Facts, fallacies, and fancies of nerve conduction studies: twenty-first annual Edward H. Lambert Lecture

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.

Modeling and Reproducibility of Twin Concentric Electrical Impedance Myography

OBJECTIVE

Electrical impedance myography (EIM) is a recent technology to assess muscle health. As of today, the clinical application of EIM has been applied only to evaluate muscle condition using non-invasive surface electrodes in contact with the skin; however, intermediate tissues at the recording site introduce confounding artifacts which reduce the technique's performance as a biomarker of neuromuscular disorders (NMD). Here, we develop and test in humans a new approach using two concentric needles for intramuscular EIM recordings.

METHODS

First, we study the recording characteristics of dual concentric needle EIM via analytical models and finite element models (FEMs). Next, the validity of the models is verified by performing experiments on saline and agar phantoms. Finally, 8 subjects with various neuromuscular diseases were studied measuring tibialis anterior, biceps, deltoid, adductor pollicis brevis, first dorsal interosseous and flexor carpi radialis muscles.

RESULTS

Analytical and FEM simulations are in good agreement with a maximum experimental discrepancy 8% and 9% using gauge needles 26 and 30, respectively. The inter-session reproducibility, as measured by the intraclass correlation coefficients for all muscles studied, was 0.926, which is comparable or exceeds the reproducibility of other well-established electrophysiological tests to assess muscle health.

CONCLUSION

The reproducibility of the technique support future clinical validation of needle EIM for assessment of disease status, either as part of standard patient care or as biomarker measure in clinical trials.

SIGNIFICANCE

Needle EIM has the potential of becoming a valuable diagnostic tool to evaluate NMD in adult population.

Reliability of phrenic nerve conduction study: In healthy controls and in patients with primary lateral sclerosis

OBJECTIVE

Phrenic nerve conduction study is a marker of hypoventilation in amyotrophic lateral sclerosis. We aimed to evaluate its intra-rater reliability in healthy subjects and in a cohort of Primary Lateral Sclerosis (PLS) patients.

METHODS

Eighteen healthy subjects and 16 PLS patients were included. All subjects underwent three phrenic nerve conduction evaluations (time interval: 1 week for healthy controls; 1 year for PLS patients). We analyzed intra-rater reliability for five parameters of the diaphragmatic motor response: latency; negative-peak duration, area and amplitude; peak-to-peak amplitude.

RESULTS

Healthy subjects showed excellent inter-test reliability for most parameters (coefficients of variation <10%). In PLS patients coefficients of variation resulted <10% for latency and peak-to-peak amplitude, <20% for remaining parameters. Inter-test reliability was excellent for latency and peak-to-peak amplitude [intra-class correlation coefficient (ICC) > 0.9] and good for negative-peak amplitude and area (ICC 0.75 ≥ 0.9); duration was not reliable (ICC = 0.383). Negative peak and peak-to-peak amplitude had the least random error (respectively ±0.136 mV and ± 0.177 mV). All parameters showed homoscedasticity (R2 < 0.1).

CONCLUSIONS

Intra-rater reliability is high for phrenic nerve study, especially for latency, peak-to-peak and negative-peak amplitude.

SIGNIFICANCE

Phrenic nerve conduction study is a reliable method to monitor respiratory function.

Patient-Reported Disability Measures Do Not Correlate with Electrodiagnostic Severity in Carpal Tunnel Syndrome

Background:

Electrophysiologic studies including electromyography and nerve conduction studies play a role in the evaluation of carpal tunnel syndrome (CTS), despite evidence that these studies do not correlate with CTS-specific symptom scores. There is a lack of evidence comparing electrophysiologic data with general measures of function.

Methods:

Fifty patients presenting for CTS treatment over an 8-month period were analyzed retrospectively. All patients completed surveys including the Quick Disabilities of the Arm, Shoulder, and Hand questionnaire (DASH) and the Medical Outcomes Study 12-Item Short-Form Survey [(physical component summary 12, mental component summary (MCS-12)]. Electromyography and nerve conduction studies were performed on all patients and compared with outcome scores.

Results:

Analysis demonstrated no relationship between DASH or MCS-12 and electrodiagnostic severity. No significant correlations were noted between DASH or MCS-12 and median motor or sensory latency. There was a moderate–weak correlation (rho = 0.34) between more severe electrophysiologic grade and better function based on physical component summary 12.

