Is the decremental pattern in Lambert–Eaton syndrome different from that in myasthenia gravis?

Electrophysiological abnormalities of the neuromuscular transmission in two patients with botulism-like syndrome following Botulinum-A muscle injections

Botulinum neurotoxin serotype A (BoNT-A) has several therapeutic indications such as spasticity and dystonia. Although its use is generally considered safe, a systemic diffusion can lead to systemic complications, and a botulism-like syndrome can occur after intramuscular injections. Herein, two adult cases who developed general muscle weakness after a BoNT-A intramuscular injection are reported. Both presented with a progressive decrement on low-frequency (LF) repetitive nerve stimulation (RNS). It is suggested that a progressive decrement on LF-RNS in muscles distant from the injection site strongly supports the diagnosis of iatrogenic botulism.

Lambert-Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disease characterized by proximal muscle weakness, loss of tendon reflexes, and autonomic dysfunction. Muscle weakness usually starts in the upper legs and can progress to oculobulbar and in severe cases respiratory muscles. P/Q-type voltage-gated calcium channels (VGCCs) localized in the presynaptic motor nerve terminal and in the autonomic nervous system are targeted by antibodies in LEMS patients. These antibodies can be detected in about 90% of patients, and the presence of decrement and increment upon repetitive nerve stimulation is also a highly sensitive diagnostic test. Rapid diagnosis is important because of the association with SCLC in 50%-60% of patients, which stresses the need for vigorous tumor screening after diagnosis. Clinical parameters can predict tumor probability and guide frequency of tumor screening. Treatment of the tumor as well as symptomatic treatment and immunosuppression can effectively control symptoms in the majority of patients.

Guideline for the management of myasthenic syndromes

Myasthenia gravis (MG), Lambert-Eaton myasthenic syndrome (LEMS), and congenital myasthenic syndromes (CMS) represent an etiologically heterogeneous group of (very) rare chronic diseases. MG and LEMS have an autoimmune-mediated etiology, while CMS are genetic disorders. A (strain dependent) muscle weakness due to neuromuscular transmission disorder is a common feature. Generalized MG requires increasingly differentiated therapeutic strategies that consider the enormous therapeutic developments of recent years. To include the newest therapy recommendations, a comprehensive update of the available German-language guideline 'Diagnostics and therapy of myasthenic syndromes' has been published by the German Neurological society with the aid of an interdisciplinary expert panel. This paper is an adapted translation of the updated and partly newly developed treatment guideline. It defines the rapid achievement of complete disease control in myasthenic patients as a central treatment goal. The use of standard therapies, as well as modern immunotherapeutics, is subject to a staged regimen that takes into account autoantibody status and disease activity. With the advent of modern, fast-acting immunomodulators, disease activity assessment has become pivotal and requires evaluation of the clinical course, including severity and required therapies. Applying MG-specific scores and classifications such as Myasthenia Gravis Activities of Daily Living, Quantitative Myasthenia Gravis, and Myasthenia Gravis Foundation of America allows differentiation between mild/moderate and (highly) active (including refractory) disease. Therapy decisions must consider age, thymic pathology, antibody status, and disease activity. Glucocorticosteroids and the classical immunosuppressants (primarily azathioprine) are the basic immunotherapeutics to treat mild/moderate to (highly) active generalized MG/young MG and ocular MG. Thymectomy is indicated as a treatment for thymoma-associated MG and generalized MG with acetylcholine receptor antibody (AChR-Ab)-positive status. In (highly) active generalized MG, complement inhibitors (currently eculizumab and ravulizumab) or neonatal Fc receptor modulators (currently efgartigimod) are recommended for AChR-Ab-positive status and rituximab for muscle-specific receptor tyrosine kinase (MuSK)-Ab-positive status. Specific treatment for myasthenic crises requires plasmapheresis, immunoadsorption, or IVIG. Specific aspects of ocular, juvenile, and congenital myasthenia are highlighted. The guideline will be further developed based on new study results for other immunomodulators and biomarkers that aid the accurate measurement of disease activity.

