[Idiopathic external hydrocephaly and familial macrocephaly in infancy].

Idiopathic external hydrocephalus is a frequent cause of macrocephaly in infants. The cranial CT scan shows an enlarged subarachnoid space, especially in the frontal area, with a prominent interhemispheric fissure and occasionally an enlargement of the ventricular system. Sixteen patients with this condition are reported; 7 of which had a family history of megalo-encephaly and 4 had delayed motor development, although it was transient in 3 of the cases. The cranial circumference was normal at about 18 months of age, but 4 patients had megalo-encephaly after 3 years of age. The radiological images were normal in all cases between 24 and 48 months of age without treatment.