Hydrocephalus is the abnormal accumulation of spinal fluid, called cerebrospinal fluid (CSF), within the brain. Hydrocephalus can be caused by a structural defect in the brain or spine that blocks CSF and causes it to accumulate. Sometimes a brain injury, especially one that causes bleeding, can interfere with the flow of CSF and cause it to build up and increase pressure. Sometimes, the brain can shrink because of brain damage and the CSF fills in the extra space. That condition does not cause pressure on the brain and does not require special treatment.