Intervention mechanisms and outcomes in somatosensory evoked potential monitoring during scoliosis surgery

Objective: Somatosensory evoked potentials (SSEPs) are now routinely used to monitor the integrity of the sensory pathways of the spinal cord during major corrective spinal deformities surgery. We have reviewed retrospectively the outcomes and course of action for a 137 electrophysiologically monitored procedures over a five year period.Methods: Responses were recorded via a bipolar epidural electrode positioned in the epidural space at levels T1/T3 prior to surgery. An initial baseline SSEP waveform was recorded early in the surgical procedure and subsequent recordings were compared with those. A decrease of 50% in amplitude or an increase of 10% in latency of the SSEP waveform was used as a threshold for intervention.Results: Our findings demonstrate that a drop in amplitude greater than 50% occurred in 46 monitored procedures and whilst the traces of 22 patients remained below the acceptable levels for the subsequent duration of the surgery, there was no evidence of any clinically detectable neurological impairment. Of the 32 interventions by the surgical team to restore the SSEP waveform, only 18 were successfully restored to above the 50% level. None of the SSEP waveforms that were not restored to acceptable levels exceeded a 75% drop in amplitude. Conclusion: There were no reports of postoperative neurological deficit and it is probable that the true boundaries at which the critical levels for intervention resides is probably between 50 % and 75%.