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Myasthenia gravis is an acquired autoimmune disease characterized by muscle weakness and progressive fatigability. The fundamental defect is failure of neuromuscular transmission due to formation of specific autoantibodies directed against postsynaptic acetylcholine receptor (AChR). These antibodies block acetylcholine-binding sites, damage the postsynaptic membrane, and accelerate receptor degradation. Synaptic transmission fails when the autoantibodies cause a critical loss of the cation channel protein, which is required to activate the muscle action potential. AChR binding antibody is useful to confirm the diagnosis of myasthenia gravis, distinguish acquired from congenital myasthenia, detect subclinical myasthenia gravis in patients with thymoma or graft versus host disease and monitor response to immunotherapy Two types of antibody exist; AChR binding and AChR modulating antibodies. They are found with approximately the same frequency in 90% patients with myasthenia gravis. Antibody levels are usually high in generalized myasthenia gravis and in myasthenia gravis associated with thymoma. In approximately 10% of patients, only one of the two tests is positive. The AChR binding antibody test is recommended as the initial case finding test because it is faster and more economical to perform. If negative, the AChR modulating test is recommended. The AChR modulating antibody test detects antibodies with the ability to cause weakness. AChR modulating antibodies are indicated when AchR binding antibody is negative and when myasthenia gravis is mild, ocular restricted, or of less than one year duration. The frequency of AChR binding antibody detection is approximately 70% in cases of myasthenia gravis restricted to ocular muscles and antibody titers are usually low (0.04 - 1.0 nmol/L). Antibody may not be detected during the first year after symptoms appear or during immunosupressant therapy. Generally, there is not a good correlation between antibody titer and the severity of weakness. Positive AChR results are found in Lambert Eaton Syndrome and autoimmune liver disease. Specimen requirement is red top of SST tube of blood. Reference range for AChR binding antibody is < 0.02 nmol/L Reference range for AChR modulating antibody 0-20%. Results are reported as the percent loss of AChR. |
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