This term, literally meaning small brain, is used to identify certain primary developmental anomalies (primary microcephaly) as well as secondary microcephaly, which results from shrinkage of the brain (in neonatal hypoxia or encephalomalacia) or physical restriction of its development. Primary microcepahly can be inherited as an autosomal recessive trait, in which case the brain at birth may weigh as little as 500 g, and may also show agyria, macrogyria, micropolygyria, corpus callosum agenesis, and neuronal heterotopias (Sugimoto et al. 1993).
Genetically determined microcephaly may show a trajectory of head growth, which drops from being in the normal range at birth to being below the third centile by the end of the first year. Many such individuals demonstrate long survival, but with moderate mental retardation, epilepsy and hyperkinetic behaviour, and variable spastic weakness of the limbs (Gabriel 1980).
Microcephaly is also seen in some chromosomal disorders associated with mental retardation, and as a result of maternal irradiation or intrauterine infection with Toxoplasma, cytomegalovirus, or rubella.