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ERROR: type should be string, got "https://profreg.medscape.com/px/getpracticeprofile.do?method=getProfessionalProfile&urlCache=aHR0cHM6Ly9lbWVkaWNpbmUubWVkc2NhcGUuY29tL2FydGljbGUvMjUwMDA0OC1vdmVydmlldw==\nDrugs & Diseases > Neurology\nAuthor: James J Reese, Jr, MD, MPH; Chief Editor: Amy Kao, MD more...\nSections Microcephaly\nMicrocephaly is broadly defined as a small head size, typically greater than two standard deviations below normal, as measured via occipital frontal circumference, a method by which a tape measure is used to assess the circumference encompassing the occiput and frontal bones (forehead). It is a clinical sign and not a disease.\nMicrocephaly has recently gained notoriety in the popular press because of its association with the Zika virus, one of many viruses that may lead to microcephaly by first damaging the brain. [4] However, causality has not definitely been proven.\nThe skull is comprised of six bones that are not fused at birth and which begin to fuse over the next few months. Head circumference typically grows because of the underlying brain growing, exerting physical force on the skull bones and causing them to expand as well. The rate of expansion is highest in the first few months and gradually plateaus over time. Various gender-specific growth charts have been published, but the Centers for Disease Control (CDC) currently recommends that children in the United States between the ages of 0 and 2 years old are tracked with the World Health Organization (WHO) growth charts, and with the CDC growth charts after children turn 2 years old. Minor differences do exist between the two charts. Clinicians can easily tell the difference because charts from WHO data tend to stop at 24 months of age, whereas the charts from CDC data extend to 36 months of age. [6, 7]\nIf the sutures fuse prematurely, the skull may take on an abnormal shape and the head circumference will grow at a much slower rate than age-matched controls. (For further information, please see the Medscape Reference article Pediatric Craniosynostosis.)\nAlternatively, if an infant’s brain has a lower volume than normal (i.e., due to a decreased number of neurons and glial cells), it may not grow as much as other infants’ brains that have a higher number of neurons and glial cells. Decreased brain growth would then lead to decreased outward force exerted on the skull, and decreased expansion of the head circumference. This process may result in a normally shaped or symmetric head, as opposed to the abnormal shape that results from craniosynostosis. Various etiologies can lead to decreased brain volume. More commonly, prenatal or perinatal insults to the brain or genetic/metabolic conditions cause poor brain growth. Less commonly, genetic neurodegenerative diseases (e.g., Rett Syndrome) will cause progressive microcephaly in infants and toddlers.\nThe etiology of microcephaly can be broadly divided into two categories: premature fusion of cranial sutures (i.e., craniosynostosis) or poor brain growth.\nCongenital microcephaly\nGenetic conditions that can cause microcephaly include the following:\nTrisomy 21, 13, 18\nCri du Chat syndrome - 5p deletion\nWolf–Hirschhorn syndrome - 4p deletion\nWilliams syndrome - 7q11.23 deletion\nSmith-Lemli-Opitz syndrome\nSeckel syndrome\nPrimary autosomal recessive microcephaly (microcephaly primary hereditary [MCPH]) - mutation in the abnormal spindle microtuble assembly (ASPM) gene, which impacts cell division of neural progenitor cells\nMaternal deprivation problems can also lead to microcephaly (e.g., folate deficiency, malnutrition, and hypothyroidism).\nPostnatal onset microcephaly\nPostanatal onset microcephaly can result from inborn errors of metabolism including congenital disorders of glycosylation, mitochondrial disorders, peroxisomal disorders, and Menkes disease. Disruptive injuries such as traumatic brain injury, hemorrhagic and ischemic stroke, and hypoxic-ischemic encephalopathy can also lead to microcephaly.