Myasthenia gravis (MG) is a neuromuscular disease leading to fluctuating muscle weakness and fatiguability. It is an autoimmune disorder, in which weakness is caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated medically with cholinesterase inhibitors or immunosuppressants, and, in selected cases, thymectomy.
Demonstration of striated muscle antibody by utilising the indirect fluorescent antibody method enables serologic assessment or possible detection of muscular diseases. The presence of a histologically defined skeletal antibody in conjunction with a positive acetylcholine receptor site assay is helpful in the confirmatory diagnosis of myasthenia gravis (MG). Similar antibodies can be found using cardiac tissue. Patients with rheumatic fever, myocardial infarction and a variety of post cardioitomy states may demonstrate antibodies detected using both skeletal and cardiac tissue. The absence of anti-striated antibodies effectively excludes thymoma (see table below). The utilisation of frozen sections of monkey skeletal muscle is the recommended substrate for IFA.
Table: Disease State vs. Patient Pop % with Striated Muscle Antibodies
· All MG patients 40%
· MG with Thymoma 90-100%
· MG without Thymoma 30%
· Thymoma without MG 25%