Anaes · Neuromuscular blockade & reversal

Suxamethonium

Also known as Succinylcholine · Sux · Depolarising neuromuscular blocker · RSI muscle relaxant · Diacetylcholine

Suxamethonium (succinylcholine, sux) is the ONLY depolarising neuromuscular blocker in clinical use — structurally it is two acetylcholine molecules joined back-to-back as a di-acetylcholine ester, and it activates the postsynaptic nicotinic (muscle) receptor to produce an initial depolarisation (the visible fasciculation) followed by a sustained depolarisation that inactivates the surrounding sodium channels and produces a phase I block and flaccid paralysis. It has the FASTEST onset of any neuromuscular blocker at 30 to 60 seconds and the SHORTEST duration at 5 to 10 minutes, which makes it the rapid-sequence induction agent of choice for the full-stomach or aspiration-risk patient and for the difficult airway (O'Connell 2026; Marcus 2026). It is not hydrolysed by synaptic acetylcholinesterase but by plasma butyrylcholinesterase synthesised in the liver, so butyrylcholinesterase deficiency — genetic (dibucaine-resistant, autosomal recessive) or acquired (liver disease, pregnancy, organophosphates) — produces prolonged apnoea lasting hours (Snak de Souza 2026; Abbasi 2026). Repeated boluses or large total doses convert the block into a phase II (desensitisation) block that resembles a non-depolariser. The exam-critical adverse-effect and contraindication list is long and high-yield: severe hyperkalaemia in burns (beyond 24 hours and up to about 2 years), denervation, spinal-cord injury, prolonged immobility and critical illness from upregulated extrajunctional acetylcholine receptors (Puxty 2026); malignant hyperthermia — sux is a potent RYR1-mediated trigger (Fang 2026); bradycardia; raised intraocular pressure (contraindicated in the open-eye injury); raised intracranial and intragastric pressure; fasciculation and postoperative myalgia; masseter spasm; anaphylaxis. Rocuronium 1.2 mg/kg reversed by sugammadex is the standard sux-sparing alternative (Kronauer 2026; Zhang 2026).