Usually unknown. When you say nph, i believe you are referring to normal pressure hydrocephalus. This is usually idiopathic and occurs in up to 500, 000 people in the U.S. The diagnosis may be made clinically when patients have the clinical triad of memory, gait, and bladder control problems. This in combination with ct/mri scans that demonstrate large cerebral ventricles. Other tests like CSF drainage may help.