Imaging description

Benign external hydrocephalus (BEH) (a.k.a. benign extra-axial fluid collections of infancy) is a clinicoradiologic entity characterized by increased head circumference and enlargement of the CSF (subarachnoid) spaces in the frontal and interhemispheric regions (Fig. 54.1). The ventricular size is often normal, although mild prominence of the ventricles and basilar cisterns may be seen. The etiology is unknown, but ineffectual resorption of CSF by immature arachnoid villi is the most widely accepted theory (1).

This is considered to be a self-limiting condition, and it does not require specific treatment. BEH usually presents around the age of 6 months, and enlargement of the head usually stabilizes by the age of 18 months (1). Some patients exhibit gross motor delay, which is often transient. Long-term follow-up studies are lacking, but one such study reported mild psychomotor delay in some patients (2).

Importance

Rapid increase in head size in infants is usually an alarming finding, which may indicate increased intracranial pressure secondary to tumor or hydrocephalus, although the most common reason is BEH. The exact incidence of BEH is not known. A review of incidental findings in a study reported that 0.6% of the children were found to have external hydrocephalus (3). Most infants with BEH are normal, although BEH is associated with increased incidence of subdural hematomas.