Conclusions:

Electrodiagnostic severity grades do not correlate with patient-reported disability, including the DASH and MCS–12 surveys. There is a counterintuitive correlation between more-severe electrodiagnostic findings and decreased physical disability. These findings indicate that disability may not correlate with electrodiagnostic severity of median neuropathy in CTS.

Adipose-Derived Stem Cells Enhance Axonal Regeneration through Cross-Facial Nerve Grafting in a Rat Model of Facial Paralysis

BACKGROUND

Cross-face nerve grafting combined with functional muscle transplantation has become the standard in reconstructing an emotionally controlled smile in complete irreversible facial palsy. However, the efficacy of this procedure depends on the ability of regenerating axons to breach two nerve coaptations and reinnervate endplates in denervated muscle. The current study tested the hypothesis that adipose-derived stem cells would enhance axonal regeneration through a cross-facial nerve graft and thereby enhance recovery of the facial nerve function.

METHODS

Twelve rats underwent transection of the right facial nerve, and cross-facial nerve grafting using the sciatic nerve as an interpositional graft, with coaptations to the ipsilateral and contralateral buccal branches, was carried out. Rats were divided equally into two groups: a grafted but nontreated control group and a grafted and adipose-derived stem cell-treated group. Three months after surgery, biometric and electrophysiologic assessments of vibrissae movements were performed. Histologically, the spectra of fiber density, myelin sheath thickness, fiber diameter, and g ratio of the nerve were analyzed. Immunohistochemical staining was performed for the evaluation of acetylcholine in the neuromuscular junctions.

RESULTS

The data from the biometric and electrophysiologic analysis of vibrissae movements, immunohistochemical analysis, and histologic assessment of the nerve showed that adipose-derived stem cells significantly enhanced axonal regeneration through the graft.

CONCLUSION

These observations suggest that adipose-derived stem cells could be a clinically translatable route toward new methods to enhance recovery after cross-facial nerve grafting.

Electrophysiologic testing in diabetic neuropathy

Electrophysiologic studies provide objective data concerning nerve and muscle function. This information enables the diagnosis of disease states and monitoring of disease progression. This chapter describes the changes in electrophysiologic function in both prediabetes and diabetes and discusses the utility of this testing in patients with diabetes. Both the strengths and limitations of electrophysiology are discussed.

The auditory space in the motor system

Sensory events in the space around us trigger specific motor patterns directed toward or away from the spatial location of the sensory source. Spatially-defined sensorimotor associations are well-known in the visual domain but less so for the auditory modality. In particular no spatially-directed audio-motor association has been described for the upper limb. We tested the instantaneous directional tuning of the corticospinal system by means of single-pulse transcranial magnetic stimulation (TMS) over the left motor cortex in 16 healthy volunteers while at rest. We recorded the lateral accelerations of the TMS-evoked movement by means of an accelerometer placed on the forearm. Acoustic stimuli (pure tone frequency=1000Hz, duration=50ms) coming from 25 different directions lying in the axial anterior half-plane at the height of the participant's ears were played on earphones. The entire set of sound directions covered a span of 160° (±80° where 0° is the frontal direction) at a fixed azimuth angle. Six different intervals between sound onset and TMS (0, 25, 50, 100, 150 and 200ms) were tested for each sound direction. Significant correlations were found between sound origin and TMS-evoked arm accelerations only when TMS was delivered 50ms prior to sound onset. We show the presence in the upper limb motor system of auditory spatial tuning. Sound information accesses the motor system at very short latency, potentially compatible with both a subcortical and a cortical origin of the response. The use of TMS-evoked accelerations allowed us to disclose a strict directional tuning in audio-motor associations.

Test-retest reliability of transcarpal sensory NCV method for diagnosis of carpal tunnel syndrome

Background:

Carpal Tunnel Syndrome (CTS) is the most frequent entrapment neuropathy affecting the upper extremity. There are a variety of electrodiagnostic methods available for documenting median neuropathy in CTS. In some studies, determining the sensory NCV across the palm-wrist segment has been introduced as the most sensitive diagnostic procedure for CTS. The aim of this study was to investigate the test-retest reliability of transcarpal median sensory NCV method for the diagnosis of CTS.

