Normal pressure hydrocephalus: correlation between CT and measurements of cerebrospinal fluid dynamics

Twenty-nine patients consecutively admitted for consideration of CSF diversion surgery for suspected communicating ("normal pressure") hydrocephalus underwent cranial computerized tomography (CT) and study of cerebrospinal fluid (CSF) absorption, the latter determined as resistance to outflow of CSF (Ro). From the CT the size of the ventricular system was determined by various linear measurements and ratios and the presence of periventricular lucencies (PVL) and the size of cortical sulci was noted. Except for absence of cortical sulci greater than 3 mm no CT feature was suggestive of compromised CSF absorption. On the other hand, cortical atrophy did not rule out increased resistance to CSF-outflow. Features thought of as characteristic for normal pressure hydrocephalus on CT: large ventricles as compared with the amount of cortical atrophy and presence of PVL, correlated poorly with measured high resistance values. There was no correlation between the size of the ventricular system and resistance to CSF outflow. In the diagnostic distinction between communicating ("normal pressure") hydrocephalus and cerebral atrophy (hydrocephalus ex vacuo) static CT cannot replace an actual determination of CSF outflow resistance.