Effect of coil orientation on motor-evoked potentials in humans with tetraplegia.

KEY POINTS: Although corticospinal function changes following spinal cord injury (SCI), the extent to which we can activate the corticospinal tract after injury remains poorly understood. To address this question, we used transcranial magnetic stimulation over the hand representation of the primary motor cortex to elicit motor-evoked potentials (MEPs) using posterior-anterior and anterior-posterior induced currents in the brain and compared them with responses evoked using lateral-medial currents in participants with and without cervical incomplete SCI during small levels of index finger abduction. We found prolonged MEP latencies in all coil orientations in SCI compared to control subjects. However, the latencies of MEPs elicited by posterior-anterior and anterior-posterior compared to lateral-medial stimulation were shorter in SCI compared to controls, particularly for MEPs elicited by anterior-posterior currents. Our findings demonstrate for the first time that corticospinal responses elicited by different directions of the induced current in the brain are differentially affected after SCI. ABSTRACT: The corticospinal tract undergoes reorganization following spinal cord injury (SCI). However, the extent to which we can activate corticospinal neurons using non-invasive stimulation after injury remains poorly understood. To address this question, we used transcranial magnetic stimulation over the hand representation of the primary motor cortex to elicit motor-evoked potentials (MEPs) using posterior-anterior (PA) and anterior-posterior (AP) induced currents in the brain and compared them with the responses evoked by direct activation of corticospinal axons using lateral-medial (LM) currents. Testing was completed during small levels of index finger abduction in humans with and without (controls) cervical incomplete SCI. We found prolonged MEP latencies in individuals with SCI in all coil orientations compared to controls. However, latencies of MEPs elicited by PA and AP stimulation relative to those elicited by LM stimulation were shorter in SCI compared to control subjects. Notably, the largest difference between SCI and control subjects was present in MEPs elicited by AP currents. Using a novel controllable pulse parameter transcranial magnetic stimulation, we also found that MEPs elicited by AP currents with 30 μs compared to 60 and 120 μs pulse width had increased latency in controls but not in SCI subjects. Our findings demonstrate that differences between corticospinal responses elicited by AP and PA induced currents were not preserved in humans with tetraplegia and suggest that neural structures activated by AP currents change largely after the injury.