The adverse effect profile of succinylcholine is the primary reason its use is restricted and requires careful patient selection. Several of these effects are mechanism-based, predictable, and life-threatening when they occur in vulnerable populations. Knowing which patients are at risk is as clinically essential as knowing the drug's pharmacokinetics.

Hyperkalemia is the most dangerous mechanism-based adverse effect of succinylcholine and arises from the efflux of intracellular potassium through depolarized end-plate ion channels. In normal subjects, this potassium release is confined to junctional nAChRs and raises serum potassium by approximately 0.5 mEq/L – a clinically trivial increase. In pathological states characterized by upregulation of extrajunctional fetal-type nAChRs, however, the depolarization extends across the entire muscle surface, and potassium efflux from millions of additional channels can raise serum potassium by 5 to 10 mEq/L or more, precipitating life-threatening arrhythmias and cardiac arrest.⁴ The at-risk populations include patients with burns (risk begins at 24 hours after injury and persists for up to one year), prolonged immobilization or disuse atrophy, upper or lower motor neuron denervation (stroke, spinal cord injury, peripheral neuropathy, Guillain-Barré syndrome), severe sepsis or septic shock with prolonged critical illness, and rhabdomyolysis. The risk window is not immediate – the upregulation of extrajunctional receptors requires time to develop, which is why a patient with an acute stroke who receives succinylcholine within the first 24 hours is at little additional risk, but the same patient after one week of hemiplegia carries a significantly elevated risk.

Malignant hyperthermia (MH) is a pharmacogenetic hypermetabolic crisis triggered by succinylcholine (and volatile anesthetic agents) in susceptible individuals. MH susceptibility is most commonly caused by autosomal dominant mutations in the ryanodine receptor gene (RYR1), less commonly by mutations in the CACNA1S gene encoding the dihydropyridine receptor. In susceptible individuals, succinylcholine triggers uncontrolled calcium release from the sarcoplasmic reticulum, leading to sustained muscle contraction, rapid oxygen consumption, hyperthermia (temperature rising as fast as 1°C every 5 minutes), hypercarbia, acidosis, rhabdomyolysis, and hyperkalemia. The condition is rapidly fatal without treatment.⁵ Dantrolene sodium, which inhibits ryanodine receptor-mediated calcium release, is the specific antidote and must be immediately available in any facility using succinylcholine or volatile agents. Initial dosing is 2.5 mg/kg intravenously, repeated every 5 minutes until the crisis is controlled, with a total dose potentially exceeding 10 mg/kg.

Additional adverse effects of succinylcholine include bradycardia, which reflects stimulation of cardiac muscarinic receptors by succinylcholine or its metabolite succinylmonocholine and is most pronounced in children and following repeated doses; increased intraocular pressure (IOP), resulting from sustained contraction of extraocular muscles, which is clinically relevant in open globe injuries where a transient IOP spike can extrude vitreous; and increased intragastric pressure, which is partially offset by a concomitant increase in lower esophageal sphincter tone, making the net aspiration risk less than historically assumed. Postoperative myalgia, occurring in 10 to 60 percent of patients and most common in ambulatory patients who are mobile within hours of receiving succinylcholine, is thought to arise from asynchronous fasciculations producing muscle micro-injury. It is more prevalent in women and in patients receiving the drug without pretreatment.¹

Pseudocholinesterase (butyrylcholinesterase) deficiency prolongs succinylcholine duration by impairing its plasma hydrolysis. Normal pseudocholinesterase activity produces the standard 8 to 12 minute block; reduced enzyme activity or genetically abnormal enzyme variants produce progressively longer block, with complete genetic deficiency of functional enzyme resulting in block lasting several hours – so-called scoline apnea. The dibucaine number is the standard clinical test for pseudocholinesterase variants: dibucaine, a local anesthetic, inhibits normal pseudocholinesterase by approximately 80 percent (dibucaine number 80), while the atypical variant most commonly responsible for prolonged block (homozygous Eu/Eu, caused by an Asp70Gly substitution) is inhibited by only 20 percent (dibucaine number 20).⁶ Heterozygotes have an intermediate dibucaine number of approximately 60 and experience moderately prolonged block of 20 to 30 minutes. Management of scoline apnea is supportive: maintain sedation and mechanical ventilation until spontaneous recovery occurs; do not attempt reversal with anticholinesterase agents, as these will worsen the block by inhibiting residual pseudocholinesterase activity.

Each non-depolarizing NMBD has a distinct pharmacokinetic profile that directly determines its clinical role. The following profiles cover the agents in routine current use. Understanding the elimination pathway for each agent is the key to rational selection in patients with organ dysfunction.

