Arachnoid cysts are cerebrospinal fluid-filled sacs 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. The majority of 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.
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 and develop as a result of head injury, meningitis, or tumors, or as a complication of brain surgery.
Arachnoid cysts are seen in 4% of the population. Only 20% of these have symptoms, usually from secondary hydrocephalus (water on the brain causing pressure inside the skull). Most individuals 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. Males are four times more likely to have arachnoid cysts than females.
The location and size of the cyst determine the symptoms and when those symptoms begin. Typical symptoms of an arachnoid cyst around the brain include:
Arachnoid cysts around the spinal cord compress the spinal cord or nerve roots and cause symptoms such as:
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.
The need for treatment depends mostly upon the location and size of the cyst. If the cyst is small, not disturbing surrounding tissue, and not causing symptoms, some doctors will refrain from treatment. In the past, doctors placed shunts in the cyst to drain its fluid. Now with microneurosurgical techniques and endoscopic tools that allow for minimally invasive surgery, more doctors are opting to surgically remove the membranes of the cyst or open the cyst so its fluid can drain into the cerebrospinal fluid and be absorbed.
Untreated, arachnoid cysts may cause permanent severe neurological damage when progressive expansion of the cyst(s) or bleeding into the cyst injures the brain or spinal cord. Symptoms usually resolve or improve with treatment.