A Multiparametric Alarm Criterion for Motor Evoked Potential Monitoring During Spine Deformity Surgery

This is a pilot study to compare changes in the amplitude, area below the curve, number of phases, duration, and latency of the intraoperative transcranial motor evoked potentials (TcMEP) for early detection of impending spinal cord injury. An empirical ratio calculated by a combination of the above-mentioned parameters was also assessed.

Intraoperative TcMEP recordings from five patients presenting with neuromuscular kyphoscoliosis, idiopathic scoliosis, achondroplasia and lumbar kyphosis, congenital kyphosis, and achondroplasia with cervical instability were reviewed. Anesthesia was sustained with propofol or sevoflurane plus remifentanil, no muscle relaxants were used after intubation. The TcMEPs to multipulse electrical stimulation were recorded from upper-limb and lower-limb muscles. To be worthy of analyses, changes should include TcMEP disappearance or amplitude decrease >-65% during surgical/force maneuverings.

Transient TcMEP changes related to surgical/force maneuvering were observed in all five patients consisting of a decrease in the empirical ratio value (>-95%), followed in magnitude by a drop in the area below the curve, amplitude (>-80%), duration, and number of phases, whereas latency increased. Changes returned to baseline when maneuverings were reverted. No hemodynamic/anesthetic factors were present during these events. After surgeries, no new neurological deficits were detected.

An empirical ratio accounting for all TcMEP parameters seems to drop more than the amplitude during an intraoperative event. As few cases were analyzed, further studies in larger series of patients will be necessary to assess empirical ratio sensitivity/specificity and to determine whether this may be a useful monitoring warning criterion not leading to unnecessary interference with surgical treatment.