Corticospinal tract and corticobulbar tract dysfunction in ALS: combined study using transcranial magnetic stimulation and diffusion tensor tractography

We evaluate the degeneration of the corticospinal tract (CST) and the corticobulbar tract (CBT) separately in amyotrophic lateral sclerosis (ALS) patients by tract-specific fractional anisotropy (FA) measurement using diffusion tensor tractography (DTT). Thirty patients with ALS [11 bulbar-onset patients (ALS-B) and 19 limb-onset (ALS-L)] and 17 healthy age-matched controls were recruited. A diffusion tensor echo-planar image with 13 axes was obtained using 1.5-T MR imager. DTT of the corticospinal tract (DTT-CST) and the corticobulbar tract (DTT-CBT) were projected from a seed at the posterior limb of the internal capsule to a target at the superior precentral gyrus (DTT-CST) or at the inferior precentral gyrus (DTT-CBT). FA was calculated at semiautomatically placed regions-of-interest (ROIs) and averaged in each tract. We measured central motor conduction time (CMCT) and cortical-brainstem conduction time (CTX-BS CT) recorded from the first dorsal interosseous (FDI) muscle. FA of DTT-CST and DTT-CBT of the patients were significantly reduced compared with those of the controls. FA of DTT-CBT was significantly lower in ALS-B than in ALS-L. CMCT and CTX-BS CT showed a significant negative correlation with the FA of DTT-CST, but not with DTT-CBT. DTT is a feasible method to quantitate individual descending motor tract damage in ALS.