Materials and Methods:

Twenty-three patients with clinical symptoms of CTS were tested two times by two different practitioners in one session and again by the first practitioner after one week. Stimulation of the median nerve was performed in the wrist and palm, with a conduction distance maximum of 7 cm, reliabilities of median nerves sensory nerve action potential latencies with stimulation at wrist and palm (W-SNAP, P-SNAP) and its transcarpal NCV were assessed with intraclass correlation coefficient (ICC).

Results:

Comparison of the obtained values, which were done by two practitioners in one session showed ICC of W-SNAP latency, P-SNAP latency and transcarpal NCV of 0.93, 0.88 and 0.87, respectively and values that were done by one practitioner in two sessions with one-week interval showed ICC of 0.60, 0.50 and 0.47, respectively.

Conclusion:

Our findings suggest excellent interpractitioner test-retest reliability of transcarpal median sensory NCV method for diagnosing CTS.

Sensitivity and specificity of diagnostic ultrasound in the diagnosis of phrenic neuropathy

OBJECTIVES

To determine the sensitivity and specificity of B-mode ultrasound in the diagnosis of neuromuscular diaphragmatic dysfunction, including phrenic neuropathy.

METHODS

A prospective study of patients with dyspnea referred to the EMG laboratory over a 2-year time frame for evaluation of neuromuscular respiratory failure who were recruited consecutively and examined with ultrasound for possible diaphragm dysfunction. Sonographic outcome measures were absolute thickness of the diaphragm and degree of increased thickness with maximal inspiration. The comparison standard for diagnosis of diaphragm dysfunction was the final clinical diagnosis of clinicians blinded to the diaphragm ultrasound results, but taking into account other diagnostic workup, including chest radiographs, fluoroscopy, phrenic nerve conduction studies, diaphragm EMG, and/or pulmonary function tests.

RESULTS

Of 82 patients recruited over a 2-year period, 66 were enrolled in the study. Sixteen patients were excluded because of inconclusive or insufficient reference testing. One hemidiaphragm could not be adequately visualized; therefore, hemidiaphragm assessment was conducted in a total of 131 hemidiaphragms in 66 patients. Of the 82 abnormal hemidiaphragms, 76 had abnormal sonographic findings (atrophy or decreased contractility). Of the 49 normal hemidiaphragms, none had a false-positive ultrasound. Diaphragmatic ultrasound was 93% sensitive and 100% specific for the diagnosis of neuromuscular diaphragmatic dysfunction.

CONCLUSION

B-mode ultrasound imaging of the diaphragm is a highly sensitive and specific tool for diagnosis of neuromuscular diaphragm dysfunction.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that diaphragmatic ultrasound performed by well-trained individuals accurately identifies patients with neuromuscular diaphragmatic respiratory failure (sensitivity 93%; specificity 100%).

Neurophysiological approach to disorders of peripheral nerve

Disorders of the peripheral nerve system (PNS) are heterogeneous and may involve motor fibers, sensory fibers, small myelinated and unmyelinated fibers and autonomic nerve fibers, with variable anatomical distribution (single nerves, several different nerves, symmetrical affection of all nerves, plexus, or root lesions). Furthermore pathological processes may result in either demyelination, axonal degeneration or both. In order to reach an exact diagnosis of any neuropathy electrophysiological studies are crucial to obtain information about these variables. Conventional electrophysiological methods including nerve conduction studies and electromyography used in the study of patients suspected of having a neuropathy and the significance of the findings are discussed in detail and more novel and experimental methods are mentioned. Diagnostic considerations are based on a flow chart classifying neuropathies into eight categories based on mode of onset, distribution, and electrophysiological findings, and the electrophysiological characteristics in each type of neuropathy are discussed.

