During prolonged anaesthesia, a patient receives repeated doses of succinylcholine. After several hours the neuromuscular blockade changes character: train-of-four shows fade, and the block is partially reversed by neostigmine. What has occurred?

• A Development of malignant hyperthermia with secondary receptor failure
• B Conversion to a phase II (desensitization-like) block resembling a non-depolarizing block ✓
• C Potentiation of depolarizing block by concurrent aminoglycoside therapy
• D Genetic pseudocholinesterase deficiency prolonging phase I block

Explanation

With prolonged or repeated exposure, succinylcholine converts from a phase I depolarizing block to a phase II block, in which the end plate behaves like a desensitized nicotinic receptor and the pattern resembles a competitive block with fade on train-of-four and partial reversibility by neostigmine. Phase I block shows no fade and is not reversed by anticholinesterases. Pseudocholinesterase deficiency simply prolongs duration without changing block character.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

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Written and medically reviewed by the StethoPrep medical team.