Study design: Spinal cord monitoring has used both spine and scalp recordings as indicators of spinal cord integrity. The relative merits of spine or scalp recordings to predict the quality of the afferent volley in the somatosensory pathway were addressed in this study by using various stimulus intensities as a way to model alterations of the size of the afferent volley.
Objectives: The results were analyzed to determine the correlation of central recordings taken at the spine or scalp with peripheral recordings.
Summary of background data: Spinal cord monitoring with somatosensory evoked potentials has been achieved with recordings of signals generated by either the spinal cord or the somatosensory cortex. Spine recordings are thought to be more stable, yet little evidence exists to document this statement.
Methods: Seven patients were studied in the course of standard intraoperative spinal cord monitoring. Responses were recorded at the popliteal fossa, thoracic epidural, cervical spine, and scalp to tibial nerve stimulation at intensities varying from 0.5 to 2.0 times muscle twitch threshold.
Results: Normalized amplitudes of the response at the popliteal fossa were used to reflect the magnitude of the afferent volley. The amplitudes of the popliteal fossa response showed a high correlation (r = 0.90) with normalized amplitudes of epidural and cervical spine responses and moderate correlation (r = 0.49) with normalized amplitudes of scalp responses. The width of the 95% confidence limits for the inverse prediction of the afferent volley from epidural and cervical responses was nearly a third narrower than that from scalp responses. At low stimulus intensities, scalp responses were consistently observed when spine responses were absent, and scalp responses had lower response thresholds than did spine responses. The latencies of the popliteal fossa responses were not well correlated with latencies of either the epidural or cervical responses.
Conclusions: These correlation and inverse prediction data suggest that the size of an afferent volley may be predicted more accurately by spine responses than by scalp responses. The presence of scalp responses at intensities too low to elicit detectable spinal-level responses suggests that scalp responses may be considered a sensitive indicator of a minimal afferent volley.