AAEM minimonograph #25: single-fiber electromyography

Evaluation and proposal for optimalization of neurophysiological tests in migraine: part 1--electrophysiological tests

Neurophysiological testing has become a valuable tool for investigating brain excitability and nociceptive systems in headache disorders. Previous reviews have suggested that most neurophysiological tests have limited value for headache diagnosis, but a vast potential for exploring the pathophysiology of headaches, the central effects of certain pharmacological treatments and phenotype-genotype correlations. Many protocols, however, lack standardization. This meta-analytical review of neurophysiological methods in migraine was initiated by a task force within the EUROHEAD project (EU Strep LSHM-CT-2004-5044837-Workpackage 9). Most of the neurophysiological approaches that have been used in headache patients are reviewed, i.e. evoked potentials, nociception-specific blink reflex, single-fibre electromyography, neuroimaging methods (functional MRI, PET, and voxel-based morphometry) and the nitroglycerin attack-provoking test. For each of them, we summarize the results, analyse the methodological limitations and propose recommendations for improved methodology and standardization of research protocols. The first part is devoted to electrophysiological methods, the second to neuroimaging techniques and the NTG test.

Concentric needle single fiber electromyography: normative jitter values on voluntary activated Extensor Digitorum Communis

Single fiber electromyography (SFEMG) is the most sensitive clinical neurophysiological test for neuromuscular junction disorders, particularly myasthenia gravis. Normal values for jitter obtained with SFEMG electrode have been published, but there are few publications for concentric needle electrode (CNE). The aim of this study was to discuss the possibilities to analyse the jitter in CNE recordings and to get normal values of jitter for voluntary activated Extensor Digitorum Communis using disposable CNE. Fifty normal subjects were studied, 16 male and 34 female with a mean age of 37.1±10.3 years (19-55). The jitter values of action potentials pairs of isolated muscular fibers were expressed as the mean consecutive difference (MCD) after 20 analysed potential pairs. The mean MCD (n=50) obtained was 24.2±2.8 µs (range of mean values in each subject was 18-31). Upper 95% confidence limit is 29.8 µs. The mean jitter of all potential pairs (n=1000) obtained was 24.07±7.30 µs (range 9-57). A practical upper limit for individual data is set to 46 µs. The mean interpotential interval (MIPI) was 779±177 µs (range of individual mean values was 530-1412); there were no potentials with impulse blocking. The present study confirms that CNE is suitable for jitter analysis although certain precautions must be mentioned. Our findings of jitter values with CNE were similar to some other few reports in literature.

Ocular myasthenia: diagnosis, treatment, and pathogenesis

BACKGROUND

Although myasthenia gravis (MG) is often considered the best-understood autoimmune disorder and effective treatments have controlled life-threatening complications, the pathogenesis of ocular myasthenia (OM) remains enigmatic, and its clinical consequences offer therapeutic challenges.

REVIEW SUMMARY

About half of patients with MG present with visual complaints of droopy eyelids or double vision, and many will remain with purely ocular muscle weakness without generalized weakness, defined as OM. OM may be confused with disorders of the brainstem, ocular motor nerves, and eye muscles. Frustrating for the clinician, confirmatory tests such as the edrophonium test, serum acetylcholine receptor antibodies, and standard electrodiagnostic evaluations may fail to positively identify the clinical suspicion of OM. Patients may derive relief from nonpharmacologic interventions and cholinesterase inhibitors, but most will desire better symptom control with corticosteroids or need other immunosuppression. Early corticosteroid therapy may reduce the probability of generalization of the disease. The reasons for ocular muscle involvement by OM include physiologic and cellular properties of the ocular motor system and the unique immunology of OM, which, when better understood, will lead to novel treatments.

CONCLUSIONS

OM is a challenging disorder for the clinician and scientist, with both learning from the other for the betterment of the patient. The future requires answers to why the ocular muscles are so frequently involved by MG, whether the generalization of the disease may be limited by early corticosteroid treatment, and what treatment options may be developed which will improve symptoms without long-term complications.

Neurophysiological and mitochondrial abnormalities in MuSK antibody seropositive myasthenia gravis compared to other immunological subtypes

OBJECTIVE

To compare the electrophysiological and histopathological features of immunological myasthenia gravis (MG) subtypes.

METHODS

Fifty MG patients underwent clinical examination, MuSK-Ab and AChR-Ab analysis. The majority underwent quantitative and single-fiber electromyography (QEMG, SFEMG), repetitive nerve stimulation and deltoid muscle biopsy. From muscle specimens with histological mitochondrial dysfunction, we amplified mitochondrial DNA (mtDNA). In specimens with mtDNA deletions, the nuclear gene POLG1 was sequenced.

RESULTS

Five AChR-Ab seropositive [AChR(+)] and 5 seronegative [AChR(-)] patients were MuSK-Ab seropositive [MuSK(+)]. Five of 7 neurophysiologically examined MuSK(+) patients (71%) had proximal myopathic pattern, compared to 7 of 31 MuSK(-)/AChR(+) patients (23%) (P=0.012). SFEMG was abnormal in all examined MuSK(+) patients. All 7 biopsied MuSK(+) and 32 MuSK(-) patients (89%) had cytochrome c oxidase (COX) negative fibers. Three of five MuSK(+) and 13 of 20 MuSK(-) patients analyzed had multiple mtDNA deletions but no POLG1 mutations.

