The cause of subdural effusion in the elderly is due to cerebral atrophy. With increasing age, the brain tissue will shrink, so the gap between the dura mater and the brain tissue will become bigger and bigger, and after the enlarged gap is filled with cerebrospinal fluid, subdural effusion will be formed. Generally, the amount of fluid is not too much, so there will not be any clinical symptoms, and only need to be observed and regularly reviewed. However, in some cases, the subdural effusion gradually increases and even forms a subdural hematoma, which requires surgery and drilling for drainage. Some subdural effusions are caused by head trauma, where the arachnoid membrane is torn and the cerebrospinal fluid enters the subdural to form a subdural effusion. Whether or not to operate depends on the amount of effusion and the patient’s specific condition to be evaluated and decided.