An arachnoid cyst is a small, benign sac that develops between the brain or spinal cord and the arachnoid membrane (which is one of three membranes that cover the brain and spinal cord). It most often occurs in the brain, where it is also called an intracranial cyst, but an arachnoid cyst can develop in the spinal cord as well.
An arachnoid cyst is not a brain tumor. It is a benign cyst, usually filled with clear cerebrospinal fluid (CSF), and is most often present at birth, although it may not be detected for years, if ever. When an arachnoid cyst is present at birth it is called a primary cyst; those that develop later in life are called secondary cysts.
An arachnoid cyst rarely causes symptoms, but if it does it is most often in the first 12 months of a child’s life, sometimes not until adolescence as the cyst grows, and almost always before age 20. An arachnoid cyst that causes symptoms needs to be evaluated for possible treatment, as it may continue to grow and cause neurological damage as it puts pressure on surrounding brain tissue.
The symptoms of an arachnoid cyst depend on where in the central nervous system it occurs — intracranial cysts cause different symptoms than spinal cysts do (see Symptoms of an Arachnoid Cyst). It occurs much more commonly in boys than in girls.
What Causes an Arachnoid Cyst?
The exact cause of a primary arachnoid cyst is unknown — it develops in a fetus during pregnancy, but nobody knows why. Secondary cysts can be caused by trauma (a fall, accident, or other injury), illness (meningitis or brain tumor), or as a complication of brain surgery.
Reviewed by: Jeffrey Greenfield, M.D., Ph.D.
Last reviewed/last updated: March 2021
Illustration by Thom Graves, CMI