An arachnoid cyst is a pocket of cerebro-spinal fluid that is surrounded by collagen and cells of the arachnoid layer of the brain or spinal cord. They develop either on the surface of the brain or one of the three arachnoid layers that cover the brain and spinal cord. They are congenital, but symptoms often don't develop until puberty.
Most (about 80%) of these cysts are asymptomatic. If discovered during searches for other conditions, they are merely monitored. However, a cyst can cause neurological symptoms if it presses against the brain or spinal cord. Although such cysts are common (occurring in about 1 out of every 25 people), they are often a zebra diagnosis as they can present with any of a number of symptoms that are more likely to occur with other conditions. They can often mimic conditions such as Meniere's disease, clinical depression, schizophrenia, and bipolar disorder.
If the cyst is symptomatic, treatment is usually recommended as the symptoms generally get worse or become permanent without treatment. The preferred treatment is a shunt to drain the cerebro-spinal fluid from the cyst. Where this is not feasible, more dangerous surgical procedures are usually required, such as opening a hole in the skull or vertebrae, or drilling a burr hole. Medication can be used to treat the headaches and seizures that often accompany the condition.