• Juan S Uribe, Robert E Isaacs, Jim A Youssef, Kaveh Khajavi, Jeffrey R Balzer, Adam S Kanter, Fabrice A Küelling, Mark D Peterson, and SOLAS Degenerative Study Group.
• Department of Neurological Surgery and Brain Repair, University of South Florida, 2 Tampa General Circle, Tampa, FL, 33606, USA, juansuribe@gmail.com.
• Eur Spine J. 2015 Apr 1; 24 Suppl 3: 378-85.

PurposeThis multicenter study aims to evaluate the utility of triggered electromyography (t-EMG) recorded throughout psoas retraction during lateral transpsoas interbody fusion to predict postoperative changes in motor function.MethodsThree hundred and twenty-three patients undergoing L4-5 minimally invasive lateral interbody fusion from 21 sites were enrolled. Intraoperative data collection included initial t-EMG thresholds in response to posterior retractor blade stimulation and subsequent t-EMG threshold values collected every 5 min throughout retraction. Additional data collection included dimensions/duration of retraction as well as pre-and postoperative lower extremity neurologic exams.ResultsPrior to expanding the retractor, the lowestt-EMG threshold was identified posterior to the retractor in 94 % of cases. Postoperatively, 13 (4.5 %) patients had a new motor weakness that was consistent with symptomatic neuropraxia (SN) of lumbar plexus nerves on the approach side. There were no significant differences between patients with or without a corresponding postoperative SN with respect to initial posterior blade reading (p = 0.600), or retraction dimensions (p > 0.05). Retraction time was significantly longer in those patients with SN vs. those without (p = 0.031). Stepwise logistic regression showed a significant positive relationship between the presence of new postoperative SN and total retraction time (p < 0.001), as well as change in t-EMG thresholds over time (p < 0.001), although false positive rates (increased threshold in patients with no new SN) remained high regardless of the absolute increase in threshold used to define an alarm criteria.ConclusionsProlonged retraction time and coincident increases in t-EMG thresholds are predictors of declining nerve integrity. Increasing t-EMG thresholds, while predictive of injury, were also observed in a large number of patients without iatrogenic injury, with a greater predictive value in cases with extended duration. In addition to a careful approach with minimal muscle retraction and consistent lumbar plexus directional retraction, the incidence of postoperative motor neuropraxia may be reduced by limiting retraction time and utilizing t-EMG throughout retraction, while understanding that the specificity of this monitoring technique is low during initial retraction and increases with longer retraction duration.

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