CONCLUSIONS

Similar degree of SFEMG abnormalities was present in proximal muscles among MuSK(+) and AChR(+) patients. Proximal myopathy was over-represented in MuSK(+) patients; however, both MuSK(+) and MuSK(-) patients had mild myopathy with frequent mitochondrial abnormalities.

SIGNIFICANCE

The weakness in MuSK(+) patients is most likely due to disturbed neuromuscular transmission. The frequently encountered mitochondrial dysfunction in MG warrants further study.

Single-fiber EMG with a concentric needle electrode: validation in myasthenia gravis

We performed a retrospective study to validate whether a disposable concentric needle electrode (CNE) can be used in place of a single-fiber (SF) electrode for jitter measurements in myasthenia gravis (MG). Normal values for voluntary contraction of orbicularis oculi (OO) and extensor digitorum communis (EDC) were collected from 20 healthy subjects. The method was validated by a retrospective analysis of 56 consecutive MG patients, the "gold standard" being a positive acetylcholine receptor (AChR) antibody titer at the time of the electrophysiological (electromyography) study and the clinical diagnosis. Receiver operating characteristic (ROC) curves were constructed to define maximal sensitivity and specificity of the technique. The sensitivity was 96.4% (95% confidence interval 87.5%-99.6%), with no false-positive results, similar to traditional SF EMG and confirming that the disposable CNE is a justifiable alternative.

Seronegative myasthenia gravis: comparison of neurophysiological picture in MuSK+ and MuSK- patients

The aim of this study was to compare the neurophysiological and clinical pictures of a large sample of seronegative myasthenia gravis (SNMG) patients with and without anti-MuSK antibodies. Fifty-two consecutive SNMG patients were retrospectively evaluated. They had undergone an extended neurophysiological evaluation: repetitive nerve stimulation (RNS), single fiber EMG (SFEMG), and electromyography (EMG) with nerve conduction study. A muscle biopsy was performed in 11 of 52 patients, the edrophonium test in 44 of 52 patients and anti-AChR antibodies and anti-MuSK antibodies were tested in all patients. Anti-MuSK antibodies were detected in 25 SNMG patients (48.1%). The number of women in the MuSK+ group was significantly higher (P = 0.01) than in the MuSK- group. Seronegative MuSK+ patients are more severely affected and the deficit often involves the bulbar and the respiratory muscles. No statistically significant differences were observed in the edrophonium test between MuSK+ and MuSK- groups. The RNS test was abnormal in a significantly higher number of MUSK- patients than MUSK+ patients (P < 0.00001). With regard to SFEMG data, MuSK- patients were characterized to have more severe neurophysiological pattern. Our observations showed several differences between the clinical and neurophysiological pictures of MUSK+ and MUSK- patients.

Clinical electrophysiological characterization of the acquired neuromyotonia phenotype of autoimmune peripheral nerve hyperexcitability

Acquired autoimmune neuromyotonia is regarded as part of the spectrum of peripheral nerve hyperexcitability disorders. We aimed to use clinical neurophysiological measurements to study the extent, distribution, and characteristics of spontaneous motor unit potentials in 11 patients with acquired neuromyotonia. Investigations revealed that most spontaneous discharges recorded were motor unit, or partial motor unit potentials of normal size. Bursts of motor unit potentials arose more commonly from distal portions of the peripheral nerve and had abnormal absolute and relative refractory periods. Spontaneous discharges in some patients occurred in semirhythmic bursts in certain muscles. No patient had neurophysiological abnormalities detectable in first-order neurons of the central nervous system when using transcranial magnetic stimulation to estimate the threshold for corticomotor excitation and determine central motor conduction time. Only patients with coexistent myasthenia gravis had neurophysiologically detectable defects in neuromuscular transmission. The pathogenic region of abnormality in peripheral nerve hyperexcitability disorders therefore seems to lie within the terminal branches of peripheral motor nerves.

Correlation between a patient-derived functional questionnaire and abnormal neuromuscular transmission in Myasthenia Gravis patients

OBJECTIVE

To correlate the patient-derived physical function score, as measured by the disease specific Myasthenia Gravis Questionnaire (MGQ), and the score obtained with commonly used generic 36-item questionnaire, the Short-Form health survey (SF-36), with the degree of abnormal neuromuscular transmission measured by Single-Fibre EMG (SFEMG) and repetitive nerve stimulation (RNS) in Myasthenia Gravis (MG) patients.

METHODS

SFEMG and RNS were performed in the deltoid muscle and SFEMG was also performed in the orbicularis oculi muscle in 45 MG patients. The patients were asked to fill out the MGQ, which has been translated and validated into Swedish, and the SF-36. The sum of MGQ items generates a global MGQ score. Items are also divided into 3 muscle specific domains: generalized, bulbar and ocular.

