The effect of temporal dispersion on compound action potential area in human peripheral nerve

Revisiting the compound muscle action potential (CMAP)

The compound muscle action potential (CMAP) is among the first recorded waveforms in clinical neurography and one of the most common in clinical use. It is derived from the summated muscle fiber action potentials recorded from a surface electrode overlying the studied muscle following stimulation of the relevant motor nerve fibres innervating the muscle. Surface recorded motor unit potentials (SMUPs) are the fundamental units comprising the CMAP. Because it is considered a basic, if not banal signal, what it represents is often underappreciated. In this review we discuss current concepts in the anatomy and physiology of the CMAP. These have evolved with advances in instrumentation and digitization of signals, affecting its quantitation and measurement. It is important to understand the basic technical and biological factors influencing the CMAP. If these influences are not recognized, then a suboptimal recording may result. The object is to obtain a high quality CMAP recording that is reproducible, whether the study is done for clinical or research purposes. The initial sections cover the relevant CMAP anatomy and physiology, followed by how these principles are applied to CMAP changes in neuromuscular disorders. The concluding section is a brief overview of CMAP research where advances in recording systems and computer-based analysis programs have opened new research applications. One such example is motor unit number estimation (MUNE) that is now being used as a surrogate marker in monitoring chronic neurogenic processes such as motor neuron diseases.

Revisiting the use of Hoffmann reflex in motor control research on humans

Research in movement science aims at unravelling mechanisms and designing methods for restoring and maximizing human functional capacity, and many techniques provide access to neural adjustments (acute changes) or long-term adaptations (chronic changes) underlying changes in movement capabilities. First described by Paul Hoffmann over a century ago, when an electrical stimulus is applied to a peripheral nerve, this causes action potentials in afferent axons, primarily the Ia afferents of the muscle spindles, which recruit homonymous motor neurons, thereby causing an electromyographic response known as the Hoffmann (H) reflex. This technique is a valuable tool in the study of the neuromuscular function in humans and has provided relevant information in the neural control of movement. The large use of the H reflex in motor control research on humans relies in part to its relative simplicity. However, such simplicity masks subtleties that require rigorous experimental protocols and careful data interpretation. After highlighting basic properties and methodological aspects that should be considered for the correct use of the H-reflex technique, this brief narrative review discusses the purpose of the H reflex and emphasizes its use as a tool to assess the effectiveness of Ia afferents in discharging motor neurones. The review also aims to reconsider the link between H-reflex modulation and Ia presynaptic inhibition, the use of the H-reflex technique in motor control studies, and the effects of ageing. These aspects are summarized as recommendations for the use of the H reflex in motor control research on humans.

Overcoming temporal dispersion for measurement of activity-related impedance changes in unmyelinated nerves

Objective.Fast neural electrical impedance tomography is an imaging technique that has been successful in visualising electrically evoked activity of myelinated fibres in peripheral nerves by measurement of the impedance changes (dZ) accompanying excitation. However, imaging of unmyelinated fibres is challenging due to temporal dispersion (TP) which occurs due to variability in conduction velocities of the fibres and leads to a decrease of the signal below the noise with distance from the stimulus. To overcome TP and allow electrical impedance tomography imaging in unmyelinated nerves, a new experimental and signal processing paradigm is required allowing dZ measurement further from the site of stimulation than compound neural activity is visible. The development of such a paradigm was the main objective of this study.Approach.A finite element-based statistical model of TP in porcine subdiaphragmatic nerve was developed and experimentally validatedex-vivo. Two paradigms for nerve stimulation and processing of the resulting data-continuous stimulation and trains of stimuli, were implemented; the optimal paradigm for recording dispersed dZ in unmyelinated nerves was determined.Main results.While continuous stimulation and coherent spikes averaging led to higher signal-to-noise ratios (SNRs) at close distances from the stimulus, stimulation by trains was more consistent across distances and allowed dZ measurement at up to 15 cm from the stimulus (SNR = 1.8 ± 0.8) if averaged for 30 min.Significance.The study develops a method that for the first time allows measurement of dZ in unmyelinated nerves in simulation and experiment, at the distances where compound action potentials are fully dispersed.

Motor Nerve Conduction Block Estimation in Demyelinating Neuropathies by Deconvolution

A deconvolution method is proposed for conduction block (CB) estimation based on two compound muscle action potentials (CMAPs) elicited by stimulating a nerve proximal and distal to the region in which the block is suspected. It estimates the time delay distributions by CMAPs deconvolution, from which CB is computed. The slow afterwave (SAW) is included to describe the motor unit potential, as it gives an important contribution in case of the large temporal dispersion (TD) often found in patients. The method is tested on experimental signals obtained from both healthy subjects and pathological patients, with either Chronic Inflammatory Demyelinating Polyneuropathy (CIDP) or Multifocal Motor Neuropathy (MMN). The new technique outperforms the clinical methods (based on amplitude and area of CMAPs) and a previous state-of-the-art deconvolution approach. It compensates phase cancellations, allowing to discriminate among CB and TD: estimated by the methods of amplitude, area and deconvolution, CB showed a correlation with TD equal to 39.3%, 29.5% and 8.2%, respectively. Moreover, a significant decrease of percentage reconstruction errors of the CMAPs with respect to the previous deconvolution approach is obtained (from a mean/median of 19.1%/16.7% to 11.7%/11.2%). Therefore, the new method is able to discriminate between CB and TD (overcoming the important limitation of clinical approaches) and can approximate patients’ CMAPs better than the previous deconvolution algorithm. Then, it appears to be promising for the diagnosis of demyelinating polyneuropathies, to be further tested in the future in a prospective clinical trial.

