Benign subarachnoid space enlargement of infancy.

Subarachnoid space enlargement is a benign clinical entity characterized by rapid head enlargement in an infant with normal neurodevelopment. We report on two infants who had rapid increases in head circumference, family histories of macrocephaly, and normal neurodevelopment. Radiologic investigations in both infants showed subarachnoid space fluid collection but normal ventricular size. They both had a benign clinical course with resolution of the subarachnoid space fluid collection by the second year of life. The head circumference, however, remained at or above the 95th percentile. There is a need for pediatricians to be aware of this clinical entity and its benign nature.