Hydrocephalus refers to a presence of liquid in abnormal quantities in the meninges, the membranes surrounding the brain, or more often the cerebral ventricles, the cavities between the brain structures. It can be sudden, and is then called acute hydrocephalus which is often a complication of a subarachnoid hemorrhage. A blood clot in this case blocks the normal flow of the cerebrospinal fluid contained within the ventricles, resulting in rapid accumulation. It can cause intracranial hypertension with compression of certain brain structures and may cause irreversible brain damage. This is an emergency requiring surgical management to shunt the flow of excess liquid. Hydrocephalus can also progress more slowly: this is called normal pressure hydrocephalus.