Nerve conductions studies in experimental models of autoimmune neuritis: A meta-analysis and guideline

Nerve conduction studies (NCS) are essential to assess peripheral nerve fiber function in research models of immune-mediated neuritis. However, the current lack of standard protocols and reference values impedes data comparability across models and studies. We performed a systematic review and subsequent meta-analysis of the last 30 years of NCS of immune-mediated neuritis in Lewis-rats. Twenty-six papers met the inclusion criteria for meta-analysis. Extracted data showed considerable heterogeneity of recorded nerve conduction velocity (NCV) and compound muscle action potential (CMAP). Studies also significantly differed in terms of technical, methodical, and data reporting issues. The heterogeneity of the underlying studies emphasizes the need for standardization when conducting and reporting NCS in rats. We provide normative values for NCS of the sciatic nerve of Lewis rats and propose seven items that should be addressed when NCS are performed when studying immune paradigms in Lewis rats.

Measuring conduction velocity distributions in peripheral nerves using neurophysiological techniques

OBJECTIVE

To determine how long it takes for neural impulses to travel along peripheral nerve fibers in living humans.

METHODS

A collision test was performed to measure the conduction velocity distribution of the ulnar nerve. Two stimuli at the distal and proximal sites were used to produce the collision. Compound muscle or nerve action potentials were recorded to perform the measurements on the motor or mixed nerve, respectively. Interstimulus interval was set at 1-5 ms. A quadri-pulse technique was used to measure the refractory period and calibrate the conduction time.

RESULTS

Compound muscle action potential produced by the proximal stimulation started to emerge at the interstimulus interval of about 1.5 ms and increased with the increment in interstimulus interval. Two groups of motor nerve fibers with different conduction velocities were identified. The mixed nerve showed a wider conduction velocity distribution with identification of more subgroups of nerve fibers than the motor nerve.

CONCLUSIONS

The conduction velocity distributions in high resolution on a peripheral motor and mixed nerve are different and this can be measured with the collision test.

SIGNIFICANCE

We provided ground truth data to verify the neuroimaging pipelines for the measurements of latency connectome in the peripheral nervous system.

Asymmetric recurrent laryngeal nerve conduction velocities and dorsal cricoarytenoid muscle electromyographic characteristics in clinically normal horses

The dorsal cricoarytenoid (DCA) muscles, are a fundamental component of the athletic horse's respiratory system: as the sole abductors of the airways, they maintain the size of the rima glottis which is essential for enabling maximal air intake during intense exercise. Dysfunction of the DCA muscle leads to arytenoid collapse during exercise, resulting in poor performance. An electrodiagnostic study including electromyography of the dorsal cricoarytenoid muscles and conduction velocity testing of the innervating recurrent laryngeal nerves (RLn) was conducted in horses with normal laryngeal function. We detected reduced nerve conduction velocity of the left RLn, compared to the right, and pathologic spontaneous activity (PSA) of myoelectrical activity within the left DCA muscle in half of this horse population and the horses with the slowest nerve conduction velocities. The findings in this group of horses are consistent with left sided demyelination and axonal loss, consistent with Recurrent Laryngeal Neuropathy (RLN), a highly prevalent degenerative disorder of the RLn in horses that predominantly affects the left side. The detection of electromyographic changes compatible with RLN in clinically unaffected horses is consistent with previous studies that identified "subclinical" subjects, presenting normal laryngeal function despite neuropathologic changes within nerve and muscle confirmed histologically.

Electrode fabrication and interface optimization for imaging of evoked peripheral nervous system activity with electrical impedance tomography (EIT)

OBJECTIVE

Non-invasive imaging techniques are undoubtedly the ideal methods for continuous monitoring of neural activity. One such method, fast neural electrical impedance tomography (EIT) has been developed over the past decade in order to image neural action potentials with non-penetrating electrode arrays.

APPROACH

The goal of this study is two-fold. First, we present a detailed fabrication method for silicone-based multiple electrode arrays which can be used for epicortical or neural cuff applications. Secondly, we optimize electrode material coatings in order to achieve the best accuracy in EIT reconstructions.

MAIN RESULTS

The testing of nanostructured electrode interface materials consisting of platinum, iridium oxide, and PEDOT:pTS in saline tank experiments demonstrated that the PEDOT:pTS coating used in this study leads to more accurate reconstruction dimensions along with reduced phase separation between recording channels. The PEDOT:pTS electrodes were then used in vivo to successfully image and localize the evoked activity of the recurrent laryngeal fascicle from within the cervical vagus nerve.

SIGNIFICANCE

These results alongside the simple fabrication method presented here position EIT as an effective method to image neural activity.

Surgical Model for Analysis of Signal Transfer Through Biologic and Synthetic Materials

BACKGROUND

High-fidelity volitional control of bioengineered prosthetic limbs with multiple degrees of freedom requires the implantation of multiple recording interfaces to detect independent control signals. However, interface utilization is complicated by interfering electrophysiological signals originating from surrounding muscles and nerves, leading to equivocal signal detection. We developed and validated a surgical model to characterize signal propagation through various biomaterials to identify insulating substrates for use in implantable interfaces. The identification of these insulating materials will facilitate the acquisition of noncontaminated prosthetic control signals, thus improving manipulation of advanced prosthetic limbs.

METHODS

Using a rat hindlimb model, 4 groups (n = 8/group) were tested. A medial gastrocnemius muscle flap was elevated, leaving the neurovascular pedicle intact. The flap was rotated into a chamber and secured to a silicone base. A stainless steel electrode was affixed to the surface of a muscle and encircled by 1-layer small intestinal submucosa (SIS), 4-layer SIS, silicone elastomer, or nothing (uninsulated). A superimposing electrode was attached, and an external silicone layer was wrapped around the construct and sutured in place. Electromyographic studies were then performed.

