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

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Abstract

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.

Introduction

In peripheral nerve entrapment syndromes, localization of the precise site of compression is best achieved by serial short incremental stimulation searching for focal conduction abnormalities (Kimura, 1979). Similarly, in compression myelopathies, the vertebral level responsible for the symptoms can be disclosed by recording the ascending axonal volley at multiple sites along the spinal cord (Kimura and Kaji, 1991; Waxman et al., 1995). For this purpose, pickup electrodes must be placed in the structures adjacent to the spinal cord itself. In this case, an abrupt reduction in size of the negative wave over a short segment is the strong evidence of a conduction block (McDonald and Sears, 1970; Cornblath et al., 1991). In practice, the change is often rendered steeper, and thereby more easily recognized by the presence of an enhanced negative wave in the lead immediately caudal to the lesion. Were it not for an enlargement of this kind, it might be more difficult to distinguish conduction block from physiologic temporal dispersion in mild cases. Despite its clinical advantage, little attention has been given to the mechanisms underlying this enhancement except in a work using bipolar epidural leads (Ohmi et al., 1991). We have therefore conducted waveform analysis to elucidate the phenomenon based on our data of spinal somatosensory evoked potentials (SSEPs) recorded during posterior operations for compression myelopathies.

Section snippets

Patients

From July 1991 to July 1996, 33 patients with compression myelopathies underwent intraoperative SSEP studies. Informed consent was obtained. We analyzed the recordings in 26 patients selected on the basis of an unequivocal focal conduction block with reduction in amplitude of the negative peak to 50% or less compared to the immediately caudal level. Myelopathy resulted from cervical spondylosis (16), ossified posterior longitudinal ligament (OPLL) (7), rheumatoid arthritis of the cervical spine

Results

Incremental studies uncovered a single site of focal conduction block designated as 0 in all patients, one each at Cl/2 and C2/3, 10 each at C3/4 and C4/5, 3 at C5/6 and one at T3/4. At this level, the amplitude of the negative component was reduced (P<0.0001) to 32±22% (mean±SD) and the area to 31±22% compared to the −4 level, which was taken as the baseline (Table 1). In contrast, the initial positivity was increased (P<0.0001) to 170±78% in amplitude and 296±201% in area (Table 2). In

Discussion

The present study was designed to document an apparent enhancement of the SSEPs recorded at the sites preceding a conduction block in chronic compression myelopathies. This is apparently paradoxical because, had it not been for the conduction block at the 0 level, the negative components would have been progressively smaller in size from −4 to −1, as predicted from physiological temporal dispersion with phase cancellation (Kimura et al., 1988). An enlargement of this kind cannot simply be

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