Influence of recording site on CMAP amplitude on its variation over a length of 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.

Neural Adjustments during Repeated Braking and Throttle Actions on a Motorcycle Setup

The aim of the study was to assess neuromuscular changes during an intermittent fatiguing task designed to replicate fundamental actions and ergonomics of road race motorcycling. Twenty-eight participants repeated a sequence of submaximal brake-pulling and gas throttle actions, interspaced by one maximal brake-pulling, until failure. During the submaximal brake-pulling actions performed at 30% MVC, force fluctuations, surface EMG, maximal M-wave (Mmax) and H-reflex were measured in the flexor digitorum superficialis. At the end of the task, the MVC force and associated EMG activity decreased (P<0.001) by 46% and 26%, respectively. During the task, force fluctuation and EMG activity increased gradually (106% and 61%, respectively) with respect to the pre-fatigue state (P≤0.029). The Mmax first phase did not change (P≥0.524), whereas the H-reflex amplitude, normalized to Mmax, increased (149%; P≤0.039). Noteworthy, the relative increase in H-reflex amplitude was correlated with the increase in EMG activity during the task (r=0.63; P<0.001). During the 10-min recovery, MVC force and EMG activity remained depressed (P≤0.05) whereas H-reflex amplitude and force fluctuation returned to pre-fatigue values. In conclusion, contrarily to other studies, our results bring forward that when mimicking motorcycling brake-pulling and gas throttle actions, supraspinal neural mechanisms primarily limit the duration of the performance.

Far-field potential of the compound muscle action potential as a reliable marker in amyotrophic lateral sclerosis

INTRODUCTION/AIMS

Reliable neurophysiological markers in amyotrophic lateral sclerosis (ALS) are of great interest. The compound muscle action potential (CMAP) amplitude has been a conventional marker, although it is greatly influenced by the electrode position. We propose the far-field potential of the CMAP (FFP-CMAP) as a new neurophysiological marker in ALS.

METHODS

Patients with ALS and age-matched healthy controls were enrolled. We used a proximal reference (pref) in addition to the conventional distal reference (dref). Routine CMAP was recorded from the belly-dref lead and FFP-CMAP from the dref-pref lead for the ulnar and tibial nerves. Multiple point stimulation motor unit number estimation (MUNE) was also examined in the ulnar nerve. Inter-rater reproducibility was evaluated by two examiners, and some patients were followed up every 3 mo for 1 y.

RESULTS

We tested 17 patients with ALS and 10 controls. The amplitudes of routine CMAP and FFP-CMAP in the ulnar and tibial nerves, and hypothenar MUNE value in the ulnar nerve were significantly decreased in ALS compared to controls. Ulnar FFP-CMAP achieved the highest inter-rater intraclass correlation coefficient (ICC) value (0.942) when compared with routine CMAP (0.880) and MUNE (0.839). The tibial FFP-CMAP had a higher ICC value (0.986) than the routine CMAP (0.697). In this way, the FFP-CMAP showed high inter-rater reproducibility because its shape was not much influenced by the electrode position. During 1-y follow-up, decline of CMAP, FFP, and MUNE showed significant correlations with the Amyotrophic Lateral Sclerosis Functional Rating Scale - Revised (ALSFRS-R).

DISCUSSION

The FFP-CMAP shows promise as a reliable marker for ALS.

Sensitivity distribution simulations of surface electrode configurations for electrical impedance myography

INTRODUCTION

Surface-based electrical impedance myography (EIM) is sensitive to muscle condition in neuromuscular disorders. However, the specific contribution of muscle to the obtained EIM values is unknown.

METHODS

We combined theory and the finite element method to calculate the electrical current distribution in a 3-dimensional model using different electrode array designs and subcutaneous fat thicknesses (SFTs). Through a sensitivity analysis, we decoupled the contribution of muscle from other surrounding tissues in the measured surface impedance values.

RESULTS

The contribution of muscle to surface EIM values varied greatly depending on the electrode array size and the SFT. For example, the contribution of muscle with 6-mm SFT was 8% for a small array compared with 32% for a large array.

CONCLUSIONS

The approach presented can be employed to inform the design of robust EIM electrode configurations that maximize the contribution of muscle across the disease and injury spectrum. Muscle Nerve 56: 887-895, 2017.

