## Depolarising vs Non-depolarising Muscle Relaxants **Key Point:** Succinylcholine is the only clinically used depolarising agent in anaesthesia. It mimics acetylcholine and causes sustained depolarisation of the motor end plate, leading to initial fasciculations followed by flaccid paralysis. ### Mechanism of Action Succinylcholine binds to nicotinic acetylcholine receptors at the neuromuscular junction and: 1. Causes initial visible fasciculations (muscle twitching) 2. Produces sustained depolarisation that prevents repolarisation 3. Results in flaccid paralysis lasting 5–10 minutes ### Clinical Characteristics | Feature | Succinylcholine | | --- | --- | | **Type** | Depolarising | | **Onset** | 30–60 seconds | | **Duration** | 5–10 minutes | | **Metabolism** | Plasma pseudocholinesterase | | **Fasciculations** | Present (visible muscle twitching) | | **Potassium release** | Yes (can cause hyperkalaemia) | **High-Yield:** Succinylcholine is the only depolarising agent in current clinical use. All other commonly used muscle relaxants (vecuronium, atracurium, cisatracurium, rocuronium, pancuronium) are non-depolarising agents. **Clinical Pearl:** The fasciculations caused by succinylcholine can lead to postoperative myalgia and a rise in serum potassium. Pre-treatment with a small dose of non-depolarising agent can prevent fasciculations. **Warning:** Succinylcholine is contraindicated in patients with: - Malignant hyperthermia susceptibility - Pseudocholinesterase deficiency - Severe hyperkalaemia or conditions predisposing to it (burns, crush injuries, denervation) - Prolonged immobilisation - Muscular dystrophies
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