To investigate the mechanisms underlying disability in multiple sclerosis (MS), 40 patients with the relapsing-remitting form of the disease and 13 patients with secondary progressive MS underwent multimodal evoked potential (EP), motor evoked potential (MEP), and spinal motor conduction time evaluation. Clinical disability was evaluated by the expanded disability status scale (EDSS) and functional system scales. In secondary progressive MS patients, magnetic resonance imaging (MRI) was used to obtain a semiquantitative estimate of the total lesion load of the brain.

Spinal motor conduction time was significantly longer in secondary progressive MS patients than controls (p < 0.001) and relapsing-remitting MS patients (p < 0.05), but did not differ between relapsing-remitting patients and controls. Spinal motor conduction times also correlated directly with EDSS scores (p < 0.001) and pyramidal functional system scores (p < 0.001). Brain lesion load (4960.3 ± 3719.0 mm2) and the total number of lesions (67.7 ± 37.0) in secondary progressive MS did not correlate with disability scores. For the following EPs, the frequencies of abnormalities were significantly higher in secondary progressive MS patients than relapsing-remitting patients: visual evoked potentials (p < 0.05), somatosensory evoked potentials and upper limb motor evoked potentials (p < 0.01), and brainstem auditory evoked potentials, lower limb somatosensory evoked potentials and lower limb motor evoked potentials (p < 0.001).

These findings suggest that disability in secondary progressive MS patients is mainly due to progressive involvement of corticospinal tract in the spinal cord.