Distinct volumetric features of cerebrospinal fluid distribution in idiopathic normal‐pressure hydrocephalus and Alzheimer’s disease

Abstract

AbstractBackgroundThe imaging characteristics of ventriculomegaly, high‐convexity tightness, and Sylvian fissure dilation has been called “disproportionately enlarged subarachnoid space hydrocephalus”.ObjectiveTo investigate relationships in cerebrospinal fluid (CSF) volumes between the lateral ventricle, high‐convexity subarachnoid space, and Sylvian fissure region in patients with idiopathic normal‐pressure hydrocephalus (INPH) and Alzheimer’s disease (AD), and to evaluate differences in these volumes among INPH, AD, and healthy controls.MethodsForty‐nine INPH patients, 59 AD patients, and 26 healthy controls were imaged with three‐dimensional MRI.ResultsINPH patients had larger lateral ventricles and CSF spaces of the Sylvian fissure region and smaller high‐convexity subarachnoid spaces than other groups, and AD patients had larger lateral ventricles and CSF spaces of the Sylvian fissure region than the control group. The INPH group showed a negative correlation between lateral ventricle and high‐convexity subarachnoid space volumes, while the AD group showed a positive correlation between lateral ventricle volume and volume for CSF spaces of the Sylvian fissure region. The ratio of lateral ventricle to high‐convexity subarachnoid space volumes yielded an area under the curve of 0.990, differentiating INPH from AD.ConclusionsAssociations between CSF volumes suggest different mechanisms between INPH and AD to explain their respective lateral ventricular dilation. In AD, lateral ventricular enlargement might be associated with brain volume loss through cortical atrophy. In INPH, the lateral ventricles may expand superiorly into the high‐convexity area, and this may further instigate cortical thickening. The ratio of lateral ventricle to high‐convexity subarachnoid space volumes distinguishes INPH from AD with good diagnostic sensitivity and specificity. We propose to refer to this ratio as the VOSS (ventricle over subarachnoid space) index.