Electrophysiological evaluation of the neuromuscular junction: a brief review

The nerve terminal and muscle membrane compose the neuromuscular junction. After opening the voltage-gated calcium channels, action potentials from the motor axons provoke a cascade for the acetylcholine release from synaptic vesicles to the synaptic cleft, where it binds to its receptor at the muscle membrane for depolarization. Low amplitude compound muscle action potential typically presents in presynaptic disorders, increasing by more than 100% after a 10-second effort in the Lambert-Eaton myasthenic syndrome and less in botulism. Needle electromyography may show myopathic motor unit action potentials and morphological instability ("jiggle") due to impulse blocking. Low-frequency repetitive nerve stimulation (RNS) is helpful in postsynaptic disorders, such as myasthenia gravis and most congenital myasthenic syndromes, where the number of functioning acetylcholine receptors is reduced. Low-frequency RNS with a decrement >10% is abnormal when comparing the 4th to the first compound muscle action potential amplitude. High-frequency RNS is helpful in presynaptic disorders like Lambert-Eaton myasthenic syndrome, botulism, and some rare congenital myasthenic syndromes. The high-frequency RNS releases more calcium, increasing the acetylcholine with a compound muscle action potential increment. Concentric needle records apparent single-fiber action potentials (spikes). A voluntary activation measures the jitter between spikes from two endplates. An electrical activation measures the jitter of one spike (one endplate). The jitter is the most sensitive test for detecting a neuromuscular junction dysfunction. Most neuromuscular junction disorders are responsive to treatment.

Practical Management for Use of Eculizumab in the Treatment of Severe, Refractory, Non-Thymomatous, AChR + Generalized Myasthenia Gravis: A Systematic Review

Myasthenia gravis (MG) is a rare autoimmune disorder caused by specific autoantibodies at the neuromuscular junction. MG is classified by the antigen specificity of these antibodies. Acetylcholine receptor (AChR) antibodies are the most common type (74–88%), followed by anti-muscle specific kinase (MuSK) and other antibodies. While all these antibodies lead to neuromuscular transmission failure, the immuno-pathogenic mechanisms are distinct. Complement activation is a primary driver of AChR antibody-positive MG (AChR+ MG) pathogenesis. This leads to the formation of the membrane attack complex and destruction of AChR receptors and the postsynaptic membrane resulting in impaired neurotransmission and muscle weakness characteristic of MG. Broad-based immune-suppressants like corticosteroids are effective in controlling MG; however, their long-term use can be associated with significant adverse effects. Advances in translational research have led to the development of more directed therapeutic agents that are likely to alter the future of MG treatment. Eculizumab is a humanized monoclonal antibody that inhibits the cleavage of complement protein C5 and is approved for use in generalized MG. In this review, we discuss the pathophysiology of MG; the therapeutic efficacy and tolerability of eculizumab, as well as the practical guidelines for its use in MG; future studies exploring the role of eculizumab in different stages and subtypes of MG subtypes; the optimal duration of therapy and its discontinuation; the characterization of non-responder patients; and the use of biomarkers for monitoring therapy are highlighted. Based on the pathophysiologic mechanisms, emerging therapies and new therapeutic targets are also reviewed.

Presynaptic Paraneoplastic Disorders of the Neuromuscular Junction: An Update

The neuromuscular junction (NMJ) is the target of a variety of immune-mediated disorders, usually classified as presynaptic and postsynaptic, according to the site of the antigenic target and consequently of the neuromuscular transmission alteration. Although less common than the classical autoimmune postsynaptic myasthenia gravis, presynaptic disorders are important to recognize due to the frequent association with cancer. Lambert Eaton myasthenic syndrome is due to a presynaptic failure to release acetylcholine, caused by antibodies to the presynaptic voltage-gated calcium channels. Acquired neuromyotonia is a condition characterized by nerve hyperexcitability often due to the presence of antibodies against proteins associated with voltage-gated potassium channels. This review will focus on the recent developments in the autoimmune presynaptic disorders of the NMJ.