Guidelines to electrode positioning for human and animal electrical impedance myography research

The positioning of electrodes in electrical impedance myography (EIM) is critical for accurately assessing disease progression and effectiveness of treatment. In human and animal trials for neuromuscular disorders, inconsistent electrode positioning adds errors to the muscle impedance. Despite its importance, how the reproducibility of resistance and reactance, the two parameters that define EIM, are affected by changes in electrode positioning remains unknown. In this paper, we present a novel approach founded on biophysical principles to study the reproducibility of resistance and reactance to electrode misplacements. The analytical framework presented allows the user to quantify a priori the effect on the muscle resistance and reactance using only one parameter: the uncertainty placing the electrodes. We also provide quantitative data on the precision needed to position the electrodes and the minimum muscle length needed to achieve a pre-specified EIM reproducibility. The results reported here are confirmed with finite element model simulations and measurements on five healthy subjects. Ultimately, our data can serve as normative values to enhance the reliability of EIM as a biomarker and facilitate comparability of future human and animal studies.

Measurement of voluntary activation based on transcranial magnetic stimulation over the motor cortex

This article reviews the use of transcranial magnetic stimulation (TMS) over the motor cortex to make estimates of the level of voluntary drive to muscles. The method, described in 2003 (Todd et al. J Physiol 551: 661-671, 2003), uses a TMS pulse to produce descending corticospinal volleys that synaptically activate motoneurons, resulting in a muscle twitch. Linear regression of the superimposed twitch amplitude and voluntary force (or torque) can generate an "estimated" resting twitch for muscles involved in a task. This procedure has most commonly been applied to elbow flexors but also to knee extensors and other muscle groups. Data from 44 papers using the method were tabulated. We identify and discuss five major technical challenges, and the frequency with which they are addressed. The technical challenges include inadvertent activation of the cortical representation of antagonist muscles, the role of antagonist torques at the studied joint, uncertainty about the effectiveness of the TMS pulse in activating the motoneuron pool, the linearity of the voluntary force (or torque) and superimposed twitch relationship, and variability in the TMS-evoked EMG and force/torque responses. The ideal situation in which the descending corticospinal volleys recruit all of the agonist motoneurons and none of the antagonist motoneurons is unlikely to ever occur, and hence results must be carefully examined to assess the authenticity of the voluntary activation estimates in the context of the experimental design. A partial compromise lies in the choice of stimulus intensity. We also identify aspects of the procedure that require further investigation.

Nerve conduction studies: clinical challenges and engineering solutions

Nerve conduction studies (NCSs) have played an important role in the evaluation of neuromuscular disease for the past 50 years. When patients present with complaints of pain, numbness, tingling, or weakness, NCS is often one of the earliest tests obtained by physicians, because it enables the quantitative assessment of peripheral nerve and muscle function and, therefore, aid the physician in identifying the physiological source of the patient's symptoms. NCSs involve the delivery of electric stimuli to peripheral nerves at accessible locations on the human body and the recording of electrophysiological responses. This article reviews how NCS is traditionally performed. This paper also examines technical challenges associated with each step of performing an NCS and describes how engineering solutions could be realized to meet these challenges. The engineering goals were several: improvement in NCS workflow, use of prefabricated electrode arrays to standardize NCS technique and reduce the errors associated with electrode placement, and improvement of the overall accuracy and reliability of NCS.

Oral high dose ascorbic acid treatment for one year in young CMT1A patients: a randomised, double-blind, placebo-controlled phase II trial

BACKGROUND

High dose oral ascorbic acid substantially improved myelination and locomotor function in a Charcot-Marie-Tooth type 1A mouse model. A phase II study was warranted to investigate whether high dose ascorbic acid also has such a substantial effect on myelination in Charcot-Marie-Tooth type 1A patients and whether this treatment is safe.

METHODS

Patients below age 25 years were randomly assigned to receive placebo or ascorbic acid (one gram twice daily) in a double-blind fashion during one year. The primary outcome measure was the change over time in motor nerve conduction velocity of the median nerve. Secondary outcome measures included changes in minimal F response latencies, compound muscle action potential amplitude, muscle strength, sensory function, Charcot-Marie-Tooth neuropathy score, and disability.

RESULTS

There were no significant differences between the six placebo-treated (median age 16 years, range 13 to 24) and the five ascorbic acid-treated (19, 14 to 24) patients in change in motor nerve conduction velocity of the median nerve (mean difference ascorbic acid as opposed to placebo treatment of 1.3 m/s, confidence interval -0.3 to 3.0 m/s, P = 0.11) or in change of any of the secondary outcome measures over time. One patient in the ascorbic acid group developed a skin rash, which led to discontinuation of the study medication.

