Single-muscle-fiber recording of the jitter phenomenon in patients with myasthenia gravis and in members of their families

Reference Jitter Values for Concentric Needle Electrode of Orbicularis Oculi and Frontalis Muscles Using Voluntary Activation Method in Sudanese Population

Single fibre electromyography is the most sensitive neurophysiological test for the diagnosis of neuromuscular junction disorders, particularly myasthenia gravis. The study aimed at establishing concentric needle (CN) normal jitter values for voluntarily activated orbicularis-oculi (V-OOc) & Frontalis (V-FRO) muscles in Sudanese population. 57 healthy volunteers (20 males & 37 females) were included in the study (mean Age 43.6 ± 14.2 years, range 18–70 years). V-OOc and V-FRO were tested in the same individual using CN. Jitter values were expressed as the mean consecutive difference (MCD) of 30 potential pairs in microseconds. The mean jitter, mean individual fibre pairs jitter & mean outliers jitter values with (upper 95% Confidence Limit-CL) for [OOc] were [26.9 ± 3.3 (31.97), 26.1 ± 8.9 (41.8) & 38.5 ± 5.7 (49.0) µs] & for [FRO] were [27.1 ± 3.0 (31.32), 26.4 ± 9.4 (42.9) & 39.9 ± 5 (49.2) µs] respectively. The suggested practical upper limits for mean jitter & for outliers were (32, 49 µs) for OOc & (31, 49 µs) for FRO. Our CN-jitter values were within the range of the few published studies. The study was unique in that it established and compared between CN reference jitter values of two voluntarily activated facial muscles (V-OOc & V-FRO) in the same individual in large number of healthy subjects.

Myasthenia gravis: State of the art and new therapeutic strategies

Myasthenia Gravis (MG) - an autoimmune neuromuscular disease - is known by the production of autoantibodies against components of the neuromuscular junction mainly to the acetylcholine receptor, which cause the destruction and compromises the synaptic transmission. This disease is characterized by fluctuating and fatigable muscle weakness, becoming more intensive with activity, but with an improvement under resting. There are many therapeutic strategies used to alleviate MG symptoms, either by improving the transmission of the nerve impulse or by ameliorating autoimmune reactions with e.g. steroids, immunosuppressant drugs, or monoclonal antibodies (rituximab and eculizumab). Many breakthroughs in the discovery of new therapeutic targets have been reported, but MG remains to be a chronic disease where the symptoms are kept in the majority of patients. In this review, we discuss the different therapeutic strategies that have been used over the years to alleviate MG symptoms, as well as innovative therapeutic approaches currently under study.

Guidelines for single fiber EMG

This document is the consensus of international experts on the current status of Single Fiber EMG (SFEMG) and the measurement of neuromuscular jitter with concentric needle electrodes (CNE - CN-jitter). The panel of authors was chosen based on their particular interests and previous publications within a specific area of SFEMG or CN-jitter. Each member of the panel was asked to submit a section on their particular area of interest and these submissions were circulated among the panel members for edits and comments. This process continued until a consensus was reached. Donald Sanders and Erik Stålberg then edited the final document.

Single-fiber electromyography in the orbicularis oculi muscle in patients with ocular myasthenia gravis symptoms: does abnormal jitter predict response to treatment?

Background

Seronegative ocular myasthenia gravis (OMG) is diagnosed by ocular symptoms with supporting SFEMG, typically of frontalis or extensor digitorum muscles. We aimed to determine the sensitivity and specificity of orbicularis oculi SFEMG to diagnose and exclude myasthenia gravis and predict response to therapy.

Methods

Orbicularis oculi SFEMG studies were conducted in 142 consecutive patients with symptoms and/or findings of OMG and negative AChR antibody during the period of 5 years. Retrospective chart review was conducted 2 years after the SFEMG to determine whether treatments were given and responses to treatment.

Results

Orbicularis oculi SFEMG was abnormal in 31 patients and normal in 111 patients. Twenty-nine patients with abnormal SFEMG were treated, and 25 had a good response. Twenty-four patients with normal SFEMG received treatment; none responded to treatment or developed generalized myasthenia.

Conclusion

An abnormal orbicularis oculi SFEMG in patients with seronegative OMG has a high predictive value for response to therapy. Our study findings may affect the treatment decisions in practice and aid better management of myasthenic patients.

