Mediastinum
Nonneoplastic
Myasthenia gravis
Author: Hanni Gulwani, M.B.B.S.
Last author update: 1 December 2012
Last staff update: 16 August 2022
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PubMed Search: Myasthenia gravis mediastinum
Table of Contents
Definition / general | Pathophysiology | Clinical features | Prognostic factors | Treatment | Differential diagnosisCite this page: Gulwani H. Myasthenia gravis. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/mediastinummyastheniagravis.html. Accessed November 26th, 2024.
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
- Mildest clinical outcome associated with normal thymus, most severe associated with thymoma (Clin Neurol Neurosurg 2013;115:432)
Treatment
- Thymectomy (regardless of presence of thymoma)
Differential diagnosis
- Lambert-Eaton syndrome:
- Muscle weakness due to antibodies to neuronal calcium channel
