| Objective: Evoked potentials have been used both experimentally and clinically to study the function of the spinal cord.The purpose of this study was to evaluate the validity and feasibility of somatosensory evoked potentials(SEP) and transcranial magnetic stimulation motor evoked potential(TMS-MEP). In the present experiment , the effect of a SCI model on evoked potentials was studied in order to use evoked potentials as a parameter for the spinal cord function. The aim of this reserch is to determine whether the EP result had strong association with spinal function ,behaviour test, pathological outcome. Additionally,to evaluate the effect of the timing of decompression after a spinal cord injury.Method:32 rabbits with a body weight between 2.0 and 2.8 kg used in these experiments fell into four groups randomly. There are several models have been designed to simulate human SCI. In the present study, the static load model were used to simulate human SCI. It causes a ischemia lesion,which frequently seen in human spinal cord injury.The rabbits were anesthetized with sodium pentobarbital by intravenous injection. The 13 thoracic lamina were removed in all rabbits, the dura being left intact. The spinal cord was injured at T13 by a apparatus was designed to put a 40g weight bar on the entire dorsal surface of cord and intact dura. The compression time was 5 to 30 minutes in this study. According to the time , the animals were dividedinto 4 groups (control,5min,15min,30min). Each group consisted 8 rabbits.Evoked potentials was used as a parameter for the spinal cord function. In this study, evoked potentials, include somatosensory evoked potentials (SEP) and transcranial magnetic stimulation motor evoked potential (TMS-MEP), were used to assess neurologic function. SEP were recorded by stimulatig the posterior tibial nerve at the left ankle. A recording needle electrode were placed in the scalp between two ears. A reference needle electrode was inserted into the scalp of forehead. A ground electrode was placed into the right forelimb. Motor evoked potentials were produced with a magnetic stimulator delivered magnetic pulse through a 9.0 cm diameter coil. Magnetic pulse were applied to the motor cortex. Levels of stimulation varied from 50% to 100% in control group to determine which level is proper. MEP were recorded with a needle electrode that was inserted into the right Gastrocnemius muscle.In control group, the EPs were recorded before and after anesthetization or operation. The EPs were recorded at 5min, 15min, 30min, 60min, 6 hours, 24 hours, 3 days, 7 days post- decompression and before spinal cord injury in injury groups. Latency and amplitude were measured for each wave. The latency of an SEP or MEP positive peak was measured from the onset of the stimulus to the peak. The amplitude of a positive peak was measured from the positive peak to the immediately preceding negative peak.Tarlov behaviour score was performed before surgery and after surgery at 1,3,7 days. Histologic studies were performed on part of animals. Pathological samples were observed by light and electron microscope.The effects of the SCI on the EPs were estimated on the changes in latency and amplitude of peak in this study. The data were analyzed by analysis of variance , t-test and linear correlation. P-value less than 0.05 was considered statistically significant.Result:EPs recorded before spinal cord compression showed a stable pattern. Spinal cord compression resulted in a gradual decrement in the peak latency and significant increment in the peak amplitude.In the control group,the wave form ,latency ,and amplitude of EPs remained quite stable throughout the whole experiment. We studied the relationship between stimulation intensity and the TMS-MEP. We found ,as the stimulus intensity was increased, the MEP latency decreased and the amplitude increased. So we use 100% intensity stimulus to obtain more stable and reliable MEP waves. In the 5min group ,a decrease in amplitude and an increase in latency of the EP were observed 5 min after decompression, the amplitude and the latency of EPs recovered quickly in this group, but the latency was more obviously. All reached preinjury value after 3 days. In the 15min group, the results showed a more pronounced decrease of amplitude and a more pronounced increase of latency, reached preinjury value after 7 days. In the 30min group, After severe spinal cord compression ,the waves disappeared totally immediately. 60min after decompression,SEP returned; 6 hours after decompression, MEP returned. Latency and amplitude never recovered to baseline levels 7 days later. The data of the 30min group showed a more pronounced decrease of amplitude and a more pronounced increase of latency. After decompression, the recovery of the latency was more obvious than the amplitude in all injury groups, EPs recovered and Tarlov improve too.The data of latency and amplitude were analyzed with analysis of variance(F-test) . In control group , no statistically significant difference in the latency and amplitude. After decompression ,there were a significantly difference compared with the baseline lantency. SEP showed obvious delay of amplitude but the varies of its latency were not obvious. By contrast, the latency and amplitude of MEP were changed dramatically. The results indicated that MEP is more sensitive and clear.
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