The consequences of stimulus intensity on sensory nerve action potentials

Although clinical references recommend evoking sensory nerve action potentials (SNAPs) using supramaximal stimulation, many patients and research subjects report that the maximal and supramaximal stimulation is uncomfortable. The purpose of this study was to determine whether using stimulation intensities at maximal and submaximal levels results in differences in SNAP latency or amplitude outcomes relative to SNAPs recorded using supramaximal stimulation. Twenty healthy male and female subjects participated. Antidromic SNAPs were evoked at two separate sites along the course of the median nerve: (1) just proximal to the wrist and (2) just proximal to the antecubital fossa in each subject. All evoked potentials were recorded from the second digit. The stimulation current that resulted in a maximal amplitude SNAP was determined and 10 SNAPs were ensemble averaged using this stimulation level. Ten SNAPs were then evoked at each of four other stimulation intensities (20% and 10% both above and below that which generated a maximal amplitude SNAP) and were ensemble averaged. For each stimulation intensity and stimulation site, SNAP amplitude and latency determined for each ensemble average. SNAP amplitudes and latencies were compared among stimulus levels and between sexes using a repeated measures analyses of variance. The results of this study suggest that stimulus intensities used to generate SNAPs at the median nerve do not need to be supramaximal in order to generate valid and reliable SNAP data, with stimulation intensities as low as 20% below maximal proving to be sufficient, particularly for the measurement of SNAP latency.

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.

Slower conduction velocity and motor unit discharge frequency are associated with muscle fatigue during isometric exercise in type 1 diabetes mellitus

Type 1 diabetes mellitus (T1DM) is associated with a peripheral neuropathy that reduces nerve conduction velocity. This may impair high motor-unit discharge frequencies (MUDF), decrease muscle activation, and curtail the ability to sustain repetitive contractile tasks. We examined (1) whether MUDF, the contractile properties of the knee extensors, and the conduction velocity of persons with T1DM differed from controls; (2) whether persons with T1DM can maintain adequate MUDF during a fatigue protocol; and (3) the relationship between these parameters and impaired glycemic control. We studied male and female subjects with T1DM and controls matched for age, height, weight, and gender. Single motor unit recordings were made from vastus lateralis during maximal and submaximal contractions and during a fatigue protocol. Glycemic control was assessed from blood glucose concentration and glycosylated hemoglobin (HbA1c). Control femoral conduction velocities were comparable to literature values and those of the T1DM subjects were slower. These values correlated with plasma glucose and HbA1c. T1DM subjects fatigued 45% sooner than controls, and time to fatigue and conduction velocity were correlated (r = 0.54, P < 0.05). Discharge frequencies tended to be slower during 50% maximal voluntary contractile force in the T1DM subjects at task failure. Persons with T1DM had slower conduction velocities and lower MUDF than their controls, which apparently leads to impaired activation of muscle and decreased endurance during isometric fatigue.

F-wave decomposition for time of arrival profile estimation

F-waves are distally recorded muscle responses that result from "backfiring" of motor neurons following stimulation of peripheral nerves. Each F-wave response is a superposition of several motor unit responses (F-wavelets). Initial deflection of the earliest F-wavelet defines the traditional F-wave latency (FWL) and earlier F-wavelet may mask F-wavelets traveling along slower (and possibly diseased) fibers. Unmasking the time of arrival (TOA) of late F-wavelets could improve the diagnostic value of the F-waves. An algorithm for F-wavelet decomposition is presented, followed by results of experimental data analysis.

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.

Test–retest reproducibility of 50kHz linear-electrical impedance myography

OBJECTIVE

Electrical impedance myography (EIM) is a method for evaluating muscle in which high-frequency, low-intensity alternating current is applied to a body region and the resulting surface voltage pattern over a muscle of interest is measured. In this study, the reproducibility for the simplest of these techniques, 50 kHz linear-EIM, was assessed for three muscles.

METHODS

Fifty kilohertz linear-EIM was performed on the biceps, quadriceps, and tibialis anterior of 30 normal subjects ranging in age from 21 to 90 years, and the major outcome variable, the spatially averaged phase (thetaavg), measured. The measurements were repeated within 250 days and comparisons between the two data sets made.

RESULTS

Reproducibility, as measured by the intraclass correlation coefficients for all three muscles, was very high at 0.970, 0.971, and 0.938 for biceps, quadriceps, and tibialis anterior, respectively. Variability between measurements was on average 4.2% for all muscle combined, with an upper limit of 16.8%.

CONCLUSIONS

Fifty kilohertz linear-EIM demonstrates excellent test-retest reproducibility.

SIGNIFICANCE

These results support the view that 50 kHz linear-EIM has the potential to be used as a simple, fast, and non-invasive measurement for the assessment of disease status, either as part of individual patient care or as a surrogate outcome measure in clinical trials work.

NC-stat sensory nerve conduction studies in the median and ulnar nerves of symptomatic patients

OBJECTIVE

This study evaluated validity and reliability of automated median and ulnar sensory nerve conduction study (NCS) measurements by the NC-stat.

