Arachnoid cysts are fluid-filled sacs that develop between the central nervous system (brain and spinal cord) and the arachnoid membrane, one of three protective membranes that surround the central nervous system. Cells from the arachnoid layer produce cerebrospinal fluid, the fluid that protects and nourishes the brain and spinal cord. Consequently, arachnoid cysts are filled with cerebrospinal fluid.

Arachnoid cysts can be primary (present at birth) or secondary (arising due to disease or injury). Sometimes, cysts will develop without leading to symptoms. Most arachnoid cysts are harmless and produce no symptoms. If the cyst is large, however, it may press on the central nervous system, which can cause headaches, lethargy, and neurological damage. In these cases, the cyst may require surgical draining.