RESULTS

The global MGQ score and generalized domain score of MGQ were strongly correlated with the degree of abnormal neuromuscular transmission, as measured by SFEMG in the deltoid muscle. Scores from ocular and bulbar domains of MGQ were not related to neurophysiological findings. Physical composite scores of SF-36 correlated significantly with the abnormal SFEMG findings in the deltoid.

CONCLUSIONS

The degree of disturbed neuromuscular transmission in a proximal limb muscle correlates with a patient's subjective experience of generalized myasthenic dysfunction.

SIGNIFICANCE

The observed correlation adds a new dimension to neurophysiological examinations in patients with MG.

Advances in the diagnosis of neuromuscular junction disorders

Disorders of the neuromuscular junction have a wide range of clinical presentations, which frequently poses a diagnostic challenge to evaluating clinicians. This article describes the tests used in the diagnosis of diseases of the neuromuscular junction, reviews the evidence supporting the use of each test, and proposes guidelines for their efficient utilization. A focused review of the literature was employed. Reports from four main categories of diagnostic tests (pharmacologic, electrodiagnostic, immunologic, and miscellaneous tests) were reviewed, and the sensitivity and specificity of each test in the diagnosis of specific neuromuscular junction diseases were examined. The clinical presentation determines which diagnostic tests should be utilized in individual cases of suspected neuromuscular junction disease. However, knowledge of the sensitivity and specificity of each test can help to focus the diagnostic evaluation and maximize the diagnostic yield of each test.

Physiologically based simulation of clinical EMG signals

An algorithm that generates electromyographic (EMG) signals consistent with those acquired in a clinical setting is described. Signals are generated using a model constructed to closely resemble the physiology and morphology of skeletal muscle, combined with line source models of commonly used needle electrodes positioned in a way consistent with clinical studies. The validity of the simulation routines is demonstrated by comparing values of statistics calculated from simulated signals with those from clinical EMG studies of normal subjects. The simulated EMG signals may be used to explore the relationships between muscle structure and activation and clinically acquired EMG signals. The effects of motor unit (MU) morphology, activation, and neuromuscular junction activity on acquired signals can be analyzed at the fiber, MU and muscle level. Relationships between quantitative features of EMG signals and muscle structure and activation are discussed.

Neuromuscular rehabilitation and electrodiagnosis. 3. Diseases of muscles and neuromuscular junction

This self-directed learning module highlights formation of a differential diagnosis as well as electrodiagnostic evaluation for those patients who present with the common complaint of weakness. It is part of the chapter on neuromuscular rehabilitation and electrodiagnosis in the Self-Directed Physiatric Education Program for practitioners and trainees in physical medicine and rehabilitation. This article specifically focuses on the common symptoms and typical clinical findings that allow the clinician to narrow the differential diagnosis. This is followed by the diagnostic evaluation, with emphasis on the technical aspects and interpretation of electrodiagnostic studies.

OVERALL ARTICLE OBJECTIVE

To summarize the clinical presentation and electrodiagnostic findings in persons with disorders of muscle or disorders of the neuromuscular junction.

Repetitive nerve stimulation in myasthenia gravis--relative sensitivity of different muscles

OBJECTIVE

To correlate repetitive nerve stimulation (RNS) decrement in different muscles with the predominant clinical presentation in myasthenia gravis (MG), and to study single fibre EMG (SFEMG) sensitivity in ocular MG.

METHODS

Sixty-nine, untreated, consecutive patients suspected for MG were observed prospectively for a minimum of 6 months. Those who improved on medical treatment were diagnosed as MG. The others, in whom the neurophysiological studies were normal and that did not improve on medical treatment served as a control group, from which normative data for RNS and SFEMG was obtained. The MG patients were further classified in 3 subgroups according to the predominant clinical presentation: group I (ocular); group b (bulbar); and group a (axial). We performed RNS in nasalis, trapezius, anconeus, and abductor digiti minimi. All patients with ocular MG underwent jitter determination of the orbicularis oculi muscle.

RESULTS

Thirty-seven patients were diagnosed as MG (group I, 15; group b, 13; group a, 9). In group I, RNS was abnormal in 33% of the patients. RNS studies disclosed at least one abnormal muscle response in every patient in groups a and b. Trapezius was significantly more sensitive in group a, and anconeus and nasalis in group b (P < 0.01). Jitter was abnormal in all patients in group I, and the most sensitive parameter was an increased number of unstable pairs, 100%.

CONCLUSIONS

Based on these observations, we recommend that a shoulder muscle, as the trapezius, should be studied first in the limb-axial presentation of MG, and the anconeus-nasalis muscles in predominant bulbar MG. In ocular MG, RNS is not sensitive and jitter should be performed in facial muscles.

SIGNIFICANCE

This paper shows the unequal sensitivity of several muscles to RNS in different forms of MG.

Electrodiagnostic studies in the management and prognosis of neuromuscular disorders

Prognosis remains a neglected aspect of modern medical care and research, behind diagnosis and treatment. The very term "electrodiagnosis" implies as much. Despite this, much has been published regarding the use and benefit of electrodiagnostic techniques in assessing prognosis and assisting in management of patients after the diagnosis has been established. This information is often hidden or otherwise not emphasized. This review summarizes the literature regarding the use of such techniques for prognosis and management of disorders of lower motor neurons, peripheral nerves, neuromuscular transmission, and muscle.

