Succinylcholine

Mechanism of action

• Structurally resembles acetylcholine and acts as an agonist at nicotinic acetylcholine receptors at the neuromuscular junction.
• It causes persistent motor end-plate depolarization:
  1. Initial depolarization produces visible muscle fasciculations.
  2. Continued depolarization prevents repolarization and sodium-channel reset, so further muscle action potentials cannot occur.
  3. This results in flaccid paralysis, including respiratory muscles.

• The end plate remains depolarized.
• Anticholinesterases generally intensify or prolong this block.

• With prolonged or repeated administration, the membrane may repolarize but becomes desensitized to acetylcholine.
• It resembles a nondepolarizing block and can cause prolonged weakness.

Uses

1. Rapid-sequence induction and endotracheal intubation
   • Particularly useful when a rapidly secured airway is required, for example in emergency surgery or a patient at aspiration risk.
2. Brief procedures requiring intense muscle relaxation
   • Such as laryngoscopy, bronchoscopy, oesophagoscopy, and some ENT endoscopic procedures.
3. Electroconvulsive therapy
   • Reduces musculoskeletal injury caused by seizure-related muscle contractions.
4. Relief of laryngospasm
   • May be used by trained clinicians in severe refractory laryngospasm.

Adverse effects

• Fasciculations followed by paralysis.
• Postoperative myalgia, especially in ambulatory and muscular patients.
• Hyperkalemia, sometimes severe and fatal.
  • Risk is markedly increased in major burns after the first 24-48 hours, crush injury or major trauma, denervation, spinal cord injury, upper motor neuron lesions, prolonged immobilization, and neuromuscular disorders.
• Bradycardia and arrhythmias, especially in children, after repeated doses, or with coexisting hyperkalemia.
• Malignant hyperthermia in genetically susceptible people, particularly with volatile anesthetics.
• Prolonged apnea/paralysis in pseudocholinesterase deficiency or reduced enzyme activity, such as in severe liver disease, pregnancy, malnutrition, or exposure to cholinesterase-inhibiting drugs.
• Raised intraocular pressure.
• Raised intragastric pressure, which may increase regurgitation risk.
• Masseter muscle rigidity.
• Anaphylaxis or other hypersensitivity reactions.
• Rarely, rhabdomyolysis, cardiac arrest, and death, particularly in children with unsuspected myopathy.

Important contraindications / avoidance

• Known or suspected malignant hyperthermia susceptibility
• Pseudocholinesterase deficiency
• Severe burns, major trauma, denervation injury, prolonged immobilization
• Known skeletal-muscle myopathies, including Duchenne muscular dystrophy
• Existing hyperkalemia
• History of severe reaction or anaphylaxis to succinylcholine