Evaluation of normal pressure hydrocephalus with diffusion tensor imaging

Abstract

Purpose: Normal Pressure Hydrocephalus (NPH) is a clinical diagnosis with no formal definition. Textbooks describe NPH as a clinical triad of gait abnormality, dementia, and urinary incontinence. Few patients present with all three symptoms, forcing the clinician to rely on a “preponderance of evidence” approach, which involves weighing triad symptoms with radiological findings, Cerebrospinal Fluid (CSF) opening pressure, response to Tap Test, external lumbar CSF drainage, lumbar infusion, and finally shunting. Radiological findings in NPH are limited to enlarged ventricles out of proportion to sulcal atrophy, callosal angles greater than 40 degrees, and ventricles with Evan’s ratios greater than 0.3. When radiologists evaluate suspected NPH patients they are limited to excluding disease, as opposed to searching for any particular finding. In this study we used Diffusion Tensor Imaging (DTI) to determine if differences can be identified on a group basis between NPH and normal groups to see if DTI (including tractography) can be a useful tool for understanding disease morphology and laying the groundwork for future clinical use of DTI for identification of NPH.

Materials and Methods: A retrospective study of patients who underwent brain MRI imaging with a Philips 3T magnet. NPH patients were classified as “definite” or “probable NPH” by their referring physicians. Normal subjects were patients found to have no anatomical brain abnormality. DTI and tractography data were acquired using Philips Fibertrak software, and post-processing was done using Tract Based Spacial Statistics (TBSS). Conclusion: NPH patients were found to have higher Fractional Anisotropy (FA) values in the upper corticospinal tract, lower FA values in the Corpus Callosum and mixed results in the internal capsule, to p ≤ 0.05 levels, consistent with previous reports. NPH tractography was also characterized with a distinct “heart-shaped” sign. Possible uses for tractography for patients under suspicion of NPH will be discussed.