RESULTS

This model was found to correspond with expected signal isolation characteristics of the nonconductive silicone group, electrically inert single and multilayer SIS group, and the uninsulated group. Signal isolation of compound muscle action potential amplitude at stimulation threshold was significantly greater using silicone (51.4%) compared with the 1-layer SIS (-6.8%), 4-layer SIS (-3.3% ), or uninsulated groups (1.2%) (P = <0.001). Isolation of the maximum compound muscle action potential peak-to-peak amplitude was also greater with silicone (56.7%) versus the 1-layer SIS (1.5%), 4-layer SIS (1.1%), or uninsulated groups (-0.7%) (P = <0.001).

CONCLUSIONS

This study demonstrates and validates a novel surgical model to characterize in vivo signal propagation and subsequently identify insulating materials for use in implantable interface systems currently in development. Improved signal isolation through the utilization of these materials stands to greatly improve control fidelity of neuroprosthetic limbs.

Investigation of potential artefactual changes in measurements of impedance changes during evoked activity: implications to electrical impedance tomography of brain function

Electrical impedance tomography (EIT) could provide images of fast neural activity in the adult human brain with a resolution of 1 ms and 1 mm by imaging impedance changes which occur as ion channels open during neuronal depolarization. The largest changes occur at dc and decrease rapidly over 100 Hz. Evoked potentials occur in this bandwidth and may cause artefactual apparent impedance changes if altered by the impedance measuring current. These were characterized during the compound action potential in the walking leg nerves of Cancer pagurus, placed on Ag/AgCl hook electrodes, to identify how to avoid artefactual changes during brain EIT. Artefact-free impedance changes (δZ) decreased with frequency from −0.045 ± 0.01% at 225 Hz to −0.02 ± 0.01% at 1025 Hz (mean ± 1 SD, n = 24 in 12 nerves) which matched changes predicted by a finite element model. Artefactual δZ reached c.300% and 50% of the genuine membrane impedance change at 225 Hz and 600 Hz respectively but decreased with frequency of the applied current and was negligible above 1 kHz. The proportional amplitude (δZ (%)) of the artefact did not vary significantly with the amplitude of injected current of 5–20 µA pp. but decreased significantly from −0.09 ± 0.024 to −0.03 ± 0.023% with phase of 0 to 45°. For fast neural EIT of evoked activity in the brain, artefacts may arise with applied current of >10 µA. Independence of δZ with respect to phase but not the amplitude of applied current controls for them; they can be minimized by randomizing the phase of the applied measuring current and excluded by recording at >1 kHz.

Why are sensory axons more vulnerable for ischemia than motor axons?

OBJECTIVE

In common peripheral neuropathies, sensory symptoms usually prevail over motor symptoms. This predominance of sensory symptoms may result from higher sensitivity of sensory axons to ischemia.

METHODS

We measured median nerve compound sensory action potentials (CSAPs), compound muscle action potentials (CMAPs), and excitability indices in five healthy subjects during forearm ischemia lasting up to disappearance of both CSAPs and CMAPs.

RESULTS

ISCHEMIA INDUCED: (1) earlier disappearance of CSAPs than CMAPs (mean ± standard deviation 30±5 vs. 46±6 minutes), (2) initial changes compatible with axonal depolarization on excitability testing (decrease in threshold, increase in strength duration time constant (SDTC) and refractory period, and decrease in absolute superexcitability) which were all more prominent in sensory than in motor axons, and (3) a subsequent decrease of SDTC reflecting a decrease in persistent Na(+) conductance during continuing depolarisation.

INTERPRETATION

Our study shows that peripheral sensory axons are more vulnerable for ischemia than motor axons, with faster inexcitability during ischemia. Excitability studies during ischemia showed that this was associated with faster depolarization and faster persistent Na(+) channel inactivation in sensory than in motor axons. These findings might be attributed to differences in ion channel composition between sensory and motor axons and may contribute to the predominance of sensory over motor symptoms in common peripheral neuropathies.

Conduction in ulnar nerve bundles that innervate the proximal and distal muscles: a clinical trial

Background

This study aims to investigate and compare the conduction parameters of nerve bundles in the ulnar nerve that innervates the forearm muscles and hand muscles; routine electromyography study merely evaluates the nerve segment of distal (hand) muscles.

Methods

An electrophysiological evaluation, consisting of velocities, amplitudes, and durations of ulnar nerve bundles to 2 forearm muscles and the hypothenar muscles was performed on the same humeral segment.

Results

The velocities and durations of the compound muscle action potential (CMAP) of the ulnar nerve bundle to the proximal muscles were greater than to distal muscles, but the amplitudes were smaller.

Conclusions

Bundles in the ulnar nerve of proximal muscles have larger neuronal bodies and thicker nerve fibers than those in the same nerve in distal muscles, and their conduction velocities are higher. The CMAPs of proximal muscles also have smaller amplitudes and greater durations. These findings can be attributed to the desynchronization that is caused by a wider range of distribution in nerve fiber diameters.

Conduction parameters of nerve fibers with different diameters in the same peripheral nerve can be estimated.

