The brain and spinal cord are surrounded and protected by three meninges or membranes: the dura mater, arachnoid mater and pia mater. Arachnoid cysts are fluid-filled sacs that form in the arachnoid space. The cysts usually accumulate cerebrospinal fluid (CNS), a protective fluid, preventing its normal drainage into the CNS system. Arachnoid cysts are most commonly congenital and present at birth (primary arachnoid cysts), but can also develop later in life (secondary arachnoid cysts).
The exact cause of primary arachnoid cysts is not clear; it may result from developmental abnormalities or genetics. Secondary arachnoid cysts may develop as a reaction to brain surgery, trauma, tumors, head injury or infection.
Symptoms can usually go undetected for years. However, if the cyst grows large enough it can compress neighboring areas of the brain or spinal cord, producing symptoms such as headache, nausea, vomiting, lethargy, dizziness, seizures, hearing problems, neurological problems, difficulties in balancing and walking, muscle spasms or weakness, bowel and bladder control problems, back pain, and tingling or lack of sensation in your arms and legs.
When you present to the clinic with these symptoms, your doctor diagnoses arachnoid cysts with imaging tests such as CT and MRI scans. Treatment depends on the location and size of the cyst, as well as on the symptoms. Asymptomatic cysts are monitored for growth and further changes, while symptomatic cysts need to be removed. Owing to the risks involved with brain surgery, cysts in the brain are treated with fenestration, where a small incision is made at the cyst and excess fluid is drained into the CNS system. Your surgeon may alternatively perform shunting, a process which involves the insertion of a catheter or narrow tube to divert excess fluid to a different part of the body. For cysts on the spinal cord, treatment involves surgical removal, shunting or fenestration.