Single Fiber EMG as an Outcome Measure in Myasthenia Gravis: Results from a Double-Blind, Placebo-Controlled Trial

In a placebo-controlled, therapeutic, pilot trial of mycophenolate mofetil (MM) in autoimmune myasthenia gravis (MG), the authors compared pretreatment and posttreatment single fiber electromyography (SFEMG) jitter measurements performed on the same muscle in a total of 11 patients. The mean jitter value decreased (improved) by an average of 15.4 micros in patients receiving MM (n = 6), compared to an increase (worsening) in mean jitter of 4.0 micros in patients receiving placebo (n = 5). This difference was statistically significant (P = 0.030). In most patients, the change in SFEMG measurements correlated with the change in clinical state as measured by quantitative testing of muscle function. The authors conclude that immunomodulation by MM improves neuromuscular junction function in MG and that SFEMG may be a useful marker of early response in future therapeutic trials in autoimmune MG.

Evaluation of jitter by stimulated single-fiber electromyography in normal dogs

Single-fiber electromyography (SFEMG), a technique used to investigate neuromuscular transmission, has been described previously in the pelvic limb of dogs. Because preferential involvement of isolated muscle groups can occur in disorders of neuromuscular transmission, SFEMG was done in the peroneus longus (PL), extensor carpi radialis (ECR), and orbicularis oculi (OO) muscles of 10 adult, clinically normal dogs. Jitter was calculated as the mean absolute value of the consecutive differences in latency of 50 single muscle fiber action potentials after stimulation of intramuscular nerve bundles at the level of the motor point in at least 20 muscle fibers per muscle. Bilateral recordings were performed in 3 dogs. Mean jitter values were determined for each muscle, and differences among muscle groups and among dogs were compared. The upper limits of mean consecutive difference (mean plus 3 standard deviations) for the PL, ECR, and OO muscles were 21.94, 22.53, and 23.39 micros, respectively, and the upper limit of mean consecutive difference for individual muscle fibers in the respective fiber pools was 28.62, 36.39, and 35.68 micros. Jitter values for the ECR and OO were significantly higher than the jitter value for the PL muscle (P < .05). Significant differences among muscles or dogs or between sides were not observed for the ECR. Significant differences among dogs were observed for OO jitter values and were attributed to extremely low jitter values in 1 dog. Significant differences were demonstrated between sides for the PL and were attributed to small sample size. Results of this study provide normative data that can be used in the application of the stimulated SFEMG technique to dogs with suspected disorders of neuromuscular transmission.

Single-fiber electromyography

Single-fiber EMG is a technique introduced in 1963 by Stålberg and Ekstedt for recording single muscle fiber action potentials by means of a specially constructed needle with a 25-microm recording surface. The needle is positioned in the muscle to record from two or more time-locked potentials belonging to the same motor unit. Jitter is the variability in the arrival time of action potentials to the recording electrode between consecutive discharges. This variability reflects end-plate conduction and is measured along with fiber density, which is the average number of fibers belonging to the same motor unit that is in the recording area. An abnormal test is one in which more than 10%, or the mean, of 20 fiber pairs has increased jitter when compared with normal reference values. Increased fiber density is seen with reinnervation. Single-fiber EMG is more sensitive than conventional EMG and is the most sensitive, but not specific, test for myasthenia gravis. Lambert-Eaton myasthenic syndrome, and other neuromusculasr junction pathology. It has been useful in the evaluation of some neuropathies and myopathies and has provided valuable information on the motor unit spatial arrangement, territory, microphysiology, and pathophysiology.

Electrodiagnostic approach to patients with weakness

Electrodiagnosis has a key role in the evaluation of patients presenting with weakness. The electrodiagnostician should maintain a broad inclusive differential diagnosis and tailor the examination using a sound conceptual framework. A clear understanding of what is normal provides the proper foundation upon which to judge electrodiagnostic findings. Many peripheral neuromuscular conditions manifest themselves in characteristic ways on NEE and nerve conduction testing, making them identifiable to the skilled electrodiagnostic medicine consultant.

20. Immunologic neuromuscular disorders

Immune-mediated disorders of each of the structural subdivisions of the nervous and neuromuscular system have been described. Despite the immune privilege of the central nervous system, and to a lesser extent the peripheral nervous system, immune dysregulation is not uncommon. Environmental, genetic, and immunologic factors have been postulated to be involved in the development of these disorders. Major immune-mediated neurologic diseases of the central nervous system include multiple sclerosis and acute disseminated encephalomyelitis. Immune-mediated diseases of the peripheral nervous system include myasthenia gravis, Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy, idiopathic polymyositis, dermatomyositis, and inclusion body myositis. Some of these disorders, such as myasthenia gravis and certain forms of acute inflammatory demyelinating polyneuropathy, are clearly autoimmune in nature, whereas the immune system plays an important role in pathogenesis in others. Understanding the immune mechanisms of disease and uncovering potential therapeutic targets are essential for the design of new treatments. The epidemiology, pathogenesis, diagnostic criteria, and current therapeutic approaches to the major neuroimmunologic diseases are reviewed.