Diagnostic utility of stimulated single-fiber electromyography of the orbicularis oculi muscle in patients with suspected ocular myasthenia

Stimulated single-fiber electromyography (SSFEMG) is a valuable diagnostic tool in cases of myasthenia gravis with limited disease. From 1990 to 2008 SSFEMG in the orbicularis oculi muscle (OO) was performed in a cohort of 456 patients referred with clinical suspicion of myasthenia gravis (MG) and exclusively ocular symptoms. A diagnosis of MG was made on clinical grounds in 103 patients. In this patient cohort, the specificity of SSFEMG for myasthenia was 97%, and the sensitivity was 80%. MG patients with a normal SSFEMG had a benign clinical course. This study confirms a high specificity and sensitivity of SSFEMG when it is performed on patients suspected to have ocular MG. In such patients, a normal SSFEMG of the OO predicts a benign clinical course.

Update on myasthenia gravis

Myasthenia gravis is an autoimmune disorder caused by autoantibodies against the nicotinic acetylcholine receptor on the postsynaptic membrane at the neuromuscular junction and characterised by weakness and fatigability of the voluntary muscles. It has a bimodal peak of incidence with first peak in the third decade and the second peak in the sixth decade. It is probably underdiagnosed in the very old population. Our understanding of the pathogenesis, immunology, and molecular biology of myasthenia gravis has greatly improved in last three decades. It is almost always possible to establish the diagnosis of myasthenia gravis with the current tests. The modern treatment is highly successful and the mortality of treated myasthenia gravis is practically zero. However, there are still important gaps in our knowledge of the origin of myasthenia gravis, the factors that contribute to chronic disease, and the way to cure the disease. In this article the current knowledge of the various aspects of myasthenia gravis are outlined.

Clinical impact of single-fiber electromyography

The major clinical impact of single-fiber electromyography has been from its role in confirming, or excluding, the diagnosis of myasthenia gravis (MG). Jitter measurements also have a clinical role in demonstrating changes in disease severity in patients with MG and Lambert-Eaton myasthenic syndrome, in demonstrating subtle changes in motor unit architecture and physiology in patients with nerve and muscle diseases, and in demonstrating the remote effects of locally injected botulinum toxin. In addition to these clinical roles, the ability to identify the activity from single muscle fibers makes it possible to mark the discharges of single motor units. This, along with information gained by jitter and fiber-density measurements, has uniquely increased our understanding of motor unit organization and function in normal and disease states.

Stimulated single fiber EMG of the frontalis muscle in the diagnosis of ocular myasthenia

We performed single fiber electromyography by axonal stimulation (SFEMG-AS) of the frontalis muscle of 16 patients with ocular myasthenia gravis (OM) and 33 controls. In the controls, values of mean consecutive differences (MCD) ranged from 5 to 55 micros (average, 14.7 +/- 2.8 micros) and mean MCD of individual MPs was 14. 6 +/- 6.8 micros. All the OM patients showed abnormal SFEMG-AS jitter before prostigmine was administered (mean MCD: 49.19 +/- 21. 82 micros, percentage of blocks: 20.97 +/- 18.53). Twenty or 30 min after prostigmine had been administered, we saw a significant improvement in jitter: mean MCD was 36.38 +/- 22.49 micros (P = 0. 005), and percentage of blocks was 10.16 +/- 18.87 (P = 0.008). The method was well tolerated. We conclude that SFEMG-AS of the frontalis muscle is a sensitive technique for the diagnosis of OM and is easy to carry out.

Single fiber electromyography in studies of neuromuscular function

Single-fiber electromyography (SFEMG) allows precise study of the microphysiology of the human motor unit under normal conditions. The physiological parameters that can be quantified include impulse transmission along the intramuscular axon collaterals, pre- and post synaptic events at the neuromuscular junction, and muscle fiber membrane properties. This chapter illustrates some of the advantages of SFEMG in studies of neuromuscular fatigue in normal muscle, as well as in disorders of neuromuscular transmission, and conditions associated with disturbed muscle fiber depolarization-repolarization.

Single motor end-plates in myasthenia gravis and LEMS at different firing rates

The jitter and frequency of blocking was studied at single motor end-plates in 10 patients with myasthenia gravis (MG) and in a patient with Lambert-Eaton myasthenic syndrome (LEMS), using single fiber EMG (SFEMG) with axonal microstimulation at rates varying from 0.5 Hz to 20 Hz. While some myasthenic motor end-plates showed lowest degrees of transmission disturbance at the lowest rates and most pronounced abnormality at the highest rates of stimulation, over one-half were most abnormal at intermediate rates and improved at higher rates. In 1 patient, all end-plates behaved in this way. On the other hand, all end-plates in the LEMS patient showed the expected improvement of the abnormal jitter and blocking on increasing the stimulation rate. It is argued that improvement of jitter and blocking at higher rates, unless dramatic, does not necessarily suggest a presynaptic abnormality.

