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Myasthenia Gravis and Lambert-Eaton Myasthenic Syndrome Review Article Copyright © American Academy of Neurology

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Myasthenia Gravis and Lambert-Eaton Myasthenic Syndrome Review Article Copyright © American Academy of Neurology Review Article 04/25/2018 on mAXWo3ZnzwrcFjDdvMDuzVysskaX4mZb8eYMgWVSPGPJOZ9l+mqFwgfuplwVY+jMyQlPQmIFeWtrhxj7jpeO+505hdQh14PDzV4LwkY42MCrzQCKIlw0d1O4YvrWMUvvHuYO4RRbviuuWR5DqyTbTk/icsrdbT0HfRYk7+ZAGvALtKGnuDXDohHaxFFu/7KNo26hIfzU/+BCy16w7w1bDw== by https://journals.lww.com/continuum from Downloaded Downloaded from Address correspondence to https://journals.lww.com/continuum Dr Michael W. Nicolle, London Health Sciences Centre, Myasthenia Gravis University Hospital, 339 Windermere Rd, London, ON N6A 5A5, Canada, and Lambert-Eaton [email protected]. Relationship Disclosure: by mAXWo3ZnzwrcFjDdvMDuzVysskaX4mZb8eYMgWVSPGPJOZ9l+mqFwgfuplwVY+jMyQlPQmIFeWtrhxj7jpeO+505hdQh14PDzV4LwkY42MCrzQCKIlw0d1O4YvrWMUvvHuYO4RRbviuuWR5DqyTbTk/icsrdbT0HfRYk7+ZAGvALtKGnuDXDohHaxFFu/7KNo26hIfzU/+BCy16w7w1bDw== Dr Nicolle has given Myasthenic Syndrome expert medical testimony in court cases for the Canadian Medical Michael W. Nicolle, MD Protective Association. Unlabeled Use of Products/Investigational ABSTRACT Use Disclosure: Dr Nicolle discusses the Purpose of Review: This article discusses the pathogenesis, diagnosis, and management of unlabeled/investigational autoimmune myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS). use of azathioprine, Recent Findings: Recognition of new antigenic targets and improved diagnostic methods mycophenolate, and rituximab for the treatment promise to improve the diagnosis of MG, although the clinical phenotypes associated with of myasthenia gravis and newer antibodies have not yet been defined. Future therapies might specifically target Lambert-Eaton myasthenic the aberrant immune response. The apparent increase in the prevalence of MG is not syndrome and the use of 3,4-diaminopyridine for the fully explained. Results of a long-awaited trial of thymectomy support the practice of treatment of Lambert-Eaton performing a thymectomy under specific conditions. myasthenic syndrome. Summary: The current treatment options are so effective in most patients with MG or * 2016 American Academy LEMS that in patients with refractory disease the diagnosis should be reconsidered. The of Neurology. management of MG is individualized, and familiarity with mechanisms, adverse effects, and strategies to manage these commonly used treatments improves outcome. Patient education is important. LEMS, frequently associated with an underlying small cell lung cancer, is uncommon, and the mainstay of treatment is symptomatic in most patients. Continuum (Minneap Minn) 2016;22(6):1978–2005. INTRODUCTION treatments in MG and LEMS.1 The Immune myasthenia gravis (MG) is the neuromuscular junction includes the pre- best delineated of human autoimmune synaptic nerve terminal, basal laminaY diseases. Fatigable weakness is the hall- containing synaptic cleft, and postsyn- mark of both MG and Lambert-Eaton aptic muscle fiber endplate. myasthenic syndrome (LEMS). The ex- Nerve depolarization allows calcium panding repertoire of antibody assays influx through nerve terminal voltage- has improved the diagnosis of MG and gated calcium channels (VGCCs). This may allow treatments to be tailored to results in release of acetylcholine into specific antibodies. Management options the synaptic cleft. Acetylcholine binds for MG include symptomatic as well as to its receptor (acetylcholine receptor immunosuppressive and immunomodu- [AChR]), opening AChR channels and latory treatments and, in select circum- producing an influx of cations, mostly stances, thymectomy. LEMS is uncommon sodium, at the endplate. Depolarization and often occurs as a paraneoplastic disorder with an underlying small cell of the muscle membrane produces an lung cancer. endplate potential, which, if of suffi- cient amplitude, results in an all or none NEUROMUSCULAR JUNCTION AND muscle fiber action potential and even- on 04/25/2018 NEUROMUSCULAR TRANSMISSION tually muscle movement. An excess of Understanding normal neuromuscular released acetylcholine and AChRs at the transmission is important to appreciate muscle endplate provides a safety fac- the rationale for diagnostic tests and tor of neuromuscular transmission. 