Rocuronium is the most widely used NDNMBD in contemporary practice and the only non-depolarizing agent whose onset speed approaches that of succinylcholine when given at high doses. At the standard intubating dose of 0.6 mg/kg, rocuronium produces intubating conditions within 60 to 90 seconds with a clinical duration of 30 to 45 minutes. At the RSI dose of 1.2 mg/kg, onset is 45 to 60 seconds – comparable to succinylcholine – but clinical duration extends to 60 to 90 minutes, requiring sugammadex 16 mg/kg for rapid reversal of deep block if intubation fails.³ Rocuronium is eliminated primarily by biliary excretion (approximately 50 percent unchanged) and hepatic metabolism, with a minor renal contribution; its duration is prolonged in hepatic failure and in elderly patients with reduced hepatic blood flow. Rocuronium does not release histamine and has minimal cardiovascular effects at clinical doses. Its lipophilicity relative to other aminosteroids accounts for its rapid onset, as greater lipophilicity facilitates faster diffusion to the NMJ.

Vecuronium is an intermediate-duration aminosteroid with a clinical duration of 25 to 40 minutes at an intubating dose of 0.1 mg/kg. It is the monoquaternary analog of pancuronium, achieved by removing one methyl group from the nitrogen in ring A, which eliminates the vagolytic effect that makes pancuronium problematic in cardiac patients. Vecuronium undergoes predominantly hepatic deacetylation to three metabolites, of which the 3-desacetyl metabolite retains approximately 50 percent of the neuromuscular blocking potency of the parent compound and accumulates in renal failure, producing prolonged block in patients receiving vecuronium infusions in the intensive care unit (ICU).⁹ This active metabolite accumulation is the principal reason cisatracurium has largely supplanted vecuronium for long-term ICU paralysis.

Pancuronium is a long-acting bisquaternary aminosteroid with a clinical duration of 60 to 90 minutes at an intubating dose of 0.1 mg/kg. Its elimination is predominantly renal (approximately 80 percent unchanged), making its duration markedly prolonged in renal failure. Pancuronium blocks cardiac muscarinic receptors, producing tachycardia, and inhibits neuronal uptake of catecholamines, resulting in a modest increase in blood pressure and heart rate – effects that were considered hemodynamically favorable in earlier eras of cardiac anesthesia but are now generally viewed as undesirable. Its use has declined substantially, and it is no longer a first-line agent in most settings. It cannot be reversed by sugammadex at clinically practical doses due to lower binding affinity compared with rocuronium and vecuronium.⁷

Atracurium is an intermediate-duration benzylisoquinolinium that undergoes elimination by two organ-independent pathways: Hofmann elimination (spontaneous chemical degradation at physiological pH and temperature, producing laudanosine and a monoquaternary acrylate) and plasma ester hydrolysis. Clinical duration at a standard dose of 0.5 mg/kg is 25 to 40 minutes. The Hofmann pathway produces laudanosine, a tertiary amine that crosses the blood-brain barrier and has been shown to cause central nervous system (CNS) excitation and seizures at high concentrations in animal models; at clinically relevant plasma concentrations in humans, laudanosine accumulation is generally not associated with detectable CNS toxicity, though it warrants monitoring in ICU patients receiving prolonged high-dose atracurium infusions.⁸ Atracurium releases histamine from mast cells in a dose-dependent fashion at doses above 0.5 mg/kg, potentially producing flushing, urticaria, bronchospasm, and hypotension; slow administration over 30 to 60 seconds attenuates but does not eliminate this effect.

Cisatracurium is one of the ten stereoisomers of atracurium and is approximately three times more potent, with markedly less histamine-releasing activity at clinical doses. Its elimination is predominantly via Hofmann degradation, with plasma esterase hydrolysis making a minor contribution; unlike the mixture of isomers in atracurium, cisatracurium generates less laudanosine per milligram of drug administered. Clinical duration at a standard dose of 0.15 mg/kg is 40 to 60 minutes. Cisatracurium's organ-independent elimination and low histamine release make it the agent of choice for patients with combined hepatic and renal failure, hemodynamic instability, and for long-term ICU paralysis in acute respiratory distress syndrome (ARDS) where prolonged infusions are required.¹⁰ Mivacurium is a short-acting benzylisoquinolinium hydrolyzed rapidly by plasma pseudocholinesterase, producing a clinical duration of 12 to 20 minutes; its use is limited by pseudocholinesterase-dependent duration variability and histamine release at higher doses.