Arachnoid Cysts

An arachnoid cyst is a noncancerous fluid-filled sac that grows on the brain or spinal cord. Symptoms include headaches and seizures, but many arachnoid cysts don’t cause symptoms. Treatment isn’t always necessary. Providers drain or remove cysts that cause symptoms. Untreated, arachnoid cysts can cause brain damage and movement problems.


What are arachnoid cysts?

Arachnoid cysts are fluid-filled sacs that grow on the brain and spine. They are not tumors, and they are not cancerous. On rare occasions, if they grow too big or press on other structures in the body, they can cause brain damage or movement problems.

Most arachnoid cysts appear at birth or after childhood head trauma, and the vast majority of cysts don’t cause symptoms. When symptoms do appear, they vary from person to person. They may include headaches, dizziness, nausea, vomiting or seizure.

Treatment depends on where the cyst is and if it’s growing or causing symptoms. If treatment is necessary, providers usually drain the cysts or open them surgically to the surrounding spaces.


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How common are arachnoid cysts?

Arachnoid cysts are the most common kind of brain cyst. They affect people of all ages, but they arise in babies and children. Males are four times more likely to develop an arachnoid cyst than females.

Healthcare providers don’t know how many people get arachnoid cysts. Most arachnoid cysts develop without causing symptoms and are found accidentally when the head is scanned for other reasons. So, it’s nearly impossible to know how many people have them.

Where do arachnoid cysts grow?

Arachnoid cysts usually grow on the brain (intracranial arachnoid cysts). Less commonly, they grow on the spinal cord (spinal arachnoid cysts). Both types develop on a thin membrane that covers and protects the brain and spinal cord. Providers call this membrane the arachnoid membrane because it looks like a spider web.

The cysts can form in several areas of the brain. Most arachnoid cysts grow in the middle fossa region, located in front of the ears. They can also grow in the suprasellar region (behind the eyes) and the posterior fossa (at the base of the skull).

These areas of the brain and spinal cord contain cerebrospinal fluid (CSF). CSF is a clear fluid that cushions the brain and spinal cord. It also delivers nutrients and removes waste from the brain. Arachnoid cysts form when cerebrospinal fluid collects and builds up inside a sac.


Symptoms and Causes

What are the symptoms of arachnoid cysts?

The vast majority of arachnoid cysts produce no symptoms. When they do, symptoms of an arachnoid cyst range from mild to severe. They depend on the location and size of the cyst and whether it’s pressing on nerves, the brain or the spinal cord. When symptoms do occur, they’re more likely to appear before age 20. They include:

  • Headaches.
  • Hydrocephalus (excess cerebrospinal fluid collects in the brain).
  • Nausea and vomiting.
  • Seizures.
  • Vertigo and dizziness.
  • Other symptoms vary depending on the location of the cyst. They include:
  • Middle fossa region: Cysts in this area of the brain can cause problems with vision, hearing, movement and balance. They can lead to fatigue and weakness or paralysis, usually on one side of the body. Some children have neurological (nervous system) symptoms. These may include delays in development and changes in behavior.
  • Suprasellar region: Cysts in this region can cause vision problems. They may also affect the endocrine system, which controls puberty and sexual development. Some people with suprasellar or posterior fossa arachnoid cysts bob their heads uncontrollably. They may move their heads side to side or in a circular motion (similar to a bobble-head doll).
  • Spinal cord: Spinal arachnoid cysts can lead to numbness and tingling in the feet and hands, muscle spasms, movement problems and paralysis. Back pain and scoliosis are also common. Some people develop urinary tract infections from a spinal cyst.

What causes arachnoid cysts?

Most arachnoid cysts appear at birth (primary arachnoid cysts). They usually occur in the uterus during the early weeks of pregnancy. Healthcare providers aren’t sure what causes arachnoid cysts to form.

Sometimes arachnoid cysts run in families, so providers think genetics may cause them to develop. People who have certain health conditions, such as arachnoiditis or Marfan syndrome, may be more likely to develop arachnoid cysts.