CONCLUSION

Oral high dose ascorbic acid for one year did not improve myelination of the median nerve in young Charcot-Marie-Tooth type 1A patients. Treatment was relatively safe.

TRIAL REGISTRATION

Current Controlled Trials ISRCTN56968278, ClinicalTrials.gov NCT00271635.

Repeatability of nerve conduction measurements derived entirely by computer methods

BACKGROUND

Nerve conduction studies are an objective, quantitative, and reproducible measure of peripheral nerve function and are widely used in the diagnosis of neuropathies. The purpose of this study is to determine the reliability of nerve conduction parameters derived entirely from computer based data acquisition and waveform cursor assignments and to quantify the relative contributions of test variability sources.

METHODS

Thirty volunteers, some with symptoms suggestive of neuropathies; of these, 29 completed the study. The median, ulnar, deep peroneal, posterior tibial, and sural nerves were evaluated bilaterally at two test sessions 3-7 days apart. Within each session, nerves were tested twice within 10 minutes. The analyzed nerve conduction parameters include motor latencies, motor conduction velocity (CV), compound muscle action potential (CMAP) amplitude, F-wave latencies (minimum, mean and maximum), sensory peak latency (DSL), sensory CV, and sensory nerve action potential (SNAP) amplitude. The primary outcome measure is variance component analysis and the corresponding coefficient of variation (CoV). The between-session-test variance is the sum of within-session variance and between-session variance, quantifying the total variation between test sessions. Additional statistical measures include the intraclass correlation coefficient (ICC) and relative interval variation (RIV).

RESULTS

Motor and sensory latencies, CV and F-wave latency parameters have low between-session-test CoVs, ranging from 4.2% to 9.8%. Amplitude parameters have a higher between-session-test CoVs in the range of 15.6--19.8%. Between-test CoVs are about 30--80% lower than between-session CoVs with the exception of F-wave latency parameters. Between-test ICC values are 0.96 or above for all parameters. Between-session ICC ranges from 0.98 for F-wave latency to 0.77 for sural sensory CV. All latency-related between-session ICCs have a value 0.83 or above. The RIVs are the tightest for F-wave latency parameters and widest for CMAP amplitude parameters. Repeatability in a sub-group of subjects with more severe symptom grades follows the same trend as the overall study population without substantial quantitative differences.

CONCLUSION

The study demonstrates the high repeatability of nerve conduction parameters acquired by modern electrodiagnostic instruments using computer based waveform cursor assignment. The reliability is comparable to benchmark studies in which the nerve conduction measurements were performed manually in controlled multi-center clinical trials. Furthermore, the ranking of reliability, whereby F-wave latencies have the best reproducibility and amplitudes the worst, is also consistent with the benchmark studies.

Interexaminer variance of median nerve compound muscle action potential measurements in hand position with and without fixation in plaster

OBJECTIVES

We examined how hand fixation affects interexaminer variance in measuring compound muscle action potential (CMAP) during nerve conduction studies of the median nerve.

METHODS

Subjects comprised 6 experienced examiners [1 woman and 5 men; mean (SD) age, 38.3 +/- 8.5 years; range, 28-46 years]. The standardized median nerve motor conduction setting was used for electromyography. The mean of 5 serial stimulations was taken as 1 examination for each subject. Distal latency, conduction velocity, and peak-to-peak amplitude of CMAP were measured as parameters. Subjects were examined without hand fixation and with hands fixed in prepared plaster molds.

RESULTS

Fixation yielded significantly decreased coefficient of variance for CMAP amplitude (5.0% +/- 3.4%) compared with measurement without fixation (8.4% +/- 3.7%; P = 0.028). No differences in distal latencies or conduction velocities were evident between fixation and nonfixation.

CONCLUSIONS

This study provides quantitative evidence for empirical observations that hand fixation might decrease CMAP variability in median nerve studies.

Correlation of median forearm conduction velocity with carpal tunnel syndrome severity

OBJECTIVE

Median nerve entrapment neuropathy at the wrist can be accompanied by slowed motor conduction within the forearm. Existing studies conflict regarding a correlation between the severity of the entrapment neuropathy in carpal tunnel syndrome (CTS) and slowing of median motor nerve conduction velocity (MNCV) in the forearm. Here, it was asked if there is a correlation between markers of CTS severity and median forearm MNCV, and if there is an explanation for the preceding conflicting results.

METHODS

Median MNCV in the forearm was correlated with neurophysiologic markers of severity of a median neuropathy at the wrist in 91 hands from 64 patients with clinical and electrodiagnostic evidence of CTS.