Literature review of the usefulness of repetitive nerve stimulation and single fiber EMG in the electrodiagnostic evaluation of patients with suspected myasthenia gravis or Lambert-Eaton myasthenic syndrome

A retrospective literature review of the electrodiagnosis of myasthenia gravis (MG) and Lambert--Eaton myasthenic syndrome (LEMS) through July 1998 was performed for the purpose of generating evidence-based practice parameters. There were 545 articles identified, of which 13 articles met at least three of the six criteria set previously by the American Association of Electrodiagnostic Medicine (AAEM). An additional 21 articles were identified from review articles or the references of these first 13 articles leading to a total of 34 articles. Results of studies utilizing repetitive nerve stimulation (RNS) showed that a 10% decrement in amplitude from the first to fourth or fifth intravolley waveform while stimulating at 2--5 HZ is valid for the diagnosis of MG. The degree of increment needed for the diagnosis of LEMS is at least 25% but most accurate when greater than 100%. Abnormal jitter or impulse blocking are the appropriate criteria for diagnosis of neuromuscular junction (NMJ) disorders when using single fiber electromyography (SFEMG). SFEMG is more sensitive than RNS for the diagnosis of disorders of neuromuscular transmission, but may be less specific and may not be available. Therefore, RNS remains the preferred initial test for MG and LEMS.

EMG signal decomposition: how can it be accomplished and used?

Electromyographic (EMG) signals are composed of the superposition of the activity of individual motor units. Techniques exist for the decomposition of an EMG signal into its constituent components. Following is a review and explanation of the techniques that have been used to decompose EMG signals. Before describing the decomposition techniques, the fundamental composition of EMG signals is explained and after, potential sources of information from and various uses of decomposed EMG signals are described.

What have we learned about cognition in myasthenia gravis?: a review of methods and results

Most individuals with myasthenia gravis (MG) complain of cognitive impairment, but empirical studies of cognition in MG have produced mixed results. In the present review, we critically examined the methodology and results of previous studies that investigated cognition in MG. Results from our review revealed that none of the studies met at least 50% of criteria under review. The most common shortcomings of previous studies included small sample size, no exclusion for visual difficulties in patients, inadequate assessment of mood, and poor control for prednisone use. Despite these methodological difficulties, mild impairments on measures of learning have been identified. These findings need to be replicated with adequate control of potential confounds before any conclusions can be made regarding cognition in this disease. Suggestions for design of future studies are provided.

Electrodiagnosis of polyneuropathy

Electrodiagnostic studies comprising electromyography (EMG) and nerve conduction studies (NCS) are well-established objective methods for the diagnosis, quantification and classification of polyneuropathies (PNP). This paper reviews examination techniques, their pathophysiological interpretation, examination strategies and diagnostic criteria for the diagnosis and classification of a PNP. The routine electrodiagnostic evaluation includes sensory NCSs performed with surface or needle electrodes, motor NCSs, F-wave studies and EMG by qualitative or quantitative techniques. Sensory NCSs and F-wave studies have a high sensitivity in PNPs and the different techniques complement each other. The distinction between a PNP with predominantly axonal loss and a PNP with predominantly demyelination is one of the major aims of the electrophysiological examination. There are, however, large variation in suggested criteria for predominantly demyelination. The degree of slowing in conduction taken to indicate demyelination varies between a decrease of 50 to 30% from mean of controls, distal latency prolongation criteria vary from 35% to 70% of mean of controls, F-wave latency prolongation criteria vary from 120% to 150% of upper limit of controls, and criteria for partial motor conduction block vary from 11 to 50% reduction of CMAP amplitude and/or area between proximal and distal stimulation. Needle EMG studies may be valuable in order to detect and quantify denervation activity, to assess chronicity by an evaluation of the extent of reinnervation, and to evaluate the topographical distribution of changes. It is concluded that electrodiagnostic studies are valuable in patients with suspected PNP and the results may have consequences for prognosis and therapy of individual patients. Large variation in examination techniques, strategies, interpretations and diagnostic criteria have been found among electromyographers and it is suggested that the value of electrodiagnostic studies may be further improved by international standardisation.

SFEMG in ocular myasthenia gravis diagnosis

BACKGROUND AND OBJECTIVE

In typical cases, the patient's history and clinical examination make it possible to diagnose ocular myasthenia gravis (OMG). But, in many cases a clear clinical picture is not present and OMG diagnosis is very difficult because gold diagnostic standard tests are not available. The diagnostic tests for OMG are usually unable to display a good sensitivity and specificity simultaneously. In this paper, we studied 86 cases submitted for suspected OMG.

METHODS

The patients were studied clinically and with various other tests used in OMG diagnosis (SFEMG, repetitive nerve stimulation, Ab anti AChR titration, tensilon test).

RESULTS AND CONCLUSION

SFEMG showed the highest sensitivity (100%) while Ab anti AChR showed the highest specificity (100%). To our knowledge this is the largest population of suspected OMG studied using most of the diagnostic parameters, reported in the literature.