METHODS

Median and ulnar distal sensory latencies (DSL) and amplitudes (SNAP) were measured in sixty subjects with the NC-stat and by a neurologist (reference) using traditional instrumentation. The median-ulnar DSL differences (MUD) was calculated. Validity was quantified by the Pearson correlation. Reliability was evaluated by the intraclass correlation coefficient (ICC), Bland-Altman analysis, and inter-rater agreement of MUD abnormalities.

RESULTS

As a result of differences in electrode placement, NC-stat and reference mean values had systematic differences. The correlation ranged from 0.70 (ulnar DSL) to 0.91 (median DSL). The ICC ranged from 0.69 (ulnar DSL) to 0.91 (median DSL). In Bland-Altman analysis of DSLs, NC-stat measurements had a bias of 0.56 ms (median) and 0.31 ms (ulnar) and precision of 0.31 and 0.30 ms. Inter-rater agreement for MUD abnormalities was 93.8% (raw) and 0.80 (Kappa).

CONCLUSIONS

NC-stat validity and reliability metrics were similar to traditional NCS. Use of the NC-stat would require applicable reference ranges.

SIGNIFICANCE

NC-stat median and ulnar NCS are valid and reliable. This device may be useful for increasing availability of NCS when clinically appropriate.

Sensitivity and specificity of electrodiagnostic criteria for CIDP using ROC curves: comparison to patients with diabetic and MGUS associated neuropathies

INTRODUCTION

Electrodiagnostic testing plays a key role in the characterization of neuropathies. To this end, sets of electrodiagnostic criteria have been proposed to define chronic inflammatory demyelinating polyneuropathy (CIDP). These criteria sets differ because of the number of data points within the sets, the number of required abnormal measures within a criterion, and the cutoff value of each measure.

OBJECTIVE

To evaluate the sensitivity and specificity of the published criteria for defining CIDP in comparison to diabetic polyneuropathy (DMPN) and monoclonal gammopathy of undetermined significance associated neuropathies (MGUS-PN).

DESIGN/METHODS

Electrodiagnostic studies of 21 patients with biopsy proven CIDP, 35 patients with MGUS-PN, and 82 patients with DMPN were analyzed. Data were compared against 4 different published criteria sets. Receiver operator characteristic (ROC) curves were used to determine the ideal threshold values for individual electrodiagnostic parameters.

RESULTS/CONCLUSION

None of the currently published criteria sets could adequately separate CIDP from DMPN and MGUS-PN. Analysis of our data using ROC curves shows that the best discrimination was achieved using the following criteria: (1) FWL>145%ULN in one nerve or 110%ULN in two nerves in separate nerve roots. (2) Motor CV<90%LLN in four nerves with at least one nerve <70%LLN. Furthermore, proximal-to-distal amplitude ratio alone is not adequate to define conduction block. Although electrodiagnostic studies are important for evaluating CIDP, these studies by themselves cannot be used to define this neuropathy.

Dose-responsiveness of electrophysiologic change in a new model of acute carpal tunnel syndrome

This study was done to determine the dose-responsiveness during the development of acute pressure-induced median neuropathy in rabbits and to develop a new animal model of acute carpal tunnel syndrome. Twenty-three rabbits were used. Carpal tunnel syndrome was induced by infusing a controlled saline solution into the carpal tunnel of rabbits using a 21-gauge needle under general anesthesia to elevate the carpal tunnel pressure. The changes in the amplitude and latency of the compound muscle action potential obtained by abductor pollicis recording were observed after a complete conduction block, and after the release of pressure at various pressure levels. Pressures greater than 30 mm Hg applied to the carpal tunnel caused a remarkable electrophysiologic change. A higher pressure level resulted in a shorter time required for a complete conduction block and also for recovery after the release of pressure. Complete conduction block occurred between 40 and 50 minutes at a pressure of 100 mm Hg. The degree of recovery after a conduction block was related inversely to the pressure-time integrals. Our animal model reflects the pathophysiology of acute carpal tunnel syndrome and shows a regular dose-responsiveness during the development of acute pressure-induced neuropathy.

Symétrie et reproductibilité temporelle des données neurographiques

The aims of the present study are to document side-to-side differences and temporal variability, between two trials (T1 and T2 at a time interval of 3 months) of nerve conduction measurements collected from 30 healthy subjects (mean age 22 +/- 2 years).

