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