Impairment of neuromuscular transmission in a subgroup of migraine patients

Neuronal voltage-dependent P/Q Ca2+ channels are genetically abnormal in many cases of familial hemiplegic migraine and possibly associated with the more common forms of migraine with and without aura. Besides the brain, these channels are found in motor nerve endings where they control stimulation-induced acetylcholine release. Using single fiber EMG recordings we were able to demonstrate subclinical abnormalities of neuromuscular transmission in a subgroup of patients suffering from migraine with aura. This could be related to genetic abnormalities of P/Q Ca2+ channels in certain patients suffering from migraine with aura, which needs to be explored by proper genetic analyses.

Stimulation single-fiber EMG in infant botulism

The principal electrodiagnostic feature of infant botulism, an incremental response on high rates of repetitive nerve stimulation, has variable sensitivity and may not always be useful as a diagnostic test given the vagaries of test timing and severity of illness. We report the use of stimulation single fiber EMG (S-SFEMG) in making this clinical diagnosis. Four infants between 1 and 5 months of age presented with rapidly progressive bulbar and limb weakness, internal and external ophthalmoplegia, areflexia, and compromised ventilation. Incremental response with high-rate repetitive nerve stimulation and a typical clinical course for infant botulism confirmed the diagnosis in all; stool toxin studies were positive for type B botulinum in 2 of the 3 cases in which they were obtained. S-SFEMG was performed by surface stimulation of median and ulnar nerves and recording with a single fiber needle in the thenar, hypothenar, or first dorsal interosseous muscles. A total of eight single fiber recordings were studied at rates of 2, 5, 10, and 20 Hz. All single fibers studies showed an improvement with higher rates of stimulation, beginning at 10 Hz and peaking at 20 Hz. Compared to baseline study at 2 Hz (100%), the mean percent changes in jitter at 5, 10, and 20 Hz were 109, 60, and 47, respectively. This is the first report of the usefulness of S-SFEMG in the diagnosis of infant botulism.

Neurophysiological assessment in the diagnosis of botulism: usefulness of single-fiber EMG

We report the clinical, serological, and neurophysiological findings in seven patients with foodborne botulism caused by ingestion of black olives in water. The clinical picture was characterized by mild symptoms with a long latency of onset and by involvement of cranial and upper limb muscles; only one patient, a child, developed respiratory failure. Spores of Clostridium botulinum were found in stools in some but not all cases. Conventional neurophysiological tests had low sensitivity; abnormal findings were present only in the patient with severe clinical involvement, in whom compound muscle action potentials (CMAPs) appeared reduced. Repetitive nerve stimulation at a high rate showed pseudofacilitation and not true posttetanic facilitation, but single-fiber electromyography (SFEMG) showed abnormalities of neuromuscular transmission in every case. Neurophysiological evaluation, particularly SFEMG, is important because it allows rapid identification of abnormal neuromuscular transmission while bioassay studies are in progress.

Satellite potentials of motor unit action potentials in normal muscles: a new hypothesis for their origin

OBJECTIVE

A satellite potential is a late component of the motor unit action potential (MUAP) that occurs both in pathologic and normal muscle. We investigated the physiological mechanisms responsible for satellite potentials in normal muscle by relating the latencies of MUAP features to the timing of the underlying electrical events.

METHODS

We analyzed 21 MUAPs with satellite potentials that had been recorded using a monopolar needle electrode from brachial biceps and tibialis anterior muscles in 10 normal subjects. We estimated the endplate-to-electrode, endplate-to-tendon, and satellite propagation times from the latencies, with respect to the MUAP onset, of the MUAP spike, terminal wave, and satellite.

RESULTS

Satellite latencies ranged from 8.8 to 32 ms, too long to be explained by mechanisms involving regenerating axons or atrophic muscle fibers. The spike-to-satellite time intervals approximated either twice the spike-to-terminal-wave interval (17 MUAPs) or twice the terminal-wave latency (4 MUAPs).

CONCLUSIONS

These results are consistent with the hypothesis that satellite potentials are due to retrograde propagation in a non-innervated muscle fiber that is connected with an innervated muscle fiber at one of the muscle/tendon junctions. Such a configuration could arise as a result of longitudinal muscle-fiber splitting.

Decomposition and quantitative analysis of clinical electromyographic signals

Procedures for the quantitative analysis of clinical electromyographic (EMG) signals detected simultaneously using selective or micro and non-selective or macro electrodes are presented. The procedures first involve the decomposition of the micro signals and then the quantitative analysis of the resulting motor unit action potential trains (MUAPTs) in conjunction with the associated macro signal. The decomposition procedures consist of a series of algorithms that are successively and iteratively applied to resolve a composite micro EMG signal into its constituent MUAPTs. The algorithms involve the detection of motor unit action potentials (MUAPs), MUAP clustering and supervised classification and they use shape and firing pattern information along with data dependent assignment criteria to obtain robust performance across a variety of EMG signals. The accuracy, extent and speed with which a set of 10 representative 20-30 s, concentric needle detected, micro signals could be decomposed are reported and discussed. The decomposition algorithms had a maximum and average error rate of 2.5% and 0.7%, respectively, on average assigned 88.7% of the detected MUAPs and took between 4 to 8 s. Quantitative analysis techniques involving average micro and macro MUAP shapes, the variability of micro MUAPs shapes and motor unit firing patterns are described and results obtained from analysis of the data set used to evaluate the decomposition algorithms are summarized and discussed.