Less often, arachnoid cysts grow after some sort of childhood head trauma, such as a brain injury, surgery or infection. When this happens, providers call them secondary arachnoid cysts.


Diagnosis and Tests

How do healthcare providers diagnose arachnoid cysts?

Signs of arachnoid cysts are often similar to symptoms of other conditions. If you or your child has symptoms of an arachnoid cyst, your provider will ask about your health history. They may recommend tests based on your symptoms.

Healthcare providers often discover arachnoid cysts when someone seeks treatment for another health concern, such as seizures. Providers use imaging studies, including MRIs and CT scans, to see pictures of the cysts.

Management and Treatment

How do healthcare providers treat arachnoid cysts?

If a cyst is causing symptoms, your provider may recommend regular imaging studies to watch the cyst and check its growth. MRIs and CT scans help your provider see if the cyst is pressing on other structures in the body, such as nerves, the brain or the spinal cord. The vast majority of cysts don’t need further treatment.

If the cyst is large and growing or causing symptoms, your provider may recommend treatment. This may include:

  • Endoscopic procedures: Depending on the location and size of the cyst, your provider may recommend an endoscopic procedure to drain the cyst or open a ‘window’ in it. Endoscopic procedures use a small, thin tube with a camera and tiny tools. These minimally invasive procedures use smaller incisions. This means you need less recovery time than traditional surgery.
  • Open craniotomy fenestration: During this procedure, your provider removes part of the skull and makes small incisions (cuts) in the cyst. These ‘windows’ allow fluid to drain from the cyst. Your body absorbs the fluid over time. Your provider replaces the skull piece and seals it in place with stitches.
  • Shunting: Your provider inserts a cyst operitoneal shunt (also called a CP shunt) into the cyst. A shunt is a device made of catheters (thin tubes) and a valve. Fluid drains through the tubes into your abdomen (belly). Your body absorbs the fluid. The shunt will remain in place so fluid can continue to drain.
  • Surgical removal: Healthcare providers usually remove spinal arachnoid cysts. Your provider makes an incision near the cyst and removes it. If it’s not possible to remove the cyst because of its location or size, your provider may recommend draining it or placing a shunt in the cyst.


Can I prevent arachnoid cysts?

It isn’t possible to prevent arachnoid cysts. Talk to your provider if you have a family history of arachnoid cysts or a condition (such as arachnoiditis) that can increase your risk.

Outlook / Prognosis

What is the outlook for people with arachnoid cysts?

Most arachnoid cysts never cause symptoms, but on the rare occasions that they do, treatment for arachnoid cysts usually relieves symptoms. But cysts can grow back or fill with fluid after treatment. If that happens, you may need another procedure to drain the fluid or remove the cyst.

Untreated, symptomatic arachnoid cysts can lead to permanent brain damage, severe pain, movement disorders and serious health problems. Rarely, untreated cysts can cause the skull to grow in an abnormal way. Complications from arachnoid cysts include:

  • Bleeding: Blood vessels on the cyst wall can tear and bleed into the cyst, which can make it grow larger. When blood vessels tear and blood pools outside of the cyst, a hematoma can form.
  • Leaking fluid: If trauma or injury damages the cyst, a CSF leak can result. The fluid can leak into other parts of the brain, causing severe health problems.

Living With

When should I see my healthcare provider about arachnoid cysts?

See your provider right away if you or your child has symptoms of an arachnoid cyst. Some cysts need immediate treatment to avoid long-term health problems. Many symptoms of arachnoid cysts are similar to signs of life-threatening problems, such as a brain tumor. It’s important to see your provider for an evaluation.

A note from Cleveland Clinic
Many arachnoid cysts don’t cause symptoms and don’t require treatment. For cysts that do cause symptoms, several treatments are available. Some of them are minimally invasive procedures that require less recovery time. Symptoms usually improve after treatment. Talk to your provider if you have a family history of arachnoid cysts. If you notice signs of an arachnoid cyst, see your provider as soon as possible. Monitoring and treating an arachnoid cyst early improves your long-term prognosis.

Medically Reviewed

Last reviewed by a Cleveland Clinic medical professional on 10/09/2021.

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