What exactly is the condition normal-pressure hydrocephalus?

An estimated 375,000 Americans have a condition called normal-pressure hydrocephalus, or NPH, which is often misdiagnosed as either Alzheimer’s disease because memory is impaired in both cases or as Parkinson’s because gait is affected in both, said Dr. Peter Black, chief of neurosurgery at Brigham and Women’s Hospital. Even CT and MRI scans often can’t tell the difference between these conditions and NPH.
Many doctors don’t know about it, but there is a way to tell the difference between Alzheimer’s and Parkinson’s, for which there is no curative treatment, and NPH, which can be reversed.