Effect of superficial peroneal nerve fascial penetration site on nerve conduction studies

Using nerve conduction studies (NCS) and ultrasonography, we sought to determine the stimulation site that corresponds to the site at which the superficial peroneal nerve (SPN) penetrates the fascia and yields the most accurate NCS results. NCS parameters of the SPN sensory nerve action potential (SNAP) were recorded from 37 legs at 14, 12, 9, 7, and 5 cm (S1, S2, S3, S4, and S5, respectively) proximal to the recording electrode, and analyzed by one-way analysis of variance. SPN penetration sites were visualized by ultrasonography. The mean SNAP amplitude/area (17.2 +/- 6.7 microV/9.6 +/- 4.6 microV/ms) at S3-S5 differed significantly from that at S1-S2 (11.6 +/- 4.7 microV/9.2 +/- 4.4 microV/ms) (F = 10.2, P < 0.001; F = 5.09, P = 0.0007). Ultrasonography showed that the SPN became subcutaneous between S3 and S4. Clinical application of these results should help in obtaining more accurate data during electrodiagnostic studies of conditions that involve the SPN.

Correlations of nerve conduction measures in axonal and demyelinating polyneuropathies

OBJECTIVE

In a considerable proportion of patients with polyneuropathy the electrophysiological distinction between primarily demyelinating or axonal pathology is not straightforward. This study aimed at determining whether the relation between the sensory nerve action potential (SNAP)/compound muscle action potential (CMAP) amplitude and conduction velocity (CV) or distal motor latency (DML) in demyelinating versus axonal polyneuropathy could be helpful in distinguishing these two pathophysiologies.

METHODS

The relation between amplitude reduction and conduction slowing was performed using regression analysis in nerve conduction studies from 53 axonal polyneuropathies and 45 demyelinating polyneuropathies. Sensory nerve conduction studies were performed using the near-nerve needle technique. Finally, needle EMG findings in 31 muscles in axonal and in 22 muscles in demyelinating polyneuropathies were compared.

RESULTS

A linear correlation between action potential amplitude and CV was seen in the majority of nerves in both axonal and demyelinating polyneuropathies. Further, an inverse linear correlation between CMAP amplitude and DML was found in most of the nerves in axonal polyneuropathies. The incidence and degree of abnormality, including decrease in action potential amplitude, was more pronounced in demyelinating than in axonal polyneuropathies, while there was no difference in EMG findings.

CONCLUSIONS

Amplitude reduction and conduction slowing were correlated in axonal as well as demyelinating polyneuropathies, and a significant reduction in SNAP and CMAP amplitudes was found in demyelinating as well as axonal polyneuropathies. The correlation in axonal polyneuropathies can be attributed to a concomitant or selective loss of large, fast conducting fibers, whereas the correlation in demyelinating polyneuropathies may be explained by temporal dispersion or secondary axonal degeneration.

SIGNIFICANCE

At present, relation between amplitude reduction and conduction slowing does not seem to be useful in revealing the primary pathophysiology of a polyneuropathy. Decrease in CV, increase in DML, increase in F-wave latency, conduction block and temporal dispersion should mainly be considered. Decrease in amplitude must be interpreted with caution.

Sensitivity of conventional motor nerve conduction examination in detecting patchy demyelination: A simulated model

OBJECTIVE

To evaluate, in 5 simulated motor nerves with patchy demyelination: (1) the sensitivity of the conventional motor conduction examination; (2) the conduction velocity of single axons (SA-CV).

METHODS

Four damaged segments were simulated in each nerve. Myelin impairment was generated by varying two parameters: (1) percent reduction in conduction velocity, i.e. degree of damage (DEGREE); (2) percentage of affected axons, i.e. extent of damage (EXTENT). Myelin impairment was simulated in axons with different diameters. We evaluated: (1) conduction velocity; (2) temporal dispersion of the negative phase of compound motor action potential (CMAP); (3) amplitude decay of CMAP; (4) SA-CV of 20 randomly-chosen axons.

RESULTS

When the damage involved both large and small axons, the conduction velocity was pathological only when severe myelin damage involved a large number of axons. Temporal dispersion and amplitude decay were more sensitive than conduction velocity in detecting the damage. In damage involving only large axons or only small axons, all parameters remained in the normal range. SA-CV evaluation was much more sensitive than the conventional studies, regardless of the diameter of the damaged axons.

CONCLUSIONS

Conventional studies are not sensitive in detecting minimal myelin damage. Decomposing the CMAPs and randomly studying 20 SA-CVs would increase the sensitivity of damage detection.

SIGNIFICANCE

These results contribute to a better understanding of the relationship between axonal properties and neurophysiological findings in motor nerve demyelination.

A new method for the estimation of motor nerve conduction block

OBJECTIVE

A method for conduction block (CB) estimation, based on compound muscle action potentials (CMAP) elicited by stimulation at sites proximal and distal to the region in which a block is suspected, which is less sensitive to temporal dispersion than methods based on area and amplitude estimation, routinely used in clinical practice.

METHODS

The method is based on deconvolution of CMAPs. It provides the delay distribution that convolved with a kernel (estimated by an optimisation method) gives a reconstruction of the CMAPs. The integral of the delay distribution was used to estimate CB. The method was tested on phenomenological signals (sum of delayed and amplitude scaled versions of the same signal), structure based simulated signals (from a plane layer generation model of surface EMG), and experimental signals (eight healthy subjects; CMAPs recorded over abductor digiti minimi; different temporal dispersions obtained comparing above-elbow stimulation of ulnar nerve with below-elbow stimulation or with wrist stimulation; conduction distances about 10 and 35 cm, respectively).

RESULTS

Deconvolution method gives more precise estimates of the simulated CB with respect to area and amplitude methods (phenomenological signals: bias in CB estimation in the worst case about 10% for deconvolution, 30% for area, 60% for amplitude). Experimental data: by increasing temporal dispersion, in the average CB estimation increases 4% for area and 10% for amplitude, no increase for deconvolution.

CONCLUSIONS

The new method for CB estimation is less sensitive to temporal dispersion than area and amplitude methods.

SIGNIFICANCE

The proposed method provides a precise CB estimation. Being stable to temporal dispersion, it allows to diagnose CB with a lower confidence threshold than in the case of area and amplitude.