Repetitive stimulation in myasthenia gravis: decrementing response revealed by anticholinesterase drugs

In myasthenia gravis (MG) a typical decrementing response is frequently revealed with repetitive stimulation (RS) in clinically unaffected muscles. However, RS is unlikely to give normal results in weak muscles. In two of our patients we found a normal response in clinically affected muscles. However, a decrementing response surprisingly appeared after the administration of anticholinesterase drugs. A possible explanation for this apparently paradoxical effect is as follows: some junctions are already blocked at rest, whereas others function normally both at rest and during RS. As a consequence, the initial compound muscle action potential (CMAP) is reduced in amplitude and shows no further decrement on RS. Anticholinesterase drugs reverse some blocks, therefore causing the CMAP increase in amplitude. However, these labile junctions are more prone than normal to synaptic fatigue, so a decrementing response is produced on RS. This behavior, though not frequently encountered, can account for some negative results of RS in MG.

Effect of electrical stimulus parameters on the development and propagation of action potentials in short excitable fibres

Intracellular action potentials (IAPs) produced by short fibres in response to their electrical stimulation were analysed. IAPs were calculated on the basis of the Hodgkin-Huxley (1952) model by the method described by Joyner et al. (1978). Principal differences were found in processes of activation of short (semilength L less than 5 lambda) and long fibres under near-threshold stimulation. The shorter the fibre, the lower was the threshold value (Ithr). Dependence of the latency on the stimulus strength (Ist) was substantially non-linear and was affected by the fibre length. Both fibre length and stimulus strength influenced the IAP amplitude, the instantaneous propagation velocity (IPV) and the site of the first origin of the IAP (and, consequently, excitability of the short fibre membrane). With L less than or equal to 2 lambda and Ithr less than or equal to Ist less than or equal to 1.1Ithr, IPV could reach either very high values (so that all the fibre membrane fired practically simultaneously) or even negative values. The latter corresponded to the first origin of the propagated IAP, not at the site of stimulation but at the fibre termination or at a midpoint. The characters of all the above dependencies were unchanged irrespective of the manner of approaching threshold (variation of stimulus duration or its strength). Reasons for differences in processes of activation of short and long fibres are discussed in terms of electrical load and latency. Applications of the results to explain an increased jitter, velocity recovery function and velocity-diameter relationship are also discussed.

The jitter in stimulated orbicularis oculi muscle: technique and normal values

A technique is described of measuring the motor end plate jitter in the orbicularis oculi muscle activated by extra-muscular nerve stimulation, standardised for routine use in diagnosis and evaluation of neuromuscular transmission disorders. Among the advantages of the technique are comparatively quick and easy sampling of adequate number of single motor end-plates, convenient control of discharge rate between less than 1 and up to 20 Hz, absence of direct muscle fibre responses, and little discomfort for the patient. The jitter in the orbicularis oculi is significantly lower than that in the extensor digitorum communis muscle. There is a positive correlation between the jitter and the latency of the individual muscle fibre responses, possibly indicating that smaller diameter muscle fibres have slightly lower safety factor of neuromuscular transmission. The data collected in a group of healthy volunteers suggest the following upper normal limits for the mean of absolute consecutive difference of the latency: 30 microseconds for individual motor end plates (one out of 20 values may be higher), and 18 microseconds for a median of 20 motor end plates.

AAEE minimonograph #25: Single-fiber electromyography in myasthenia gravis

Single-fiber electromyography (SFEMG) demonstrates abnormal jitter in virtually all (99%) patients with myasthenia gravis (MG). One muscle, the extensor digitorum communis, is abnormal in most patients with this disease, but to obtain the maximum diagnostic sensitivity, it may be necessary to examine other muscles, especially ones that are more involved clinically. There is no one muscle that will be more abnormal in every patient with MG. The muscle(s) to be tested must be selected based on the distribution of weakness in the individual patient. Abnormal jitter is also seen in diseases of nerve and muscle; these diseases must be excluded by other electrophysiologic and clinical examinations before diagnosing MG. If neuronal or myopathic disease is present, increased jitter does not indicate that MG is also present. However, if jitter is normal in a muscle with definite weakness, the weakness is not due to MG. When abnormal neuromuscular transmission has been demonstrated by repetitive nerve stimulation, the finding of abnormal jitter does not add to the diagnosis, though it may be useful in providing baseline values for comparison with the results of subsequent studies. SFEMG is most valuable clinically in the patient with suspected MG in whom other tests of neuromuscular transmission and antiacetylcholine receptor antibody titers are normal. Serial measurements of jitter can be useful in following the course of disease and in assessing the effect of treatment, but the results from these studies must always be interpreted in light of the overall clinical picture.

