The ventricular system within the brain and the subarachnoid space outside the brain are filled with a colorless, clear fluid, cerebrospinal fluid. Cerebrospinal fluid is produced from blood, specifically by filtration and secretion from a tissue within the ventricles called the choroid plexus. The generated cerebrospinal fluid first fills the ventricles, then flows into the subarachnoid space through the ventricular and external foramina, and finally is absorbed by the villi-like granules of the arachnoid membrane into the venous system, thus completing the circulation of cerebrospinal fluid. After a subarachnoid hemorrhage, the subarachnoid space is filled with blood. Especially in case of heavy bleeding, clots can block the passage between the ventricles and the outside, and the stale blood closes the arachnoid granules, the absorbing structures of cerebrospinal fluid. As a result, it is possible that the cerebrospinal fluid that is continuously generated within the ventricles may not flow out, and even if it does, it may not be absorbed, thus causing a dramatic increase in intracerebroventricular pressure and causing ventricular enlargement, which is called hydrocephalus. Hydrocephalus in the acute phase of subarachnoid hemorrhage is sometimes very serious, causing a significant increase in intracranial pressure along with hemorrhage and cerebral edema, and thus can be life-threatening to the patient. After the subarachnoid hemorrhage has healed, some patients continue to have progressive ventricular dilatation during the recovery period and longer thereafter. This condition is usually due to blockage of the arachnoid granules and formation of arachnoid adhesions following the hemorrhage, allowing poor reflux and malabsorption of cerebrospinal fluid in the subarachnoid space. Although the ventricles are open to the external passages, the ventricles dilate progressively to compensate for the slowly increasing pressure. In this case, the measured cerebrospinal fluid pressure is usually not high, so it is called normal pressure hydrocephalus. The chronic dilatation of the ventricles causes progressive atrophy of the brain, and the patient may present with progressive dementia, impaired mobility of both lower extremities, and urinary disturbances.