The patient was a 16 y/o female who presented to the operating room with idiopathic scoliosis. She was in good health with no neurologic deficit. Her x-rays demonstrated a severe thoracolumbar “S” shaped curve. The surgeon performed a T2-L2 posterior spinal fusion with instrumentation. Motor and somatosensory evoked potential monitoring was utilized to assess lower extremity function intraoperatively. These modalities were used to replace the traditional wake-up test.
For motor evoked potentials (MEP’s), transcranial electrical stimulation of the motor cortex was used to produce a response that was recorded at the muscle level. Bipolar subdermal electrodes were placed in the abductor pollicus brevis muscle in the hand and the tibialis anterior muscles on the anterior shin bilaterally. Subdermal stimulating corkscrew electrodes were inserted at specific cortical locations on the scalp. MEP transcranial stimulus levels were achieved at a threshold of 310 Volts. The Anesthesiologist employed a strict, standardized protocol with no neuromuscular blockade. Baseline MEP waveforms showed excellent morphology and repeatability for bilateral lower extremities, with the hand response present and useful as a control. No change was seen in the latency or amplitude of the tibialis anterior responses for the entire length of the procedure, confirming intact lower extremity motor function.
SSEP studies were performed as an adjunct to the new and evolving MEP modality. Bipolar surface stimulating electrodes were placed over the surface of the posterior tibial nerve at the medial ankle bilaterally. Electrical stimulation was delivered at the ankle using a threshold of 38 mA. Recording electrodes consisted of subdermal needles placed at the subcortical cervical spine level and cortically at specific scalp locations; a reference needle electrode was placed at Fpz. Baseline lower extremity responses were within normal limits with large amplitude values. No change was seen in the latency or amplitude of the SSEP responses for the entire length of the surgical procedure, confirming intact somatosensory function.