Hydrocephalus may be either communicating or non-communicating. Communicating is when the CSF is able to flow from ventricle to ventricle and circulate in its normal procedure, however, the problem is one of re-absorption (at the subarachnoid cisternae, also referred to as the arachnoid granulations). Non-communicating hydrocephalus is caused by an obstruction within the ventricular system.

Both communicating and non-communicating hydrocephalus can be subdivided into congenital and acquired types. One form of communicating hydrocephalus seen in infants is Benign External Hydrocephalus. The cause is unclear, but CSF flow studies suggest a disturbance in absorption. This condition usually corrects itself without surgery by 18 months of age.

Another type of communicating hydrocephalus is Normal Pressure Hydrocephalus. This condition normally strikes those who are middle age and older. The classic signs of this condition are dementia, incontinence, and a gait (walking) disturbance. Because the syndrome develops gradually and signs of increased intracranial pressure are absent, the diagnosis may not be made for a long time, and in many cases, the problem is misdiagnosed as Dementia, Alzheimer's, or Parkinson's. (Please refer to the Normal Pressure Hydrocephalus Section.)

Non-communicating generally indicates a blockage at a site within the ventricular system or between it and the subarachnoid space. The most common type of non-communicating hydrocephalus is Aqueductal Stenosis.