Serial SFEMG studies of orbicularis oculi muscle after the first administration of botulinum toxin

Serial single fiber electromyography (SFEMG) examinations of orbicularis oculi muscle in patients with blepharospasm or hemifacial spasm treated with botulinum toxin injections were performed. The aim of the study was to evaluate the impairment of neuromuscular transmission, to follow reinnervation after botulinum toxin administration and to find out whether there was a relationship between SFEMG parameters and clinical symptoms. Examinations were performed before injection, during early and late remission of symptoms, and after recurrence of the involuntary movement. Severe impairment of neuromuscular transmission, as revealed by increased jitter and increased presence of abnormal potential pairs and pairs with blocking, was found in early remission, but fiber density remained unchanged when compared with pretreatment values. In late remission, increased fiber density was registered for the first time. The recurrence of involuntary movements was related to the further increase of fiber density and tendency to normalization of jitter parameters. The study therefore suggests that formation of new neuromuscular junctions and their functional maturation is responsible for muscle recovery after botulinum toxin administration.

Detecting single fiber contributions to motor unit action potentials

The ability to detect muscle fiber action potential (MFAP) contributions to motor unit action potentials (MUAPs) measured using single fiber (SF) and concentric needle (CN) electrodes was studied using simulated MUAPs. Various MFAP-acceleration thresholds were used to define significant fiber contributions. Attempts to detect the significant MFAP contributions, by locating peaks in filtered MUAPs or MUAP accelerations using various MUAP-based thresholds, were then made. Considering filtered MUAPs and a significant contribution threshold of 7.5 kV/s2, and using fiber-density peak-detection criteria, at best 46% and 50% of significant MFAP contributions were detected for the SF and CN MUAPs, respectively. Considering MUAP accelerations and a significant contribution threshold of 7.5 kV/s2, 80% and 84% of significant MFAP contributions could be detected, respectively. Most significant contributions were created from fibers located within approximately 350 microm of the electrode. The results suggest that significant peaks, defined using MUAP-based thresholds, within the acceleration of CN MUAPs can strongly correspond to individual fiber activity and may be useful for measuring fiber density and neuromuscular jitter.

Single fibre electromyography in multifocal motor neuropathy with persistent conduction blocks

OBJECTIVE

To study the process of denervation-reinnervation in multifocal motor neuropathy with persistent conduction blocks in clinically affected and unaffected muscles.

METHOD

Volitional single fibre electromyography (SFEMG) was performed in the extensor digitorum communis (EDC) of seven patients. The jitter, the fibre density, and the mean interpotential interval were determined. The results before and after treatment with intravenous immunoglobulin (IVIg) between the unaffected EDC and affected EDC examined during the same SFEMG session were also compared. In addition the values of jitter, fibre density, and mean interpotential interval were analysed for correlation with the strength score on the MRC scale, the duration of the neuropathy, the number of IVIg treatment periods, and the radial nerve conduction block values.

RESULTS

Mean jitter, percentage of jitters >60 micros, and impulse blocking percentage, were higher than normal in both the affected EDCs and to a lesser degree in unaffected EDCs. Jitter decreased significantly after IVIg and correlated only with the MRC score. Fibre density and mean interpotential interval were higher than normal equally in the affected EDC and unaffected EDCs, but no correlation was found with strength, duration of the neuropathy, number of treatment periods, and conduction block values.

CONCLUSION

The major finding is the presence of SFEMG abnormalities in clinically unaffected EDCs. This shows a process of denervation-reinnervation even in the absence of clinical symptoms, probably more frequent than commonly supposed in this neuropathy. The rapid clinical improvement after IVIg infusions could be due to remyelination after demyelination and to an interference of IVIg with the blocking effect of antibodies on the Na+ channels at the motor nerve endings.

Motor end-plate jitter in myasthenia gravis at different firing rates

Twenty patients with generalized myasthenia gravis (MG) were studied to assess firing rate-dependent changes in end-plate jitter. Stimulated single-fiber electromyography (EMG) was performed in the extensor digitorum communis muscles at stimulation frequencies of 1, 2, 5, and 10 Hz for all the sampled fibers and at 20 Hz for a few ones. In all 137 end-plates were obtained, of which 43 showed blocking. Statistical analysis demonstrated significant increases in jitter and the blocking rate from 1 to 2 and 2 to 5 Hz, whereas there were decreases from 5 to 10 and 10 to 20 Hz. Further analysis showed that the decrement in jitter from 5 to 10 Hz was greater in end-plates with preexisting low safety factors. In contrast, even though the safety factor of long-duration MG end-plates was lower than that of the short-duration group, the decrement in jitter was insignificant in the long-duration MG end-plates. These results suggest that the intratetanic facilitation effect begins at 10 Hz and is more prominent in end-plates with a preexisting low safety factor. Long-lasting MG, however, eliminates this effect.