RESULTS

Median MNCV within the forearm segment was negatively correlated with the median nerve distal motor latency (r=-0.64, P<0.001, n=91) and positively correlated with the CMAP amplitude of the abductor pollicis brevis muscle (r=0.45, P<0.001, n=91). These correlations only occurred in patients with a prolonged median distal motor latency. Previous investigations that failed to find such correlations used variable or non-standardized methods or analyzed smaller numbers of patients.

CONCLUSIONS

Slowing of median MNCV in the forearm is related to the severity of the entrapment of median motor fibers at the wrist.

SIGNIFICANCE

Slowed forearm median MNCV can be a marker of motor nerve injury at the wrist.

Contribution of reference electrode to the compound muscle action potential

In compound muscle action potential (CMAP) recording, the contribution by the reference electrode is considered to be much smaller than that of the active electrode. We tested this assumption by making quantitative measurements of the signals recorded individually by the active and reference electrodes. In the thenar (median nerve) and extensor digitorum brevis (peroneal nerve) muscles, the reference electrode did contribute less. In the hypothenar muscle (ulnar nerve), however, the signals recorded by active and reference electrodes were of similar amplitude. In tibial nerve conduction studies (NCS), the CMAP from the abductor hallucis (AH) muscle was recorded mainly by the reference electrode; the large-amplitude signal recorded by the reference electrode is attributed to volume-conducted activity from other muscles stimulated during the study. The onset latency of the potential recorded by the active and reference electrodes was similar despite significantly different distances from the stimulating site. Hence, the merits of using anatomic landmarks for defining the distal stimulation site are assessed. When the reference electrode makes a large contribution, the CMAP amplitude may not decrease commensurate with any wasting of the muscle under the active recording electrode, and the need to use another muscle for recording the CMAP for that nerve should be considered.

Criterion validity of the NC-stat automated nerve conduction measurement instrument

The purpose of this study is to assess the criterion validity of peroneal and posterior tibial nerve conduction measurements obtained with the NC-stat system. Sixty patients referred to the Boston VA EMG laboratory were enrolled. Each subject had a full study of the lower extremity performed using traditional EMG equipment prior to obtaining the NC-stat measurements. These included peroneal and posterior tibial distal motor latency (DML), amplitude (AMP) and F-wave latency (FLAT) measurements. Excellent criterion validity was demonstrated for the peroneal and posterior tibial FLATs and the peroneal AMP. Acceptable criterion validity was identified in the peroneal DML and the posterior tibial AMP. The validity of the posterior tibial DML could not be demonstrated. With the exception of the peroneal DML, criterion validity was maintained in a sub-group analysis of the 50% most abnormal parameter values. The comparability of NCS performed with the NC-stat and in traditional settings has been demonstrated for motor studies of the median and ulnar nerves in previous studies. This study shows that the technology used by the NC-stat for studying the peroneal and posterior tibial nerves compares favorably as well with that obtained with traditional EMG equipment used under neurologist supervision.

Repeatability of nerve conduction measurements using automation

Objective

To quantify nerve conduction study (NCS) reproducibility utilizing an automated NCS system (NC-stat^®, NeuroMetrix, Inc.).

Method

Healthy volunteers without neuropathic symptoms participated in the study. Their median, ulnar, peroneal, and tibial nerves were tested twice (7 days apart) by the same technician with an NC-stat^® instrument. Pre-fabricated electrode arrays specific to each nerve were used. Both motor responses (compound motor action potential [CMAP] and F-waves –all nerves) and sensory responses (sensory nerve action potentials [SNAP] –median and ulnar nerves only) were recorded following supramaximal stimuli. Automated algorithms determined all NCS parameters: distal motor latency (DML), mean F-wave latency (FWL), distal sensory latency (DSL), CMAP amplitude, and SNAP amplitude. Latency was adjusted for skin temperature deviation from reference. Pearson correlation coefficient (CC), intraclass correlation coefficient (ICC), coefficient of variance (CoV), and relative intertrial variation (RIV) were calculated.

Results

Fifteen subjects participated in either upper or lower extremity studies with nine participating in both. With the exception of CMAP amplitude, all parameters had CoV less than 0.06. Upper extremity amplitude parameters had CCs greater than 0.85. CCs for latencies were greater than 0.80 except for the median nerve FWL (CC = 0.69). For lower extremity nerves, ICCs were highest for mean FWL (>0.90), followed by DML (>0.82) and then CMAP (peroneal 0.33, tibial 0.73). The 10th to 90th RIV percentiles were bounded by ±7% for F-wave latencies; ±9% for all DSLs; and ±11% for DML (except peroneal at 15%).

