INTRAOPERATIVE DERMATOMAL EVOKED-POTENTIAL MONITORING FAILS TO PREDICT OUTCOME FROM LUMBAR DECOMPRESSION SURGERY

Study Design. Thirty-three patients with single-level, unilateral lumb osacral radiculopathy underwent micro-decompression and intraoperative dermatomal evoked potential monitoring. Side-to-side latency asymmetr y was calculated. A criteria for ''abnormal'' was defined. Intraoperat ive dermatomal evoked potentials were obtained before and after decomp ression. The changes were correlated with clinical outcome at the 8-mo nth follow-up examination. Objectives. To determine whether intraopera tive dermatomal evoked potential latency asymmetry confirms nerve root compression and whether an improvement of latency asymmetry after dec ompression predicts a good clinical outcome. Summary of Background Dat a. Intraoperative dermatomal evoked potential has been proposed as a t est to assess the adequacy of nerve root decompression. Initial report s suggested improvement of dermatomal evoked potential amplitude and l atency after decompression. The clinical efficacy is controversial bec ause of its technical difficulty and inherent variation. Methods. Cerv ical recording was chosen to reduce the effects of anesthesia. The asy mptomatic nerve root was used as a control. Quality of the tracings wa s determined by evoked potentials-to-noise amplitude ratio. Clinical o utcome was based on patient's pain relief and satisfaction. Results. T racings of acceptable quality were obtained at baseline in 57.6% (19 o f 33) of patients. A side-to-side latency asymmetry > 5% was defined a s abnormal. Before decompression, 68.4% (13 of 19) of patients had an abnormal dermatomal evoked potential. After decompression, latency asy mmetry returned to normal in every patient. Clinical outcome was good or excellent in 13 patients, fair in four patients, and poor in two pa tients. Dermatomal evoked potential latency improvements were not rela ted to variation in clinical outcome. Conclusions. Intraoperative derm atomal evoked potential monitoring is technically demanding. Finding r eproducible potentials is difficult. More research is necessary before general use of dermatomal evoked potentials for monitoring nerve root decompression.