cochlea is filled with a watery liquid, the  endolymph , which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, the cochlear partition (basilar membrane and organ of Corti) moves; thousands of  hair cells  sense the motion via their  stereocilia , and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells. These primary auditory neurons transform the signals into electrochemical impulses known as  action potentials , which travel along the auditory nerve to structures in the brainstem for further processing.


The  stapes  (stirrup) ossicle bone of the middle ear transmits vibrations to the  fenestra ovalis  (oval window) on the outside of the cochlea, which vibrates the perilymph in the  vestibular duct  (upper chamber of the cochlea). The ossicles are essential for efficient coupling of sound waves into the cochlea, since the cochlea environment is a fluid–membrane system, and it takes more pressure to move sound through fluid–membrane waves than it does through air. A pressure increase is achieved by reducing the area ratio from the tympanic membrane (drum) to the oval window ( stapes  bone) by 20. As pressure = force/area, results in a pressure gain of about 20 times from the original sound wave pressure in air. This gain is a form of  impedance matching  – to match the soundwave travelling through air to that travelling in the fluid–membrane system.


At the base of the cochlea, each 'duct' ends in a membranous portal that faces the middle ear cavity: The  vestibular duct  ends at the  oval window , where the footplate of the  stapes  sits. The footplate vibrates when the pressure is transmitted via the ossicular chain. The wave in the perilymph moves away from the footplate and towards the  helicotrema . Since those fluid waves move the cochlear partition that separates the ducts up and down, the waves have a corresponding symmetric part in perilymph of the tympanic duct, which ends at the round window, bulging out when the oval window bulges in.


The perilymph in the vestibular duct and the  endolymph  in the cochlear duct act mechanically as a single duct, being kept apart only by the very thin  Reissner's membrane .
The vibrations of the endolymph in the cochlear duct displace the basilar membrane in a pattern that peaks a distance from the oval window depending upon the soundwave frequency. The  organ of Corti  vibrates due to  outer hair cells  further amplifying these vibrations.  Inner hair cells  are then displaced by the vibrations in the fluid, and depolarise by an influx of K+ via their  tip-link -connected channels, and send their signals via neurotransmitter to the primary auditory neurons of the  spiral ganglion . [ 8 ]


The hair cells in the organ of Corti are tuned to certain sound frequencies by way of their location in the cochlea, due to the degree of stiffness in the basilar membrane. [ 9 ]  This stiffness is due to, among other things, the thickness and width of the basilar membrane, [ 10 ]  which along the length of the cochlea is stiffest nearest its beginning at the oval window, where the stapes introduces the vibrations coming from the eardrum. Since its stiffness is high there, it allows only high-frequency vibrations to move the basilar membrane, and thus the hair cells.  The farther a wave travels towards the cochlea's apex (the  helicotrema ), the less stiff the basilar membrane is; thus lower frequencies travel down the tube, and the less-stiff membrane is moved most easily by them where the reduced stiffness allows: that is, as the basilar membrane gets less and less stiff, waves slow down and it responds better to lower frequencies. In addition, in mammals, the cochlea is coiled, which has been shown to enhance low-frequency vibrations as they travel through the fluid-filled coil. [ 11 ]   This spatial arrangement of sound reception is referred to as  tonotopy .


For very low frequencies (below 20 Hz), the waves propagate along the complete route of the cochlea – differentially up vestibular duct and tympanic duct all the way to the  helicotrema . Frequencies this low still activate the organ of Corti to some extent but are too low to elicit the perception of a  pitch .  Higher frequencies do not propagate to the  helicotrema , due to the stiffness-mediated tonotopy.


A very strong movement of the basilar membrane due to very loud noise may cause hair cells to die. This is a common cause of partial hearing loss and is the reason why users of firearms or heavy machinery often wear  earmuffs  or  earplugs .


