Gender, head size and disease: a hypothesis related to posterior fossa growth.

This hypothesis proposes that the difference in head size between the genders has a bearing on disease incidence. Smaller head size predisposes to spina bifida and larger head size predisposes to autism. In the fetus, the posterior fossa has a regulatory influence on maximum intracerebral cerebrospinal fluid (CSF) pressure. CSF pressure pulsations are generated by movements in the chest and abdomen and transmitted to the intracranial CSF spaces via the posterior fossa. The efficiency of transmission of pressure to the head is influenced by the size of posterior fossa CSF spaces. The larger posterior fossa in the male fetus allows higher peaks of pressure in the lateral ventricles than the female, resulting in larger head size. Restriction of posterior fossa growth has a greater influence on females than males. This leads to a greater incidence of spina bifida in females, where reduced head size is characteristic in the fetal stages. Fetal surgery for spina bifida expands posterior fossa CSF spaces and improves transmission of pressure waves generated in the fetal abdomen and thorax into the head. This results in an increase in head circumference following surgery. Males are susceptible to autism where head growth is accelerated in early childhood. This is because they have a larger average head size.