What are 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. 

Most arachnoid cysts form outside the temporal lobe of the brain in an area of the skull known as the middle cranial fossa. Arachnoid cysts involving the spinal cord are rarer. The location and size of the cyst determine the symptoms and when those symptoms begin.   

Symptoms

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.

Who is more likely to get arachnoid cysts?

Males are four times more likely to have arachnoid cysts than females. 

How are arachnoid cysts diagnosed and treated?

Diagnosing arachnoid cysts

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

Treating arachnoid 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.  

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

• Accumulation of excessive cerebrospinal fluid in the brain (hydrocephalus), resulting in increased intracranial pressure
• Malformation of certain cranial bones, resulting in macrocephaly, which is an abnormally enlarged head (only in rare cases)
• Neurological signs such as developmental delays, behavioral changes, inability to control voluntary movements (ataxia), difficulties with balance and walking, cognitive impairment, and weakness or paralysis on one side of the body (hemiparesis)