What are arachnoid cysts?Arachnoid cysts are fluid-filled sacs that occur on the arachnoid membrane that covers the brain (intracranial) and the spinal cord (spinal). Arachnoid cysts are collections of CSF cerebrospinal fluid contained within the arachnoidal lining of the brain. They represent approximately one percent of brain mass lesions. Most common in infants, they may also
present in adulthood. There are three membranes covering these components of the central nervous system: dura mater, arachnoid, and pia mater. Arachnoid cysts appear on the arachnoid membrane, and they may also expand into the space between the pia mater and arachnoid membranes (subarachnoid space). The most common locations for intracranial arachnoid cysts are the middle fossa (near the temporal lobe), the suprasellar region (near the third ventricle) and the posterior fossa, which contains the cerebellum, pons, and medulla oblongata. In infants, they cause head enlargement or signs of obstructive hydrocephalus such as headache, lethargy, double vision, and developmental delay. In adults, they are more likely to cause seizures, particularly those of the middle cranial fossa. Arachnoid cysts are readily seen on CT and MRI scans as non-enhancing cystic masses of near CSF density/intensity. Treatment options include fenestration of the cyst into normal CSF pathways, either endoscopically or by open craniotomy, or shunting of the cyst and/or ventricles into the peritoneal cavity. Successful cystic opening or fluid diversion is usually effective treatment.