To the Editor: This is the first report of the use of magnesium sulfate to treat Irukandji syndrome.

A previously well 26-year-old commercial diver was stung on the neck by a jellyfish while collecting sea cucumbers in Barrier Reef waters off Townsville in February 2003. As is typical for an Irukandji syndrome, he was asymptomatic for about 30 minutes, after which he developed back and abdominal pain, nausea and headache.

He was retrieved from the scene by helicopter and arrived in the emergency department (ED) two hours after the onset of symptoms. On retrieval he had a blood pressure of 150/90 mmHg, agitation, marked diaphoresis, piloerection and some dyspnoea. A typical carybdeid jellyfish sting mark was present on the neck. The cardiac troponin I level was elevated from the time of admission.

En route and in the ED he was treated with intravenous morphine and diazepam. Skin scrapings were taken for nematocyst identification. After he had received 27.5 mg of morphine and 15 mg of diazepam, his pain settled somewhat, but abdominal discomfort, agitation and profuse diaphoresis persisted. Despite the dyspnoea, he showed no other overt clinical signs of cardiac failure.

Concern with the patient's increasing hypertension (170/100 mmHg five hours after envenomation) led to his being transferred to the high dependency unit (HDU) six hours after envenomation. It was decided to try a therapeutic trial of magnesium sulfate for this patient in an attempt to control the hypertension. This decision was taken on the basis of:

• the unsatisfactory results of measures taken thus far,

• the postulated hyperadrenergic basis of hypertension in Irukandji syndrome,

• the known 20%–30% fall in systemic vascular resistance associated with magnesium administration in hyperadrenergic states,1 and

• considerable experience within the HDU with managing severe pre-eclampsia.

Intravenous magnesium sulfate was administered as a loading dose of 10 mmol followed by an infusion of 5 mmol per hour.

Sympathetic features and agitation resolved, and pain nearly completely resolved towards the end of the loading dose. Of note, an early reduction in the rate of magnesium sulfate infusion resulted in recrudescence of hypertension, back pain and piloerection. The infusion was uneventfully reduced to 3 mmol per hour at 11 hours after envenomation, and discontinued at 20 hours after envenomation. No adverse effects related to the magnesium infusion were noted. The patient subsequently remained well. The troponin I level rose to a peak of 6.4 μg/L, and an echocardiogram was normal at 20 hours after envenomation.

Irukandji syndrome is produced by carybdeid jellyfish envenomation2 and has been shown (in animals) to be associated with dramatically elevated serum noradrenaline levels.3 Severe hypertension in Irukandji syndrome can be difficult to treat and has been associated with two deaths from intracranial haemorrhage. The origin of the extensive, severe pain associated with the syndrome is unknown. Postulated mechanisms include ischaemia from widespread small vessel vasoconstriction resulting from a hyperadrenergic state, and sodium channel opening in afferent pain fibres. Other mechanisms are equally likely. Induced catecholamine release or direct toxicity have been proposed as the cause of myocardial injury. This may produce overt, and occasionally severe, cardiac failure.

Magnesium decreases both catecholamine release and sympathetic terminal receptivity to catecholamines1 via multiple sites of action, including most calcium channel subtypes (both at the cell membrane and intracellularly), as well as modifying other cation fluxes. It reduces catecholamine-induced myocardial necrosis in phaeochromocytoma (Professor M James, Department of Anaesthesia, University of Cape Town, personal communication) and is widely used in other hyperadrenergic states, such as phaeochromocytoma and pre-eclampsia.1

The apparent efficacy of intravenous magnesium in our patient suggests the need to further investigate this therapy. A larger case series, a multicentre randomised trial of magnesium sulfate administration in Irukandji syndrome and a dose-finding study are under way.