Signal Modeling and Simulation of Temporal Dispersion and Conduction Block in Motor Nerves

OBJECTIVE

Electroneurography is a well-established diagnostic test for supporting the diagnosis of disorders of myelinated peripheral nerves. Neurophysiological quantities are automatically calculated and are used to determine the pathology of the nerve (axonal damage) or its sheath (myelin damage). Specific differential diagnostic criteria are derived from time-domain normative data, which result primarily from a computer simulation in the early 1990s based on animal data, namely rats. However, the rat signals studied differ significantly from those of humans because of anatomical differences.

METHODS

We present a model-based simulation of nerve conduction in healthy and pathological motor nerves. In contrast to earlier simulations, the present model is based on motor unit action potentials extracted from real human measurements facilitating the generation of realistic signals, starting from a conduction velocity distribution. In addition to the modeling of healthy nerves, we model a hereditary peripheral nerve disease as well as an acute and a chronic inflammatory demyelinating condition.

RESULTS

Quantitative signal differences based on standard variables in the time-domain are presented. The findings for the demyelinating conditions demonstrate amplitude reductions of 71% and 65% between the distal and proximal responses, which result from an increase in the variance of the nerve fiber conduction velocities.

CONCLUSION

The simulation results closely match those of empirical measurements, indicating that the signal model captures relevant pathological mechanisms. An amplitude reduction of more than 50% in demyelinating conditions is in accordance with routine measurements and shows that temporal dispersion is quite well-modeled compared to previous simulation models.

SIGNIFICANCE

The simulation outcomes can serve as the basis for an improved pathophysiological understanding of peripheral nerve disorders and should aid neurophysiologists to refine their diagnostic armamentarium resulting in a more precise differential diagnosis.

Functional importance of degenerative spondylolisthesis in cervical spondylotic myelopathy in the elderly

STUDY DESIGN

A correlation was studied between degenerative spondylolisthesis (DSL) of the cervical spine and spinal-evoked potentials intraoperatively recorded in elderly patients who had surgical treatment for cervical spondylotic myelopathy.

OBJECTIVE

To investigate the functional importance of cervical DSL in elderly patients with cervical spondylotic myelopathy.

SUMMARY OF BACKGROUND DATA

Cervical DSL has received insufficient attention in contrast to the lumbar DSL. The authors are unaware of any journal article in which this condition has been evaluated electrophysiologically.

METHODS

This study investigated 47 patients with 68 DSL of 2 mm or more (3.1 +/- 0.9 mm; range, 2-6 mm) who underwent serial intervertebral recording of spinal-evoked potentials from either the intervertebral disc or the ligamentum flavum after epidural stimulation.

RESULTS

All the study patients had unequivocal evidence of a focal conduction block, with the area of negative evoked potential peak reduced to less than 60% that of the immediately caudal level: 31 at C3-C4, 12 at C4-C5, and 1 each at C1-C2, C2-C3, C5-C6, and C6-C7. The site of conduction block matched the level of DSL in 30 patients, but not in 17 patients. The DSL accompanied by conduction block had significantly greater displacement with greater angular mobility than that without conduction block.

CONCLUSIONS

A significant association between DSL and conduction block in the face of a relatively wide canal indicates the functional importance of DSL in elderly patients with cervical spondylotic myelopathy. In this age group, a high incidence of both DSL (81%) and focal conduction block (91%) at the upper cervical level (C3-C4 or C4-C5) is of clinical interest.

The motor-evoked potentials elicited from the deltoid muscle by transcranial magnetic stimulation with a standardized facilitation: the potential diagnostic utility for C5 radiculopathy

STUDY DESIGN

We investigated the effects of standardized facilitation on stability in amplitude of the motor-evoked potentials from the deltoid muscle after magnetic stimulation over the scalp in normal study participants.

OBJECTIVES

To test the potential of this technique in evaluating deltoid weakness and recovery after C5 radiculopathy.

SUMMARY OF BACKGROUND DATA

Most previous motor-evoked potential studies have emphasized the value of the latency rather than amplitude, probably because the amplitude varies much more than the latency, even in the same individual.

METHODS

Twenty right-handed healthy men underwent deltoid motor-evoked potential studies on three successive occasions at intervals of >2 weeks. A digital readout of the shoulder abduction force allowed the study participant to adjust voluntary contraction to 20% of the maximal force at the moment of stimulation. The deltoid motor-evoked potentials were compared with the M responses recorded from the relaxed deltoid after electrical stimulation of the brachial plexus.

RESULTS

Despite considerable individual variability in the amplitude of deltoid motor-evoked potentials, a standardized facilitation yielded a relatively small variation of amplitude between sessions in the same individual, showing an intersession variability index of 9.1 +/- 7.9%. The side-to-side amplitude ratio, averaging 82 +/- 12%, also provides a more reliable measure in evaluating unilateral deltoid weakness than the absolute value. The measures on the left showed significantly higher amplitude than on the right for the deltoid motor-evoked potentials but not for the M responses.

CONCLUSIONS

The changes of the motor-evoked potential amplitude exceeding 25% between sessions and side-to-side amplitude ratios <60% can be outside the normal limits. The asymmetry of deltoid motor-evoked potentials in favor of the nondominant side is of physiologic interest.

Electrophysiologic assessment of shoulder girdle weakness in patients with cervical spondylosis: prognostic value of supraclavicular stimulation

OBJECTIVES

We tested the use of supraclavicular stimulation as a prognostic measure of deltoid function after cervical spondylotic radiculopathy.

METHODS

Electrophysiological study consisted of supramaximally stimulating the brachial plexus and recording compound muscle action potentials (CMAPs) from the deltoid in 32 patients with unilateral weakness of 3 months or shorter duration as compared with 69 healthy subjects.

RESULTS

Despite considerable individual variability and age-related reduction in amplitude and area of deltoid CMAPs, the side-to-side ratios yielded useful, reproducible values. The measures recorded initially correlated significantly with the eventual recovery of deltoid strength but, interestingly, not with the initial degree of weakness. In particular, the CMAPs greater than 50%, even in the face of severe weakness, predicted a nearly complete return of function.

CONCLUSIONS

Supraclavicular stimulation provides a useful measure in monitoring deltoid weakness and predicting its recovery in patients with cervical spondylosis.

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.

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

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.

Multifocal motor neuropathy: response to human immune globulin

Multifocal motor neuropathy (MMN) is a progressive disorder producing asymmetrical weakness and muscle wasting. Case reports suggest that patients with MMN improve after cyclophosphamide therapy, but not after prednisone or plasmapheresis. Because MMN is likely to be immune mediated, we investigated the therapeutic response to human immune globulin (HIG) in an open, uncontrolled trial. Nine patients, ages 28 to 58 years, had chronic, progressive, asymmetrical, predominantly distal, limb weakness for 5 to 18 years. Sensation was normal, and reflexes were reduced asymmetrically. All had physiological evidence of multifocal motor demyelination with partial motor conduction block, and 7 had elevated serum titers of anti-GM1 IgM antibody. All patients were treated with HIG, 1.6 to 2.4 gm/kg, given intravenously over 3 to 5 days. Strength improved in all patients 3 to 10 days after treatment, with improvement peaking at 2 weeks and lasting for an average of 2 months. The range of functional improvement varied from dramatic to mild. The degree of partial motor conduction block was reduced, at least partially, in 7 of 8 patients. The serum anti-GM1 antibody titers did not change. Repeated courses of HIG resulted in similar improvements. We conclude that HIG may be an effective therapy for patients with MMN.