Sensitivities of Sensory Nerve Conduction Study Parameters in Carpal Tunnel Syndrome

It is generally accepted that median sensory nerve conduction studies are more sensitive than motor nerve conduction studies in the electrodiagnostic evidence of carpal tunnel syndrome (CTS). This study was conducted to compare the sensitivities of various parameters of sensory nerve conduction studies in the diagnosis of CTS. This prospective study included 88 consecutive patients (151 hands) with CTS and 106 control subjects. CTS was diagnosed clinically by two neurologists. Median sensory nerve responses with wrist stimulation were determined. The onset and peak latencies, peak-to-peak amplitudes, negative peak duration, and area were measured. The differences between the peak and onset latencies were also calculated as a measure of waveform temporal dispersion. Among each measured parameter, values between the 2.5th and the 97.5th percentile range of the control subjects served as the normal limits. Among the 151 hands with suspected CTS, five (3.3%) had normal electrodiagnostic studies and 146 (96.7%) had at least one abnormal electrodiagnostic study. Among the 146 hands with an abnormality, 138 had abnormal onset latency, 143 had abnormal peak latency, and 88 had abnormal difference between peak and onset latency. In addition, 87 had abnormal amplitude, 70 had abnormal duration, and 59 had abnormal area. The sensitivity was 91.4% for onset latency, 94.7% for peak latency, 58.3% for difference between peak and onset latency, 57.6% for amplitude, 46.4% for duration, and 39.1% for area. Our study shows that in patients with CTS, the most sensitive sensory nerve conduction parameter is the peak latency. Studying various additional sensory nerve conduction parameters did not significantly increase the diagnostic yield.

Amplitude, area and duration of the compound muscle action potential change in different ways over the length of the ulnar nerve

OBJECTIVE

To study physiological changes of the compound muscle action potential (CMAP) obtained from stimulation at different sites over the full length of a motor nerve and to study possible effects of anthropometrical factors.

METHODS

Multicentre study of ulnar motor nerve conduction in five segments to Erb's point performed bilaterally on 100 healthy subjects aged 17-83 years.

RESULTS

CMAP amplitude decreased linearly with conduction distance (0.31%/cm) from wrist to Erb's point. CMAP area decreased with the square of conduction distance. Decrease in area was smaller than decrease in amplitude especially distally. CMAP duration increased linearly (0.17%/cm). Amplitude decay correlated with age, height and BMI and dispersion correlated with age and height. There were no correlations between area decay and anthropometrical factors. There was no significant inter-examiner variation.

CONCLUSIONS

Area decay may be preferred to amplitude decay in the evaluation of conduction block over short segments due to smaller physiological changes and independence of anthropometrical factors. The absence of inter-examiner variation indicates that the results are robust and may be used by other laboratories.

SIGNIFICANCE

This study provides knowledge of physiological changes of CMAP parameters that may be of importance in the evaluation of nerve pathology, in particular conduction block.

Interexaminer Repeatability of Antidromic Ulnar Sensory Nerve Conduction Velocity Measurements

OBJECTIVE

To determine the interexaminer repeatability of the ulnar antidromic sensory nerve conduction velocity (NCV).

DESIGN

Test-retest design. Based on a randomization list of various combinations and sequences from 2 of a total of 3 examiners, the measurement was repeated within half an hour by a second examiner blinded to the results of the first examiner.

SETTING

Outpatient department.

PARTICIPANTS

Twenty-four consecutive healthy subjects (mean age, 38 y).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

The intraclass correlation coefficient (ICC) and the coefficient of repeatability (CR) were determined for the below elbow to wrist (BE-to-W), above elbow to below elbow (AE-to-BE), and axilla to above elbow (AX-to-AE) segments.

RESULTS

The ICC was .42 for the BE-to-W, .15 for the AE-to-BE, and -.05 for the AX-to-AE segment. The CR was 12.2m/s for the BE-to-W, 16.2m/s for the AE-to-BE, and 21.4m/s for the AX-to-AE segment.

CONCLUSIONS

During the assessment of the antidromic sensory NCV of the ulnar nerve, a moderate amount of interexaminer variability must be taken into account for the BE-to-W segment. More proximally, an extremely large amount of interexaminer variability must be taken into account. This calls into question the usefulness of the antidromic ulnar sensory NCV for the AX-to-AE and AE-to-BE segments.

Compound sensory action potential in normal and pathological human nerves

The compound sensory nerve action potential (SNAP) is the result of phase summation and cancellation of single fiber potentials (SFAPs) with amplitudes that depend on fiber diameter, and the amplitude and shape of the SNAP is determined by the distribution of fiber diameters. Conduction velocities at different conduction distances are determined by summation of SFAPs of varying fiber diameters, and differ in this respect, also, from the compound muscle action potential (CMAP) for which conduction velocities are determined by the very fastest fibers in the nerve. The effect and extent of temporal dispersion over increasing conduction distance is greater for the SNAP than CMAP, and demonstration of conduction block is therefore difficult. In addition, the effect of temporal dispersion on amplitude and shape is strongly dependent on the number of conducting fibers and their distribution, and, with fiber loss or increased conduction velocity variability changes of the SNAP may be smaller than expected from normal nerve. The biophysical characteristics of sensory and motor fibers differ, and this may to some extent determine divergent pathophysiological changes in sensory and motor fibers in different polyneuropathies. In this review, different factors that characterize sensory fibers and set the SNAP apart from the CMAP are discussed to emphasize the supplementary and complementary information that can be obtained from sensory conduction studies. Sensory conduction studies require particular effort and attention to theory and practical detail that may be time consuming.

