PHARMACOLOGICAL Anticholinesterase drugs are used to confirm diagnosis.
Tensilon (edrophonium) - short action, 2-A minutes, given i.v. 2-10 mg slowly, with atropine available to counter muscarinic side effects (nausea and bradycardia). This is positive when noticeable improvement in weakness occurs on objective testing. A control injection of saline is useful, especially when assessing limb weakness only. The Tensilon test may be negative in ocular myasthenia and give a false positive in the Lambert-Eaton syndrome.
Acetylcholine receptor antibodies arc detected in 90% of patients and are virtually specific to this disease. In ocular myasthenia, only 60% show antibodies. Magnitude of titres correlates with disease severity.
Other antibodies e.g. microsomal, colloid, rheumatoid factor, gastric parietal cell antibody - are occasionally found. These reflect the overlap between myasthenia gravis and other autoimmune disorders.
Anti striated muscle antibodies are found in 30% of all patients and in 90% of those with thymoma.
ELECTROPHYSIOLOGICAL Reduction of the amplitude of the compound muscle action potential evoked by repetitive supramaximal nerve stimulation - 'the decrementing response'.
Various rates of stimulation;
even as low as 3/second I nnJiJIliiJIIlh!
may produce a decrementing response.
Single fibre electromyography - measure of 'Jitter' - the time interval variability of action potentials from two single muscle fibres of the same motor unit - is a more sensitive index of neuromuscular function and is increased (95% of mild cases are abnormal).
Chest X ray will show a large mediastinal mass but will not exclude a small thymoma. CT of chest should be performed in all newly diagnosed cases.
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