A patient given succinylcholine 1.5 mg/kg for rapid sequence intubation develops generalised muscular rigidity, inability to open the mouth, and hyperthermia within 5 minutes. Volatile anaesthetic was not used. The MOST likely diagnosis and explanation is:

• A Masseter muscle spasm only (isolated trismus); a benign succinylcholine side effect not requiring treatment
• B Succinylcholine-induced hyperkalaemia causing widespread skeletal muscle tetany
• C Malignant hyperthermia; succinylcholine triggers RYR1 mutation-mediated sarcoplasmic Ca2+ release ✓
• D Phase II block from overdose producing paradoxical sustained depolarisation

Explanation

Succinylcholine is a recognised and potent trigger for malignant hyperthermia (MH) equal in potency to the volatile agents. In susceptible patients with mutations in RYR1 (ryanodine receptor type 1) or CACNA1S, succinylcholine causes uncontrolled calcium release from the sarcoplasmic reticulum, producing hypermetabolism, hyperthermia, generalised rigidity, and eventually rhabdomyolysis. Masseter spasm following succinylcholine may be an early MH sign and must never be dismissed as benign. The absence of a volatile agent does not preclude MH when succinylcholine is used.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.