Significance of periventricular hemodynamics in normal pressure hydrocephalus.

To delineate the pathophysiology of periventricular hemodynamics in normal pressure hydrocephalus, we performed quantitative and three-dimensional measurements of cerebral blood flow (CBF) by using xenon-enhanced computed tomographic scans. Measurements were made on 7 patients in whom normal pressure hydrocephalus after subarachnoid hemorrhage had been confirmed by clinical improvement after shunting. We compared mean CBF values in the white matter and cortex of the frontal, temporal, parietal, and occipital lobes and in the thalamus before and after shunting, with an evaluation of dementia and the extent of ventricular dilation and periventricular lucency on computed tomographic scans. CBF returned to within normal limits in the white matter of the frontal and temporoparieto-occipital lobes. CBF restoration closely correlated with clinical improvement and reduction in ventricular dilation and periventricular lucency. We speculate that ischemia occurs initially in the periventricular white matter as a result of diffused cerebrospinal fluid and then extends of the cortex and to the thalamus, causing a "misery perfusion" state with neuronal dysfunction. Incomplete improvement of dementia and CBF in the cortex and thalamus may be explained by preexisting arteriosclerosis in aged patients, coexisting brain damage caused by subarachnoid hemorrhage and subsequent surgical insult in aneurysm patients, and delayed recovery of cortical function that has been secondarily impaired by the periventricular lesions.