Assessment of Corticospinal Function in Spinal Cord Injury Using Transcranial Motor Cortex Stimulation: A Review

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Abstract
Other than clinical examination, few methods exist for assessing the functional condition of descending long tracts of the spinal cord in humans. This review covers neurophysiological examination of the corticospinal system using transcranial electrical and magnetic motor cortex stimulation. The neurophysiological basis for the motor evoked potentials (MEPs) and the differences between the two methods are discussed followed by a review of their use in individuals with spinal cord injury (SCI). Transcranial motor cortex stimulation is used to monitor descending spinal cord tract condition during spinal surgeries and could be useful for assessing central nervous system trauma, especially in the unconscious multitrauma patient. In the chronic phase of SCI, recordings of MEPs have enabled the estimation of central conduction times that relate to the condition of axons passing through the injured segment of the spinal cord. They were found to correlate well with clinical examination scores but as predictors of outcome, the reports have been mixed. The use of transcranial motor cortex stimulation to modify segmental reflexes and in combination with volitional attempts have also provided evidence of conduction across the lesion in paralyzed SCI subjects. However, MEPs can be absent in some SCI individuals who may be able to volitionally activate muscles below the level of the spinal cord lesion. Such findings are useful in elucidating the neural mechanisms underlying the performance of a volitional movement and may serve to guide and monitor the effects of future treatments for paralysis in SCI and other neurological disorders.