\nMicrocephaly is not a common condition. State birth defects tracking systems have estimated that microcephaly ranges from 2 babies per 10,000 live births to about 12 babies per 10,000 live births in the Unites States. [5] Two large population-based studies have found similar incidences of microcephaly in newborns: 0.56% and 0.54%, respectively. [1, 2]\nSimilar studies for postnatal onset microcephaly could not be found and are not mentioned in the American Academy of Neurology’s (AAN) Practice Parameter: Evaluation of the child with microcephaly (an evidence-based review). [3]\nMicrocephaly itself does not necessarily predict someone’s functional outcome but can be associated with many other clinical problems, depending on the etiology of the microcephaly. For example, seizures, cerebral palsy, developmental delay, hyperactivity, and other neurodevelopmental problems occur more commonly in children with microcephaly.\nVargas JE, Allred EN, Leviton A, Holmes LB. Congenital microcephaly: phenotypic features in a consecutive sample of newborn infants. J Pediatr. 2001 Aug. 139 (2):210-4. [Medline].\nDolk H. The predictive value of microcephaly during the first year of life for mental retardation at seven years. Dev Med Child Neurol. 1991 Nov. 33 (11):974-83. [Medline].\nAshwal S, Michelson D, Plawner L, Dobyns WB, Quality Standards Subcommittee of the American Academy of Neurology and the Practice Committee of the Child Neurology Society. Practice parameter: Evaluation of the child with microcephaly (an evidence-based review): report of the Quality Standards Subcommittee of the American Academy of Neurology and the Practice Committee of the Child Neurology Society. Neurology. 2009 Sep 15. 73 (11):887-97. [Medline].\nWorld Health Organization. Assessment of infants with microcephaly in the context of Zika virus: Interim guidance. World Health Organization. Available at http://apps.who.int/iris/bitstream/10665/204475/1/WHO_ZIKV_MOC_16.3_eng.pdf?ua=1. March 4, 2016; Accessed: May 4, 2016.\nNational Birth Defects Prevention Network. Major birth defects data from population-based birth defects surveillance programs in the United States, 2006-2010. Birth Defects Research (Part A). Clinical and Molecular Teratology. 2013. 97:S1-S172.\nNational Center for Health Statistics. Clinical Growth Charts. Centers for Disease Control and Prevention. Available at http://www.cdc.gov/growthcharts/clinical_charts.htm. Accessed: May 5, 2016.\nNational Center for Health Statistics. WHO Growth Charts. Centers for Disease Control and Prevention. Available at http://www.cdc.gov/growthcharts/who_charts.htm#The WHO Growth Charts. Accessed: May 5, 2016.\nIllustration of typical head size, Microcephaly, and Severe Microcephaly\nJames J Reese, Jr, MD, MPH Assistant Professor of Neurology, Director of Child Neurology Residency, Department of Neurology, Division of Child Neurology, University of New Mexico School of Medicine\nJames J Reese, Jr, MD, MPH is a member of the following medical societies: American Academy of Neurology, Child Neurology Society\nAmy Kao, MD Attending Neurologist, Children's National Medical Center\nAmy Kao, MD is a member of the following medical societies: American Academy of Neurology, American Epilepsy Society, Child Neurology Society\nDisclosure: Have stock (managed by a financial services company) in healthcare companies including Allergan, Cellectar Biosciences, CVS Health, Danaher Corp, Johnson & Johnson.\nencoded search term (Microcephaly) and Microcephaly\nFast Five Quiz: Do You Know the Zika Virus Guidelines?\nFast Five Quiz: Mosquito-Borne Diseases\nClinical Trends for November 2016\nViral Infections and Pregnancy\nFast Five Quiz: Are You Familiar With Conditions Responsible for Recent Outbreaks?\nFast Five Quiz: Bites and Stings\nIn Utero SARS-CoV-2 Transmission Unlikely, but Neonates May Be Unprotected\nPreventing Vectorborne Transmission of Zika Virus Infection During Pregnancy\nCOVID-19 Vaccine FAQ: Safety, Side Effects, Efficacy\nInfant Outcomes of Zika in Pregnancy: New Data\nDiseases & Conditions Zika Virus\nDengue: Global Update on Outbreaks, Screening, and Vaccination 1.0 CME Credits Clinical Review\nDengue: Global Update on Outbreaks, Screening, and Vaccination\nJournal Article Women's Awareness and Healthcare Provider Discussions About Zika Virus During Pregnancy, United States, 2016–2017\nJournal Article Systematic Hospital-Based Travel Screening to Assess Exposure to Zika Virus"