Patient characteristics and anesthetic technique are additive but not synergistic predictors of successful motor evoked potential monitoring

Background: Spinal cord monitoring is associated with a significantly lower rate of neurologic deficits after deformity surgery, and has been shown to have predictive value in cervical, thoracic, and lumbar surgery. Lower extremity motor evoked potentials (MEPs) are particularly sensitive to anesthetics and physiologic change, and can be difficult to obtain at baseline. The anesthesiologist is often required to modify the maintenance anesthetic to facilitate signal attainment. Although intuitive, the predictive significance of increasing age, body mass index (BMI), presence of diabetes and/or hypertension, surgical procedure, and anesthetic technique has not been well delineated. Methods: We conducted a retrospective chart review of the anesthetic records of all patients who underwent spine surgery and MEP monitoring of the lower extremities from August 1, 2001 to December 31, 2005. Patients with preexisting paralysis of the lower extremities were excluded. Univariate analysis was performed to examine the distribution of diabetes, hypertension, anesthesia technique, age, gender, BMI, and surgical procedure. The χ test and the 2-sample t test were used to test associations between MEP status and potential risk factors. Cochran-Armitage test was used to analyze trends in BMI and age by quartile. The effects of diabetes and hypertension, compared with patients with neither, were presented for each anesthetic technique. Bivariate analysis of the data was performed to analyze a potentially synergistic deleterious effect of diabetes, hypertension, and anesthetic technique using the Breslow-Day test for homogeneity of the odds ratios. Logistic regression analysis through stepwise selection was performed to form a model of the data. Results: Two hundred fifty-six charts were reviewed. The univariate analysis showed that diabetes, hypertension, anesthesia technique, age, and BMI were significantly associated with failure to obtain MEP signals. None of the variables were found to have a synergistic effect on MEP signal attainment in the bivariate analysis. Hypertension, diabetes, and anesthetic technique were independent factors for MEP failure and their joint effects were additive not synergistic. Conclusions: Diabetes, hypertension, and anesthetic technique were the most important patient risk factors associated with failure to obtain lower extremity MEP signals. These results will improve anesthesiologists' ability to tailor anesthetic regimen to patient comorbidity when MEP monitoring is planned.