Motor unit number estimation in the evaluation of focal conduction block

Temporal dispersion and phase cancellation limit the utility of amplitude reduction in compound muscle action potential (CMAP) as a measure of focal conduction block but may not affect motor unit number estimation (MUNE). Hence, MUNE offers the potential of a specific measure of conduction block. We investigated the role of MUNE in 11 patients with ulnar neuropathy and conduction block at the elbow and also in 8 normal subjects. MUNE failed to detect motor unit dropout in the patient group because reduced values for surface-recorded motor unit potentials (SMUPs) were obtained at proximal locations, suggesting that focal compression selectively damages larger motor axons, an hypothesis that has support from animal studies. We conclude that, because MUNE is affected by the physiological characteristics of functional axons surviving the underlying pathological process, the utility of MUNE is limited to diseases in which the expected pathology affects motor axons uniformly.

Two approaches to measure amplitude changes of the sensory nerve action potential over a length of nerve

Changes in amplitude of sensory nerve action potentials (SNAPs) over a length of nerve have not been used for diagnostic purposes. We measured such changes for median and ulnar nerves in 25 healthy subjects in two ways. The multiple-stimulation method involved three stimulation sites (palm, wrist, elbow) and one recording site on a finger. The multiple-recording method involved stimulation at the wrist and simultaneous recording from two adjacent sites on a finger. Percentile changes in amplitude could be measured in all subjects for the median nerve, but not for the ulnar nerve due to submaximal stimulation in the palm. For the median nerve, the multiple-recording method resulted in a mean amplitude change of 79% +/- 13%, whereas comparison of wrist to palm stimulation in the multiple-stimulus method resulted in a change of 73% +/- 10%. These values are high enough to encourage study into practical applications of such methods in detecting peripheral nerve disease.

Electrophysiologic measures of diabetic neuropathy: mechanism and meaning

Whole nerve electrophysiologic procedures afford a battery of measures that can provide a noninvasive and objective index of the onset and progression of diabetic polyneuropathy (DPN). Advances in physiologic procedures, digital hardware, and mathematical models have allowed assessment of activity in slower conducting fibers, as well as measures that reflect changes in refractory periods and threshold excitability. These expanded options can augment standard measures of maximal conduction velocity and compound amplitude and greatly enhance the sensitivity of whole nerve measure to both structural (e.g. demyelination) and "nonstructural" (e.g. redistribution of ion channels) deficits associated with DPN. The mechanisms underlying the physiologic events in DPN are multifactorial and their sequence in complex, with different mechanisms contributing to change at overlapping, but distinct points in the progression. Factors influencing early change in velocity may differ from those contributing to chronic deficits and these mechanisms may also differ in their response to various putative therapies. This review attempts to summarize the pattern of whole nerve electrophysiologic change associated with DPN, outlines the strengths and limitations of the various measures that are feasible, and discusses the specific impact of know pathophysiologic mechanisms on these end points.

Role of primary afferents in spinal cord stimulation-induced vasodilation: characterization of fiber types

Selected patients with peripheral vascular disease can be treated with spinal cord stimulation (SCS) to improve blood flow in the limbs. However, the mechanisms producing these effects remain unclear. The present study was designed to investigate if SCS produces cutaneous vasodilation via antidromic activation of the unmyelinated C-fibers and/or the small myelinated fibers. SCS was applied to anesthetized rats with a ball electrode at the L2-L3 spinal level. In Protocol 1, effects of capsaicin were examined. Blood flow changes in the hindpaw induced by SCS were measured in the footpad with laser Doppler flowmeters. Topical application of capsaicin (1%) on the tibial nerve did not affect SCS-induced vasodilation at 30 and 60% of motor threshold (MT). However, the duration of vasodilation induced by SCS at 90% MT and at 10 times MT was significantly reduced after capsaicin application on the tibial nerve. In Protocol 2, antidromic compound action potentials (CAPs) of the tibial nerve were recorded in response to SCS. CAPs of the large and the small myelinated afferent fibers were observed in response to SCS at all intensities. However, even with SCS at ten times MT, CAPs of C-fibers could not be detected in the tibial nerve. In Protocol 3, antidromic CAPs of the dorsal root were measured in response to SCS. Antidromic CAPs of C-fibers in dorsal roots were evoked by SCS at >or=90% of MT. It is concluded that SCS-induced vasodilation at <or=60% of MT may be mediated via only the myelinated fibers, whereas vasodilation at >or=90% of MT may also involve antidromic activation of some unmyelinated C-fibers.

Estimation of the conduction velocity distribution of human sensory nerve fibers

A new method for estimating the distribution of conduction velocities (DCV) of peripheral nerve fibers has been developed. It also enables estimation of single nerve fiber action potential (SFAP), which agrees with the physiological knowledge. Two compound nerve action potentials (CAPs) elicited by electrical stimulation of a nerve bundle were recorded at different conduction distances. The distances between the stimulation and recording electrodes were measured on the skin surface along the nerve bundle. Starting with an arbitrary SFAP, the first estimated DCV was calculated from a CAP by the regularized non-negative least squares method. The next SFAP was then calculated by deconvolution of the other CAP and the estimated DCV. A lowpass filter with an appropriate cutoff frequency was used to obtain better conversion. The process was iterated until the CAP error defined as /CAP(calculated)-CAP/(2) was small enough. The conduction distances contained errors in measurement, especially in the distal segment, that distorted the estimated results. The Fibonacci search, therefore, was adopted to optimize the distance according to the CAP error. The accuracy of this method was demonstrated by a simulation study performed with two CAPs calculated from an arbitrary bimodal DCV and a biphasic SFAP to which a Gaussian white noise was added. The reliability of this method was checked in normal subjects by recording a pair of CAPs elicited by stimulation of the median nerve at the wrist and the elbow.

