These are congenital lesions which arise during
the development of the nervous system, from a split in the membrane
surrounding and investing the brain. They are located most
commonly in the middle fossa (near the temporal lobe), although
they may be located in other portions of the brain as well.
They may present with headaches, seizures or weakness on one side
of the body. These cysts are often watched, but may be operated
upon if they become symptomatic.
In the CT of the head seen here, an arachnoid
cyst is seen sitting in front of the left temporal lobe. The cyst
is also seen in the operating room.