Conclusions

The reproducibility of NCS parameters obtained with an automated NCS instrument compared favorably with traditional electromyography laboratories. F-wave latencies had the highest repeatability, followed by DML, DSL, SNAP and CMAP amplitude. Given their high reproducibility, automated NCS instrument may encourage wider utilization of NCS in clinical and research applications.

Comparison of CMT1A and CMT2: similarities and differences

To evaluate the clinical and electrophysiological similarities and differences between two large groups of patients with Charcot-Marie-Tooth disease, i.e. CMT1A and CMT2, we performed a post hoc comparison of clinical and electrophysiological data. Most CMT1A and CMT2 patients had the classical CMT phenotype. Age of onset was significantly later in CMT2. Total areflexia was present in approximately half of the CMT1A patients whereas it was rare in CMT2. Foot deformities and weakness of knee extensor and foot dorsal flexor muscles were more frequent in CMT1A. Median nerve motor nerve conduction velocities (MNCV) were always less than 38 m/s in CMT1A patients, whereas this was also the case in 16% of the CMT2 patients. Sensory nerve conduction velocities showed less overlap. In both CMT1A and CMT2 CMAP and SNAP amplitudes were often reduced or not obtainable in the legs. In CMT1A, SNAP amplitude was more reduced and SNAP duration more prolonged than in CMT2. We conclude that there are no robust clinical signs or symptoms that differentiate between CMT1A and CMT2 patients. Electrodiagnostical studies show a length-dependent motor and sensory axonal dysfunction in both CMT-types. Additional SNAP and SNCV evaluation may be helpful in focusing molecular genetic analysis in the occasional case of CMT2 showing slow motor nerve conduction velocities overlapping with CMT1A values. The reduction of CMAP and SNAP amplitudes in CMT1A is probably a combined effect of demyelination and axonal dysfunction.

An Electrodiagnostic Evaluation of the Effect of Pre-existing Peripheral Nervous System Disorders in Patients Treated with the Novel Proteasome Inhibitor Bortezomib

AIMS

Bortezomib (Velcade), a novel proteasome inhibitor, has shown promise in the treatment of malignancies, including multiple myeloma and non-Hodgkin's lymphoma. Several studies have identified neuropathy as a potentially dose-limiting side effect of treatment with bortezomib. We report the clinical and electrodiagnostic data from four patients who developed signs and symptoms of peripheral neuropathy from treatment with bortezomib.

MATERIALS AND METHODS

Patients were included if they were enrolled in active phase 2 trials of bortezomib for non-Hodgkin's lymphoma or prostate cancer, developed signs and symptoms of peripheral neuropathy, and were referred for electrodiagnostic evaluation.

RESULTS

Four patients, including two with non-Hodgkin's lymphoma and two with prostate cancer, underwent electrodiagnostic testing. Electrodiagnostic evaluation showed pre-existing peripheral nervous system disorders in three out of four patients. Multiple peripheral nervous system disorders were present in two out of four patients.

CONCLUSIONS

Bortezomib can cause a predominately sensory axonal polyneuropathy. Pre-existing peripheral nervous system disorders, such as neuropathy and radiculopathy, are common in patients with cancer, and may pre-dispose to the development of symptomatic neuronal toxicity when treated with bortezomib. Baseline electrodiagnostic evaluation may identify patients with pre-existing peripheral nervous system disorders at risk for additive neuronal toxicity from neurotoxic chemotherapeutic agents.

Glutamine as a Neuroprotective Agent in High-dose Paclitaxel-induced Peripheral Neuropathy: A Clinical and Electrophysiologic Study

AIMS

The appearance of peripheral neuropathy is the dose-limiting toxicity in many chemotherapy protocols, and glutamine has been proposed as a potentially neuroprotective agent in patients receiving paclitaxel.

MATERIALS AND METHODS

In this non-randomised study, we assessed neurologic signs and symptoms, and changes in nerve-conduction studies in 46 consecutive patients given high-dose paclitaxel either with (n=17) or without (n=29) glutamine. Neurological assessments and electrodiagnostic studies were carried out at baseline and at least 2 weeks (median 32 days) after treatment.

