What Are Arachnoid Cysts?

Medically Reviewed by Neha Pathak, MD on June 07, 2021
3 min read

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

Doctors aren't sure why these cysts happen. But they may 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 tell exactly how common they are. About 3 in every 100 children in the U.S. are thought to have them. Males are four times more likely to get them than females.

Most don't cause any problems. You may not know you have one unless your doctor is checking you for another issue, like a seizure or head injury.

Sometimes, though, a cyst gets big enough to press on your brain, spinal cord, or a cranial nerve, which can lead to a variety of symptoms. If you're going to have symptoms, they'll probably show up during childhood.

The 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
  • Lumps on your head or spine
  • Developmental delays in children
  • Hormonal issues like early puberty
  • Uncontrolled head bobbing
  • Vision problems
  • Fluid build-up in the brain (hydrocephaly)

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).

Your doctor will do a CT scan or a brain MRI to diagnose an arachnoid cyst. These tests also show the cyst's location and size, which will help your doctor figure out how to treat it.

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

If your cyst is small and not causing symptoms, your doctor may recommend just leaving it alone.

Surgical treatments include:

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

This can be a quick and simple procedure. But with this method, there’s a higher chance that your cysts (and symptoms) will come back.

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

After fenestration, it’s rare for the arachnoid cyst to fill with fluid again.

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

Shunting. Your surgeon inserts a tube, or shunt, into the cyst. This allows the fluid to drain and be absorbed into your body. But because the shunt remains in place, you may 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.

Arachnoid cysts that don't lead to symptoms don’t need to be treated. But if you notice symptoms, tell a doctor as soon as possible.

If a cyst that's causing symptoms is left untreated, it can get bigger. Or an injury might make it leak or start bleeding. These things could lead to permanent nerve damage to your spinal cord or brain.