An arachnoid cyst is a congenital abnormality within the brain or the spinal canal. It occurs during the development of the arachnoid membrane before birth. Thus, it is not a tumor. It occurs in males four times more frequently than in females, and more in the left side of the brain than the right side of the brain. It is typically an incidental finding, meaning it rarely causes symptoms.

The arachnoid membrane is one of the covering layers of the brain that also extends down and covers the spinal cord as well. An arachnoid cyst occurs when the developing arachnoid membrane splits and forms a fluid filled cavity or cyst. This is usually filled with cerebrospinal fluid which also surrounds the brain and spinal cord. As the child matures the brain develops around the cyst. There may be a skull protrusion or a bump over the location of the cyst.

Arachnoid cysts are mostly asymptomatic, but symptoms depend on the location of the cyst within the brain. Possible symptoms of increased pressure in the brain include headaches, nausea and vomiting, and lethargy. Other symptoms include seizures, hemorrhage, focal skull protrusions, hydrocephalus (water on the brain), endocrine dysfunction, head bobbing, and visual impairment.

Cysts are typically found incidentally on CT or MRI scan, which show size and location. If concerned about the flow of cerebrospinal fluid, special cerebrospinal fluid flow studies can be performed to see if there is obstruction of the flow.

If the cyst is asymptomatic, it can be followed by a repeat imaging study in 6-8 months to check for any changes. If any symptoms arise, or if it progresses in size, definitive treatment may be undertaken.  Treatment options include drainage, shunting, and fenestration.  Fenestration can be performed endoscopically (through a small incision, using an endoscope) or through a craniotomy.  This is performed by opening the skull, and making several small holes in the cyst wall, allowing it to communicate with the normal flow of cerebrospinal fluid.

Lindsey Parker PA-C; Justin F. Fraser M.D.