General MedicineTier 1Disease (DEADMAN)

Hypotension or shock - anaphylactic

Description

Distributive shock from IgE-mediated (or non-IgE mast cell/basophil activation) systemic hypersensitivity reaction
Anaphylaxis = allergic reaction + hypotension, airway compromise, or significant respiratory/GI/other organ involvement

Epidemiology

Common triggers: food (peanut, tree nut, shellfish - commonest in children/young adults), insect venom, drugs (beta-lactams, NMBAs, contrast, biologics)
Rising incidence, particularly food-related anaphylaxis in children

Aetiopathogenesis

Allergen cross-links IgE on mast cells/basophils -> histamine, tryptase, leukotriene, prostaglandin release -> vasodilation, increased capillary permeability, bronchoconstriction, increased mucus secretion
Distributive shock from profound vasodilation + capillary leak (relative + absolute hypovolaemia)
Biphasic reactions - recurrence of symptoms hours after apparent resolution, mechanism not fully understood

Diagnosis

Clinical - acute onset with skin/mucosal involvement PLUS respiratory compromise, hypotension, or persistent GI symptoms, typically within minutes of exposure
Serum tryptase (elevated within 1-2h of onset) supports retrospective diagnosis, especially if clinical picture atypical
Exclude mimics - vasovagal (bradycardia, no urticaria/respiratory involvement), ACE-inhibitor angioedema (no urticaria, doesn't respond typically)

Management

1. IM adrenaline - immediate, first-line, repeatable

0.5mg (0.5mL of 1:1000) IM anterolateral thigh, repeat every 5 minutes as needed
IV adrenaline infusion for refractory shock (specialist/critical care setting)

2. Supportive

High-flow oxygen, supine positioning with legs raised (unless respiratory distress - then sit up), large-bore IV access, crystalloid fluid bolus for hypotension
Remove/stop the trigger where identifiable (stop the infusion, remove the stinger)

3. Adjuncts (not substitutes for adrenaline)

Antihistamines and corticosteroids - symptomatic relief only, do not treat the airway/haemodynamic compromise acutely, corticosteroids may reduce biphasic reaction risk

4. Observation and follow-up

Minimum 4 hours observation post-resolution (longer for severe reactions) for biphasic reaction
Discharge with adrenaline autoinjector, written action plan, allergy/immunology referral for confirmatory testing and allergen identification

Associations

Biphasic anaphylaxis (recurrence without re-exposure), asthma as a risk factor for more severe respiratory involvement
Mastocytosis - predisposes to more frequent/severe reactions
Idiopathic anaphylaxis - no identifiable trigger in a proportion of cases

Natural history & complications

Rapid response to prompt adrenaline administration; delayed/withheld adrenaline is associated with worse outcomes and fatality
Biphasic reactions occur in a minority but justify the extended observation period
Risk of future reactions persists with continued/inadvertent allergen exposure - allergen avoidance education and an action plan are essential for secondary prevention

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