Mediastinum
Thymus
Myasthenia gravis (MG)

Author: Hanni Gulwani, M.D. (see Authors page)

Revised: 14 June 2018, last major update December 2012

Copyright: (c) 2003-2018, PathologyOutlines.com, Inc.

PubMed Search: Myasthenia gravis[TI] mediastinum

Cite this page: Gulwani, H. Myasthenia gravis (MG). PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/mediastinummyastheniagravis.html. Accessed September 21st, 2018.
Definition / general
  • Defect in nicotinic acetylcholine receptor (AChR) present in subsynaptic membrane of neuromuscular junction (at motor end plate), due to circulating autoantibodies to receptor
  • Acetylcholine receptor also present in normal thymus, in myoid type cells
  • Thymus may contain ectopic germinal centers with B cells producing pathogenic antiacetylcholine receptor antibodies (Ann N Y Acad Sci 2008;1132:135)
Pathophysiology
  • May be due to T cells attacking myoid cells, then T cells induce B cells to produce autoantibodies; physiological connection with thymomas is unclear
  • Two step hypothesis: hyperplastic medullary thymic epithelial cells are involved in provoking infiltration and thymic myoid cells (with intact AChR) are involved in germinal center formation (Am J Pathol 2007;171:893)
  • MG patients have high number / ratio and abnormal distribution of thymic dendritic cells, which may be actively involved in pathogenesis (Zhonghua Yi Xue Za Zhi 2008;88:3349)
  • Autoimmunity may be related to increased toll-like receptor 4 expression in thymus of some myasthenic patients (Am J Pathol 2005;167:129)
Clinical features
  • 12% of myasthenia gravis (MG) patients have other autoimmune diseases, including Graves disease, rheumatoid arthritis
  • MG patients with thymomas may have autoantibodies to titin or other striated muscle antigens
  • 65% of patients have thymic hyperplasia, 25% normal thymus, 10% thymomas; risk factors for thymoma are males with initial MG symptoms age 50+ years
  • Present or develops in 30 - 45% of patients with thymomas, usually months / years after excision of thymoma
  • Lymphoid follicles in thymoma or adjacent thymus indicates higher risk for MG
  • MG associated thymomas are morphologically similar to non-MG associated thymomas
Prognostic factors
Treatment
  • Thymectomy (regardless of presence of thymoma)
Microscopic (histologic) images

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Thymic follicular hyperplasia

Differential diagnosis
  • Lambert-Eaton syndrome: muscle weakness due to antibodies to neuronal calcium channel