Motor evoked potential as a predictor of recovery in chronic spinal cord injury

In injury to the spinal cord, motor function is the most critical system affected. The introduction of motor evoked potential (MEP) monitoring techniques over the past 5 years has opened new possibilities for the assessment of motor function. We examined the MEP change in 30 cats that received weight drop injuries to the thoracic spinal cord and correlated it with function. The MEPs were recorded above and below the injury and in the sciatic nerves. In these animals, we found that the peripheral nerve response to the MEP was the most sensitive to injury, disappearing immediately upon weight drop. The MEP at the spinal cord level was somewhat more sensitive to injury than the SSEP at the cord level, and the SSEP at the cortex was the least sensitive to injury. The MEP spinal cord signal below the lesion showed both a latency increase and a substantial amplitude decrease after impact. The latency increase, however, was of relatively small absolute magnitude making the amplitude change easier to monitor. The animals were followed chronically, and 17 of the 30 regained ambulation. In all animals (followed up to 2 months), the peripheral nerve signals returned at or immediately before the time of ambulation. In none of the animals who failed to regain a peripheral nerve response was ambulation attained. This supports a strong correlation of the peripheral nerve response with walking. However, the peripheral nerve response was only a short term predictor. Examination of the spinal cord signal showed that the MEP spinal cord signal below the lesion as a percentage above the lesion was a significant correlate of current ambulation recovery, with a correlation coefficient of 0.55. This suggests that evaluation of the SSEP and MEP spinal cord signals may be able to predict longer term recovery in animals and perhaps humans.