Epidural Recordings of Electrical Events Produced in the Spinal Cord by Segmental, Ascending and Descending Volleys

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Abstract
Epidural electrodes implanted for a percutaneous trial of therapeutic spinal cord stimulation were used to record electrical events evoked by the stimulation of peripheral nerves or of the spinal cord itself. The data collected in patients with no neurological deficit were analyzed in order (1) to check the consistency between epidural and surface recordings, (2) to get information on the genesis of such potentials, and (3) to demonstrate the feasibility of complex neurophysiological studies by means of epidural electrodes. Spinal cord potentials evoked by segmental volleys were recorded at cervical levels with the recording electrodes anterior, lateral and posterior to the spinal cord. The refractory period of the evoked potentials has been studied as well. Responses to stimulation of the tibial nerve were obtained at T11–12 vertebral level with posterior epidural electrodes. Segmental cervical potentials were characterized by a P10, N11, N13/P13 followed by a slow positivity/negativity. A response of similar waveform, but with different peak latencies, was recorded at segmental levels following tibial nerve stimulation. Such a response showed an increasing number of spikes while ascending along the spinal cord. Maximum conduction velocities in the cord were between 65 and 85 m/s. Our epidural recordings are similar to those obtained from the skin, but with a greater amplitude and waveform resolution. Furthermore, the use of epidural electrodes made it feasible to perform complex examinations of sensory function (i.e., the study of orthodromic and antidromic conduction along the dorsal cord and of the influence of a single dorsal cord volley on the segmental cervical potential). Finally, the genesis of the potentials recorded is discussed.

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