Hydrocephalus is the condition where the fluid spaces in the brain (Ventricles) become enlarged.
There are three basic types:

1. Communicating Hydrocephalus: Where the Cerebro-Spinal Fluid (CSF) inside of the ventricles communicates or is open to, the fluid spaces surrounding the brain. This type usually occurs as a result of some sort of dysfunction of the absorption channels known as the Arachnoid Villi. This can be a result of infection such as meningitis; Blockage with blood from ruptured aneurysms (see SAH); or from abnormal aging and absorption failure as seen in Normal Pressure Hydrocephalus (NPH).

2. Non-communicating Hydrocephalus: This results from an actual mechanical obstruction somewhere along the connections of the 4 Ventricles in the brain. Examples include Colloid Cysts which obstruct the 3rd ventricle, and tumors of the brainstem with compress the channel between the 3rd and 4th ventricle (Aqueduct of Sylvius).

3. Hydrocephalus Ex-Vacuo: Sometimes the brain will shrink in size (as is seen in Alzheimer’s Disease), and as a result, the ventricles will enlarge to compensate. Thus though the ventricles are enlarged, they are not under any pressure.

To learn more about hydrocephalus: Hydrocephalus Association – Support, Education and Advocacy

Neurological and Neurosurgery Institute Beth Israel Medical Center, NY