A 54-year-old man presented with a 4-month history of gait difficulty and progressive leg weakness, predominantly on the left side. A neurological examination revealed slurred speech, tongue fasciculations, spastic weakness and hyper-reflexia of both lower extremities. A left Babinski sign and ipsilateral foot drop were also noted. Positive sharp waves, fasciculations and fibrillation potentials indicative of active and chronic denervation were seen on electromyography. An MRI of the brain revealed bilateral signal hyperintensities along the corticospinal tracts (CST; figure 1A,C). Tractography showed volume loss of the CST with a …