Anti-idiotypic antibodies, acetylcholine receptor antibodies and disturbed neuromuscular function in healthy relatives to patients with myasthenia gravis

Fifty-eight first-degree relatives to 40 patients with myasthenia gravis were investigated regarding presence of acetylcholine receptor antibodies, anti-idiotypic antibodies against the receptor antibodies and clinical and electrophysiological signs of disturbed neuromuscular function. No relative had clinical signs of muscle weakness. The prevalence of low concentrations of receptor antibodies was 54%, of anti-idiotypic antibodies 37% and of pathological and borderline single fibre EMG 45%. No sibling, only 2/11 children and 3/14 parents were normal in all three tests. A combination of receptor antibodies and anti-idiotypic antibodies was the most common finding and was especially frequent in children. In female siblings and children there was a positive correlation between the presence of HLA-antigen A1 and/or B8 and that of receptor antibodies and anti-idiotypic antibodies. Male siblings and children showed no such correlation but had a higher frequency of pathological single fibre EMG than females.

Clinical pharmacology of pyridostigmine and neostigmine in patients with myasthenia gravis

Determination of plasma concentration of pyridostigmine in 20 myasthenic patients on maintenance therapy revealed rather small intraindividual variations within a dose interval. The predose concentration varied considerably between different patients and up to seven fold in patients on the same daily dose. No pharmacokinetic interaction between pyridostigmine and neostigmine was found in five patients studied. In six patients the decrement in the deltoid muscle was studied in parallel with determination of the plasma concentrations following administration of pyridostigmine or neostigmine. In these patients the existence of a "bell-shaped" dose response curve is suggested with the maximal effect at a concentration of 30-60 ng/ml for pyridostigmine and 5-15 ng/ml for neostigmine.

Cephalic tetanus studied with single fibre EMG

In a case of cephalic tetanus with left facial spasms and trismus, the repetitive stimulation of the left facial nerve at 3, 10 and 20 Hz showed no facilitation or decrement. The amplitudes of the blink reflex were 50% lower on the affected side. The silent period of the masseter muscles was shortened. Concentric needle examination of the masseters and left facial EMG of the left frontalis muscle showed increased jitter and blocking in a significant proportion of the recorded potentials. Both jitter and blocking improved on higher innervation rates. All electrophysiological findings were normal on the second examination when the patient was asymptomatic. The single fibre EMG findings point to a presynaptic defect in the neuromuscular transmission in human tetanus.

Clinical electrophysiology in myasthenia gravis

Effective diagnostic methods are of great importance in order to recognise myasthenic patients among those with muscle fatigability. Intracellular recordings are useful for research work within the field and for detailed description of the motor end-plate's physiology in the individual case. The method is not used for the routine diagnosis of myasthenia gravis. The decrement of the electrical muscle response with nerve stimulation is the most commonly used method. The diagnostic yield is higher in proximal muscles, in warmed muscles, after exercise, and after ischaemia. A significant number of patients may be undiagnosed with this technique. The mechanical response with nerve stimulation shows the same type of decrement but also an abnormal response to long stimulation. The diagnostic value of this is under dispute. Single fibre ENG needs more patient cooperation than do these tests. The diagnostic yield is significantly higher. Some patients considered to have myasthenia gravis do not show any abnormalities with this technique, particularly those with the pure ocular form. Conventional EMG is not useful for the diagnosis of myasthenia, but may be indicated in these patients when concurrent nerve or muscle disease is in question. Tests for eye movement fatique have not proved useful. Stapedius reflex fatigability is demonstrated in about the same proportion of patients as have positive SFEMG findings. The technique is not uncomfortable for the patient and requires minimal cooperation. The general usefulness must be assessed by further routine use. Even with the advent of immunological tests, neurophysiological investigations are indispensable in helping establish the diagnosis of myasthenia gravis. Discrepancies between the results comparing electrophysiological and immunological tests may indicate that myasthenia gravis is a heterogenous entity within which subgroups may be identified.

