Central motor conduction time in progressive multiple sclerosis. Correlations with MRI and disease activity.

The purpose of this study was to relate abnormalities of motor conduction time to the presence of spinal cord MRI lesions in progressive multiple sclerosis and to investigate the relationship between changes in motor conduction over time and clinical and MRI changes. Central motor conduction time (CMCT), serial MRI of the brain and spinal cord, and clinical evaluations were carried out in 20 patients with primary and secondary progressive multiple sclerosis. CMCT was carried out at the beginning and end of the study whilst the clinical and MRI examinations occurred at monthly intervals for 12 months. Median CMCT to abductor pollicis brevis was 14.8 ms (range 8.8-27.4 ms). The response latency to tibialis anterior correlated with disability measured on the Expanded Disability Status Scale. Latencies to upper limb muscles correlated with cervical MRI lesion load and the presence of atrophy of the cervical cord. Over the 12-month study period, 15 of 19 patients deteriorated clinically. However, an increase in motor response latencies occurred only in the four patients who had developed new cord lesions. The results suggest that prolonged CMCT is related to spinal cord lesion load and that, over time, changes in the CMCT occur only when spinal cord lesion load increases. Clinical change in progressive multiple sclerosis may therefore occur without either the development of new lesions on MRI scans or an increase in motor conduction time. This suggests that clinical deterioration in these patients may occur by a mechanism other than increasing demyelination. This may be progressive axonal degeneration.