Arachnoid cysts are congenital anomalies that occur (usually) as focal expansions of the subarachnoid space, contained within separated layers of arachnoid membrane.
A common site is in the middle cranial fossa (the area of the skull that lies just ahead of the ear), and typically the adjacent temporal lobe is somewhat underdeveloped, or atrophic (this is not normally associated with any intellectual or neurological impairment because the brain develops alongside the developing cyst, adjustments in brain function occur, compensating for the underdeveloped area).
Cysts can occur throughout the intracranial space, and possible effects are varied.
Occasionally arachnoid cysts are associated with focal neurological signs and symptoms, or more general symptoms such as headaches. In infants, large cysts may lead to asymmetrical head shape. However, the majority of arachnoid cysts are incidental, asymptomatic findings, which are unlikely to require intervention, although a period of observation may be recommended, to ensure that the cyst is static in size.
If treatment is required, options include inserting a shunt tube (with valve) from the cyst to the peritoneal cavity, or by performing a mini craniotomy and creating pathways which communicate the cyst with the normal CSF cisterns at the base of the brain.