The motor nerve simulator

OBJECTIVE

The aim of the study was to develop a mathematical model of the motor nerve and the action potentials generated from its axons, in order to simulate conditions seen in neurography. The model should be used for the detailed study of the relationship between various nerve characteristics and the electrophysiological recordings obtained.

METHODS

The model was developed as a software tool. The signals from individual motor units were real recordings using conventional surface electrodes. There was good agreement between the constructed compound muscle action potential and the one recorded live for the subject from whom the individual signals were obtained.

RESULT

A number of physiological characteristics can be changed, including the number of axons, their conduction properties, excitability properties, degree of proximo-distal velocity slowing. F-waves and A-waves can be generated.

CONCLUSION

The model gives a good similarity to findings obtained in live recordings. A number of physiological characteristics can be studied individually, something that cannot be done in live recordings. The model can be used in teaching and in research studies of the relationship between nerve properties and neurography parameters.

Reduced Ia-afferent-mediated Hoffman reflex in streptozotocin-induced diabetic rats

In addition to reduced nerve conduction velocity, diabetic neuropathic patients often exhibit a reduction in the amplitude of the compound muscle action potential elicited by stimulation of the Ia-afferent-mediated reflex pathway (Hoffman or H wave) that can contribute to diminished or absent tendon reflexes. In contrast to nerve conduction velocity deficits, changes in H-wave amplitudes have not been reproduced in diabetic animal models. Using electrophysiological techniques developed for repeated recordings in individual animals, we report H-wave deficits in streptozotocin (STZ)-treated insulin-dependent diabetic rats. After 4 weeks of diabetes induced by STZ treatment, a 47% reduction in the H-wave amplitude was demonstrated by recording compound muscle action potentials in foot muscles after stimulation of Ia afferents. Interestingly, we also demonstrate that the H-wave amplitude gradually recovers to a 26% deficit after 12 weeks of experimental diabetes. The recovery of the H wave in STZ-treated rats distinguishes this deficit mechanistically from other STZ-induced electrophysiological changes and may model a similar recovery of the H wave reported in diabetic patients.

Length dependence of variables associated with temporal dispersion in human motor nerves

Temporal dispersion in motor nerves is associated with changes of amplitude, area, duration, and Fourier spectra of compound muscle action potentials (CMAPs) when comparing responses to proximal and distal stimulation. These changes depend on the length of the nerve segment. To quantitatively assess this dependence, motor conduction studies of nerve segments of various lengths were performed in the median, ulnar, and tibial nerves of 86 test subjects, aged 4 to 73 years. Amplitude, area, duration, and spectral energy above 49 Hz of CMAPs were measured. Values after distal and proximal stimulation of each nerve segment were compared to determine amplitude decay, area decay, protraction, and high-frequency attenuation. A significant length dependence of amplitude decay was found in the tibial and ulnar nerves, of area decay in the median and ulnar nerves, and of CMAP duration in the ulnar and tibial nerves. The length dependence of the high-frequency attenuation was significant in all nerves studied. This report provides normative data for variables associated with temporal dispersion.

Ulnar 14-cm and 7-cm antidromic sensory studies to the fifth digit: reference values derived from a large population of normal subjects

The ulnar antidromic sensory conduction study to the fifth digit is commonly performed in clinical electrodiagnosis. Several authors have published normal ranges for this study, but these published reports have been limited by generally small sample sizes. The purpose of this article is to present a large database of normal ranges for this nerve study. After obtaining Institutional Review Board approval, 258 asymptomatic subjects were tested bilaterally with an ulnar antidromic sensory technique recording from the fifth digit. Stimulation was performed 7 cm and 14 cm proximal to the recording electrode. Onset latency, peak latency, onset-to-peak amplitude, peak-to-peak amplitude, area, duration, and rise time were recorded. Side-to-side and 14- to 7-cm comparisons were made. A repeated measures analysis of variance was performed to determine whether any of the subjects' demographic characteristics of age, race, gender, height, or body mass index (kg/m2) were associated with different results for the nerve conduction measures. Increasing age and body mass index were found to correlate with decreasing amplitude and area. No other correlations were noted between the results and the physical characteristics. Mean onset latency was 1.4 +/- 0.2 ms at 7-cm and 2.6 +/- 0.2 ms at 14-cm stimulation. Mean peak latency was 2.0 +/- 0.2 ms at 7-cm and 3.4 +/- 0.3 ms at 14-cm stimulation. Mean onset-to-peak amplitude was 32 +/- 20 microV at 7-cm and 33 +/- 17 microV at 14-cm stimulation. Mean negative-to-positive-peak amplitude was 55 +/- 36 microV at 7-cm and 50 +/- 32 microV at 14-cm stimulation. Mean side-to-side difference for onset latency was 0 +/- 0.2 ms. Additional data is presented in the study.

Median 14-cm and 7-cm antidromic sensory studies to digits two and three

The purpose of this study was to derive a normative database for the median digital sensory conduction study using a large and varied subject population. Two hundred fifty-eight asymptomatic volunteers were tested with antidromic sensory technique at 14- and 7-cm distances to digits 2 and 3. Onset latency, peak latency, onset-to-peak amplitude, peak-to-peak amplitude, area, rise time, and duration were recorded. A repeated analysis of variance was performed, with the nerve conduction study measures as the dependent variables and age, race, gender, body mass index, and height as the independent variables. Factors that were significant at the P < or = 0.01 level were used to create separate normal ranges. Both increasing age and increasing body mass index correlated with decreasing amplitudes and area. No other correlations were noted between the results and the physical characteristics. For digit 3, the mean 14-cm onset latency was 2.7 +/- 0.3 ms and mean peak latency was 3.4 +/- 0.3 ms. The mean onset-to-peak amplitude was 41 +/- 20 microV for all subjects taken together. Mean rise time was 0.7 +/- 0.1 ms and mean duration was 2.1 +/- 0.4 ms. Mean side-to-side difference in onset and peak latencies was 0.0 +/- 0.2 ms. The upper limits of normal side-to-side differences in amplitudes and area were approximately 50%-60%.

