Microcephaly

Microcephaly is a descriptive term meaning a small head and is associated with numerous disorders of diverse aetiology. It is usually associated with microencephaly (small brain). For the purpose of this article the two will be used interchangeably.

As with most terms which 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 two standard deviations expected for gestation or falling under the 3rd percentile. The same definition is sometime applied to children and adults. It is important to not that this means that the majority of these individuals are actually normal, but small. 

Other authors advocate the use of three standard deviations which increases specificity and narrows the diagnosis to less than 1% of the population and more closely correlates with abnormal development 6

Microcephaly can be divided into primary (or true) or secondary microcephaly. In primary microcephaly the brain never forms normally whereas in secondary microcephaly normal continued brain development is arrested by some defined insult / event. The terms congenital and acquired can also be used which only partially overlap with primary and secondary. 

The estimated incidence various according to definition but when pathological it usually result from deficient brain development. As it is seen in a very large number of conditions (see below) it therefore a similarly large demographic spread. 

Most cases of microcephaly (regardless of associated anomalies) have mental retardation. Recognised associations with microcephaly are protean and include

Many gene products and some genes have been identified which result in microcephaly, and have aided the development of a more detailed classification 7.

Antenatal ultrasound

The detection of microcephaly usually becomes easier as the gestation advances. The small head is best assessed by using the head circumference (HC). Measurement of the biparietal diameter (BPD) is not helpful in diagnosing microcephaly as 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. 

CT and MRI

Both MRI and to a lesser degree CT 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.


Malformations of the central nervous system
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Article Information

rID: 14623
Tag: cases
Synonyms or Alternate Spellings:
  • Microencephaly
  • Microcephaly

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