To transmit the sensation of sound to the brain, where it can be processed into the perception of  hearing , hair cells of the cochlea must convert their mechanical stimulation into the electrical signaling patterns of the nervous system. Hair cells are modified  neurons , able to generate action potentials which can be transmitted to other nerve cells. These action potential signals travel through the  vestibulocochlear nerve  to eventually reach the anterior  medulla , where they  synapse  and are initially processed in the  cochlear nuclei . [ 12 ]


Some processing occurs in the cochlear nuclei themselves, but the signals must also travel to the  superior olivary complex  of the  pons  as well as the  inferior colliculi  for further processing. [ 12 ]


Not only does the cochlea "receive" sound, a healthy cochlea  generates  and amplifies sound when necessary.  Where the organism needs a mechanism to hear very faint sounds, the cochlea amplifies by the reverse  transduction  of the OHCs, converting electrical signals back to mechanical in a positive-feedback configuration.  The OHCs have a protein motor called  prestin  on their outer membranes; it generates additional movement that couples back to the fluid–membrane wave. This "active amplifier" is essential in the ear's ability to amplify weak sounds. [ 13 ] [ 14 ]


The active amplifier also leads to the phenomenon of soundwave vibrations being emitted from the cochlea back into the ear canal through the middle ear (otoacoustic emissions).


Otoacoustic emissions  are due to a wave exiting the cochlea via the oval window, and propagating back through the middle ear to the eardrum, and out the ear canal, where it can be picked up by a microphone.  Otoacoustic emissions are important in some types of tests for  hearing impairment , since they are present when the cochlea is working well, and less so when it is suffering from loss of OHC activity. Otoacoustic emissions also exhibit sex dimorphisms, since females tend to display higher magnitudes of otoacoustic emissions. Males tend to experience a reduction in otoacoustic emission magnitudes as they age. Women, on the other hand, do not experience a change in otoacoustic emission magnitudes with age. [ 15 ]


Gap-junction proteins, called  connexins , expressed in the cochlea play an important role in auditory functioning. [ 16 ]  Mutations in gap-junction genes have been found to cause syndromic and nonsyndromic deafness. [ 17 ]   Certain connexins, including  connexin 30  and  connexin 26 , are prevalent in the two distinct gap-junction systems found in the cochlea.  The epithelial-cell gap-junction network couples non-sensory epithelial cells, while the connective-tissue gap-junction network couples connective-tissue cells. [ 18 ]   Gap-junction channels recycle potassium ions back to the endolymph after  mechanotransduction  in  hair cells . [ 19 ]   Importantly, gap junction channels are found between cochlear supporting cells, but not auditory  hair cells . [ 20 ]


Damage to the cochlea can result from different incidents or conditions like a severe head injury, a  cholesteatoma , an infection, and/or exposure to loud noise which could kill hair cells in the cochlea.


Hearing loss associated with the cochlea is often a result of outer hair cells and inner hair cells damage or death. Outer hair cells are more susceptible to damage, which can result in less sensitivity to weak sounds. Frequency sensitivity is also affected by cochlear damage which can impair the patient's  ability to distinguish between spectral differences of vowels. The effects of cochlear damage on different aspects of hearing loss like temporal integration, pitch perception, and frequency determination are still being studied, given that multiple factors must be taken into account in regard to cochlear research. [ 21 ]


In 2009, engineers at the  Massachusetts Institute of Technology  created an  electronic chip  that can quickly analyze a very large range of  radio frequencies  while using only a fraction of the power needed for existing technologies; its design specifically mimics a cochlea. [ 22 ] [ 23 ]


The coiled form of cochlea is unique to  mammals . In birds and in other non-mammalian  vertebrates , the compartment containing the sensory cells for hearing is occasionally also called "cochlea," despite not being coiled up. Instead, it forms a blind-ended tube, also called the cochlear duct. This difference apparently  evolved  in parallel with the differences in  frequency range of hearing  between mammals and non-mammalian vertebrates. The superior  frequency range  in mammals is partly due to their unique mechanism of pre-amplification of sound by active cell-body vibrations of outer  hair cells . Frequency resolution is, however, not better in mammals than in most lizards and birds, but the upper frequency limit is – sometimes much – higher. Most bird species do not hear above 4–5 kHz, the currently known maximum being ~ 11 kHz in the barn owl. Some marine mammals hear up to 200 kHz. A long coiled compartment, rather than a short and straight one, provides more space for additional octaves of hearing range, and has made possible some of the highly derived behaviors involving mammalian hearing. [ 24 ]


As the study of the cochlea should fundamentally be focused at the level of hair cells, it is important to note the anatomical and physiological differences between the hair cells of various species.  In birds, for instance, instead of outer and inner hair cells, there are tall and short hair cells.  There are several similarities of note in regard to this comparative data.  For one, the tall hair cell is very similar in function to that of the inner hair cell, and the short hair cell, lacking afferent auditory-nerve fiber innervation, resembles the outer hair cell.  One unavoidable difference, however, is that while all hair cells are attached to a  tectorial membrane  in birds, only the outer hair cells are attached to the tectorial membrane in mammals.


