An arachnoid cyst is actually an intracranial sac containing cerebrospinal fluid with an arachnoid membrane wall (all of the brain has a membrane on its outer surface called the arachnoid). The cerebrospinal fluid inside the cyst is generally not directly the same as the cerebrospinal fluid in other parts of the skull outside the cyst, so most of the cyst has a certain amount of pressure that can put pressure on the surrounding brain tissue or skull. It can cause headache, dizziness or epilepsy. However, many patients are asymptomatic and are mostly found by chance due to a cranial CT or MRI performed after trauma or headache after a cold. Arachnoid cysts need to be treated in the following two situations: 1) if they cause significant headache, nausea and vomiting and other symptoms of cranial hypertension; 2) if it is determined that the arachnoid cyst is causing epilepsy. Other than this, there is no need for surgery or medication. Many patients are usually asymptomatic and are found by chance due to trauma. The main surgical procedures are: 1) cystic window drainage, in which a portion of the cyst wall is removed to communicate with the subarachnoid space within the skull; 2) cystic ventriculoperitoneal shunt, in which a silicone tube is used to drain the fluid from the cyst into the abdominal cavity, and the fluid is absorbed by the peritoneum within the abdominal cavity.