Dr. Schor of the National Institutes of Health and Deputy Director of the National Institute of Neurological Disorders and Stroke has no relevant financial relationships to disclose.)
This article includes discussion of microcephaly, micrencephaly, microcephalus, autosomal dominant microcephaly, autosomal recessive microcephaly, idiopathic microcephaly, and X-linked recessive microcephaly. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
In this updated article, the author includes updated information on congenital Zika syndrome and new roles of microcephaly causing genes.
• Microcephaly is defined as a head circumference of less than 3 standard deviations from the population mean for age and gender.
• Microcephaly is a clinical finding that can result from a variety of causes. There are both genetic and nongenetic causes of microcephaly.
• Advances in neuroimaging and genetic testing have value for the diagnosis and prognosis of microcephaly.
• The American Academy of Neurology and Child Neurology Society have published an evidence-based review with recommendations for the evaluation of a child with microcephaly.
Historical note and terminology
Microcephaly is a descriptive term that refers to a cranial vault that is significantly smaller than the standard for the person's age and sex. Microcephaly should be considered as a clinical finding rather than a disorder, and it may stem from a variety of causes that lead to the disruption of various stages of brain development. In clinical settings, microcephaly is determined by measuring one's head circumference, which is known to correlate well with cranial volume (Bray et al 1969). When the head circumference is smaller than 2 standard deviations below the mean for the age and sex, microcephaly is present. However, most researchers use head circumference of 3 standard deviations below the mean as the definition of microcephaly (Barkovich et al 1998; Woods 2004; Mochida 2009).
Microcephaly has been observed and recognized for a long time, but it was not until the late 19th century that it started to attract scientists' attention. Many theories concerning the pathogenesis of microcephaly have been proposed. Some authors viewed microcephaly as a form of atavism, and others thought that it was due to mechanical compression of the fetal brain by contraction of the uterus. Giacomini, in 1885, proposed a classification of microcephaly, which is of historical significance. He divided microcephaly into 3 categories. The first was named "microcephalia vera," which means "true" microcephaly, in which there is no gross pathological abnormality other than smallness of the brain. Giacomini thought that this condition was due to pure inhibition of brain development. The second category was termed "microcephalia spuria," which refers to "pseudo" microcephaly, in which the pathological process or its residuum is identifiable. The third was "microcephalia combinata," which was thought to be a combination of both inhibitions of developmental and pathological processes. The term "microcephalia vera" is still occasionally used in clinical settings, and refers to patients who have microcephaly with no identifiable gross central nervous system pathology and little or no involvement of the organ systems other than the CNS. Since then, many different classifications and terminologies of microcephaly have emerged. Genetic and nongenetic microcephaly is classification according to etiology. Primary microcephaly is often used synonymously as congenital microcephaly, in which microcephaly is present at the time of birth. In contrast, secondary microcephaly or acquired microcephaly is used when head circumference is normal at birth, and subsequently falls to the microcephalic range (Opitz and Holt 1990).
The term micrencephaly refers to "small brain," and this is a pathological term. Although the presence of microcephaly usually implies that the brain is small, normal head circumference does not guarantee a normal-sized brain. Hence, microcephaly and micrencephaly are not interchangeable.
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