History of Myasthenia Gravis Revisited

The first description of myasthenia gravis (MG) was given by Thomas Willis in 1672. MG was the focus of attention after mid-nineteenth century and a great amount of information has been accumulated in a span of 150 years. The aim of this review is to convey this information according to a particular systematic and to briefly relate the experience of Istanbul University. MG history was examined in four periods: 1868-1930, 1930-1960, 1960-1990, and 1990-2020. In the first period (1868-1930), all the clinical characteristics of MG were defined. Physiological/pharmacological studies on the transmission at the neuromuscular junction were initiated, and the concept of repetitive nerve stimulation emerged. A toxic agent was believed to be the cause of MG which appeared to resemble curare intoxication. Association of MG with thymus was noticed. No noteworthy progress was made in its treatment. In the second period (1930-1960), acetylcholine was discovered to be the transmitter at the neuromuscular junction. Repetitive nerve stimulation was used as a diagnostic test. The autoimmune nature of MG was suspected and experiments to this end started to give results. The hallmark of this period was the use of anticholinesterases and thymectomy in the treatment of MG. The third period (1960-1990) can probably be considered a revolutionary era for MG. Important immunological mechanisms (acetylcholine receptor isolation, discovery of anti-acetylcholine receptor antibodies) were clarified and the autoimmune nature of MG was demonstrated. Treatment modalities which completely changed the prognosis of MG, including positive pressure mechanic ventilation and corticosteroids as well as plasma exchange/IVIg and azathioprine, were put to use. In the fourth period (1990-2020), more immunological progress, including the discovery of anti-MuSK antibodies, was achieved. Videothoracoscopic thymectomy reduced the morbidity and mortality rate associated with surgery. New drugs emerged and clinical trials were performed. Valuable guidelines were published. In the last part of the review, the experience in MG of Istanbul University, a pioneer in Turkey, is related.

MuSK-Associated Myasthenia Gravis: Clinical Features and Management

Muscle-specific tyrosine kinase (MuSK) myasthenia gravis (MG) is a rare, frequently more severe, subtype of MG with different pathogenesis, and peculiar clinical features. The prevalence varies among countries and ethnic groups, affecting 5–8% of all MG patients. MuSK-MG usually has an acute onset affecting mainly the facial-bulbar muscles. The symptoms usually progress rapidly, within a few weeks. Early respiratory crises are frequent. The disease may lead to generalized muscle weakness up to muscle atrophy. The main bulbar involvement, the absence of significant thymus alterations, and the association with HLA class II DR14, DR16, and DQ5 alleles have been confirmed. Atypical onset, such as ocular involvement, lack of symptom fluctuations, acetylcholinesterase inhibitors failure, and negative results of electrophysiologic testing, if not specifically performed in the mainly involved muscle groups, makes MuSK-MG diagnosis challenging. In most cases, steroids are effective. Conventional immunosuppressants are not commonly able to replace steroids in maintaining a satisfactory long-term control of symptoms. However, the majority of MuSK-MG patients are refractory to treatment. In these cases, the use of rituximab showed promising results, resulting in sustained symptom control.

Lambert-Eaton myasthenia syndrome: specified description of a response pattern to low-frequency repetitive nerve stimulation

OBJECTIVE

We aimed to specify and quantify the characteristics of the decrement in low-frequency repetitive nerve stimulation response in Lambert-Eaton myasthenia syndrome (LEMS) and compare it to those of myasthenia gravis (MG).

PATIENTS AND METHODS

We retrospectively reviewed 18 patients with LEMS and 24 patients with MG. Ten consecutive stimulations were applied at 3 Hz to the abductor pollicis brevis. We determined the position of the smallest wave in the stimulation sequence, and we calculated the decrement and recovery.

RESULTS

The median sequential order of the minimum wave was 8 in the LEMS group and 5 in the MG group (p < 0.001). The median decrement in the LEMS group was 36.7%, while that in the MG group was 21.0% (p = 0.047). The recovery percentage was 1.4% in the LEMS group and 3.5% in the MG group (p = 0.001). The area under the curve for the sequential order of the minimum wave was 0.90, and the reciprocal optimum cut-off point was 6.5.

CONCLUSIONS

We elucidated a pattern with a delayed nadir and subsequent poor recovery, featuring a low-frequency decrement; furthermore, we determined the most likely sequential order of the minimum wave in patients with LEMS, and the indicator was useful for differentiation.

β2-Adrenergic receptor agonists ameliorate the adverse effect of long-term pyridostigmine on neuromuscular junction structure

