Arachnoid cyst

In medicine, arachnoid cysts are "intracranial or spinal cavities containing a cerebrospinal-like fluid, the wall of which is composed of arachnoidal cells. They are most often developmental or related to trauma. Intracranial arachnoid cysts usually occur adjacent to arachnoidal cistern and may present with hydrocephalus; headache; seizures; and focal neurologic signs."

Arachnoid cysts may be classified as:
 * Type I cysts are extradural without involving of nerve-root fibers
 * Type II cysts, also called Tarlov cysts, are extradural and involve nerve-root fibers
 * Type III cysts are intradural