Arachnoid Cysts

What are arachnoid cysts?

Arachnoid cysts are the most common brain cysts. They are not tumors but sacs filled with cerebrospinal fluid (the protective fluid that coats and protects the brain and spinal cord). Arachnoid cysts happen when the arachnoid membrane, one of the three membranes that cover the brain and spinal cord, splits. When the membrane breaks, fluid collects inside, forming a cyst. Most arachnoid cysts don’t cause symptoms.

The location and size of an arachnoid cyst determines if and when a person may develop symptoms. Typically, arachnoid cysts are located in the middle cranial fossa at the base of the skull near the temporal lobe. The middle cranial fossa processes memories, sights, and sounds. Less frequently, arachnoid cysts can form near the spinal cord. 

Most arachnoid cysts are present at birth (called primary arachnoid cysts). They happen because of unusual brain or spinal cord development during the early weeks of pregnancy. Some develop later in life after a head injury, infection, tumor, or surgery (called secondary arachnoid cysts).  

Symptoms of arachnoid cysts

Most people with arachnoid cysts don’t have any symptoms. However, people who are born with arachnoid cysts and experience symptoms tend to experience them early in life. Children often develop symptoms within a year of birth, and most people develop symptoms before age 20 (if they are going to). 

When arachnoid cysts press on the brain or spinal cord, they can cause symptoms. The most common symptoms of an arachnoid cyst around the brain include:

• Headache
• Nausea and vomiting
• Seizures
• Problems with hearing and vision
• Vertigo
• Problems with balance and walking

Arachnoid cysts can press parts of the spinal cord, or nerve roots, closer together. This can cause back and leg pain and tingling or numbness in the legs or arms.

Who is more likely to get arachnoid cysts?

The causes of arachnoid cysts are not completely understood. However, some people are more likely to get arachnoid cysts than others. 

Men and boys are four times more likely to have arachnoid cysts than women and girls. Additionally, people with certain disorders, like Aicardi syndrome, may be more likely to get arachnoid cysts. In rare cases, changes in a person’s DNA, called variants, may make arachnoid cysts more likely.

How are arachnoid cysts diagnosed and treated?

Diagnosing arachnoid cysts

Diagnosis of arachnoid cysts usually involves a CT (computed tomography) or MRI (magnetic resonance imaging) scan of the brain or spinal cord, which helps distinguish arachnoid cysts from other cysts. Since arachnoid cysts often do not cause symptoms, many people learn that they have an arachnoid cyst during treatment for other disorders that require brain or spine imaging.

Treating arachnoid cysts

Only some arachnoid cysts need treatment. Small cysts that do not disturb surrounding tissue and do not cause symptoms may not need treatment. However, it is recommended that arachnoid cysts be monitored over time to make sure they don’t grow or begin causing symptoms. Sometimes, cysts can injure the brain or spinal cord if they get bigger. Head trauma near a cyst can cause it to grow or cause bleeding into the cyst.

Usually, treating arachnoid cysts makes symptoms go away or improve. Treatment for arachnoid cysts involves either draining the fluid in the cyst or removing it altogether. Some surgeries for arachnoid cysts cut the cyst open so that the fluid can drain out, a process called cyst fenestration. After surgery, the body absorbs the fluid released from the cyst. 

In rare cases, a doctor may place a shunt, a device that drains the fluid from the cyst into a different part of the body. Treatment for a severe arachnoid cyst may require more extensive surgery to remove the walls of the cyst entirely. In rare cases, arachnoid cysts may come back after surgery.

How can I or my loved one help improve care for people with arachnoid cysts?

Consider participating in a clinical trial, so clinicians and scientists can learn more about arachnoid cysts and related disorders. Clinical research with human study participants helps researchers learn more about a disorder and perhaps find better ways to safely detect, treat, or prevent disease.

All types of participants are needed — those who are healthy or may have an illness or disease — of all different ages, sexes, races, and ethnicities to ensure that study results apply to as many people as possible and that treatments will be safe and effective for everyone who will use them.

For information about participating in clinical research, visit NIH Clinical Research Trials and You. Learn about clinical trials currently looking for people with arachnoid cysts at ClinicalTrials.gov, a searchable database of current and past clinical studies and research results.