Myasthenia Gravis

Myasthenia Gravis is a topic covered in the Washington Manual of Medical Therapeutics.

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General Principles

Definition

Myasthenia gravis (MG) is an autoimmune disorder that involves antibody-mediated postsynaptic dysfunction of the neuromuscular junction of skeletal muscle resulting in fatigable weakness.

Classification

  • Generalized disease is most common and affects a variable combination of ocular, bulbar, respiratory, and appendicular muscles.
  • Ocular MG is confined to eyelid and oculomotor function. It accounts for 10%–40% of all MG cases. The longer a patient with ocular MG goes without evidence of generalization, the less likely he or she is to develop generalized MG (<5% will advance to generalized MG if there are no symptoms of generalization by 2 years).

Epidemiology

Bimodal distribution with peak incidence in women in the second and third decades and in men in the sixth and seventh decades.

Pathophysiology

MG is an acquired autoimmune disorder resulting from the production of autoantibodies against the postsynaptic acetylcholine receptor (AChR) or, less commonly, against receptor-associated proteins, including muscle-specific receptor tyrosine kinase (MuSK) or low-density lipoprotein receptor-related protein 4 (LRP4). However, despite the identification of these additional antigens, “seronegative” forms still account for up to 10% of MG patients. One must consider hereditary/congenital forms of MG and avoid mistakenly lumping these patients or the anti-LRP4 or anti-MuSK patients into this group.

Associated Conditions

  • MG is often associated with thymus hyperplasia; 10% may have a malignant thymoma. Hyperplasia is more common in those under 40 years of age. Thymoma is more common in MG patients over 30 years of age. There are data to support thymectomy in all patients with generalized MG under the age of 65, regardless of the presence of thymic hyperplasia or thymoma.1 Thymectomy for non–thymoma-associated generalized MG in patients over the age of 65 years is generally reserved for treatment refractory cases and can be considered on a case-by-case basis.
  • Autoimmune thyroiditis (hyper- more common than hypo-) is present in approximately 15% of patients with MG. MG patients also have an increased risk of other autoimmune diseases including lupus, rheumatoid arthritis, polymyositis, and pernicious anemia.

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General Principles

Definition

Myasthenia gravis (MG) is an autoimmune disorder that involves antibody-mediated postsynaptic dysfunction of the neuromuscular junction of skeletal muscle resulting in fatigable weakness.

Classification

  • Generalized disease is most common and affects a variable combination of ocular, bulbar, respiratory, and appendicular muscles.
  • Ocular MG is confined to eyelid and oculomotor function. It accounts for 10%–40% of all MG cases. The longer a patient with ocular MG goes without evidence of generalization, the less likely he or she is to develop generalized MG (<5% will advance to generalized MG if there are no symptoms of generalization by 2 years).

Epidemiology

Bimodal distribution with peak incidence in women in the second and third decades and in men in the sixth and seventh decades.

Pathophysiology

MG is an acquired autoimmune disorder resulting from the production of autoantibodies against the postsynaptic acetylcholine receptor (AChR) or, less commonly, against receptor-associated proteins, including muscle-specific receptor tyrosine kinase (MuSK) or low-density lipoprotein receptor-related protein 4 (LRP4). However, despite the identification of these additional antigens, “seronegative” forms still account for up to 10% of MG patients. One must consider hereditary/congenital forms of MG and avoid mistakenly lumping these patients or the anti-LRP4 or anti-MuSK patients into this group.

Associated Conditions

  • MG is often associated with thymus hyperplasia; 10% may have a malignant thymoma. Hyperplasia is more common in those under 40 years of age. Thymoma is more common in MG patients over 30 years of age. There are data to support thymectomy in all patients with generalized MG under the age of 65, regardless of the presence of thymic hyperplasia or thymoma.1 Thymectomy for non–thymoma-associated generalized MG in patients over the age of 65 years is generally reserved for treatment refractory cases and can be considered on a case-by-case basis.
  • Autoimmune thyroiditis (hyper- more common than hypo-) is present in approximately 15% of patients with MG. MG patients also have an increased risk of other autoimmune diseases including lupus, rheumatoid arthritis, polymyositis, and pernicious anemia.

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