In essence, these cysts are cerebrospinal fluid-filled sacs which form
between the brain (or spinal cord) and a layer that surrounds the brain known as the arachnoid membrane. Primary arachnoid cysts are the most common, arising en utero during development. On occasion, these cysts can also form later in life as a result of meningitis, tumor, trauma, or following brain surgery. Many people are asymptomatic, although those who develop symptoms usually do so prior to age 20.