Arachnoid Cysts

An arachnoid cyst is a congenital (present from birth) finding. The cyst contains cerebrospinal fluid (CSF) trapped in a pocket of the membrane which holds it.

Arachnoid cysts are most often in the front part of the temporal lobe, but can occur elsewhere in the brain or spine. They are benign and usually do not change or grow. Their size can vary quite a bit. Most arachnoid cysts do not cause any symptoms and are usually found by chance on an imaging study.

What’s in this section?

Signs and Symptoms

In some cases, an arachnoid cyst may grow and put pressure on the brain or spine. When this happens, signs and symptoms can develop. Usually this occurs during childhood. Signs and symptoms are based on the location of the cyst such as:

  • Brain
    • Headaches
    • Nausea and vomiting
    • Seizures
    • Loss of balance or trouble walking
  • Spine
    • Weakness in the legs or arms
    • Back pain
    • Changes in bowel function
    • Trouble urinating

Rarely, hemorrhage may occur in an arachnoid cyst, usually as a result of trauma.


Arachnoid cysts are recognized on computed tomography ( CT ) or magnetic resonance imaging ( MRI ) scans. Most are found by chance.

If a cyst is found and is not causing any problems, it will be imaged again in about a year with either a CT or MRI of the brain or spinal cord to make sure that the cyst has not changed. Research has shown that arachnoid cysts in the brain are more likely to change in children under 5 years of age. If a cyst is diagnosed in a child under 5 years of age, he/she will have yearly scans until they reach age 5. If there has been no change in the cyst and the child has no symptoms no further scanning is needed. In children diagnosed after age 5, no further scans are needed if there are no changes between the first scan and a follow-up scan.

Treatment Options

Treatment recommendations are based on symptoms caused by the arachnoid cyst:

  • Observation If the cyst is not causing symptoms, observation is typically recommended. Research has shown that arachnoid cysts in the brain are more likely to change in children under 5 years of age. Yearly scans (MRI or CT) to look for growth may be recommended until age 5. If there are no changes or symptoms, no further scans are needed. For asymptomatic patients who are older at the time of diagnosis, no further scans are needed if there is no change on a one-year follow-up scan.
  • Cyst shunt CSF from the cyst may be drained to another location, such as the abdominal cavity, where it will then be absorbed. The shunt system is similar to the one used to treat hydrocephalus.
  • Fenestration. This surgery may be done through an open craniotomy or an endoscopic approach. Windows are surgically created in the wall of the arachnoid cyst. This allows the trapped CSF to flow into the normal CSF spaces and circulate in its regular pattern.

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