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Microcephaly (plural: microcephalies) is a descriptive term meaning a small head associated with numerous disorders of diverse etiology. It is usually associated with microencephaly (plural: microencephalies) (small brain). For the purpose of this article, the two terms will be used interchangeably.
As with most terms that describe a relative size, the definition varies from publication to publication.
In fetal imaging, it is usually defined as fetal head measurements (e.g., head circumference) falling under 2 standard deviations expected for gestation or falling under the 3rd percentile. The same definition is sometimes applied to children and adults. It is important to note that this means that the majority of these individuals are actually normal but small.
Other authors advocate using three standard deviations, which increases specificity and narrows the diagnosis to <1% of the population and more closely correlates with abnormal development 6.
Congenital microcephaly is divided into two main types 8:
primary (genetic) microcephaly: small but architecturally normal brain; the brain never forms normally
could be familial (autosomal dominant or recessive) or syndromic (e.g., Edward or Down)
some show an abnormally simplified gyral pattern without thickening of the cerebral cortex, known as microcephaly with a simplified gyral pattern (MSG) 8
secondary microcephaly: normal continued brain development is arrested by some defined insult such as a hypoxic-ischemic insult, TORCH infection, or metabolic disease
The estimated incidence varies according to definition, but when pathological it usually results from deficient brain development. As it is seen in a very large number of conditions, it therefore has a similarly large demographic spread.
Recognized associations with microcephaly are protean and include:
due to in utero substance / drug exposure
in utero infections
other isolated features
Most cases of microcephaly (regardless of associated anomalies) have an intellectual disability.
Many gene products and some genes have been identified which result in microcephaly, and have aided the development of a more detailed classification 7.
The detection of microcephaly usually becomes easier as the gestation advances. The small head is best assessed by using the head circumference (HC). The biparietal diameter (BPD) measurement does not help diagnose microcephaly as the head shape can be misleading 1.
In addition, the diagnosis should be reserved for cases where there is a significant discrepancy between the head size and the rest of the body.
Both MRI and CT (to a lesser degree) are able to assess the underlying brain. The forebrain is usually most affected and often a degree of simplification of cortical structure (shallow sulci, reduced in number/abnormal sulcation) is evident 6,7.
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