METHODS

The protocol at T1 consisted of motor nerve conduction studies of median, ulnar, peroneal and tibial nerves bilaterally, with measurement of (a) motor response size (amplitude and area); (b) terminal latency; (c) minimal, mean and maximal F-wave latency; (d) motor conduction velocity; and (e) F-wave occurrence. T1 also involved sensory nerve conduction studies of median, ulnar, radial, lateral and medial cutaneous, sural and superficial peroneal nerves bilaterally, with measurement of sensory potential size (amplitude and area) and computation of sensory conduction velocity. The protocol at T2 consisted of identical measurements from the dominant side.

RESULTS AND CONCLUSION

There was a negative relationship between the variability of parameters evaluating nervous conduction and the length of the nerve segment under study. Thus, the smallest side-to-side and temporal variabilities are measured for minimal F-wave latencies (on average 2-3%). The limits of symmetry and temporal variability are particularly useful for diagnosis of unilateral peripheral neuropathy or neurophysiological follow-up of patients with neuropathy, when the variability of the parameter under study is weak and when there is a high correlation between values recorded on the left and on the right or at T1 and T2. This was the case for motor response size of tibial and ulnar nerves, sensory potential size of radial nerve and minimal F-wave latencies from each studied motor nerve.

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.

Inherited polyneuropathy in Leonberger dogs: a mixed or intermediate form of Charcot-Marie-Tooth disease?

A spontaneous distal, symmetrical polyneuropathy in related Leonberger dogs with onset between 1 to 9 years of age was characterized clinically, electrophysiologically, histologically, and morphometrically. Exercise intolerance and weakness was associated with a high-steppage pelvic-limb gait, a loss or change in the pitch of the bark, and dyspnea. Neurological examination revealed marked atrophy of the distal limb muscles, depressed spinal and cranial nerve reflexes, and weak or absent movement of the laryngeal and pharyngeal muscles. Electrophysiological evaluation was consistent with denervation and was characterized by loss or marked attenuation of compound muscle action potentials and slowed motor nerve conduction velocity. Muscle biopsy specimens showed neurogenic atrophy. Chronic nerve fiber loss associated with decreased myelinated fiber density and a shift of the axonal size-frequency distribution toward smaller fibers was the predominant finding in peripheral nerve specimens. Pedigree analysis of a large multigenerational family, including nine sibships with at least one affected individual, suggested X-linked inheritance. Mutational and linkage analysis of this family may aid in identification of the chromosomal loci and gene responsible for this inherited axonal neuropathy. Further characterization of this inherited axonal neuropathy may establish the Leonberger dog as a spontaneous animal model of inherited axonal neuropathy and possibly lead to the discovery of a new gene or genes associated with axonal variants.

Individual attributes versus composite scores of nerve conduction abnormality: sensitivity, reproducibility, and concordance with impairment

Composite scores may be more sensitive and reproducible than single attributes of nerve conduction for detection of peripheral neuropathy, but this requires validation in large patient cohorts. Also, the concordance of individual attributes versus composite scores with clinical measures of severity has not been tested. Here, we study these issues in prospectively studied cohorts: diabetic patients from Rochester, Minnesota (RDNS; n = 396); chronic inflammatory demyelinating polyneuropathy (CIDP) patients (n = 55); and multifocal motor neuropathy (MMN) patients (n = 18). With specificity fixed at the 97.5 percentile, we found that, in generalized polyneuropathies (diabetic and CIDP), composite scores (especially ones including conduction velocity, distal latencies, and F-waves) of individual or multiple nerves tended to be more sensitive than individual attributes. By contrast, for multiple mononeuropathies, some individual attributes or composite scores of individual nerves were more sensitive than composite scores. In diabetic polyneuropathy, composite scores tended to be more reproducible than individual attributes of nerve conduction. Highly significant correlations were found between individual attributes or composite scores and neurologic impairment in diabetic polyneuropathy and in CIDP; in general, correlation coefficients were higher for composite scores. These correlations were higher for amplitudes than for conduction velocities or distal latencies. We conclude that, with the availability of microprocessors and normative databases, electromyographers may increasingly seek to express nerve conduction abnormality also as composite scores of individual or several nerves.

