Somatosensory Evoked Responses to Dermatomal Stimulation in Cervical Spinal Cord Injured and Normal Subjects

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Abstract
This exploratory study investigates dermatomal evoked potential patterns in the upper extremities of normal and spinal cord injured subjects. Fifteen normal subjects without neurologic deficits and twelve patients with partial or complete spinal cord injuries were tested at dermatomal levels C5, C6, C7, C8, and T1, and also at median and ulnar nerve sites. Responses were recorded at the scalp. Analyses of evoked response patterns included measurement and comparison of peak and interpeak latencies and amplitudes as well as blind ratings of the degree of abnormality of evoked potential waveforms. Analyses were also made of relationships between evoked potential data and neurological findings on clinical examination. There appeared to be a fairly consistent SEP response among normals when dermatomes C6 through C8 are stimulated. Less consistent responses are observed when C5 and T1 are stimulated. In general, spinal cord injured subjects as compared to normal subjects had evoked responses with less consistent peaks, more amplitude diminution, and greater diffuseness and overall pattern abnormality even at dermatomal levels that were intact on clinical neurologic examination. There was also a distinct progression of overall SEP abnormality in dermatomes with impaired vibration, light touch, and position sense. There were no consistent differences in interpeak latencies between SEPs of normal and spinal cord injured subjects at intact dermatomes, but there were significant differences in EP abnormalities (EPA scores). Possible reasons for the differences in the SEP responses between normals and spinal cord injured subjects include spinal cord injury not detectable by clinical exam. Difficulty in obtaining objective and accurate sensory reports also contributes to data unreliability. In conclusion, we believe that stimulation of specific sensory dermatomes merits further study, as it has a number of possible clinical uses. These include: (1) surgical monitoring at more specific levels than monitoring with mixed nerve root stimulation, (2) for study of specific nerve root injury, and (3) as an aid in examining the neurological status in acutely injured spinal cord patients who are unable to cooperate adequately during examination, such as the very young or those with lowered levels of awareness associated with head injury. Additional information which could be useful to obtain is cervical dermatomal stimulation with spinal recording sites.