Specific Imaging Findings

Benign external hydrocephalus (BEH) is characterized by an increased amount of fluid in the subarachnoid spaces (SAS), with density and signal intensity following CSF, typically in the bilateral frontal regions and along the interhemispheric fissure. Enlarged SAS follow the contour of gyri and appear symmetric between the two hemispheres. Additional imaging features include widening of the suprasellar cistern and normal to slightly increased ventricular size. The distance between the frontal gyri and the skull and between the mesial frontal gyri and the anterior interhemispheric fissure measures more than 5 mm. MRI or Doppler ultrasound can confirm the presence of transversing leptomeningeal vessels within the extra-axial collections. MRI can rule out associated chronic subdural hematomas or hygromas.

Pertinent Clinical Information

Increased head circumference above the 95th percentile, full fontanelles, and frontal bossing are the most common clinical findings. Patients are usually referred for imaging when the head size has rapidly increased over the course of months. A family history of macrocephaly is often present. BEH is usually diagnosed between 3 and 8 months of age and is more common in male infants. Transient gross motor delay can be observed and is usually attributed to the added head weight in the macrocephalic infant. Otherwise BEH is usually not associated with developmental delay or neurological deficit. Once the diagnosis of BEH is established, clinical follow-up is required to confirm normal development and no treatment is necessary. In selected cases, follow-up imaging at 18–24 months of age can be considered and should show resolution of the abnormality.