The  cochlear duct  (a.k.a. the  scala media ) is an  endolymph  filled cavity inside the  cochlea , located between the  tympanic duct  and the  vestibular duct , separated by the  basilar membrane  and the  vestibular membrane  (Reissner's membrane) respectively. The cochlear duct houses the  organ of Corti . [ 1 ]


The cochlear duct is part of the  cochlea . It is separated from the  tympanic duct  (scala tympani) by the  basilar membrane . [ 2 ]  It is separated from the  vestibular duct  (scala vestibuli) by the  vestibular membrane  (Reissner's membrane). [ 2 ]  The  stria vascularis  is located in the wall of the cochlear duct. [ 2 ]


The cochlear duct develops from the ventral  otic vesicle  (otocyst). [ 3 ]  It grows slightly flattened between the middle and outside of the body. [ 3 ]  This development may be regulated by the genes  EYA1 ,  SIX1 ,  GATA3 , and  TBX1 . [ 3 ]  The  organ of Corti  develops inside the cochlear duct. [ 4 ]


The cochlear duct contains the  organ of Corti . [ 2 ] [ 5 ]  This is attached to the basilar membrane. [ 5 ]  It also contains  endolymph , which contains high concentrations of K +  for the function of  inner hair cells  and  outer hair cells  in the organ of Corti. [ 2 ]


Drugs delivered directly to the  tympanic duct  will spread to all of the  cochlea  except for the cochlear duct. [ 6 ]  Rarely, the cochlear duct may develop to have the wrong shape. [ 3 ]


A  cochlear implant  ( CI ) is a surgically implanted  neuroprosthesis  that provides a person who has moderate-to-profound  sensorineural hearing loss  with sound perception. With the help of therapy, cochlear implants may allow for improved speech understanding in both quiet and noisy environments. [ 1 ] [ 2 ]  A CI bypasses acoustic hearing by direct electrical stimulation of the auditory nerve. [ 2 ]  Through everyday listening and auditory training, cochlear implants allow both children and adults to learn to interpret those signals as speech and sound. [ 3 ] [ 4 ] [ 5 ]


The implant has two main components. The outside component is generally worn behind the ear, but could also be attached to clothing, for example, in young children. This component, the sound processor, contains microphones, electronics that include  digital signal processor  (DSP) chips, battery, and a coil that transmits a signal to the implant across the skin. The inside component, the actual implant, has a coil to receive signals, electronics, and an array of  electrodes  which is placed into the  cochlea , which stimulate the  cochlear nerve . [ 6 ]


The surgical procedure is performed under  general anesthesia . Surgical risks are minimal and most individuals will undergo  outpatient surgery  and go home the same day. However, some individuals will experience  dizziness , and on rare occasions,  tinnitus  or facial nerve bruising.


From the early days of implants in the 1970s and the 1980s, speech perception via an implant has steadily increased. More than 200,000 people in the  United States  had received a CI through 2019. Many users of modern implants gain reasonable to good hearing and speech perception skills post-implantation, especially when combined with lipreading. [ 7 ] [ 8 ]  One of the challenges that remain with these implants is that hearing and speech understanding skills after implantation show a wide range of variation across individual implant users. Factors such as age of implantation, parental involvement and education level, duration and cause of hearing loss, how the implant is situated in the cochlea, the overall health of the cochlear nerve, and individual capabilities of re-learning are considered to contribute to this variation. [ 9 ] [ 10 ] [ 11 ] [ 12 ]


André Djourno and Charles Eyriès invented the original cochlear implant in 1957. Their design distributed stimulation using a single channel. [ 13 ]


