Normal pressure hydrocephalus: new findings and old questions.

Normal pressure hydrocephalus (NPH), a form of communicating hydrocephalus, is characterized by normal mean CSF pressure. MR imaging is essential to establish the presence of ventricular di-latation and to exclude other conditions that also may be responsible for the clinical triad of gait disorder , mental slowing, and urinary incontinence. Clinical features that support the diagnosis of NPH include a recent or remote history of subarachnoid bleeding or meningitis and mental changes of a ''subcortical dementia,'' typified by slowness of responses (bradyphrenia), and dullness and apathy, different from early Alzheimer's disease. The gait is typically slow and unsteady with magnetic responses (as if the feet are stuck to the floor) and gait apraxia. The ''CSF tap test,'' with which some CSF is removed by means of lumbar puncture, may be of value despite false-negative and false-positive tests, but only if the test is well standardized (such as comparing the time required to walk a standard distance before and after large volume CSF drainage). Lumbar puncture is essential to establish that the CSF pressure is normal and to exclude a chronic meningitis that can be responsible for large ven-tricles and the clinical features of NPH. An elevated protein (Յ3000 mg/dL) may be the sole indicator of a spinal neoplasm responsible for the syndrome of NPH. The major diagnostic challenge is to differentiate NPH from cerebral atrophy and deep white matter ischemic, both far more common causes of the clinical triad than NPH. Although NPH and cerebral ischemic changes often concur, the clinician's major question for the neuroradiologist is whether the ventricular enlargement observed is consonant with the degree of enlargement of the cortical sulci (ie, is it hydrocephalus ex vacuo due to cerebral atrophy or is it communicating hydrocephalus?). Unfortunately , many radiologists fail to address this question in their clinical reports. Other MR imaging features of NPH that have been proposed include the ''aqueductal flow void sign'' and increased CSF stroke volume (1), but these remain controversial (2). Isotope cisternography has also been proposed for the evaluation of NPH. Despite the occurrence of ventricular reflux of isotope in NPH, its presence does not reliably predict which patients will have favorable outcomes after shunt surgery; similarly, its absence does not preclude a favorable response to shunt surgery. MR imaging in cases of hydrocephalus also reveals thinning of the periventricular white matter associated with hyperintensities of the subependy-mal and deep white matter. …