Microcephaly (plural: microcephalies) is a descriptive term meaning a small head and is 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 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 two standard deviations expected for gestation or falling under the 3 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 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 .
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 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 (see below) it, therefore, a similarly large demographic spread.
Most cases of microcephaly (regardless of associated anomalies) have an intellectual disability. Recognized associations with microcephaly are protean and include
- aneuploidic syndromic
- non-aneuploidic syndromic
- Coffin-Lowry syndrome
- Coffin-Siris syndrome
- Cornelia de Lange syndrome
- Feingold syndrome
- Freeman Sheldon syndrome
- holoprosencephaly spectrum
- due to in utero substance / drug exposure
- in utero infections
- Miller-Dieker syndrome
- Neu-Laxova syndrome
- Rett syndrome
- Roberts syndrome
- Rubinstein-Taybi syndrome
- Ruvalcaba-Myhre-Smith syndrome
- Seckel syndrome
- Smith-Lemli-Opitz syndrome
- Walker-Warburg syndrome
- other isolated features
Many gene products and some genes have been identified which result in microcephaly, and have aided the development of a more detailed classification .
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 the head shape can be misleading .
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 .