Central nervous system injury, by accident or disease, is a devastating problem whose treatment could be furthered by an electrical monitoring system that would indicate the status of the various motor pathways. This would aid investigative research, clinical management, operating room treatment of CNS problems and provide prognostic information for the patient. Somatosensory evoked potentials which travel in the dorsal columns have been used extensively for this purpose. However, they are limited by having a different location and blood supply from the motor pathways. They have been inaccurate in trauma for predicting motor recovery in a number of circumstances. We have developed a motor evoked potential using transcranial stimulation of the motor cortex through the scalp and skull between a surface anode and subcranial cathode in the palatal area. This produces an evoked potential in the brainstem, spinal cord and peripheral nerve which can be detected by modern evoked potential signal averaging devices and produces movement at higher current levels. The current required in the cat is in the 7 to 30 milliampere range. Lesioning studies show that this follows classical expectations for corticospinal activation, in that a large component of the signal travels in both dorsolateral and ventral cord. Initial acute lesioning studies show that the peripheral nerve signal is much more sensitive than the cord signal to spinal cord impact, and very sensitive to middle cerebral artery occlusion.
We aim to develop this test for investigative and clinical use based on the cat as an animal model.
We aim to 1) specify the pathways involved, 2) to assess the safety of the electrical stimulation upon the cortex when done transcranially, and 3) to identify the parameters of the motor evoked potential which are most useful in predicting recovery from spinal cord injury and stroke in chronic models. We then would use those parameters to test the realiability of the evoked potential in recovery of function in a number of cats. We will assess both electrical and magnetic stimulation. In addition, we have identified a separate evoked potential activating non-pyramidal pathways, resulting from stimulation of the cerebellum, and a further separate evoked potential from stimulating the pharynx or posterior palate area. These may have uses of their own. It is necessary that they be characterized to make clear their separation from the transcranial motor evoked potential and establish their value. This work should provide a defined status for the pathway, the safety, and the predictive ability of this test.
