Myotonia congenita (MC) is caused by a defect in the skeletal muscle chloride channel function, which may cause sustained membrane depolarisation. We describe a previously healthy 32-year-old woman who developed a life-threatening muscle spasm and secondary ventilation difficulties following a preoperative injection of suxamethonium. The muscle spasms disappeared spontaneously and the surgery proceeded without further problems. When subsequently questioned, she reported minor symptoms suggesting a myotonic condition. Myotonia was found on clinical examination and EMG. The diagnosis MC was confirmed genetically. Neither the patient nor the anaesthetist were aware of the diagnosis before this potentially lethal complication occurred. We give a brief overview of ion channel disorders including malignant hyperthermia and their anaesthetic considerations.