Correlation of phasic muscle strength and corticomotoneuron conduction time in multiple sclerosis

Central motor conduction times for the adductor pollicis muscle, the twitch force of that muscle to scalp magnetic motor cortex stimulation, and the maximum force of phasic voluntary contraction of the same muscle were measured in 15 patients with multiple sclerosis. Two tests of manual dexterity of the same hand also were studied: the Purdue pegboard test, and the maximal frequency of a scissors movement of the thumb and index finger. The patients had normal strength or minimal weakness of the intrinsic muscles of the hand on clinical examination. The mean central motor conduction times for the adductor pollicis muscle for the patients were longer than normal, the peak twitch force for the adductor pollicis muscle evoked by cortical stimulation and the maximum for ce of a phasic voluntary contraction of the adductor pollicis muscle were s maller than normal. There were strong correlations between all these measu res. Central motor conduction time in the patients was inversely correlate d with voluntary phasic force and the twitch force after cortical stimulation. That is, the longer the central motor conduction time, the we aker the force. Prolonged central motor conduction time is likely to be ac companied by conduction block in corticomotoneuron pathways. The correlati on of central motor conduction time with voluntary phasic force and the twi tch force most likely reflects the degree of conduction block and temporal dispersion rather than delay in conduction per se. These results indicate that objective assessments of phasic muscle strength may reveal correlation s with central motor conduction time that are not evident on conventional c linical examination which assesses tonic muscle contraction strength. Ther e also was some correlation of central motor conduction time, twitch force, and voluntary phasic force with the performance of tests of manual dexterit y. Since the voluntary phasic strength and twitch force to cortical stimul ation were similarly weakened, the findings would also be consistent with t he cortical stimulus and voluntary contraction using the same descending mo tor tracts.