Normal pressure hydrocephalus: new concepts on etiology and diagnosis.

Normal pressure hydrocephalus (NPH) is remarkable for two reasons: 1) it is one of the few treatable causes of dementia, and 2) neuroradiologists are usually involved in making the diagnosis. Hakim and Adams (1) are generally credited with the initial description of NPH, although it may actually have been described under a different name earlier by McHugh (2). It consists of the clinical triad of gait disturbance, dementia, and incontinence in a patient who radiographically has communicating hydrocephalus, ie, ventricles dilated out of proportion to any sulcal enlargement (which distinguishes it from atrophy) (3). Over the 35 years since it was first described, the definition of NPH has been expanded. Initially it was considered to be idiopathic (4, 5); at present, common usage includes any form of chronic, communicating hydrocephalus (6, 7), and even a few noncommunicating forms such as aqueductal stenosis (8). Because all these patients may present with a similar clinical triad, and they may all be treated with a ventriculoperitoneal (VP) shunt, this expansion of the definition is probably appropriate, although certain secondary features distinguish the idiopathic form from communicating hydrocephalus with known causes. For example, the idiopathic form of NPH tends to present in the elderly (9), whereas patients with chronic communicating hydrocephalus from prior subarachnoid hemorrhage, meningitis, neurosurgery, or head trauma tend to present at an earlier age. Also, response to shunting seems to be worse (30–50%) for patients with the idiopathic form than for patients with a known cause of communicating hydrocephalus (50–70%) (10–12). Depending on the specific diagnostic criteria used, one half of the cases of NPH are considered to be idiopathic and one half result from a known insult; thus, NPH probably represents the final common pathway for a number of different disease processes (13–15). The symptom complex of NPH has been explained on the basis of both mechanical (16) and ischemic factors (17–21). It has been suggested that the ventricular enlargement leads to vascular stretching (22), and the decreased compliance (23) and high pulse pressure leads to local ‘‘barotrauma’’ (20) or ‘‘tangential shear stress’’ (16). It has been postulated that the purpose of the shunt is to