Disappearing corticospinal tract on routine MRI: dynamic signal evolution in primary lateral sclerosis

Primary lateral sclerosis (PLS) may show corticospinal tract (CST) hyperintensity on fluid-attenuated inversion recovery and motor cortex hypointensity on susceptibility-weighted imaging (SWI); however, its longitudinal evolution remains poorly understood. Here, we describe two cases with definite PLS, who were followed up for 15 and 6 years and assessed using qualitative visual magnetic resonance imaging (MRI) scores. Both patients initially exhibited CST hyperintensity. Despite progressive clinical deterioration due to wheelchair/walker dependence, serial MRI demonstrated complete CST normalisation (score 0/16). Concurrently, SWI revealed progressive motor cortex hypointensity, consistent with iron deposition. These cases illustrate a possible dissociation between conventional and susceptibility-based MRI markers, suggesting dynamic pathophysiological processes and potentially early inflammation followed by gliotic remodelling, although technical factors cannot be excluded. A normal-appearing CST should not exclude advanced PLS, and progressive motor cortex hypointensity may provide a more stable marker. Prospective studies with standardised protocols are required to validate these observations.