AN EVALUATION OF MOTOR-EVOKED POTENTIALS FOR DETECTION OF NEUROLOGIC INJURY WITH CORRECTION OF AN EXPERIMENTAL SCOLIOSIS

Study Design. Controlled correction of scoliosis in a rat model was us ed to assess the accuracy of intraoperative motor-evoked potential mon itoring. Objectives. The purpose of this study was to develop a model in which motor-evoked potential changes could be compared with neurolo gic function after surgery, such that a threshold for responding to mo tor-evoked potential changes may be established. Summary of Background Data: Intraoperative motor-evoked potential monitoring has become tec hnically feasible. Clinical application now depends on the development of useful interpretation parameters and correlation with neurologic s equelae. Methods. Experimental scoliosis was produced in 30 rat pups. After growth, the rats underwent correction of their scoliosis by dist raction. Changes in tcMMEP onset latency and amplitude were measured. Distraction was applied either until a 10% delay in tcMMEP onset laten cy (Group 1), until tcMMEP responses were ablated (Group 2), or for 10 minutes after the loss of transcranial magnetic stimulation response (Group 3). Results. In Group 1 (n = 10), all animals had tcMMEP with n ormal onset latency and normal neurologic examinations 24 hours after surgery. In Group 2 (n = 10), tcMMEP were normal in four rats, markedl y delayed in three rats, and absent in three rats 24 hours after surge ry. Neurologic examination was normal in the four rats with normal tcM MEP. Moderate deficit was noted in two of the three rats with prolonge d onset latency 24 hours after surgery; the third was intact. Moderate neurologic injury was noted in two of three rats with absent tcMMEP 2 4 hours after surgery; the third rat was paralyzed. In Group 3 (n = 10 ), vertebral dislocation was noted on lateral radiographs in eight of 10 animals. Twenty-four hours after surgery, tcMMEP remained absent, a nd paralysis was noted in the eight rats with dislocation. The two rat s without dislocation had delayed tcMMEP but some return of neurologic function. Conclusions. Comparison of the three groups show a signific ant correlation between tcMMEP and endpoint neurologic outcome. None o f the rats in Group 1 had a neurologic deficit after surgery as oppose d to five of 10 rats in Group 2 and 10 of 10 rats in Group 3 with sign ificant neurologic injury. These findings suggest that a 10% delay in onset latency would be an appropriate threshold for responding to chan ges in tcMMEP.