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Intraoperative Dermatomal Evoked Potential Monitoring Fails to Predict Outcome From Lumbar Decompression Surgery

Tsai, Richard Yen-Chun, MD*; Yang, Rong-Sen, MD, PhD*; Nuwer, Marc R., MD, PhD; Kanim, Linda E. A., MA*; Delamarter, Rick B., MD*; Dawson, Edgar G., MD*

Diagnostic Assessment

Study Design. Thirty-three patients with single-level, unilateral lumbosacral radiculopathy underwent microdecompression and intraoperative dermatomal evoked potential monitoring. Side-to-side latency asymmetry was calculated. A criteria for "abnormal" was defined. Intraoperative dermatomal evoked potentials were obtained before and after decompression. The changes were correlated with clinical outcome at the 3-month follow-up examination.

Objectives. To determine whether intraoperative dermatomal evoked potential latency asymmetry confirms nerve root compression and whether an improvement of latency asymmetry after decompression predicts a good clinical outcome.

Summary of Background Data. Intraoperative dermatomal evoked potential has been proposed as a test to assess the adequacy of nerve root decompression. Initial reports suggested improvement of dermatomal evoked potential amplitude and latency after decompression. The clinical efficacy is controversial because of its technical difficulty and inherent variation.

Methods. Cervical recording was chosen to reduce the effects of anesthesia. The asymptomatic nerve root was used as a control. Quality of the tracings was determined by evoked potentials-to-noise amplitude ratio. Clinical outcome was based on patient's pain relief and satisfaction.

Results. Tracings of acceptable quality were obtained at baseline in 57.6% (19 of 33) of patients. A side-to-side latency asymmetry > 5% was defined as abnormal. Before decompression, 68.4% (13 of 19) of patients had an abnormal dermatomal evoked potential. After decompression, latency asymmetry returned to normal in every patient. Clinical outcome was good or excellent in 13 patients, fair in four patients, and poor in two patients. Dermatomal evoked potential latency improvements were not related to variation in clinical outcome.

Conclusions. Intraoperative dermatomal evoked potential monitoring is technically demanding. Finding reproducible potentials is difficult. More research is necessary before general use of dermatomal evoked potentials for monitoring nerve root decompression.

From *the University of California, Los Angeles, Comprehensive Spine Center and Department of Orthopaedic Surgery and †the Department of Neurology, University of California, Los Angeles, School of Medicine, Los Angeles, California.

Acknowledgment date: May 20, 1996.

First revision date: August 20, 1996.

Second revision date: October 21, 1996.

Acceptance date: October 30, 1996.

Device status category: 11.

Address reprint requests to: Edgar G. Dawson, MD; Clinical Professor of Orthopaedic Surgery; UCLA School of Medicine; 100 UCLA Medical Plaza, Suite 755; Los Angeles, California 90095

© Lippincott-Raven Publishers.