Summary:Intracranial arachnoid cysts have been found in 0.3% of computed tomography (CT) scans and in 0.1% of brain autopsy specimens, more often in children than in adults. Intracranial arachnoid cysts occur prevalently in males, on the left side, and in the temporal fossa. Their occasional association with other hereditary disorders suggests a genetic factor. The majority of temporal arachnoid cysts are of small size and remain so; large arachnoid cysts appear to exhibit a tendency to grow. Communication studies have suggested an inverse correlation between the size of the cyst and the extent of communication with surrounding cerebrospinal fluid (CSF) spaces. The small cysts, in particular, tend to be asymptomatic. Symptomatology may be aspecific, such as headache and epileptic seizures, or focal, including hemiparesis. Progressive symptoms of increased intracranial pressure may be the result of expansion of the cyst, intracystic hemorrhage progressing to subdural hemorrhage (especially with temporal fossa cysts), or obstructive hydrocephalus (associated with cysts at the posterior fossa and intraventricular locations). In children, the presenting symptoms are usually enlargement of the head, seizures, and developmental retardation. In the elderly, arachnoid cysts may present with the symptoms common to normal pressure hydrocephalus. On CT or magnetic resonance imaging (MRI) scans arachnoid cysts have a fluid content identical to CSF, and they are typically extraaxially located. Temporal fossa arachnoid cysts of medium size may show the typical straight-edged triangular or rectangular shape. Communication studies are recommended in the management of arachnoid cysts at the less frequent locations. Intraaxial cysts containing CSF-like fluid, which are not of the ex vacuo type in our classification, may be arachnoid or neuroepithelial cysts, and histological verification of biopsy specimens, including immunocytochemical studies and electron microscopy, may be required. Indications for treatment are the progression of symptoms, and the presence of severe functional disorders, such as hemiparesis, cranial nerve palsies, and endocrinological dysfunction. In children, enlargement of the head and the space-occupying nature of the cyst, which interferes with brain development, may be considered indications for intervention. As to the less specific symptoms of headache and seizures, surgical treatment appears to achieve amelioration in the majority of patients, although seizures may occasionally remain unchanged or even deteriorate. There is considerable controversy as to the primary intervention of choice: shunting or craniotomy with fenestration or excision of the cyst wall. In the presence of hydrocephalus, shunting should involve both the cyst and the ventricular system, using an interconnected shunt. Follow-up of treated patients may include sequential scans because improvement appears to parallel morphological normalization.