Waveform changes due to conduction block and their underlying mechanism in spinal somatosensory evoked potential: a computer simulation

Abstract

Based on a square-wave solid-angle analysis, a simplified mathematical model was produced for computing a sequence of potential change in a volume conductor generated by an impulse traveling along a nerve fiber. A conduction block was simulated as a phenomenon in which a depolarization wavefront stops traveling when it reaches a certain point, although the following repolarization wavefront continues to travel until it reaches the same point. The spinal somatosensory evoked potential (SSEP) was produced as an algebraic sum of simulated nerve fiber action potentials (NFAPs). With a conduction block, an NFAP that was normally triphasic showed a positive-negative diphasic wave with reduced negativity at the point of the block, diphasic waves with enhanced negativity at points immediately preceding the block, and initial-positive waves alone or abolition of any wave at points beyond the block. The absence of their terminal-positive phases paradoxically enhanced the negative peak of the spinal SSEPs in a partial block that involved only the constituent fastest fibers, because phase cancellation of the phases between the terminal-positive phases of the fastest fibers and the negative phases of the slower fibers, which normally happens, failed to occur. At the points immediately preceding the block, the identical mechanism sustained the spinal SSEP enhancement even when every fiber was included in the block. The computer model predicted that localization of the precise site of conduction block can be achieved by demonstrating an abrupt reduction in the amplitude of the spinal SSEP, which is accompanied by an increased negative wave caudally and an enhanced monophasic positive wave rostrally.

Intraoperative electroneurography in the assessment of the level of operation for cervical spondylotic myelopathy in the elderly

“…In the measurement of SCEPs, the combination of two opposite changes in waveform, namely an abrupt reduction in size of the negative peak and a concomitant augmentation of the initial positive peak, identified the site of a conduction block. [16][17][18]27,28 Whereas compression of the cord may appear without functional change, the presence of conduction abnormalities implies a sufficient degree of compression, usually at the level of the smallest AP diameter and cross-sectional area of the cord. In the patients with the same degree of compression at more than one level, MRI by itself was insufficient to localise the site primarily responsible for myelopathy (Figs 1 and 2).…”

Evaluation of time-dependent spread of tissue damage in experimental spinal cord injury by killed-end evoked potential: effect of high-dose methylprednisolone

“…In some cats, we confirmed that the latency of the KEEP, which was recorded using a depth electrode placed in the injury site of the posterolateral funiculus, demonstrated a decrease in amplitude with a similar time course (data not shown). Although the amplitude of KEEP is influenced by many variables, 22,28 most are related to events occurring at a point of conduction block, such as the injury current. 27 Because there was no significant change in amplitude of the largest monophasic positivity during the experimental period, its observed decrease at the injury site does not appear to be attributable to changes in the KEEP as the source of volume-conducted potential.…”

Paradoxical enhancement of spinal-cord-evoked potentials rostral and caudal to the site of progressive cord compression in the cat

“…Computer modelling, however, disputed the validity of this interpretation predicting that partial conduction block could cause an increase in the size of SCEPs instead. 10,11 Furthermore, an animal experiment employing constant-speed progressive cord compression showed a transient increase in the amplitude of ascending SCEPs at the compression site. 12 Therefore, an enhancement of SCEPs should be interpreted with caution, since it may not necessarily imply functional recovery.…”

“…A computer model of a progressive conduction block supports this finding as well as predicts a similar phenomenon after the preferential block of fast fibers. 10,11 How, then, do conduction abnormalities produce an enhancement of N1 peaks? It is known that SCEPs represent a linear summation of the nerve fiber action potentials (NFAPs) arising from constituent nerve fibers of different diameters.…”

“…Rostral to the compression site, the impulse in a blocked fiber approaches without reaching the recording site. Impulses arriving at the site of conduction block will give rise to a killed-end effect with a local volumeconducted initial-positive wave alone 10,11,[21][22][23] ( Figure 4b), which will quickly diminish in size rostrally; that is, removing both the negative and positive peaks. In this case, the loss of the negative peaks from blocked fibers would reduce the N1 peak.…”