hydrocephalus is when there is increased pressure in the ventricles pushing OUT at the brain. it is caused by a blockage within the ventricular system.
IIH is when there is increased pressure all around the outside of the brain, pushing IN. it is generally caused by poor CSF drainage at the arachnoid granulations, due to an abnormal pressure gradient from CSF to the venous system. what causes that may be different in different people, and not always discernable.