Ulnar nerve motor conduction to the abductor digiti minimi

Ulnar motor study to the abductor digiti minimi is commonly performed, but a more extensive database of normative values using modern electrodiagnostic and statistical techniques and temperature control is needed for this test. Demographic subgroups of age, gender, and height should be evaluated using a large subject population to determine whether separate normal ranges should be created for subsets of the general population. In this study, 248 volunteers were tested to measure ulnar motor latency, amplitude, area, duration, and nerve conduction velocity. Side-to-side and distal-to-proximal variability was analyzed. A repeated measures analysis of variance was performed with the waveform measures as the dependent variables and age, gender, and height as independent variables. None of the results were found to vary significantly (at the P < or = 0.01 level) with the subjects' physical characteristics, and thus, the data for all subjects were pooled to create a normative database. The normal range was derived as mean +/- 2 standard deviations and at the 97th (third) percentile of observed values. Mean latency was 3.0 +/- 0.3 ms, and amplitude was 11.6 +/- 2.1 mV. Mean nerve conduction velocity was 61 m/s across all segments tested. The upper limit of normal side-to-side variability (mean + 2 standard deviations) for latency was 0.6 ms; for amplitude, it was 3.6 mV. The upper limit of normal drop in conduction velocity across the elbow was 15 m/s (at the 97th percentile). Additional data are presented for all variables measured, as well as for side-to-side variability and distal-to-proximal change.

"Pseudo-conduction block" in vasculitic neuropathy

The predominant electrophysiologic feature of vasculitic mononeuropathy multiplex is axonal loss. Electrophysiologic findings interpreted as conduction block have, however, also been reported to occur in neuropathy secondary to necrotizing vasculitis. We report 3 patients with mononeuropathy multiplex and biopsy proven vasculitis in whom eight nerves met criteria for conduction block. In each circumstance, serial study demonstrated conversion of the electrophysiologic findings to those most consistent with severe axonal loss. "Conduction block" in vasculitic mononeuropathy multiplex is secondary to focal axonal conduction failure presumably related to infarctive axonal injury. The term conduction block should be used with caution in this disorder and only if serial studies demonstrate findings consistent with this electrophysiologic diagnosis.

Assessment of temporal dispersion in motor nerves with normal conduction velocity

Demyelinated nerves attenuate high-frequency components of propagating action potentials. In order to study if there is diagnostic use of this in motor nerves, the spectral energy above 49 Hz, amplitude, area, and duration of the compound muscle action potentials were measured; values after distal and proximal stimulation of posterior tibial nerves were compared. Normative data were collected in 48 control subjects. The same measurements were made in 20 patients with polyneuropathy and reduced motor nerve conduction velocity, in 21 patients with mild polyneuropathy but normal motor nerve conduction velocity, and in 8 patients with myasthenia gravis. Overall, high-frequency attenuation was closely correlated with amplitude decay (r = 0.63, P<10(-19)) and with increase of action potential duration (r = 0.34, P = 10(-5)). In the group of patients with normal NCV, high-frequency attenuation was abnormal in 9 (43%), amplitude decay was abnormal in two (10%), and area decay was abnormal in one (5%) patient. The action potential duration was normal in all of these patients. High-frequency attenuation was not influenced by stimulus intensity, thus it is not changed by conduction block, and it was not influenced by impaired neuromuscular transmission. Hence, high-frequency attenuation, both sensitively and specifically does indicate abnormal temporal dispersion. In conclusion, the simple measurement of high-frequency attenuation markedly improves detection and characterization of demyelination of human motor fibers.

The diagnostic sensitivity of different F wave parameters

OBJECTIVE

To examine the relative diagnostic sensitivity of various F wave parameters.

METHODS

Normal values for minimum, mean, and maximum F wave latency, chronodispersion, and persistence in the four major motor nerves were established and systematically applied to at least four separate categories of patients (radiculopathies, polyneuropathies, mononeuropathies, and others). F Waves were studied both isolated and in comparison with other motor nerve conduction parameters.

RESULTS

F Chronodispersion was the most often abnormal parameter, particularly in lumbosacral radiculopathies. Minimum F wave latency was more useful in polyneuropathies. Compared with minimum F wave latency, F chronodispersion was able to identify most additional cases.

CONCLUSIONS

F Wave studies should include minimum F wave latency and chronodispersion.

Anatomical and electrophysiological determinants of the human thenar compound muscle action potential

Clinical interpretation of the compound muscle action potential (CMAP) requires a precise understanding of its underlying mechanisms. We recorded normal thenar CMAP5 and motor unit action potentials using different electrode configurations and different thumb positions. Computer simulations show that the CMAP has four parts: rising edge, negative phase, positive phase, and tail which correspond to four distinct stages of electrical activity in the muscle: initiation at the end-plate, propagation, termination at the muscle/tendon junctions, and slow repolarization. The shapes of volume-conducted signals recorded beyond the muscle are also explained by these four stages. Changes in CMAP shape associated with thumb abduction are due to changes in termination times resulting from changes in muscle-fiber lengths. These findings demonstrate that the negative and positive phases of the CMAP are due to different mechanisms, and that anatomical factors, particularly muscle-fiber lengths, play an important role in determining CMAP shape.