Myasthenia Gravis

Myasthenia Gravis is a topic covered in the 5-Minute Clinical Consult.

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Basics

Description

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

Epidemiology

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

Incidence
Estimated annual incidence 2 to 21/1 million

Prevalence
In the United States, 200/1 million; increasing over the past 5 decades

Pediatric Considerations
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])

Genetics
  • 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.

Risk Factors

  • 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%)

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Citation

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TY - ELEC T1 - Myasthenia Gravis ID - 116396 ED - Baldor,Robert A, ED - Domino,Frank J, ED - Golding,Jeremy, ED - Stephens,Mark B, BT - 5-Minute Clinical Consult, Updating UR - https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116396/all/Myasthenia_Gravis PB - Wolters Kluwer ET - 27 DB - Medicine Central DP - Unbound Medicine ER -