Acetylcholine receptor deficiency is the most common form of the congenital myasthenic syndromes, a heterogeneous collection of genetic disorders of neuromuscular transmission characterized by fatiguable muscle weakness. Most patients with acetylcholine receptor deficiency respond well to acetylcholinesterase inhibitors; however, in some cases the efficacy of acetylcholinesterase inhibitors diminishes over time. Patients with acetylcholine receptor deficiency can also benefit from the addition of a β2-adrenergic receptor agonist to their medication. The working mechanism of β2-adrenergic agonists in myasthenic patients is not fully understood. Here, we report the long-term follow-up for the addition of β2-adrenergic agonists for a cohort of patients with acetylcholine receptor deficiency on anticholinesterase medication that demonstrates a sustained quantitative improvement. Coincidently we used a disease model to mirror the treatment of acetylcholine receptor deficiency, and demonstrate improved muscle fatigue, improved neuromuscular transmission and improved synaptic structure resulting from the addition of the β2-adrenergic agonist salbutamol to the anticholinesterase medication pyridostigmine. Following an initial improvement in muscle fatiguability, a gradual decline in the effect of pyridostigmine was observed in mice treated with pyridostigmine alone (P < 0.001). Combination therapy with pyridostigmine and salbutamol counteracted this decline (P < 0.001). Studies of compound muscle action potential decrement at high nerve stimulation frequencies (P < 0.05) and miniature end-plate potential amplitude analysis (P < 0.01) showed an improvement in mice following combination therapy, compared to pyridostigmine monotherapy. Pyridostigmine alone reduced postsynaptic areas (P < 0.001) and postsynaptic folding (P < 0.01). Combination therapy increased postsynaptic area (P < 0.001) and promoted the formation of postsynaptic junctional folds (P < 0.001), in particular in fast-twitch muscles. In conclusion, we demonstrate for the first time how the improvement seen in patients from adding salbutamol to their medication can be explained in an experimental model of acetylcholine receptor deficiency, the most common form of congenital myasthenic syndrome. Salbutamol enhances neuromuscular junction synaptic structure by counteracting the detrimental effects of long-term acetylcholinesterase inhibitors on the postsynaptic neuromuscular junction. The results have implications for both autoimmune and genetic myasthenias where anticholinesterase medication is a standard treatment.

Lambert-Eaton Myasthenic Syndrome

Lambert-Eaton myasthenic syndrome is a paraneoplastic or primary autoimmune neuromuscular junction disorder characterized by proximal weakness, autonomic dysfunction and ariflexia. The characteristic symptoms are thought to be caused by antibodies generated against the P/Q-type voltage-gated calcium channels present on presynaptic nerve terminals and by diminished release of acetylcholine. More than half of Lambert-Eaton myasthenic syndrome cases are associated with small cell lung carcinoma. Diagnosis is confirmed by serologic testing and electrophysiologic studies. 3,4-diaminopyridine is effective symptomatic treatment of LEMS.

Myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease caused by antibodies against the acetylcholine receptor (AChR), muscle-specific kinase (MuSK) or other AChR-related proteins in the postsynaptic muscle membrane. Localized or general muscle weakness is the predominant symptom and is induced by the antibodies. Patients are grouped according to the presence of antibodies, symptoms, age at onset and thymus pathology. Diagnosis is straightforward in most patients with typical symptoms and a positive antibody test, although a detailed clinical and neurophysiological examination is important in antibody-negative patients. MG therapy should be ambitious and aim for clinical remission or only mild symptoms with near-normal function and quality of life. Treatment should be based on MG subgroup and includes symptomatic treatment using acetylcholinesterase inhibitors, thymectomy and immunotherapy. Intravenous immunoglobulin and plasma exchange are fast-acting treatments used for disease exacerbations, and intensive care is necessary during exacerbations with respiratory failure. Comorbidity is frequent, particularly in elderly patients. Active physical training should be encouraged.

Clinical neurophysiology of neuromuscular junction disease

The neuromuscular junction (NMJ) is a cholinergic synapse where quantal release of acetylcholine (ACh) from motor nerve terminals generates a local endplate potential (EPP) on the muscle fiber. EPPs that reach threshold depolarize the entire muscle fiber and initiate the process of excitation-contraction coupling. Deficits of neuromuscular transmission result in clinical weakness that is fatigable and may fluctuate. Repetitive nerve stimulation (RNS) testing can unmask the reduced safety factor common to all NMJ disorders via depletion of immediate ACh stores at the presynaptic motor nerve terminal with decremental responses to low-frequency RNS (LF-RNS). The facilitated responses characterizing presynaptic NMJ disorders can be revealed by brief exercise or high stimulation rates that augment presynaptic calcium levels. Activation with isometric exercise may increase the sensitivity of RNS testing. Attention to technical detail and reproducibility of findings are essential in generating valid results in RNS testing. Motor unit potential (MUP) instability or jiggle is the main finding seen in NMJ disorders on conventional needle EMG and reflects the moment-to-moment variability in the number and synchrony of muscle fiber action potentials (MFAPs) that compose a MUP. Single fiber EMG (SFEMG) is a highly selective technique that assesses jitter, the temporal variability in MFAPs generated in response to motor nerve action potentials.