William House  also invented a cochlear implant in 1961. [ 14 ]  In 1964, Blair Simmons and  Robert L. White  implanted a single-channel electrode in a patient's cochlea at Stanford University. [ 15 ]  However, research indicated that these single-channel cochlear implants were of limited usefulness because they cannot stimulate different areas of the cochlea at different times to allow differentiation between low and mid to high frequencies as required for detecting speech. [ 16 ]


The next step in the development of the CI was its clinical trial on a cohort of patients. In the late 1960's Robin Michelson and colleague Melvin Bartz construct a cochlear device with biocompatible materials that can be implanted in human patients. This system is implanted in 4 patients, and the report of the hearing results represent a watershed for clinically applicable cochlear implants. [ 17 ]  Robin Michelson, Robert Schindler, and Michael Merzenich at the University of California, San Francisco, conducted these experiments in 1970 and 1971. Michelson, a clinical pioneer, and Merzenich, a talented basic scientist with a solid foundation in neurophysiology, was an integral element in the development of the UCSF cochlear implant team. Michelson was recognized for implanting a single-channel device into a congenitally deaf woman. She demonstrated auditory sensations from stimulation, as well as pitch perception for stimulus frequencies less that 600 Hz. Unfortunately, This patient had no word recognition. His pioneering work was presented, but not well-received at the 1971 annual meeting of the American Otological Society (Michelson, 1971 and Merzenich et al., 1973). [ 18 ]  In 1973, the first international conference on the "electrical stimulation of the acoustic nerve as a treatment for profound sensorineural deafness in man" was organized in San Francisco.  [ 19 ]


NASA  engineer Adam Kissiah started working in the mid-1970s on what would become the modern cochlear implant. Kissiah used his knowledge learned while working as an electronics instrumentation engineer for NASA. This work took place over three years, when Kissiah would spend his lunch breaks and evenings in  Kennedy Space Center's  technical library, studying the impact of engineering principles on the inner ear. In 1977, NASA helped Kissiah obtain a patent for the cochlear implant; Kissiah later sold the patent rights. [ 20 ]


The modern multi-channel cochlear implant was independently developed and commercialized by two separate teams—one led by  Graeme Clark  in Australia and another by  Ingeborg Hochmair  and her future husband,  Erwin Hochmair  in Austria, with the Hochmairs' device first implanted in a person in December 1977 and Clark's in August 1978. [ 21 ]


Cochlear implants bypass most of the  peripheral auditory system  which receives sound and converts that sound into movements of  hair cells  in the  cochlea ; the deflection of  stereocilia  causes an influx of  potassium ions  into the hair cells, and the depolarisation in turn stimulates calcium influx, which increases release of the  neurotransmitter   glutamate . Excitation of the  cochlear nerve  by the neurotransmitter sends signals to the brain, which creates the  experience  of sound. With an implant, instead, the devices pick up sound and digitize it, convert that digitized sound into electrical signals, and transmit those signals to electrodes embedded in the cochlea. The electrodes electrically stimulate the cochlear nerve, causing it to send signals to the brain. [ 22 ] [ 23 ] [ 24 ]


There are several systems available, but generally they have the following components: [ 22 ] [ 24 ]


External:


Internal:


A  totally implantable cochlear implant (TICI)  is currently in development. This new type of cochlear implant incorporates all the current external components of an audio processor into the internal implant. The lack of external components makes the implant invisible from the outside and also means it is less likely to be damaged or broken. [ 25 ]


Most modern cochlear implants can be used with a range of  assistive listening devices  (ALDs), which help people to hear better in challenging listening situations. These situations could include talking on the phone, watching TV or listening to a speaker or teacher. With an ALD, the sound from devices including mobile phones or from an external microphone is sent to the audio processor directly, rather than being picked up by the audio processor's microphone. This direct transmission improves the sound quality for the user, making it easier to talk on the phone or stream music.


