Clinical scenario
A 48-year-old man with cutaneous mastocytosis (urticaria pigmentosa) is brought to the emergency department 10 minutes after a wasp sting. He complains of severe dizziness, nausea, and dyspnoea. He carries an epinephrine autoinjector but did not use it as the initial rash was not severe. His known baseline tryptase is 32 mcg/L.
Clinical examination
General appearance: pallid, diaphoretic, lethargic. Blood pressure: 72/40 mmHg. Heart rate: 120 bpm. Respiratory rate: 24/min. SpO2: 93% on room air. Skin: small brownish macules over trunk (urticaria pigmentosa lesions) with faint urticaria. Lungs: faint wheeze. Abdomen: nausea.
Labs & imaging
Acute serum tryptase (on arrival): 78 mcg/L. Known baseline tryptase: 32 mcg/L.
Given acute anaphylaxis following a wasp sting in a patient with mastocytosis with an acute tryptase of 78 mcg/L and a baseline tryptase of 32 mcg/L what is the most appropriate immediate treatment and the most important long-term management step after stabilisation?
Mastocytosis is an independent risk factor for severe anaphylaxis following Hymenoptera stings because excess mast cells release massive quantities of inflammatory mediators | Acute tryptase of 78 mcg/L compared to a baseline of 32 mcg/L confirms anaphylaxis per the EAACI formula: 78 > (1.2 × 32) + 2 = 40.4 | Immediate treatment is unchanged: intramuscular epinephrine into the mid-thigh immediately with fluids | The strategic priority after stabilisation is Hymenoptera venom immunotherapy (VIT) which is the gold standard for prevention of recurrent anaphylaxis in Hymenoptera venom allergy per EAACI guidelines | EAACI 2021 guidelines clearly state: patients with mastocytosis and any prior systemic reaction have an absolute indication for epinephrine autoinjector prescription | VIT in mastocytosis patients should be administered lifelong as the risk of recurrence does not resolve | Differential diagnosis for elevated tryptase includes: systemic mastocytosis (confirmed here by history) hereditary alpha-tryptasaemia and acute anaphylactic reaction itself | The critical error is substituting antihistamine prophylaxis for VIT | Observation must be 12-24 hours given haemodynamic compromise | The epinephrine autoinjector must be prescribed at a dose of at least 0.3 mg for adults per guideline recommendations