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Primary disorder of neuromuscular transmission characterized by fluctuating muscle weakness:
- Ocular myasthenia gravis (MG) (15%): weakness limited to eyelids and extraocular muscles
- Generalized MG (85%): commonly affects ocular as well as a variable combination of bulbar, proximal limb, and respiratory muscles
- 50% of patients who present with ocular symptoms develop generalized MG within 2 years.
- Onset may be sudden and severe, but it is typically mild and intermittent over many years, maximum severity reached within 3 years for 85%.
- System(s) affected: neurologic, hematologic, lymphatic, immunologic, musculoskeletal
Occurs at any age but a bimodal distribution to the age of onset:
- Female predominance: 20 to 40 years
- Male predominance: 60 to 80 years
Estimated annual incidence 2 to 21/1 million
In the United States, 200/1 million; increasing over the past 5 decades
A transient form of neonatal MG seen in 10–20% of infants born to mothers with MG. It occurs as a result of the transplacental passage of maternal antibodies that interfere with function of the neuromuscular junction; resolves in weeks to months. Autoimmune juvenile MG makes up 10–15% of cases of MG in North America.
Etiology and Pathophysiology
- Reduction in the function of acetylcholine receptors (AChR) at muscle endplates, resulting in insufficient neuromuscular transmission
- Antibody-mediated autoimmune disorder
- Antibodies are present in most cases of MG.
- Seropositive/antiacetylcholine receptor (anti-AChR): a humoral, antibody-mediated, T-cell–dependent attack of the AChRs or receptor-associated proteins at the postsynaptic membrane of the neuromuscular junction. Found in 85% of generalized MG and 50% of ocular MG. Thymic abnormalities common (1)
- Muscle-specific kinase (MuSK). 5% of generalized MG patients. Typically females. Is a severe form, respiratory and bulbar muscles involved. Thymic abnormalities are rare (1).
- In remainder of seronegative, 12–50% with anti-LRP4, a molecule that forms a complex with MuSK, mild generalized weakness most common (1)
- Seronegative MG (SNMG): 5%; may have anti-AChR detectable by cell-based assay. Clinically similar to anti-AChR, thymic hyperplasia may be present (1).
- Also documented immediately after viral infections (measles, Epstein-Barr virus [EBV], HIV, and human T-lymphotropic virus [HTLV])
- Congenital MG syndrome describes a collection of rare hereditary disorders. This condition is not immune-mediated but instead, results from the mutation of a component of the neuromuscular junction (autosomal recessive).
- Familial predisposition is seen in 5% of cases.
- Familial MG
- D-penicillamine (drug-induced MG)
- Other autoimmune diseases
Commonly Associated Conditions
- Thymic hyperplasia (60–70%)
- Thymoma (10–15%)
- Autoimmune thyroid disease (3–8%)