1978 www.ContinuumJournal.com December 2016 Copyright © American Academy of Neurology. Unauthorized reproduction of this article is prohibited. KEY POINTS Acetylcholine binds to its receptor tran- in a decrement with low-frequency re- h The function of siently and either diffuses out of the petitive nerve stimulation studies and acetylcholine at the neuromuscular junction or is hydro- weakness clinically. Second, during neuromuscular junction is lyzed by acetylcholinesterase, anchored high-frequency (20 Hz to 50 Hz) stimu- influenced by several other in the basal lamina. This terminates the lation or after brief (10 seconds) maximal key proteins, which are effects of nerve depolarization. voluntary contraction, intracellular cal- now known to be targets AChR localization at the neuromus- cium is increased in the presynaptic nerve of the autoimmune response in some patients terminal, increasing acetylcholine release. cular junction and its function are in- with myasthenia gravis. fluenced by several other proteins at the Maximal voluntary contraction mimics h Antibodies against the muscle endplate. Agrin, secreted by the high-frequency stimulation, as the nerve most common antigenic nerve terminal, interacts with muscle- fires at approximately 20 Hz during a target in myasthenia specific tyrosine kinase (MuSK) via its maximal contraction. The increased in- gravis, the acetylcholine coreceptor, low-density lipoprotein tracellular calcium may overcome defec- receptor, are found in receptor-related protein 4 (LRP4). The tive neuromuscular transmission; in 85% of patients with resultant MuSK-LRP4 complex results in MG, this results in postexercise repair generalized myasthenia the activation and clustering of AChRs at on repetitive nerve stimulation studies, gravis and 50% of the neuromuscular junction. MuSK also and in LEMS, this results in facilitation or patients with ocular myasthenia gravis and anchors acetylcholinesterase at the syn- increment after high-frequency stimu- disrupt neuromuscular aptic basal lamina. In MG, antibodies lation or maximal voluntary contraction. transmission through against the AChR or other neuromuscu- This also explains the return of a deep several mechanisms. lar junction targets reduce the number, tendon reflex after maximal voluntary function, or clustering of AChRs at the contractioninLEMS.Third,longerpe- neuromuscular junction. In LEMS, anti- riods of exercise result in less well bodies against the VGCC inhibit calcium understood pre- and postsynaptic ef- influx into the nerve terminal and re- fects, which also worsen neuromuscular duce acetylcholine release into the syn- transmission and cause postexercise aptic cleft after nerve depolarization. exhaustion as is seen in repetitive nerve Three other phenomena during nor- stimulation studies or fatigable weak- mal neuromuscular transmission are ness, which is seen clinically. important to understand; these phenom- ena are seen electrophysiologically and MYASTHENIA GRAVIS clinically in MG and LEMS. First, during MG is a disorder where neuromuscular low-frequency (2 Hz to 5 Hz) stimulation, transmission is disrupted as a result of an acetylcholine release gradually declines autoimmune attack on postsynaptic anti- as presynaptic stores are depleted, with genic targets, producing weakness of ske- a nadir at the fourth or fifth stimulation letal muscles. The clinical hallmark of the in a train of stimuli. In healthy individ- weakness is its variability and fatigability. uals, this never reaches significance be- cause of the safety factor. However, when Pathophysiology the amount of acetylcholine available for In MG, antibodies against proteins at release is diminished (as in LEMS) or the the neuromuscular junction interfere number of available AChRs is reduced with neuromuscular transmission. Anti- (as in MG), muscle fiber depolarization bodies against the most common anti- may fail. If this fails at a single fiber, this is genic target in MG, the AChR, are found detected on single fiber EMG as blocking in 85% of patients with generalized MG or is detected as jitter if the endplate and 50% of patients with ocular MG and potential amplitude is sufficient to pro- disrupt neuromuscular transmission duce a delayed muscle fiber depolariza- through several mechanisms. They can tion. When many fibers fail, this results reversibly block acetylcholine binding, Continuum (Minneap Minn) 2016;22(6):1978–2005 www.ContinuumJournal.com 1979 Copyright © American Academy of Neurology. Unauthorized reproduction of this article is prohibited. Myasthenia Gravis and LEMS KEY POINTS 9 h In more than one-half of cross-link adjacent cell surface AChRs the pathogenesis of MG. Although not the remaining patients resulting in their internalization into the a hereditary disorder in the mendelian who have acetylcholine muscle cell, or, for some immunoglo- sense, about 3% to 5% of patients with receptorYnegative bulin subclasses, cause complement MGwillhaveafamilymemberwithim- generalized myasthenia fixation and destruction of the neuro- mune MG, similar to the data from more gravis, antibodies target muscular junction.2,3 A mixture of anti- than 800 patients in the author’s MG other proteins at the bodies with each of these mechanisms clinic.10 The prevalence of other autoim- neuromuscular junction. occurs in an individual patient, although mune disorders, especially thyroid, is The first described and still
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