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Simeon A. Boyadjiev Boyd

, MD, University of California, Davis

Last full review/revision May 2020| Content last modified May 2020
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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, alcohol, or radiation exposure, prenatal infections (eg, TORCH [toxoplasmosis, other pathogens, rubella, cytomegalovirus, and herpes simplex] and Zika virus), and poorly controlled maternal phenylketonuria. Microcephaly also is a feature of > 400 genetic syndromes.

The consequences of microcephaly itself include neurologic and developmental disorders (eg, seizure disorders, intellectual disability, spasticity).

Prenatally, the diagnosis of microcephaly sometimes is made with ultrasonography done in the late 2nd or early 3rd 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.

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.

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