Idiopathic external hydrocephalus: Natural history and relationship to benign familial macrocephaly

External hydrocephalus was identified in 63 infants. The 36 infants in whom external hydrocephalus was idiopathic constitute the study population. The group was homogeneous. Although not all were macrocephalic at birth, the head circumference exceeded the 95th percentile in all cases by 1 year of age, and subsequent head growth was parallel to the 95th percentile. The head computed tomographic scan showed a characteristic picture of a prominent interhemispheric fissure, a collection of subarachnoid fluid over the frontal convexities, and prominent basal cisternae. Mild ventriculomegaly was present in ten cases. These abnormalities resolved after 18 to 24 months of age. No infant required therapy for the condition. Development was normal in 32 infants, suspect in three, and clearly abnormal in only one of the 36 infants. However, of the 32 infants with normal development, 14 were found to be delayed in gross motor development and five in language development at a prior time. There was a family history of macrocephaly in 88% of cases. Several families had a mixture of members with external hydrocephalus and benign familial macrocephaly throughout several generations. These findings suggest that idiopathic external hydrocephalus is a relatively benign, self-limited condition that resolves without treatment and is closely related to benign familial macrocephaly.