- National Institute of Health
Arachnoid cysts are sacs filled with cerebrospinal fluid located between the brain or spinal cord and the arachnoid membrane, one of three membranes that cover the brain and spinal cord. Primary arachnoid cysts are present at birth and are the result of developmental disorders in the brain and spinal cord that occur during the first weeks of pregnancy. 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. 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. 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. Males are four times more likely to develop arachnoid cysts than females.
Typical symptoms of an arachnoid cyst around the brain are headache, nausea and vomiting, seizures, hearing and visual disturbances, dizziness, and problems with balance and walking. Arachnoid cysts around the spinal cord compress the spinal cord or nerve roots and cause symptoms such as progressive back and leg pain and tingling or numbness in the legs or arms. Diagnosis usually involves a brain scan or spine scan using diffusion-weighted magnetic resonance imaging (MRI) that helps distinguish fluid-filled arachnoid cysts from other types of cysts.
There has been active debate about the treatment of arachnoid cysts. The need for treatment largely depends on the location and size of the cyst. If the cyst is small, does not interfere with surrounding tissue, and is not causing symptoms, some doctors will forgo treatment. In the past, doctors placed shunts in the cyst to drain the fluid. Now with micro neurosurgical techniques and endoscopic instruments that allow minimally invasive surgery, more doctors are choosing to surgically remove the cyst's membranes or open the cyst so that the fluid can drain into the cerebrospinal fluid and be absorbed.
Untreated arachnoid cysts can cause permanent severe neurological damage when progressive expansion of the cyst (s) or bleeding into the cyst damages the brain or spinal cord. Symptoms usually disappear or improve with treatment.
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