[Post-operative improvement of 14 cases who were considered iNPH despite Evans' index of 0.3 or less]

Abstract

Background and purpose: The diagnosis of idiopathic normal pressure hydrocephalus (iNPH) is based on ventricular enlargement with the Evans' index (>0.3). Cerebrospinal fluid (CSF) shunt surgery was performed for 14 patients (5 men and 9 women; mean age, 73.8 years; range, 61-80 years) who showed improvement after CSF tap test despite Evans' index of 0.3 or less. We evaluated Evans' index, sylvian fissure, and cerebral ventricular size on the coronal sections obtained perpendicularly to the antero-posterior commissure line before and 3 months after surgery.

Results: Of the 14 patients, 9 (64%) had marked improvement and 5 (36%) showed mild improvement. Evans' index remained unchanged in patients with marked improvement. The sizes of Sylvian fissures and lateral cerebral ventricles on the coronal sectional imaging were markedly reduced.

Discussion and conclusion: Some patients with Evans' index of 0.3 or less might show improvements in symptoms after the surgery;thus, the CSF tap test should be performed when iNPH is suspected based on the symptoms and medical history. Characteristic imaging findings of iNPH include enlarged Sylvian fissure, tight high-convexity, and small callosal angle on the coronal section passing through the posterior commissure, indicating that the lateral cerebral ventricle is located disproportionately in the upper medial part. Cerebral ventricles might enlarge in the direction of the long axis instead of externally in iNPH. In iNPH, Evans' index, which indicates external enlargement, is not appropriate for evaluating ventricular enlargement;alternatively, the size of cerebral ventricles estimated by coronal sections can be used.