The answer is build up of metabolic waste.
The cerebrospinal fluid (CSF) is a viscous liquid the surrounds and suspends the brain in it. The CSF serves the function of protecting the brain from physical damage by acting as a cushion. It also provides nourishment to the brain tissues, and removes metabolic waste generated by the cells in the brain. The choroid plexuses, ventricles, and ependymal cells produce the cerebrospinal fluid. A damage to these cells will lead to the loss of CSF production causing a build up of waste within the brain.