RESULTS

Patients who received glutamine developed significantly less weakness (P = 0.02), less loss of vibratory sensation (P = 0.04) and less toe numbness (P = 0.004) than controls. The per cent change in the compound motor action potential (CMAP) and sensory nerve action potential (SNAP) amplitudes after paclitaxel treatment was lower in the glutamine group, but this finding was not statistically significant in these small groups.

CONCLUSIONS

In this study, serial neurologic assessment of patient symptoms and signs seemed to be a better indicator of a possible glutamine effect than sensory- or motor-nerve-conduction studies. Prospective randomised trials are needed to clarify the effect of glutamine on paclitaxel and other types of chemotherapy-induced neuropathy.

Physiologic changes of compound muscle action potentials related to voluntary contraction and muscle length in carpal tunnel syndrome

The affect of muscle length and voluntary contraction upon compound muscle action potentials (CMAPs) in subjects with carpal tunnel syndrome (CTS) has been evaluated. Twenty-five hands in a CTS patient group and 29 hands in a normal subject control group were studied. The CMAPs from the abductor pollicis brevis induced by median nerve stimulation at the wrist were obtained for five thumb positions: neutral, abduction for shortening with and without contraction, and adduction for lengthening with and without contraction. Upon muscle shortening with relaxation, CMAP duration decreased in both groups, whereas waveform amplitude increased in the control group and showed no significant change in the CTS group. Muscle shortening with contraction afforded decreased CMAP duration and increased CMAP amplitude in both groups. Upon muscle lengthening with relaxation, both groups showed a reduction in CMAP amplitude and an increase in CMAP duration. Upon lengthening with contraction, CMAP duration decreased in the control group; in contrast, the CTS group showed further amplitude reduction and the waveform duration returned to the neutral value. These results demonstrate that, in patients with CTS, physiologic CMAP summations by muscle shortening or contraction may be less effective, whereas decreases in amplitude and increases in duration may be accentuated by lengthening and contraction.

Clinical disease severity and axonal dysfunction in hereditary motor and sensory neuropathy Ia

BACKGROUND

Hereditary motor and sensory neuropathy type Ia (HMSN Ia) is known as a primarily demyelinating peripheral nerve disease. Evidence is accumulating that axonal involvement determines the course of the disease process.

METHODS

Fifty-one patients were investigated. Physical disability and impairments were scored. Nerve conduction velocities (NCVs) were used as indirect measures for myelination status and compound muscle/sensory nerve action potential (CMAP/SNAP) amplitudes served as indirect measures for axonal function.

RESULTS

Median age was 39 years (range 6-69). Muscle weakness and sensory dysfunction was more severe in the legs than in the arms and distally more than proximally. However, more than 40% of the patients had proximal muscle weakness in the legs. Three point grip was used as representative of combined distal arm muscle groups. CMAP amplitude was the most important independent variable in a multiple linear regression model (forward selection) to explain the relation between three point grip strength and four different features, i. e., CMAP amplitude of the abductor pollicis brevis, median nerve MNCV, gender, and duration of signs and symptoms. The severity of axonal dysfunction was nerve length-dependent and was related to the myelination status. The mild physical disability due to both muscle weakness and sensory dysfunction was also related to axonal dysfunction.

CONCLUSIONS

In HMSN Ia, clinical disease severity at the impairment and disability levels is related to the severity of axonal dysfunction. Our data support the hypothesis that the myelination status is one of the factors that determine the extent of axonal dysfunction later in life. Proximal weakness of the legs is encountered in a considerable proportion of our patients.

‘Neurographic’ palmaris brevis sign in type II° ulnar neuropathy at wrist

OBJECTIVE

To investigate the source of an unusual and previously unreported volume conducted potential on motor nerve conduction studies. In a case of subacute ulnar neuropathy at wrist (UNW) selectively involving the deep motor branch, we recorded from the hypothenar eminence a large positive wave (2.5 ms-2 mV) preceding the negative takeoff of the delayed distal ulnar motor response.

METHODS

We performed multiple channels motor and sensory ulnar nerve (UN) conduction studies; these included selective electrical stimulation and anaesthetic block of UN branches and also selective recording of motor responses by single fibre needles; data were confirmed by an intraoperative neurophysiological study and correlated with MRI and surgical findings.

RESULTS

Detailed neurophysiological investigation demonstrated the generation of this waveform from the palmaris brevis (PB) muscle. MRI and surgical exploration documented a hypertrophy of this muscle.

CONCLUSIONS

In type II degrees UNW, depolarization of a spared palmaris brevis muscle may be recorded as a positive wave preceding the delayed abductor digiti minimi motor response.