Characterization of the relationship between motor end-plate jitter and the safety factor

A mathematical model developed for a personal computer was used to simulate the jitter phenomenon in neuromuscular transmission in order to characterize the relationship between jitter and the safety factor. Four models of normal and abnormal neuromuscular transmission were investigated. In a human experiment, the mean consecutive difference (MCD) values for stimulated single-fiber electromyography at firing rates of 1, 2, 5, and 10 Hz were measured in 137 motor end-plates of myasthenic patients. Results of the computer simulations show that the relationship between jitter and the safety factor is exponential like. Variations in jitter are most prominent in end-plates with low safety factors. This relationship agrees with results of human end-plate studies. Changes in the MCD values caused by presynaptic depression or facilitation are linearly correlated to the initial jitter, whereas the logarithmic values of MCD are not. It is very important to keep in mind this nonlinear relationship when relating single-fiber jitter to the safety factor.

Stimulated single-fiber electromyography in the rat

We constructed an animal model of stimulated single-fiber electromyography (SFEMG) by testing Wistar rats under anesthesia. Stimuli of 1 Hz were applied to the sciatic nerve through an insulated monopolar needle electrode. Single-fiber action potentials were acquired from the gastrocnemius muscle. Jitter was assessed by the mean consecutive difference (MCD). Eighty-seven fibers were obtained from 12 rats. Their MCDs ranged from 2 to 72 micros (17.7+/-13.4). Seven of these values were less than or equal to 5 micros, and three exceeded 50 micros. Neuromuscular blocking agents injected into some of the rats induced considerable increases in jitter and blocking. A rat with one fiber with an MCD less than 5 micros also received an injection of curare. The jitter showed the same pattern of increment, evidence that the small jitter was not attributable to direct muscle stimulation. These results show that SFEMG can be used on rats. In addition, jitter reflects the changes in motor end-plate function. The findings also suggest the presence of an extremely high safety factor in rat neuromuscular junctions.

[Importance of neuromuscular "jitter" under stimulation in the diagnosis of myasthenia gravis]

Ninety five patients with global muscular weakness or purely extraocular weakness were included in a retrospective study. Electrical micro stimulation and single fiber electromyography were performed in all, for neuromuscular jitter evaluation in myasthenia gravis diagnosis. In our study, increased jitter was more often present (70% of generalised myasthenia gravis and 57% of ocular myasthenia gravis) than decrement after repetitive nerve stimulations (58% of generalised myasthenia gravis and 14% of ocular myasthenia gravis). Increased jitter was also found in non-myasthenic patients. With the aim of a better sensitivity and specificity of the electrophysiological diagnosis for myasthenia gravis a protocol is described.

The role of electromyography in neurology

A review is given of the role of electromyography (EMG) for diagnosis, pathophysiological description and monitoring of patients with disorders of the peripheral nervous system. Various EMG methods are presented and their principal differences are discussed. The usefulness of these methods varies depending on the pathology to be studied. With modern quantitative methods for analysis, EMG has become more sensitive and accurate and is therefore an important part in the evaluation of the neurologic patient. EMG results are usually combined with findings from other neurophysiological investigations (neurography, evoked potentials), histochemistry, biochemistry and most importantly with the clinical signs to give as complete a picture of the condition as possible. The usefulness of EMG depends on a number of factors other than the quality of the investigation as such. These aspects are discussed briefly.

Acquired myasthenia gravis

Myasthenia gravis, an antibody-mediated disorder of neuromuscular transmission that produces clinical weakness, may be ocular or generalized. Clinical diagnostic evaluation may be supplemented by electrophysiologic studies and antibody testing. Therapeutic options, including anticholinesterase inhibitors, immunosuppressive agents, plasmapheresis and thymectomy, are tailored for the individual patient. This article emphasizes the key aspects of the clinical evaluation, diagnosis, and therapy.

Myasthenia gravis

Adult-onset myasthenia gravis is an acquired autoimmune disorder of neuromuscular transmission in which acetylcholine receptor antibodies attack the postsynaptic membrane of the neuromuscular junction. Although the cause of this disease is unknown, the role of immune responses in its pathogenesis is well established. Circulating acetylcholine receptor antibodies are present in 80% to 90% of patients with the generalized form of myasthenia gravis. Most patients have ptosis, diplopia, dysarthria and dysphagia. The weakness and fatigue worsen on exertion and improve with rest. Respiratory muscle and limb weakness are rare at the onset of the disease. For the past two decades, there has been considerable progress in understanding the diagnosis and management of myasthenia gravis. The diagnosis is based on clinical presentation, neurologic examination, and confirmation by means of electrophysiologic testing and immunologic studies. Myasthenia gravis mimics many neuromuscular diseases and even illnesses such as depression and chronic fatigue syndrome. One should always exclude drug-induced myasthenia gravis for all patients. With the introduction of new modalities of treatment, particularly immunosuppressive or immunomodulating drugs, plasma exchange and thymectomy, the morbidity and mortality of myasthenia gravis have decreased dramatically to the point that myasthenia gravis should not be considered as serious a disease as it once was. Although the several therapeutic options are usually effective and have meant independence in daily life to many patients with myasthenia gravis, well-designed, controlled, prospective studies are still lacking.