Arachnoid cysts represent a fluid collection located somewhere between the pia and the arachnoid. Some have suggested calling them "intra-arachnoid" cysts. The majority of arachnoid cysts are congenital and asymptomatic. However, secondary or "acquired" arachnoid cysts occur in association with extraaxial intracranial masses (usually neoplasms like meningioma and Schwannoma) and after trauma. The most common location is the middle cranial fossa - lateral to the temporal lobe.

On imaging, arachnoid cysts should be virtually identical to CSF in attenuation on CT and signal intensity on MR. To distinguish and arachnoid cyst from a similar lesion (e.g. an epidermoid inclusion cyst) you may use MR to image the lesions with FLAIR or diffusion-weighted sequences.

When they are symptomatic they can present like any other mass lesion depending on location in the brain. On CT these masses have well delineated borders, thin walls and look like cerebrospinal fluid. They are best demonstrated on MRI.

Most arachnoid cysts are aymptomatic and generally should not be treated surgically unless there is increased intracranial pressure or visual field defects .