Short-interval intracortical inhibition with incomplete spinal cord injury

ObjectiveShort-interval intracortical inhibition (SICI) in leg and hand muscles was characterized in individuals with incomplete spinal cord injury (SCI) to understand how such inhibition limits corticospinal drive after spinal insult.MethodsWe compared SICI during a voluntary contraction in 16 SCI and 14 control subjects, the latter group tested over a larger range of conditioning and test stimulus (CS and TS) intensities to best match the SCI data.ResultsThe average peak SICI in the tibialis anterior muscle was typically 3–4 times lower in the SCI subjects compared to controls. When matched for absolute TS intensity, in terms of maximum stimulator output, both U-shaped SICI recruitment curves were produced by similar CS intensities. SICI in the first dorsal interosseous muscle of the hand tended to be larger than in the ankle flexor.ConclusionsIncomplete SCI reduces SICI compared to controls, but the absolute CS intensities that produce the U-shaped SICI recruitment curves are unchanged.SignificanceThese findings suggest that although the relative excitability profile of cortical SICI networks is unchanged after SCI, the effective inhibition of corticospinal tract output by these neurons is reduced.

► In this study we examined if impairment of ascending sensory transmission to the motor cortex after incomplete spinal cord injury affects the excitability of circuits mediating short-interval intracortical inhibition (SICI) of corticospinal pathways to muscles of the leg and hand. ► SICI was reduced in chronically injured subjects compared to uninjured controls; however, when the absolute stimulation intensities were matched between groups (in terms of Maximum Stimulator Output), both U-shaped SICI recruitment curves were produced by similar conditioning stimulation intensities. ► We discuss some the problems in measuring the absolute excitability levels of SICI networks by paired-pulse stimulation techniques in patients with altered transmission to the corticospinal tract.