Merck Manual

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Simeon A. Boyadjiev Boyd

, MD, University of California, Davis

Reviewed/Revised Sep 2022
Topic Resources

Microcephaly is a head circumference < 2 standard deviations below the mean for age.

In microcephaly, the head is disproportionately small in relation to the rest of the body. Microcephaly has many chromosomal or environmental causes, including prenatal drug Prenatal Drug Exposure Alcohol and illicit drugs are toxic to the placenta and developing fetus and can cause congenital syndromes and withdrawal symptoms. Prescription drugs also may have adverse effects on the fetus... read more , alcohol Fetal Alcohol Syndrome Alcohol exposure in utero increases the risk of spontaneous abortion, decreases birth weight, and can cause fetal alcohol syndrome, a constellation of variable physical and cognitive abnormalities... read more , or radiation exposure Exposure to Teratogens Risk factors for complications during pregnancy include Preexisting maternal disorders Physical and social characteristics (eg, age) Problems in previous pregnancies (eg, a previous history... read more Exposure to Teratogens , prenatal infections (eg, TORCH [ toxoplasmosis Toxoplasmosis Toxoplasmosis is infection with Toxoplasma gondii. Symptoms range from none to benign lymphadenopathy, a mononucleosis-like illness, to life-threatening central nervous system (CNS) disease... read more Toxoplasmosis , other pathogens, rubella Congenital Rubella Congenital rubella is a viral infection acquired from the mother during pregnancy. Signs are multiple congenital anomalies that can result in fetal death. Diagnosis is by serology and viral... read more , cytomegalovirus Congenital and Perinatal Cytomegalovirus Infection (CMV) Cytomegalovirus infection may be acquired prenatally or perinatally and is the most common congenital viral infection. Signs at birth, if present, are intrauterine growth restriction, prematurity... read more Congenital and Perinatal Cytomegalovirus Infection (CMV) , and herpes simplex Neonatal Herpes Simplex Virus (HSV) Infection Neonatal herpes simplex virus infection is usually transmitted during delivery. A typical sign is vesicular eruption, which may be accompanied by or progress to disseminated disease. Diagnosis... read more Neonatal Herpes Simplex Virus (HSV) Infection ] and Zika virus Zika Virus (ZV) Infections The Zika virus is a mosquito-borne flavivirus that is antigenically and structurally similar to the viruses that cause dengue, yellow fever, and West Nile virus. Zika virus infection is typically... read more ), and poorly controlled maternal phenylketonuria Phenylketonuria (PKU) Phenylketonuria is a disorder of amino acid metabolism that causes a clinical syndrome of intellectual disability with cognitive and behavioral abnormalities caused by elevated serum phenylalanine... read more . Microcephaly also is a feature of > 400 genetic syndromes.


  • Prenatally, ultrasound

  • Postnatally, physical examination, including measurement of head circumference and cranial imaging

  • Genetic testing

Prenatally, the diagnosis of microcephaly sometimes is made with ultrasonography done in the late second or early third trimester.

Postnatally, evaluation should include detailed prenatal history to identify risk factors, developmental and neurologic assessment, and brain MRI or CT. Primary autosomal recessive microcephaly may involve a defect in one or more of at least four genes.

A clinical geneticist should evaluate affected patients even in cases of apparent isolated congenital anomaly.

Among the genetic syndromes to be considered are Seckel syndrome, Smith-Lemli-Opitz syndrome, syndromes due to defective DNA repair (eg, Fanconi and Cockayne syndromes), and Angelman syndrome. For parents of an affected child, risk of the disorder appearing in subsequent offspring may be as high as 25%, depending on which syndrome is present, and thus clinical genetic assessment is necessary. A gene panel test specific for microcephaly is available through several diagnostic laboratories. Chromosomal microarray analysis Diagnosis Chromosomal abnormalities cause various disorders. Abnormalities that affect autosomes (the 22 paired chromosomes that are alike in males and females) are more common than those that affect... read more , or broader gene panel tests also should be considered in the evaluation of patients with microcephaly. If the results of these tests are nondiagnostic, whole exome sequencing analysis is recommended.


  • Symptomatic treatment

Symptoms resulting from brain damage are treated. Some disorders causing microcephaly can be treated.

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