Paraspinal stimulation to elicit somatosensory evoked potentials: an approach to physiological localization of spinal lesions.

Somatosensory evoked potentials (SEPs) were elicited by stimulation of the paraspinal region. Simultaneous bilateral stimulation, 2 cm lateral to the midline, sufficient to induce a visible muscle twitch, was applied opposite vertebral levels L3, T12, T6 and T1 and intervening segments in some subjects. The potentials were recorded over the scalp (Cz-Fz). The stimulus excludes most of the peripheral nervous system; the volley being initiated in the cutaneous branches of the primary dorsal root rami with some contribution from paraspinal muscle Ia afferents. In normal subjects, paraspinal evoked SEPs are easily elicitable and measurable. Mean spinal cord conduction velocity between T12 and T1 measured 64.1 m/sec (N = 25). The upper thoracic cord propagated faster than the lower thoracic cord which conducted faster than the lumbar segment. The technique was used to confirm the approximate level of radiologically visible spinal lesions that were surgically treated and to identify diffuse, focal or multisegmental spinal conduction slowing in patients devoid of radiologically visible lesions. The method has potential for intraoperative spinal cord monitoring.