Learning About Arachnoid Cysts

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A cross-section of the brain and spinal cord in the head, with detail of the brain, the arachnoid membrane, and the skull.

What is an arachnoid cyst?

An arachnoid cyst is a fluid-filled sac that grows between the arachnoid membrane and the brain or spinal cord. The arachnoid is one of the layers of tissue (membranes) that cover the brain and spinal cord.

This type of cyst can form as a baby grows in the uterus. Or the cyst may form later in life from an illness or head injury.

What are the symptoms?

Some arachnoid cysts don't cause any symptoms. If you do have symptoms, they may include:

  • A headache.
  • Nausea and vomiting.
  • Seizures.
  • Problems with hearing and vision.
  • Dizziness.
  • Trouble with balance and walking.

If the cyst presses on the spinal cord, the symptoms might include:

  • Back and leg pain.
  • Tingling or numbness in the legs or arms.
  • Weakness in the legs that may get worse over time.

How is it treated?

Treatment may not be needed if there aren't any symptoms. You may have follow-up appointments and regular MRI tests. This is to see if the cyst is growing.

Some cysts are large or are in a place where they cause symptoms. A cyst that puts pressure on the brain or spinal cord may cause permanent injury. Treatment may be a surgery to place a shunt or to remove part or all of the cyst. This may allow the fluid to drain.

Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor if you are having problems. It's also a good idea to know your test results and keep a list of the medicines you take.




The Health Encyclopedia contains general health information. Not all treatments or services described are covered benefits for Kaiser Permanente members or offered as services by Kaiser Permanente. For a list of covered benefits, please refer to your Evidence of Coverage or Summary Plan Description. For recommended treatments, please consult with your health care provider.