SIGNIFICANCE

We underline the peculiar localizing value of this volume conducted 'meaningful artefact' in that particular setting. It actually represented an early neurographic analogue of what is known as the clinical 'Palmaris Brevis Sign' in long standing type II degrees UNW.

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.

Electrophysiologic endpoint measures in a multicenter ALS drug trial

We report the analysis of a battery of secondary electrophysiologic measurements to assess the progression of amyotrophic lateral sclerosis (ALS) in a two center, six month, double-blind, three arm trial comparing branched chain amino acids to L-threonine with pyridoxal 5-phosphate to placebo. The endpoint measurements were chosen to separately assess the effects of lower motor neuron loss and collateral reinnervation. For tests of inter-center reliability, we found no differences that could not be readily explained by variations in electrophysiologic testing techniques. Since the drug study was negative for the primary endpoint measure (muscle strength), we combined data from both centers and the three treatment arms. For measures of progression, all measures changed in the expected direction during the 6 months of the trial. We conclude that a battery of electrophysiologic measures can be used in a multicenter ALS drug trial to provide information on changes in lower motor neuron numbers and the effects of collateral reinnervation.

Repetitive nerve stimulation: effects of recording site and the nature of 'pseudofacilitation'

OBJECTIVES

To describe changes in the waveform of the compound muscle action potential (CMAP) during repetitive nerve stimulation for various recording sites.

METHODS

Responses to trains of 10 stimuli given at 0.1, 1, 3, 5, 10 and 30 Hz to the ulnar nerve were recorded simultaneously from 8 hand sites in 15 healthy subjects. Percentile changes of amplitude, duration and area of both negative and positive phases were analyzed.

RESULTS

Duration consistently decreased during the trains. At 30 Hz, the mean amplitude of the negative phase increased on 5 sites but decreased on 3. Area consistently decreased, but least for hypothenar sites. Repeated stimulation causes an alteration in the waveform of the CMAP that consists of 4 elements: (1) shorter duration; (2) changed amplitude of the negative phase (up or down); (3) merging of bifid peaks; (4) changes were more pronounced for positive than negative phases.

CONCLUSIONS

As the term 'pseudofacilitation' implies an increase in amplitude, it is often not appropriate. Increased muscle fiber conduction velocity can explain most of the waveform alterations. Movement and shortening of muscles may play additional roles. Consequences for diagnostic yield await a comparison with disease groups.

'Rules of conduct': some practical guidelines for testing motor-nerve conduction

The testing of nerve conduction using electromyography (EMG) is a frequently used diagnostic method for the identification of various neuropathies. The present article illustrates a variety of conditions on the basis of clinical data, and suggests how one can obtain the best results by observing a few simple rules.

CMAP amplitude cartography of muscles innervated by the median, ulnar, peroneal, and tibial nerves

The spatial and temporal distribution of compound muscle action potential (CMAP) amplitudes was mapped using 1 x 1-cm grids over thenar, hypothenar, dorsal foot, and foot sole muscles (seven maps each). The high-amplitude zone (HAZ, area where amplitudes were over 80% of the maximum amplitude) denoted susceptibility to changes in recording site. Thenar maps had one peak (spatially and temporally) with a HAZ of 3.5 +/- 2.3 cm2. Hypothenar maps had two peaks (spatially and temporally) with a HAZ of 7.7 +/- 3.6 cm2. Dorsal foot maps had one temporal peak, which could be split up spatially; the HAZ was smallest, at 1.7 +/- 1.7 cm2. Foot sole muscles had one peak (spatially and temporally), with the largest HAZ at 18.4 +/- 6.1 cm2. Wave-form differences were ascribed to differences in muscle anatomy, architecture, and variability. These explain differences in amplitude reproducibility between nerves and the differing effect that increasing electrode size has on reproducibility.

Influence of electrode site and size on variability of magnetic evoked potentials

Successive magnetic evoked potentials (MEPs) concern varying motor neurons. We investigated whether this MEP-specific source of variability depends on electrode site and size. Amplitude variability (standard deviation) was largest over the center of the hypothenar muscles. Latencies were longer at distal and proximal sites than at the center site. Large electrodes (10 cm2) did not decrease this source of amplitude variability compared with EEG electrodes, in contrast to other sources of variability.

Compound muscle action potential cartography of an accessory peroneal nerve

In daily practice, accessory peroneal nerves (APNs) are detected in less than the 18-25% of legs, as revealed by systematic searches. In one APN case, compound muscle action potential cartography showed that the APN was only apparent when the recording electrode was placed over a small lateral region of the extensor digitorum brevis muscle. Effects of recording site can explain why many APNs go unrecognized.