ALDs come in many forms, such as neckloops, [ 26 ]  pens, [ 27 ]  and specialist battery pack covers. [ 28 ]  Modern ALDs are usually able to receive sound from any Bluetooth device, including phones and computers, before transmitting it wirelessly to the audio processor. Most cochlear implants are also compatible with older ALD technology, such as a telecoil. [ 29 ]


Implantation of children and adults can be done safely with few surgical complications and most individuals will undergo outpatient surgery and go home the same day. [ 30 ] [ 31 ] [ 32 ]


Occasionally, the very young, the very old, or patients with a significant number of medical diseases at once may remain for overnight observation in the hospital. The procedure can be performed in an ambulatory surgery center in healthy individuals. [ 33 ]


The surgical procedure most often used to implant the device is called  mastoidectomy  with facial recess approach (MFRA). [ 24 ]


The procedure is usually done under general anesthesia. Complications of the procedure are rare, but include  mastoiditis ,  otitis media  (acute or with effusion), shifting of the implanted device requiring a second procedure, damage to the  facial nerve , damage to the  chorda tympani , and wound infections. [ 34 ]


Cochlear implantation surgery is considered a clean procedure with an infection rate of less than 3%. [ 35 ]  Guidelines suggest that routine prophylactic antibiotics are not required. [ 36 ]  However, the potential cost of a postoperative infection is high (including the possibility of implant loss); therefore, a single preoperative intravenous injection of antibiotics is recommended. [ 37 ]


The rate of complications is about 12% for minor complications and 3% for major complications; major complications include infections, facial paralysis, and device failure.


Although up to 20 new cases of post-CI bacterial meningitis occur annually worldwide, data demonstrates a reducing incidence. [ 38 ]  To avoid the risk of bacterial meningitis, the CDC recommends that adults and children undergoing CI receive age-appropriate vaccines that generate antibodies to Streptococcus pneumoniae. [ 39 ]


The rate of transient facial nerve palsy is estimated to be approximately 1%. Device failure requiring reimplantation is estimated to occur 2.5–6% of the time. Up to one-third of people experience disequilibrium, vertigo, or vestibular weakness lasting more than one week after the procedure; in people under 70 these symptoms generally resolve over weeks to months, but in people over 70 the problems tend to persist. [ 24 ]


In the past, cochlear implants were only approved for people who were deaf in both ears; as of 2014 [update]  a cochlear implant had been used experimentally in some people who had acquired deafness in one ear after they had learned how to speak, and none who were deaf in one ear from birth; clinical studies as of 2014 [update]  had been too small to draw generalizations. [ 40 ]


Other approaches, such as going through the  suprameatal triangle , are used. A systematic literature review published in 2016 found that studies comparing the two approaches were generally small, not randomized, and retrospective so were not useful for making generalizations; it is not known which approach is safer or more effective. [ 34 ]


With the increased utilization of  endoscopic ear surgery  as popularized by professor Tarabichi, there have been multiple published reports on the use of endoscopic technique in cochlear implant surgery. [ 41 ]  However, this has been motivated by marketing and there is clear indication of increased morbidity associated with this technique as reported by the pioneer of  endoscopic ear surgery . [ 42 ]


As cochlear implant surgical techniques have advanced over the last four decades, the global complication rate for CI surgery in both children and adults has decreased from more than 35% in 1991 to less than 10% at present. [ 43 ] [ 44 ] [ 45 ]  The risk of postoperative facial nerve injury has also decreased over the last several decades to less than 1%, most of which demonstrated complete return of function within six months. The rate of permanent paralysis is approximately 1 per 1,000 surgeries and likely less than that in experienced CI centers. [ 45 ]


The majority of complications following CI surgery are minor requiring only conservative medical management or prolongation of hospital stay. Less than 5% of all complications are major resulting in surgical intervention or readmission to the hospital. [ 45 ]  Reported rates of revision cochlear implant surgery vary in adults and children from 3.8% to 8% with the most common indications being device failure, infection, and migration of the implant or electrode. [ 46 ]  Disequilibrium and vertigo after CI surgery can occur but the symptoms tend to be mild and short-lived. [ 47 ]  CI rarely results in significant or persistent adverse effects on the vestibular system when hearing conservation surgical techniques are practiced. Moreover, gait and postural stability may actually improve post-implantation. [ 48 ]


Cochlear implant outcomes can be measured using  speech recognition  ability and functional improvements measured using patient reported outcome measures. [ 49 ] [ 50 ]  While the degree of improvement after cochlear implantation may vary, the majority of patients who receive cochlear implants demonstrate a significant improvement in speech recognition ability compared to their preoperative condition. [ 49 ]