Motor cortical mapping of proximal upper extremity muscles following spinal cord injury

Focal transcranial magnetic stimulation was used to map the motor cortical representations of the relaxed and gently contracted biceps brachii, deltoid and triceps muscles in 22 subjects comprised of 12 controls, five subjects with complete and five with incomplete cervical spinal cord lesions (SCI). Motor evoked potentials (MEPs) were rarely observed during the resting condition (3/30 muscles tested; SCI group) which precluded detailed analysis of these data. With background facilitation, the mean number of scalp stimulation sites producing MEPs varied according to muscle (P<0.001); biceps yielded the largest maps and triceps the smallest. The cortical representations of proximal upper extremity 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 MEP) was always surrounded by an area producing submaximal MEPs, but was variable across subjects and groups. There was extensive overlap in the motor cortical representation areas corresponding to the three muscles of interest. Following maximal intensity stimulation at the optimal site, the mean MEP amplitudes (normalized) were largest for the biceps muscle and smallest or absent in triceps (P<0.02). No differences were 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 lower thresholds than triceps (P<0.001). The findings suggest that cortical map areas and MEP characteristics are not significantly altered in gently contracting muscles innervated by nerve roots rostral to the lesion. Only activation thresholds are higher following SCI, particularly incomplete lesions, although there is no apparent association with sensorimotor function. The inability to elicit MEPs in the relaxed muscles of patients with SCI fail to support previous reports of expanded motor cortical representations associated with muscles innervated by roots rostral to the lesion.