Arachnoid cysts: what you need to know

Arachnoid cysts are the most common type of brain cyst. These are not tumors, but cysts filled with cerebrospinal fluid (a clear fluid that surrounds your brain and spinal cord). Most people who have arachnoid cysts are born with them. You can also get one after a head injury, meningitis, or brain surgery.

Doctors do not know why these cysts occur. But they can be caused by damage to the arachnoid membrane, one of the protective layers that surround your brain and spinal cord.

Since they usually don’t have any symptoms, it’s hard to say exactly how common they are. About 3 in 100 children in the United States are thought to have them. Men are four times more likely to have them than women.

What are the symptoms of arachnoid cysts?

Most pose no problem. You may not know you have one, unless your doctor is examining you for another problem, such as a seizure or head injury.

Sometimes, however, a cyst grows large enough to press on your brain, spinal cord, or cranial nerve, which can lead to various symptoms. If you are going to have symptoms, they will probably start in childhood.


Symptoms vary from mild to severe, depending on the size and location of the cyst. They may include:

  • Headache
  • Nausea
  • Vomiting
  • Feeling tired and lethargic
  • Seizures
  • Bumps on the head or spine
  • Developmental delays in children
  • Hormonal problems like precocious puberty
  • Uncontrolled head movements
  • Sight problems
  • Fluid buildup in the brain (hydrocephalus)

Rarely, you may have weakness or paralysis on one side of your body. This is called hemiparesis. Another rare complication in children is an enlarged head (macrocephaly).

How are they diagnosed and treated?

Your doctor will do a computed tomography or a brain MRI to diagnose an arachnoid cyst. These tests also show the location and size of the cyst, which will help your doctor determine how to treat it.

Doctors treat arachnoid cysts with brain surgery. The type of surgery will depend on the severity of your cyst and how it affects your brain or spine. The goal is to drain the fluid from the cyst to relieve pressure on the surrounding tissue.


If your cyst is small and isn’t causing symptoms, your doctor may recommend that you leave it alone.

Surgical treatments include:

Drainage. In this procedure, a neurosurgeon pierces the cyst with a small needle to drain the fluid. Or they can opt for a burr hole drainage procedure. For this operation, your surgeon drills a small hole in your skull to relieve the pressure from the build-up of fluid.

It can be a quick and easy procedure. But with this method, there is a higher chance that your cysts (and symptoms) will come back.

Endoscopic fenestration. Your surgeon inserts an endoscope, a small device that includes a camera, into a small incision near the cyst. Then, they make openings in the wall of the cyst (called fenestration) to allow cerebrospinal fluid to drain.

After fenestration, it is rare for the arachnoid cyst to refill with fluid.

Craniotomy for fenestration. In this advanced procedure, your surgeon opens your skull to reach and open the cyst. You might get it if the size or location of your cyst means that endoscopic fenestration is not possible.


Referral. Your surgeon inserts a tube, or shunt, into the cyst. This allows the liquid to drain and be absorbed into your body. But because the shunt stays in place, you can become dependent on it to keep your symptoms from coming back. Shunts can also cause complications like infections or blockages.

Resection. If you have an arachnoid cyst on your spine, your surgeon may be able to remove it completely. This usually ends your symptoms.

What is Outlook?

Arachnoid cysts that do not cause symptoms do not need to be treated. But if you notice any symptoms, talk to a doctor as soon as possible.

If a cyst that is causing symptoms is left untreated, it can get bigger. Or an injury could cause it to leak or start to bleed. These things could lead to permanent nerve damage to the spinal cord or brain.

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