Directional diffusion of corticospinal tract supports therapy decisions in idiopathic normal-pressure hydrocephalus

Gait disturbance in patients with idiopathic normal pressure hydrocephalus (iNPH) may be caused by alterations of the corticospinal tract that we aimed to measure with diffusion tensor imaging (DTI). The directional diffusion parameters axial diffusivity and fractional anisotropy (FA) reflect axon integrity, whereas mean diffusivity, radial diffusivity and magnetization transfer ratio (MTR) reflect myelin content. Twenty-six patients with probable iNPH were grouped into drainage responders (n = 12) and drainage non-responders (n = 14) according to their improvement on gait assessment tests after a 3-day lumbar CSF drainage. We measured DTI and MTR of the corticospinal tract and, as reference, of the superior longitudinal fascicle before and after CSF withdrawal in iNPH and in ten age-matched controls. Drainage responders were re-examined after ventricoperitoneal shunting. Differences before any intervention and changes upon CSF withdrawal were evaluated. Axial diffusivity in corticospinal tract and superior longitudinal fascicle was higher in both patient groups compared to controls (p < 0.001). Only in the corticospinal tract of drainage responders was FA higher compared to controls, and both FA and axial diffusivity decreased after shunting. For axial diffusivity upon CSF drainage, a decrease of > 0.7 % discriminated drainage responders from drainage non-responders with 82 % sensitivity, and a decrease of > 1 % predicted overall improvement after shunting with 87.5 % sensitivity and 75 % specificity. The specificity to discriminate responders/non-responders was low for all DTI values (max. 69 % for FA values). High values of directional diffusion parameters in the corticospinal tract are found in iNPH patients indicating affection of its axons. Increased values and their decrease upon CSF drainage may facilitate treatment decisions in clinically uncertain cases.