Insect-sting hypersensitivity — formative SAQs

Question 1 (10 marks)

A nine-year-old boy is stung on the forearm by a wasp at a picnic. Within four minutes he is flushed, wheezy and complaining his throat feels tight, and his lips are swelling. On arrival his oxygen saturation is 91 per cent in air, his respiratory rate is 34 with accessory-muscle use, and his blood pressure is 88/50. Outline your immediate and subsequent management, including the definitive workup and prevention package. [2]

Model answer

Immediate recognition and resuscitation (4 marks). This is systemic anaphylaxis to a wasp sting with both respiratory and cardiovascular compromise (Brown grade 3). Call for help, lie him flat with legs raised (he is hypotensive), give high-flow oxygen, and give intramuscular adrenaline into the anterolateral thigh without waiting for any further sign — 300 micrograms (he is around 30 kg). Remove any retained stinger by scraping sideways. Repeat adrenaline every five minutes if there is no response, and give an intravenous crystalloid bolus of 10 mL/kg for shock. Add an inhaled bronchodilator for refractory wheeze after adrenaline. Antihistamines and corticosteroids are adjuncts only. [1] [2]

Observation and disposition (2 marks). Arrange timed serum tryptase on arrival, at one to two hours, and at baseline at least twenty-four hours later; admit overnight because he has respiratory and cardiovascular compromise and needed resuscitation. Observe for the biphasic reaction, which can recur one to seventy-two hours later. Discharge once fully recovered and stable. [2] [5]

Definitive workup and prevention (4 marks). Refer to the allergy clinic at four to six weeks for venom-specific immunoglobulin E and skin testing to confirm the culprit (vespid), a baseline serum tryptase to stratify severity and screen for clonal mast-cell disease, and component-resolved diagnostics if double-positive. Refer for venom immunotherapy, which gives around 90 to 98 per cent protection once maintenance is reached over three to five years — indicated because he had a systemic reaction. Discharge with an adrenaline autoinjector (300 micrograms) — prescribe two if rural or remote — a written action plan, medical identification, and family and school education on avoidance and device use. The autoinjector rescues the next reaction; venom immunotherapy prevents it. [3] [4]

Question 2 (10 marks)

A seven-year-old girl with known systemic mastocytosis is stung by a bee in the garden. Her mother finds her collapsed and pale three minutes later, with no rash and no wheeze. Discuss the recognition, the role of baseline tryptase and the long-term prevention strategy. [9]

Model answer

Recognition (3 marks). This is venom anaphylaxis with cardiovascular collapse despite the absence of skin signs. In clonal mast-cell disease, up to a fifth or more of systemic sting reactions have no urticaria or flushing, and collapse minutes after a sting is anaphylaxis until proven otherwise. Lie her flat, give intramuscular adrenaline immediately — 150 micrograms if she is under 30 kg — remove the embedded honeybee sac, give oxygen and an intravenous fluid bolus, and repeat adrenaline every five minutes. Escalate to an intravenous adrenaline infusion if refractory. Never delay adrenaline for a rash or a test. [2] [9]

Role of baseline tryptase (3 marks). Baseline serum tryptase is the strongest predictor of sting-anaphylaxis severity and is characteristically elevated in systemic mastocytosis, reflecting the expanded hyper-reactive mast-cell clone driven by the KIT mutation. Measure it at the workup visit; an elevated value identifies the severe-reactor phenotype, prompts a KIT D816V blood test and bone-marrow assessment when indicated, and marks the child out for the most aggressive prevention. A normal acute tryptase never excludes anaphylaxis. [5] [9]

Long-term prevention (4 marks). Refer for lifelong venom immunotherapy using the honeybee extract, because mastocytosis confers a high risk of severe, recurrent sting anaphylaxis; premedicate and use a slower build-up to reduce reactions to the immunotherapy itself. Provide an adrenaline autoinjector (two devices), a written action plan, medical identification, and explicit education that the absence of a rash does not exclude anaphylaxis. Optimise any co-morbidity and review the medication list for beta-blockers, which blunt adrenaline and require glucagon in refractory disease. Venom immunotherapy gives around 90 to 98 per cent protection and is continued indefinitely in mastocytosis. [3] [11]