Primary microcephaly: microcephalin and ASPM determine the size of the human brain

In microcephaly (small head), the size of the head as measured by the occipito-frontal circumference of an affected individual is greater than three standard deviations below the population age-related mean. The cranial vault in a microcephaly patient is smaller than normal relative to the facial skeleton and the rest of the body. The small cranial capacity results from underlying hypoplasia of the cerebral cortex rather than abnormal development of the overlying skull and there is no major abnormality in cortical architecture (Jackson et al 1998; Mochida and Walsh 2001). Microcephaly is known to have a heterogeneous etiology with environmental and genetic causes. Among the environmental causes are intrauterine infections, drugs (alcohol) taken during pregnancy, prenatal radiation exposure, maternal phenylketonuria and birth asphyxia. All of these except birth asphyxia are known to be rare causes of microcephaly. The majority of microcephalic cases are caused by a variety of genetic mechanisms including cytogenetic abnormalities and single-gene disorders (Jackson et al 1998).