Waveform analysis of spinal somatosensory evoked potential: paradoxically enhanced negative peaks immediately caudal to the site of conduction block.

OBJECTIVES

We studied waveform changes associated with a focal conduction block in compression myelopathies.

DESIGN AND METHODS

A total of 26 patients underwent serial intervertebral recording of spinal somatosensory evoked potentials (SSEPs) after epidural stimulation. The site of compression identified by abrupt reduction in size of the negative peak was designated as '0' level with the other levels numbered in order of distance assigning a minus sign caudally.

RESULTS

Considering the response recorded at '-4' as baseline (100%), SSEPs showed a progressive increase rostrally, reaching an average of 154% in amplitude and 216% in area at '-1' followed by an abrupt decline to 32% and 31% at '0'. The incremental change of the negative peak was accompanied by a small reduction in area of the initial positive component to 90% at '-1' considering the value at '-4' as baseline (100%).

CONCLUSIONS

The theory of solid angle approximation and the concept of phase cancellation best explain the apparently paradoxical enhancement of the negative peak which characterize typical waveform changes at the site of conduction block.