Arachnoid Cysts

Arachnoid cysts are sacs filled with spinal fluid that are located between the brain or spinal cord and the arachnoid membrane, one of the three membranes that cover the brain and spinal cord.   

• Primary arachnoid cysts are present at birth and are the result of developmental abnormalities in the brain and spinal cord that arise during the early weeks of gestation.  
• Secondary arachnoid cysts are not as common as primary cysts; they develop as a result of head injury, meningitis or tumors, or as a complication of brain surgery. 

These brain cysts are typically congenital and detectable at birth. These fluid-filled sacs may resemble tumors, are often asymptomatic and can also form as a result of head injury or trauma. In some cases, arachnoid cysts may result from head injury or trauma. 

Most instances of arachnoid cysts are sporadic, with the causes unknown. Some research shows that there may be a genetic predisposition for cysts forming, and males are four times more likely to have arachnoid cysts than females.

Arachnoid cysts are detected using magnetic resonance imaging (MRI). These scans can help distinguish these fluid-filled cysts from other types. As causes are unknown at this time, there are no known means of prevention. 

Most people with arachnoid cysts develop symptoms before the age of 20, and especially during the first year of life, but some people with arachnoid cysts never have symptoms. Typical symptoms of an arachnoid cyst around the brain include:

• Headache
• Nausea and vomiting
• Seizures
• Hearing and visual disturbances
• Vertigo
• Difficulties with balance and walking

Arachnoid cysts around the spinal cord press parts of the spinal cord, or nerve roots, closer together. This causes symptoms such as back and leg pain and tingling or numbness in the legs or arms.

If arachnoid cysts are not treated, they may cause serious, permanent nerve damage if the cyst(s) injures the brain or spinal cord. This can happen if the cyst(s) get larger or if there is bleeding into the cyst. Treating the symptoms of arachnoid cysts usually makes the symptoms go away or improve.

Diagnosis usually involves a brain scan or spine scan using diffusion-weighted MRI, which helps distinguish fluid-filled arachnoid cysts from other types of cysts.

The need for treatment depends mostly on the location and size of the cyst. If the cyst is small, it does not disturb surrounding tissue, and is not causing symptoms, some doctors will decide not to treat it.

Modern techniques and tools allow for surgery that is less invasive (in other words, surgery that involves using smaller cuts and fewer stitches). So, more doctors are choosing to remove the membranes of the cyst with surgery, or to open the cyst so its fluid can drain into the spinal fluid and be absorbed. 

If arachnoid cysts are not treated, they may cause serious, permanent nerve damage if the cysts injure the brain or spinal cord. This can happen if the cysts get larger or if there is bleeding into the cyst. Treating the symptoms of arachnoid cysts usually makes the symptoms go away or improve.

To further your understanding of your diagnosis and to contribute to cutting-edge research, consider participating in a clinical trial so clinicians and scientists can learn more about causes, symptoms, treatment and prevention. Clinical research uses human volunteers to help researchers learn more about a disorder and perhaps find better ways to safely detect, treat or prevent disease.

All types of volunteers 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 participants at Clinicaltrials.gov.