Benign enlargement of the subarachnoid space (BESS) in infancy, also called benign external hydrocephalus or extraventricular hydrocephalus, is the most common cause of macrocephaly.

Large head circumference, normal or minor motor and linguistic delay, increased cerebrospinal fluid (CSF) in the subarachnoid space, and normal ventricles or mild ventriculomegaly are some of its clinical characteristics.

Although the etiology of increased subarachnoid spaces is unknown, arachnoid villi that have not fully developed have been believed to be the reason and by the age of two years, the majority of cases resolve on their own.

The diagnosis of BESS may be complicated by other mimicking diagnoses such as:

• brain atrophy: Symmetrical enlargement of frontal, parietal, temporal, and occipital lobes subarachnoid space, and the cerebral sulcus widens without increasing the size of the head.
• subdural collection: Displace bridging veins from the inner table.
• hydrocephalus: Considerable enlargement of the ventricles, but there is no sign of obstruction, periventricular lucency, or transependymal edema.