Diagnosis of myasthenia

Edrophonium is a short acting anticholinesterase used in the diagnosis of myasthenia in patients with no previous history of myasthenia gravis. In myasthenic patients with an acute deterioration the test may distinguish a myasthenic from a cholinergic crisis. In cholinergic crisis there is a possibility of further deterioration and atropine and facilities for urgent intubation and ventilation should be available. An initial dose of 2 mg IV is given. If there are no cholinergic side-effects a further 8 mg may be given. A positive test is judged by improvement of weakness within 3 min of injection. The test may be combined with objective assessment of respiratory function by measuring the FVC response or by assessing the response to repetitive stimulation with an EMG.

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