Motor evoked potentials of the respiratory muscles in tetraplegic patients

We studied the respiratory muscles with magnetic transcranial stimulation (TCS) in four spinal cord injured (SCI) patients as compared to age-matched controls from a database of 40 healthy subjects. These SCI patients all had spinal cord lesions above C6 level with a clinically incomplete tetraplegia. One patient was artificially ventilated. Motor evoked potentials (MEPs) were recorded from the diaphragm, the scalenes, the parasternal intercostals and the expiratory rectus abdominis during inspiration and expiration. In patients with incomplete tetraplegia MEP latency times were significantly prolonged in the scalenes and the parasternal intercostals, both during inspiration and expiration, and were nearly normal for the diaphragm, which was found to be more or less preserved. The mean MEP amplitudes in these patients for all inspiratory muscles studied were significantly decreased in tetraplegic patients, in part due to a decreased number of innervating axons and muscle hypotrophy. No MEPs could be obtained from the abdominal muscles, except in one C3 tetraplegic patient, in whom only a very small response was seen during expiration, with a very delayed latency time. The much lower location of their innervating nerve roots (T10) and the much longer distance of their spinal exit zone from the level of injury at the cervical spinal cord might at least partially explain this phenomenon. In the ventilator-dependent tetraplegic patient no MEPs could be obtained from any of the muscles studied. Thus, magnetic TCS is a painless and easily applicable technique to investigate the central motor conduction properties of the respiratory muscles, both in healthy humans and in tetraplegic patients.