MOTOR CORTICAL MAPPING OF PROXIMAL UPPER EXTREMITY MUSCLES FOLLOWING SPINAL-CORD INJURY

Focal transcranial magnetic stimulation was used to map the motor cort ical representations of the relaxed and gently contracted biceps brach ii, deltoid and triceps muscles in 22 subjects comprised of 12 control s, five subjects with complete and five with incomplete cervical spina l cord lesions (SCI). Motor evoked potentials (MEPs) were rarely obser ved during the resting condition (3/30 muscles tested; SCI group) whic h precluded detailed analysis of these data. With background facilitat ion, the mean number of scalp stimulation sites producing MEPs varied according to muscle (P<0.001); biceps yielded the largest maps and tri ceps the smallest. The cortical representations of proximal upper extr emity muscles were largest for the control group and smallest for the incomplete SCI group although differences were not significant (P>0.09 ). The optimal site of stimulation (that which produced the largest ME P) was always surrounded by an area producing submaximal MEPs, but was variable across subjects and groups. There was extensive overlap in t he motor cortical representation areas corresponding to the three musc les of interest. Following maximal intensity stimulation at the optima l site, the mean MEP amplitudes (normalized) were largest for the bice ps muscle and smallest or absent in triceps (P<0.02). No differences w ere detected between groups (P>0.50). The threshold stimulus intensity was highest for those with incomplete SCI and lowest amongst control subjects (P<0.05), with biceps then deltoid muscles generally having l ower thresholds than triceps (P<0.001). The findings suggest that cort ical map areas and MEP characteristics are not significantly altered i n gently contracting muscles innervated by nerve roots rostral to the lesion. Only activation thresholds are higher following SCI, particula rly incomplete lesions, although there is no apparent association with sensorimotor function. The inability to elicit MEPs in the relaxed mu scles of patients with SCI fail to support previous reports of expande d motor cortical representations associated with muscles innervated by roots rostral to the lesion.