A myasthenia gravis patient on pyridostigmine presents with worsening weakness. The resident gives intravenous edrophonium. Muscle strength improves markedly over the next minute. This response indicates:

• A Cholinergic crisis requiring reduction of anticholinesterase dose
• B Myasthenic crisis requiring increase of anticholinesterase dose ✓
• C Depolarising neuromuscular blockade requiring ventilatory support
• D Lambert-Eaton syndrome requiring guanidine therapy

Explanation

Edrophonium is a short-acting acetylcholinesterase inhibitor. In myasthenic crisis, relative deficiency of acetylcholine at the end plate means extra acetylcholine from enzyme inhibition strengthens the contraction, so weakness improves. In cholinergic crisis, additional acetylcholine worsens weakness through depolarising desensitisation. The improvement seen here therefore confirms myasthenic crisis and the need for a higher anticholinesterase dose. Lambert-Eaton involves presynaptic calcium channel antibodies and responds poorly to this test.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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