Assessment of experimental autoimmune neuritis in the rat by electrophysiology of the tail nerve

The assessment of experimental autoimmune neuritis (EAN) by electrophysiological studies of the sciatic innervation of the plantar muscle may be complicated by local inflammation. We therefore utilized the tail nerve-muscle system to monitor disease progression in 20 rats with EAN and 10 control rats. Early changes were detected in motor nerve conduction velocity (32.06 +/- 1.85 m/s versus 43.57 +/- 3.98 m/s in controls, P < 0.001) at 15 days postimmunization (DPI), and conduction block (70.6 +/- 9.4% compared to 12.4 +/- 3.4%, P < 0.001) at 22 DPI. No consistent conduction block (22.4 +/- 10.4%) was found in the plantar muscle measurements. The tail nerve response of EAN rats demonstrated severe temporal dispersion at 43 DPI, which returned to normal at 135 DPI, although motor nerve conduction velocity values were still lower than in controls (24.4 +/- 0.9 m/s, P < 0.001). The tail nerve may be a useful addition to electrophysiological studies in this model of the Guillain-Barré syndrome.

The spectrum of chronic inflammatory demyelinating polyneuropathy

Research criteria for the diagnosis of chronic inflammatory demyelinating polyneuropathy (CIDP) were proposed by an Ad Hoc Subcommittee of the American Academy of Neurology (AAN) in 1991, and since then these criteria have been widely used in clinical studies. We have been impressed by the frequent finding of electrophysiological changes of a demyelinating neuropathy in patients whose clinical presentation does not conform to the usually accepted clinical phenotype of CIDP. To determine the clinical spectrum of CIDP, we conducted a retrospective review of patients of the peripheral electrophysiology laboratory of the University of Miami-Jackson Memorial Medical Center. Diagnostic criteria for acquired demyelination of an individual nerve were adapted from the AAN research criteria for the diagnosis of CIDP (1991). Patients were accepted for inclusion when such evidence was demonstrated in at least one motor nerve or at least two sensory nerves. We then reviewed the clinical phenotype and the underlying etiology of the neuropathy in these cases. Eighty-seven patients, 63 male and 24 female, age of onset 4-84 (mean 49.3) years, met these inclusion criteria. Forty-seven patients (54%) had distinct features outside the usual clinical presentation of CIDP. Of these, 15 (17%) had predominantly distal features, 13 (15%) had exclusively sensory polyneuropathy; seven (8%) had markedly asymmetric disease, seven (8%) had associated CNS demyelination, four (5%) had predominant cranial nerve involvement, and one (1%) had only the restless legs syndrome. An associated medical condition that may have been responsible for the acquired demyelinating neuropathy was present in 60% of the patients. We conclude that spectrum of CIDP is broader than would be indicated by the strict application of the AAN research criteria, and that many of the cases meeting more liberal criteria frequently respond to immunosuppressive therapy.

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.

Peroneal nerve motor conduction to the extensor digitorum brevis

Peroneal motor studies to the extensor digitorum brevis are commonly performed in electrodiagnosis. They have been investigated by many authors to derive the normal ranges for latency, amplitude, and nerve conduction velocity. Many of these studies, particularly the older ones, have methodological limitations, especially in light of modern technique and statistical applications. They often used small sample sizes. The objective of this study was to generate an expanded database of normative values for the peroneal nerve. In this study, 242 asymptomatic subjects without risk factors for neuropathy were tested, and their peroneal motor response was analyzed for latency, amplitude, area, duration, and nerve conduction velocity. Side-to-side and proximal-to-distal variation was recorded. Mean +/- 2 standard deviations (SD) and percentiles of normality are presented. Mean onset latency was 4.8 ms (SD, 0.8). Mean amplitude was 6.8 mV (SD, 2.5) and 5.1 mV (SD, 2.5) for the younger and older than 40-yr age groups, respectively. Mean nerve conduction velocity ranged from 44 to 49 m/s, depending on the demographic group (SD, 4-5). The upper limit of normal side-to-side latency variation was 1.4 ms (mean + 2 SD). The upper limit of normal drop in nerve conduction velocity from the low leg to the across knee segment was 10 m/s (mean - 2 SD) or 12% (97th percentile). The upper limit of normal amplitude difference from side to side was 61 % (at the 97th percentile), and the upper limit of normal drop in amplitude from below to above the fibular head stimulation was 25% (at the 97th percentile). The other measures are presented in the article.