The influence of active electrode placement on CMAP amplitude

The compound muscle action potential (CMAP) is a measure of the number of axons in a nerve. Placement of the active recording electrode over the motor point of a muscle is thought to give the maximal response, but there is considerable variation in amplitude among initially negative CMAP wave forms. Ten examiners of varied training backgrounds and experience placed the active electrode as they usually do over the thenar, hypothenar, abductor hallucis, and extensor digitorum brevis muscles in the same normal subject. There was variability of the CMAP amplitude recorded over each muscle; the lowest value recorded from a muscle was 57% of the maximum value, and the lowest median value was 77%. There was no relation between examiner background or level of training and recording the maximal response. Higher amplitude CMAPs were associated with steeper wave form slopes, but the range of correlations between amplitude and slope was 0.42 to 0.92. We conclude that when it is important to record the maximal CMAP response, empirical assessment by moving the active electrode is necessary.

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.

Repetitive CMAPs: mechanisms of neural and synaptic genesis

Repetitive compound muscle action potentials (R-CMAPs) occur when a single nerve shock excites muscle fibers repeatedly. "Double discharges" are due to intramuscular nerve reexcitation. "Synaptic" R-CMAPs, due to excess acetylcholine in the neuromuscular synapse, can occur in congenital myasthenia, the slow-channel syndrome, and acetylcholinesterase inhibition. Secondary nerve excitation can reexcite muscle fibers. Synaptic R-CMAPs in a patient consisted of two discharges. The second diminished during repetitive stimulation and began 3.5-4.0 ms after the first, which is slightly longer than the synapse-muscle refractory period. Neural R-CMAPs, due to ectopic nerve activity, occur in neuromyotonia (NMT). R-CMAPs in a patient consisted of about 20 discharges at 200-300 Hz. Studies in healthy subjects showed that such trains represent added single CMAPs. Impulse frequency in the patient lied close to the threshold of refractoriness. Refractoriness of the synapse-muscle cell assembly determines the characteristics of R-CMAPs regardless of the primary cause.

Large electrodes improve nerve conduction repeatability in controls as well as in patients with diabetic neuropathy

Recording site is an important cause of variability of compound muscle action potential (CMAP) and conduction parameters, which can be reduced by using large electrodes. Repeatability of CMAP and conduction parameters of conventional and large electrodes was compared in 16 controls and 17 diabetic neuropathic patients, using defined recording sites linked to anatomical landmarks. Right-sided median, peroneal, and tibial nerves were investigated twice with a 1-2 week interval by the same examiner. Compared to previous studies, conventional electrodes on strictly defined recording sites resulted in better repeatability: intraindividual coefficients of variation (CV) varied between 4% and 14.4% for all parameters. CV of conduction parameters, not published previously, was smaller than CV of CMAP parameters. The use of large electrodes improved repeatability further: large electrodes resulted in substantially smaller CV for duration, amplitude, area, and changes of amplitude and area over a length of nerve, which were reduced by 10%, 31%, 29%, 27%, and 16%, respectively. Patients had higher CV than controls; large electrodes reduced patient CV more than control CV, resulting in less contrast between groups. Strictly defined recording sites and large electrodes improve repeatability of motor conduction studies to relevant degrees: all CMAP and conduction parameters are suitable for longitudinal studies of neuropathic patients.

CMAP variability as a function of electrode site and size

The site of the recording electrode influences the amplitude of the compound muscle action potential (CMAP) and its variation over a length of nerve. The effects of large electrodes on this source of intraindividual variability were assessed. Right median nerves of 20 healthy subjects were studied, and recordings made at three sites (at 1-cm intervals) using five electrode sizes (0.01, 1, 2, 4, and 10 cm2). Site-induced variability was defined as the standard deviation (SDi) and coefficient of variation (CVi) of the measurements of the three sites. Site induced variability of all parameters (latency, duration, amplitude, area, MNCV, and the percentile changes of duration, amplitude, and area over the forearm) decreased significantly with electrode size. Decreases were most pronounced for amplitude and area: CVi fell from 29% and 30% (0.01-cm2 electrode) to 10% and 8% (10 cm2). It is argued that large electrodes record activity of more motor units than small electrodes, and that their measurement fields overlap to a greater extent. The use of large electrodes is recommended in order to reduce site-induced CMAP variability.