A 62-year-old man with myasthenia gravis controlled on pyridostigmine is admitted with urosepsis. The physician wants a Gram-negative cover. Why should an aminoglycoside be avoided or used only with extreme caution?

• A Aminoglycosides inhibit acetylcholinesterase and worsen weakness by increasing acetylcholine at the motor end plate
• B Aminoglycosides bind pyridostigmine irreversibly, causing therapeutic failure
• C Aminoglycosides competitively block presynaptic calcium-dependent acetylcholine release and postsynaptic sensitivity to acetylcholine ✓
• D Aminoglycosides depolarise the motor end plate like succinylcholine and precipitate malignant hyperthermia

Explanation

Aminoglycosides reduce both the calcium-mediated presynaptic release of acetylcholine and the postsynaptic sensitivity of the receptor, producing curare-like neuromuscular blockade. This effect is minor normally but can precipitate respiratory paralysis in myasthenia gravis or after high doses, and it is potentiated by neuromuscular blockers. It is reversed by calcium gluconate and neostigmine. They do not inhibit cholinesterase and are not depolarising agents.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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