High T2 Signal in Primary Lateral Sclerosis Supports the Topographic Distribution of Fibers in the Corpus Callosum: Assessing Disease in the Primary Motor Segment

SUMMARY: PLS is a disease of the UMN, distinguished from ALS in prognosis and absence of LMN signs. We present, to our knowledge, the first conventional MR imaging visualization of the callosal motor segment, a concept previously supported by primate models, electrophysiologic studies, and postmortem examinations. Modification of the Witelson topographic scheme of the CC is supported by MR tractography. On the basis of 2 cases of PLS, we present conventional imaging confirmation of the revised topographic scheme of fiber distribution across the CC. ABBREVIATIONS: ALS amyotrophic lateral sclerosis; CC corpus callosum; CST corticospinal tract; DTI diffusion tensor imaging; FA fractional anisotropy; FLAIR fluid-attenuated inversion recovery; LMN lower motor neuron; PLS primary lateral sclerosis; UMN upper motor neuron PLS is a progressive pure UMN disorder first described inthe late 1800s by Charcot and Erb; it can be distinguished from ALS by its characteristic absence of LMN signs. Both conditions arise sporadically, and both have been linked to specific mutations,1 though PLS is far less common than ALS, constituting as little as 1 % of motor neuron diseases.2 Since diagnostic criteria for PLS were first proposed,3 reports hav