Single fibre electromyographic jitter in multiple sclerosis

Recent histological and electrophysiological reports have given evidence for peripheral nervous system (PNS) involvement in multiple sclerosis. We have applied the single fibre electromyography (SFEMG) technique to 15 patients with multiple sclerosis. Six patients had clearly abnormal jitter and two of these had previously undiagnosed coexistent peripheral neuropathy. A further five patients had borderline abnormalities of SFEMG. The mean jitter for each patient was abnormal in 10 patients. This was clear evidence for PNS involvement in this disease. Theoretically, the site of the abnormality could be in the terminal nerve network or at the neuromuscular junction, but this technique cannot distinguish between these sites.

Evaluation of the jitter phenomenon in myasthenic patients and their relatives

Single fiber electromyography with jitter measurements and blocking evaluation was used for investigating 23 myasthenic patients, their 29 healthy relatives, coming from 10 families, and a control group of 10 subjects. Blocking was never seen in the healthy controls, and the jitter averaged some 30 microsec. Blocking was present in the myasthenic patients in 25.4% of the recorded potential pairs and there was a pathological jitter, which often extended over a few hundred microsec. The records of ten healthy relatives of patients were suggestive of slight but unquestionable abnormalities of neuromuscular transmission, viz. blocking and a jitter of over 60 microsec or more. The results demonstrate subclinical derangements of neuromuscular transmission in families of myasthenic patients, which points to a familial factor as involved in the pathomechanism of the disease.

Myasthenia gravis: review of diagnosis and management

Although the cause of myasthenia gravis is still unknown, its pathogenesis appears clear: immunologic attack on synaptic receptors in muscle causes receptor deficiency, decreased miniature endplate potentials, and decrements in the compound action potentials evoked from muscles on repetitive stimulation of peripheral nerves. In addition to the involvement of skeletal muscle, some MG patients may manifest subtle alterations of the function of heart, lung, smooth muscle, and CNS, indicating that this is truly a systemic disorder. Modern therapy involves adjusting treatment to the needs of individual patients. Anticholinesterases, calcium, ephedrine, potassium, and germine partially correct the defect in neuromuscular transmission; prednisone, ACTH, cytotoxic drugs, antilymphocyte serums, gamma globulin, thoracic duct drainage, plasmapheresis, and thymectomy partially modify the abnormalities of the immune system.

Morphological observations on motor end-plates in rabbits with experimental myasthenia

The morphology of motor end-plates in rabbits immunized with Torpedo nicotinic acetylcholine receptor (nAChR) has been studied by light and electron microscopy. Rabbits were studied either after one period of paralysis, some in parallel with electrophysiological recordings of MEPPs and EPPs and of Naja naja alpha-neurotoxin binding properties or after recovery followed by a second paralysis. Changes in the sub-neural apparatus were noted after cholinesterase staining only in the latter group. Ultrastructurally, however, most end-plates in both groups contained a wide range of abnormalities. Many were similar in appearance to those observed in human myasthenia gravis (MG). This further supports the theory that immunized rabbits can be used as a model for myasthenia gravis. In the rabbits with 1 period of paralysis an acute stage of influence on the neuromuscular junction seemed to be present while simplified motor end-plates typical for human MG were mostly found in rabbits with 2 periods of paralysis. Short post-synaptic folds in conjugation with thickeneed membrane-bound vesicles at their tops, inside the basement membrane, were frequently observed. These were interpreted as if the crests of the folds containing nAChR had degenerated and had been budded off. If so, a large number of receptor sites had been lost which would be one possible explanation for the lowered capacity of the muscles to bind Naja naja alpha-neurotoxin. Membrane thickenings with projections and striations were interpreted as reflecting ACh receptors and were observed in the post-junctional membrane without proximity to the nerve terminal. The degeneration of the top of the post-synaptic folds and the occurrence of receptors at other locations within the motor end-plate will result in a widened distance between the nerve terminal and the receptors, which can explain previous interpretations of a presynaptic defect in MG.

The results to be expected from electrical testing in the diagnosis of myasthenia gravis

The purpose of this study was to derive numerical data concerning the probability of diagnosing myasthenia gravis by electrical tests, employing repetitive stimulation of motor nerves. This was done by recording the compound muscle action potential (CMAP) from 80 patients with clear-cut myasthenia gravis, using a number of different, reportedly useful, testing techniques. In 95% of these patients, the diagnosis could be documented by the careful application of rather simple methods, providing several muscles, including a proximal one such as deltoid, were studied.