Definition

1. Multisystem severe hypersensitivity reaction of sudden onset (or rapidly progressive).
2. Requires at least 1 of the following 3 criteria:
   1. Acute onset of mucocutaneous signs AND 1 of the following:
      1. respiratory compromise (wheezing-bronchospasm,
      2. dyspnea, stridor, hypoxemia),
      3. ↓BP (syncope),
      4. hypotonia.
   2. Rapid onset of 2 of the following after exposure to likely allergen:
      1. mucocutaneous signs,
      2. respiratory compromise,
      3. hypotension,
      4. persistent gastrointestinal symptoms.
   3. Hypotension after exposure to a known allergen.

Pathophysiology

1. Allergic: Type 1 hypersensitivity reaction: allergen crosslinks specific IgE antibodies on mast cells and basophils → rapid release (degranulation) of stored histamine, LTs, PAF, cytokines & chemotactic factors →capillary leakage, mucosal oedema →ultimately shock and asphyxia. Usually immediate (<1hr), occasionally biphasic & rarely delayed.
2. Anaphylactoid: degranulation via non-IgE mediated pathway.

Causes

1. Idiopathic.
2. Foods: (peanuts, other nuts, shellfish, fish, eggs, milk, strawberries, mushrooms)
3. Venom: (bee/wasp stings) or antivenoms
4. Drugs (incl Abx, opioids, NSAIDs, IV contrast, muscle relaxant, streptokinase)
5. Others: Latex, heat/cold, exercise.

Presentation

1. History: Previous reaction, new exposure to a drug / food.
2. Mucocutaneous: Urticaria, rhinitis, conjunctivitis and angio-oedema.
3. Respiratory: itching of the palate / external auditory meatus, dyspnoea, stridor / wheezing.
4. CVS: Palpitations, tachycardia, hypotension, syncope, collapse
5. GIT: Nausea, vomiting, abdominal pain, diarrhoea
6. Other: Sense of impending doom

Management

1. Attach monitoring, take vital signs, ECG.
2. Remove allergen, sting, wash mouth out etc.
3. Airway: Consider suction, intubation (adrenaline 1:1000 5ml neb may help if poor view, but don’t delay if worsening) – beware hypotension may be exacerbated with drugs.
4. CPR if cardiac arrest
5. High flow O2
6. Adrenaline
   1. 0.3–0.5mg (0.3–0.5ml of 1:1000) [child 10mcg/kg or 0.01ml/kg 1:1000] IM stat
   2. If unresponsive to 2 x IM doses or moribund, consider IV options:
      1. Bolus of 0.1ml/kg 1:100,000 IV over 5–10min [1mcg/kg]
         1. Dilute 1ml of 1:10,000 [100mcg] in 10ml NS to get 1:100,000 [10mcg/ml]
   3. Infusion of 0.1mcg/kg/min–1mcg/kg/min, titrating from lower dose
      1. Dilute 6ml of 1:1000 [6mg] in 1L NS [6mcg/ml] start @ 1ml/kg/hr [0.1mcg/kg/min]
      2. If fluid an issue use 100ml NS [10mcg/ml] & start @ 0.1ml/kg/hr [0.1mcg/kg/min]
   4. If resistant to adrenaline (e.g. on –blockers), try 1–2mg glucagon IV over 5min
7. IVC & Fluids:
   1. 0.9% Saline or colloid 500ml–1L [child 10–20ml/kg] boluses.
8. Other therapies:
   1. Salbutamol 5mg [2.5mg<20kg] if bronchospasm only.
   2. Steroids (?may ↓delayed/biphasic reactions) if asthmatic or severe: hydrocortisone 200mg [child:4mg/kg] IV or prednisolone 50mg [1mg/kg] PO
   3. Antihistamines for skin manifestations. Avoid IV route as can ↓BP. H1±H2 blockers:
      1. Promethazine 10–25mg [child: 0.125–0.5mg/kg] PO or non-sedating **loratidine 10mg [child: 5mg if ≤6yr, 2.5mg if 1–2yr] PO
      2. Ranitidine 150mg [child: 3mg/kg] PO or 50mg [1mg/kg] IV

Investigations

1. Consider serial serum tryptase levels (immediately, 2hr & 24hr post-exposure) – if high suggests degranulation by mast cells (low level doesn’t exclude anaphylaxis)
2. Outpatient allergy testing

Disposition

1. Observation for at least 6hrs and admit if:
   1. Asthmatic component to their anaphylactic reaction
   2. Previous history of biphasic reactions
   3. Possibility of continuing absorption of allergen
   4. Poor access to emergency care
   5. Presentation in the evening or at night
   6. Severe reactions with slow onset caused by idiopathic anaphylaxis
2. On discharge:
   1. Organise prescription & education on usage of an EpiPen® (adult 300μg 1:1000 adrenaline, child<20kg 150μg 1:2000 adrenaline)
   2. Encourage patient to wear a Medic alert bracelet/necklace endorsed by doctor.
   3. Consider 3 day course of antihistamines and oral steroids.