The influence of stimulus presentation rate on the cortical amplitude and latency of intraoperative somatosensory-evoked potential recordings in patients with varying degrees of spinal cord injury.

Fourteen patients whose posterior tibial nerve somatosensory cortical-evoked potentials (SCEPs) were monitored for spinal surgery, underwent sequential recordings at different stimulus rates (6.1, 5.1, 4.1, 3.1, 2.1, and 1.1 Hz), after induction of anesthesia and patient positioning but before the incision was made. A markedly detrimental influence of higher (5.1 and 6.1 Hz), yet commonly used, stimulus rates on the amplitude of the evoked potential waveform was noticed. This tendency was most pronounced in patients with an antecedent spinal cord injury. In three patients with severe incomplete cord injuries, the amplitude of the cortical-evoked potential was sufficiently attenuated to preclude reliable intraoperative monitoring when stimulus rates of 5.1 Hz and greater were used. By contrast, evoked potentials could be monitored reliably in all patients when stimulus rates of 1.1 or 2.1 Hz were used. Six of seven patients (87%) whose evoked response amplitude showed a major reduction with higher stimulus frequencies had either a discrete sensory level or bilateral lower extremity weakness or both. Only two of seven patients (29%) whose evoked potential remained minimally affected by the stimulus rate had similar neurologic impairment. The authors conclude that, particularly in the setting of severe incomplete spinal cord injuries, the higher rates of stimulus presentation commonly used may result in failure to obtain waveforms that are adequate for intraoperative SCEP monitoring. The use of unconventionally low stimulus rates may greatly facilitate SCEP monitoring in such situations.