= BENIGN ENLARGEMENT OF SUBARACHNOID SPACES = BENIGN EXTRAAXIAL COLLECTIONS OF INFANCY = EXTERNAL HYDROCEPHALUS

Cause: defective reabsorption of CSF at arachnoid villi; commonly familial with autosomal dominant inheritance

Age: presentation between 3 and 12 months

• infant with macrocephaly (head circumference >90^th percentile)
• delayed motor development, hypotonia (in up to 30%)

Location: bilateral frontoparietal area + interhemispheric fissure + sylvian fissure + basal cisterns

• enlarged subarachnoid spaces
• “floating” cortical veins
• NO / mild ventricular enlargement

Cx: subdural hematoma in response to minor impacts

Prognosis: self-limiting transient development that usually resolves by 2–3 years

DDx:

1. Cerebral atrophy (diffuse sulcal prominence not localized to frontoparietal area)
2. Spontaneous subdural hematoma (12%)