Relationships between the changes in compound muscle action potentials and selective injuries to the spinal cord and spinal nerve roots

Objective: Compound muscle action potentials (CMAPs) evoked by transcranial electrical stimulation have been widely introduced to monitor motor function during spinal surgery. They may reflect segmental injuries as well as injuries to motor-related tracts in the spinal cord. However, we have experience with some patients who developed postoperative segmental motor weakness without any potential changes during surgery. To evaluate the efficacy of this method, we used a cat model to observe the relationships between potential changes and selective injuries to the white and gray matters of the spinal cord and spinal nerve roots. Methods: Ten CMAPs were obtained before and after injury to the spinal cord and spinal nerve roots in 20 cats. Changes in the amplitude, latency, and duration of CMAPs were analyzed. Results: CMAPs decreased in amplitude significantly after the insult to the motor-related tracts in the spinal cord in all cats, while the potentials did not always change when the insult was restricted to a limited area in the anterior horn of the spinal cord or to the single spinal nerve root. Conclusions: CMAPs may not exactly reflect segmental injury, and careful attention should be paid to the interpretation of CMAPs.