Myasthenia gravis

Clifton L Gooch MD (Dr. Gooch of the University of South Florida College of Medicine has received consulting fees from Baxalta and CSL Behring.)
Salvatore DiMauro MD, editor. (Dr. DiMauro, Director Emeritus of H Houston Merritt Clinical Center for the Study of Muscular Dystrophy and Related Diseases at Columbia University, has no relevant financial relationships to disclose.)
Originally released December 7, 1994; last updated January 22, 2016; expires January 22, 2019

This article includes discussion of myasthenia gravis, autoimmune myasthenia gravis, Erb-Goldflam syndrome, myasthenia gravis pseudoparalytica, bulbar myasthenia gravis, congenital myasthenia gravis, familial infantile myasthenia gravis, generalized myasthenia gravis, myasthenia gravis with thymoma, neonatal myasthenia gravis, ocular myasthenia gravis, oculobulbar myasthenia gravis, and oculo-bulbar myasthenia gravis. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.

Overview

Myasthenia gravis is a potentially fatal neuromuscular disorder, but myasthenic patients typically lead normal lives when properly diagnosed and managed. In this article, the author reviews the immunopathogenesis, clinical features, diagnostic evaluation, and treatment of myasthenia gravis. The full range of diagnostic methods, including electrophysiologic and immunologic tests, is detailed along with surgical and pharmacologic treatments. Developments include the growing use of monoclonal antibody therapy, including rituximab and eculizumab, which have now received Orphan Drug Designation from the FDA for the treatment of myasthenia.

Key points

 

• Myasthenia gravis is fatal in up to one third of patients if untreated.

 

• The most dangerous manifestation of myasthenia is respiratory muscle weakness.

 

• Hospitalization and observation with respiratory monitoring are essential in rapidly progressive disease.

 

• Acute temporizing therapy is best achieved with IVIG or plasma exchange.

 

• Chronic immunomodulatory therapy can effectively control symptoms in the vast majority of patients.

 

• Ten percent of patients with myasthenia gravis will have a thymoma.

Historical note and terminology

As early as 1904, Elliot proposed that neurotransmitter release at the neuromuscular junction could mediate muscle contraction (Aarli 1982). In 1934, specific release of acetylcholine at the neuromuscular junction was demonstrated (Nachmansohn 1939). During this same period, a number of reports of pathologic thymic abnormalities in myasthenic patients and of symptomatic improvement following thymectomy appeared, prompting Blalock to further investigate and ultimately recommend removal of the thymus as a primary therapy (Bell 1917; Blalock 1944). In 1960, Simpson proposed an autoimmune pathogenesis for myasthenia gravis based on the high prevalence of immunologic disorders in myasthenic patients, the transient neonatal form of the disease, and the well-described thymic abnormalities.

Later studies demonstrated antibodies in the sera of affected patients that reacted with the cross striations of skeletal muscle, as well as muscle membrane damage following the application of myasthenic sera to nerve-muscle preparations. In 1962, alpha-bungarotoxin (a snake alpha-toxin) was found to specifically bind and irreversibly inactivate the acetylcholine receptor (AChR) in skeletal muscle. The density of AChRs is particularly high in the electric organs of the Torpedo marmorata electric fish (Richman and Agius 1994), providing a rich source of AChRs for basic scientific investigation. In 1973, a group of rabbits was immunized with solubilized membranes from torpedo electric organs in an attempt to create anti-AChR antibodies for labeling studies. These animals developed a syndrome that closely paralleled human myasthenia gravis (Patrick and Lindstrom 1973). The detection of antibodies in these animals that cross-reacted with rabbit AChRs confirmed the first animal model of experimental allergic myasthenia gravis. In 1974, Alman, Andrew, and Appel identified anti-AChR antibodies in human sera (Almon et al 1974), further opening a promising new immunologic frontier in the pathogenesis of human disease. Subsequent animal models have been created in rats, mice, goats, monkeys, frogs, and hens (Lindstrom et al 1988). Passive transfer has also been accomplished by injection of human myasthenia gravis IgG into mice and of experimental allergenic myasthenia gravis sera and purified monoclonal anti-AChR antibodies into normal mice or rats (Toyka et al 1978). The data from these early experiments confirmed an autoimmune pathogenesis for myasthenia gravis, satisfying the criteria proposed by Milgrom and Witebsky for an autoimmune etiology (Milgrom and Witebsky 1962).

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