Tibial nerve motor conduction to the abductor hallucis

Tibial motor studies to the abductor hallucis are commonly performed in electrodiagnosis. Numerous authors have investigated this nerve to derive the normal ranges for latency, amplitude, and nerve conduction velocity (NCV). Many of the studies were performed without regard to temperature control, fixed distance measurement, and demographic characteristics such as age, gender, and height, which are known to affect nerve conduction studies. They often used small sample sizes, and some did not use true normal controls. This study was performed to create an expanded database of normative values for the tibial nerve. In this study, 250 asymptomatic subjects without risk factors for neuropathy were recruited and tested for their tibial motor response. Latency, amplitude, area, duration, and NCV were recorded. A repeated measures analysis of variance was performed with the waveform measures as the dependent variables and age, gender, and height as independent variables. Factors that were significant at the P < or = 0.01 level were used to create separate normative databases. Age was found to be associated with different results for amplitude, area, and NCV. Height was found to be associated with different results for NCV. Once these statistically significant factors were determined, Tukey-adjusted pair-wise comparisons of least squares means were used to collapse categories (by decade for age) that were not significantly different from each other at the P < or = 0.05 level. Side-to-side and proximal-to-distal differences were analyzed. The normal range was derived as mean +/- 2 standard deviations (SD) and at the 97th (third) percentiles of observed values. Mean onset latency was 4.5 ms (SD, 0.8). Mean amplitude was 15.3 mV (SD, 4.5), 12.9 mV (SD, 4.5), and 9.8 mV (SD, 4.2) for the respective age groups of 19-29, 30-59, and 60-79 yr. Nerve conduction velocity ranged from a mean of 44 to 51 m/s depending on the demographic groups of various ages and heights. The upper limit of side-to-side latency variation was 1.4 ms (at the 97th percentile), and the upper limit of normal amplitude difference from side to side was 50% (at the 97th percentile). The other measures are presented in the article.

Reliability of nerve conduction studies among active workers

Nerve conduction studies play an important role in clinical practice and research. Given their widespread use, reliability of tests merits careful attention. We assessed interexaminer and intraexaminer reliability of median and ulnar sensory nerve measures of amplitude, onset latency, and peak latency. In a two-phase cross-sectional study, two examiners tested 158 workers. Reliability was assessed with intraclass correlations (ICC) and kappa statistics. Median nerve measures were more reliable (ICC range, 0.76 to 0.92) than ulnar measures (ICC range, 0.22 to 0.85). Ulnar-onset latencies had the worst reliability. The median-ulnar peak latency difference was a particularly stable measure (ICC range, 0.79 to 0.92). The median-ulnar peak latency difference had high interexaminer reliability (kappa range, 0.71 to 0.79) for normal tests defined by cut points of 0.8 ms and 0.5 ms. Intraexaminer reliability was higher with the 0.8-ms cut point (kappa = 0.90 and kappa = 0.85 for examiners 1 and 2, respectively). Rather than absolute cut points to describe normality, a more rational interpretation of results can be made with ordered categories or continuous measures.

A method to improve the estimation of conduction velocity distributions over a short segment of nerve

Accurate, noninvasive determination of the distribution of conduction velocities (DCV) among fibers of a peripheral nerve has the potential to improve both clinical diagnoses of pathology and longitudinal studies of the progress of disease or the efficacy of treatments. Current techniques rely on long distances of propagation to increase the amount of temporal dispersion in the compound signals and reduce the relative effect of errors in the forward model. The method described in this paper attempts to reduce errors in DCV estimation through transfer function normalization and, thereby, eliminate the need for long segments of nerve. Compound action potential (CAP) signals are recorded from several, equally spaced electrodes in an array spanning only a 10-cm length of nerve. Relative nerve-to-electrode transfer functions (NETF's) between the nerve and each of the array electrodes are estimated by comparing discrete Fourier transforms of the array signals. NETF's are normalized along the array so that waveform differences can be attributed to the effects of temporal dispersion between recordings, and more accurate DCV estimates can be calculated from the short nerve segment. The method is tested using simulated and real CAP data. DCV estimates are improved for simulated signals. The normalization procedure results in DCV's that qualitatively match those from the literature when used on actual CAP recordings.

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.

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.