Distinguishing Features of the Repetitive Nerve Stimulation Test Between Lambert-Eaton Myasthenic Syndrome and Myasthenia Gravis, 50-Year Reappraisal

OBJECTIVE

To reappraise the distinguishing features of the repetitive nerve stimulation (RNS) tests in the abductor digiti quinti muscle between myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) 50 years after the 1965's Lambert seminal paper.

METHODS

The various parameters of the RNS test were compared between 34 patients with LEMS and 140 patients with MG to assess their diagnostic sensitivity.

RESULTS

RNS test was abnormal in all (100%) patients with LEMS and 76 (54%) patients with MG. The diagnostic hallmark of LEMS, ≥60% increment at high-rate stimulation or postexercise facilitation, was present in 33 (97%) of patients. The diagnostic hallmark of MG, decrement at low-rate stimulation (LRS), was present in 65 (46%) of patients with MG. The most prominent difference in the various parameters of RNS tests was noted between LEMS versus MG and normal controls.

CONCLUSIONS

Distinguishing features of the RNS test in LEMS and MG are confirmed in this direct comparison study: the low compound muscle action potential amplitude, decrement at LRS, and increment at high-rate stimulation or postexercise facilitation for LEMS and normal compound muscle action potential amplitude and decrement at LRS for MG.

[Decrement pattern of M-response amplitude in the low-frequency repetitive nerve stimulation in the muscles of patients with myasthenia gravis and Lambert-Eaton myasthenic syndrome]

AIM

To investigate the pattern of decrement in the muscles of patients with myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS).

MATERIAL AND METHODS

Twenty-seven patients with MG and 39 patients with LEMS were studied using low frequency repetitive nerve stimulation (3/ sec).

RESULTS AND CONCLUSION

The decrease of safety factor of neuromuscular transmission was equal in both groups. At the same time, a significant difference in the decrease of pattern of the amplitude compound of muscle action potential (CMAP) was found. In LEMS, by contrast with MG, another sequence of amplitude variability of CMAP from the second stimulus to the first and from the fifth stimulus to the fourth was noted. In LEMS patients, progressive decrement, manifesting by increasing ratios of late A9/A1 to early A4/A1 was found, whereas the MG patients showed retrogressive decrement expressed by the reduction in decrement ratio (from late to early). These differences reflect the mechanisms and status of acetylcholine mobilization and release from the axon terminal.

Modelling the response to low-frequency repetitive nerve stimulation of myasthenia gravis and Lambert-Eaton myasthenic syndrome

Myasthenia gravis (MG) is an autoimmune postsynaptic disorder of neuromuscular transmission caused, in most patients, by antibodies against postsynaptic acetylcholine receptors. Lambert-Eaton myasthenic syndrome (LEMS) is a presynaptic autoimmune disease in which there is a reduction in Ca²⁺ entry with each impulse due to the action of antibodies against Ca²⁺ channels. These diseases have a distinct pattern of response to low-frequency repetitive nerve stimulation which allows its recognition in a particular subject. Nevertheless, the physiologic basis of this response is not entirely known. A model of the time-course of release probability of neuromuscular junctions that incorporates facilitation and a depression-recovery mechanism has been developed with the aim to investigate these response patterns. When the basal value of release probability was in the physiologic range, as in MG, release probability showed an increment after its initial decrease only if the recovery from depression was accelerated by presynaptic residual Ca²⁺. Otherwise, when the basal release probability was low, as in LEMS, a progressive reduction in the release probability without any late increase was only obtained if the efficacy of Facilitation and Ca²⁺-dependent recovery from depression were reduced.

Review of the Diagnostic Challenges of Lambert–Eaton Syndrome Revealed Through Three Case Reports

Lambert–Eaton syndrome (LES) is a rare immune-mediated disorder characterized by proximal leg weakness, autonomic symptoms and hypoactive tendon reflexes. The paraneoplastic form is associated with small-cell lung cancer in 50-60% of cases, whereas the remaining cases are found in younger adults with a higher likelihood of coexisting autoimmune disease. The early recognition of LES is crucial for improving clinical outcomes but remains a major challenge. In this review, we analyze the clinical characteristics and diagnostic considerations in treating LES through a series of three case studies, one of which showed definitive response to pyridostigmine and corticosteroid combination therapy, followed by spontaneous remission. Patients were assessed by image-based screening, serological testing and electrophysiological evaluations, which included respiratory and autonomic testing. A better understanding of the common pitfalls in the clinical, serological and neurophysiologic diagnosis of LES through assessment of typical LES dysfunction throughout the nervous system should enable improved recognition and treatment of this syndrome.