Analysis of motor and somatosensory evoked potentials during thoracic and thoracoabdominal aortic aneurysm repair.

OBJECTIVES: Use of motor evoked potentials (MEP) and somatosensory evoked potentials (SSEP) monitoring during thoracic and thoracoabdominal aortic surgery is controversial. This study evaluated the intraoperative use of SSEP and MEP during thoracoabdominal repair and assessed their role in decreasing the risk of spinal cord ischemia and paralysis.

METHODS: We conducted paired SSEP and MEP monitoring to assess agreement between the methods and their ability to predict neurologic outcome in 233 patients. Changes in SSEP and MEP monitoring were classified as no change, reversible change, or irreversible change during the intraoperative period and by the conclusion of surgery. Agreement between the methods was computed using the Cohen kappa statistic. Sensitivity, specificity, and positive and negative predictive values were computed for each method on the immediate and delayed neurologic deficit.

RESULTS: Immediate neurologic deficit, determined immediately upon awakening from anesthesia and confirmed by a neurologist, occurred in eight of 233 (3.4%) patients. For any change (reversible plus irreversible), agreement between MEP and SSEP was relatively low (kappa = 0.53), despite being highly statistically significant (P < .001). MEP tended to overestimate SSEP for immediate neurologic deficit, demonstrating a 53% false-positive rate, compared with a 33% false-positive rate for SSEP (specificity ratio, 1.42; P < .0001). With irreversible change, agreement between the methods was 90% (kappa = 0.896, P < .0001). Only irreversible change was significantly associated with neurologic outcome (odds ratio, 21.9; P < .00001 for SSEP; 60.8, P < .0001 for MEP), but sensitivity and positive predictive values were low (37% and 33% for SSEP; 22% and 45% for MEP, respectively). Reversible changes in neurophysiologic monitoring were not significantly associated with immediate neurologic deficit. Negative predictive values for all negative evoked potential findings were >98% for immediate deficit. No evoked potential variables were associated with delayed deficit.

CONCLUSIONS: SSEP and MEP monitoring were highly correlated only when intraoperative changes were irreversible. Reversible changes were not significantly associated with immediate neurologic deficit. Irreversible changes were significantly associated with immediate neurologic deficit, and the findings were identical for SSEP and MEP in this variable, indicating that the more complex MEP measures do not add further information to that obtained from SSEP. Normal SSEP and MEP findings had a strong negative predictive value, indicating that patients without signal loss are unlikely to awake with neurologic deficit.