Benign external hydrocephalus or Benign enlargement of the subarachnoid spaces (BESS) is typically characterized clinically by macrocrania and frontal bossing.

BESS is more common in males and there is often a family history of macrocrania.

It is theorized to be due to a delay in development of arachnoid villi in the saggital sinus leading to faulty CSF resorption.

Imaging features include:

• widening of bifrontal CSF spaces > 5mm
• no flattening of adjacent gyri
• enlarged anterior fontanelle
• normal ventricle size and no atrophy
• no blood products
• no cortical vein displacement

The enlarged subarachnoid fluid collection usually resolves by age 2, however, the macrocrania may persist.

While BESS is typically benign, there have been reported associations with increased risk of subdural hemorrhage, communicating hydrocephalus, and psychomotor delay.