Paediatrics chapters
Anaphylaxis in children
A sudden, life-threatening allergic reaction: common triggers in children, how it presents, how to tell it from look-alikes, the emergency steps led by intramuscular adrenaline, observation for late reactions, and prevention.
- Anaphylaxis is a sudden, life-threatening, IgE-mediated (type I) reaction with mast cell mediator release, causing urticaria and angioedema, airway obstruction or bronchospasm, and circulatory collapse; common triggers are drugs (antibiotics), foods (peanuts, eggs, milk), insect stings and latex.
- The first and most important drug is intramuscular adrenaline 0.01 mg/kg (0.01 mL/kg of 1:1000, maximum 0.5 mL), repeated if needed; then oxygen, lie the child flat, 0.9% saline 20 mL/kg for shock, salbutamol for wheeze; antihistamines and steroids are second-line.
- Observe for at least 12 hours because of biphasic reactions; prevent with trigger avoidance, an adrenaline autoinjector and an emergency plan.
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Overview
Anaphylaxis is an acute, severe, life-threatening systemic allergic reaction. It starts within minutes (sometimes up to a few hours) of exposure to a trigger and can kill through airway obstruction or shock. Every doctor giving injections, vaccines or antibiotics must recognise it at once and give intramuscular adrenaline without delay.
Mechanism
- Anaphylaxis is a type I (immediate) hypersensitivity reaction. The allergen cross-links IgE on mast cells and basophils, which release:
- Histamine, tryptase, leukotrienes, prostaglandins, platelet-activating factor and cytokines.
- These cause vasodilatation, capillary leak, bronchoconstriction, mucosal oedema and itch.
- Anaphylactoid reactions look the same but are not IgE-mediated: mast cells are triggered directly (for example opiates, radiocontrast media) or through complement products C3a and C5a (anaphylatoxins, for example with blood transfusion).
Treatment is the same for anaphylactic and anaphylactoid reactions.
Causes in children
| Group | Examples |
|---|---|
| Foods (commonest in children) | Peanuts, tree nuts, milk, eggs, fish, shellfish, seeds, fruits |
| Drugs | Penicillins and cephalosporins, sulphonamides, NSAIDs, opiates, muscle relaxants, vancomycin, local anaesthetics, insulin |
| Insect stings | Bees, wasps, hornets, fire ants |
| Latex | Gloves, catheters |
| Vaccines and biologicals | Reaction to a previous dose or a component (for example gelatin, neomycin); immunoglobulin, blood products |
| Others | Radiocontrast media, exercise (sometimes after a specific food), cold, allergen immunotherapy |
| Idiopathic | No trigger found |
- The parenteral route (injection, sting) gives the fastest and most severe reactions; ingestion is also common; inhalation is less common.
- A history of anaphylaxis to a previous vaccine dose or vaccine component is a contraindication to further doses of that vaccine.
Clinical features
The faster the onset, the more severe the reaction usually is.
| System | Features |
|---|---|
| Skin (the commonest) | Urticaria (itchy wheals), angioedema, flushing, itching, warmth |
| Upper airway | Lip, tongue and laryngeal oedema, hoarseness, stridor |
| Lower airway | Wheeze, cough, breathlessness, cyanosis |
| Cardiovascular | Tachycardia, hypotension, shock, arrhythmia, collapse |
| Gastrointestinal | Nausea, vomiting, crampy abdominal pain, diarrhoea |
| Nervous system | Dizziness, syncope, seizures, sense of impending doom |
| Nose and eyes | Sneezing, rhinorrhoea, nasal congestion |
No rash does not mean no anaphylaxis. Nelson notes sudden cardiovascular collapse can occur without skin signs.
Akbar's typical case
A child on intravenous ampicillin for meningitis develops urticaria, wheeze, tachycardia and breathlessness within 10 minutes of the infusion. This is anaphylaxis to a beta-lactam drug: stop the infusion and give IM adrenaline at once.
Biphasic and protracted reactions
- Up to 20% of patients have a biphasic or protracted course (Nelson).
- Biphasic: symptoms return after an initial recovery, usually 8-72 hours later.
- Protracted: symptoms last for hours or days despite treatment.
Observe every child with anaphylaxis for at least 12 hours (Akbar), because the reaction can recur after apparent recovery.
Diagnosis
- Clinical: sudden onset of skin and/or mucosal signs with airway, breathing or circulation problems after a likely trigger.
- Serum tryptase helps when the diagnosis is uncertain:
- Peaks 1-1.5 hours after onset; take blood about 1-2 hours after the start.
- A normal tryptase does not exclude anaphylaxis.
- Histamine is less useful because it is cleared very quickly.
- Later, allergy referral for skin prick or specific IgE testing to identify the trigger.
Differential diagnosis
| Condition | Clue |
|---|---|
| Vasovagal faint | Bradycardia, pallor, sweating, no urticaria, wheeze or stridor; quick recovery lying flat |
| Acute asthma | Wheeze without urticaria, hypotension or angioedema |
| Foreign body aspiration | Sudden choking while eating or playing; no skin signs |
| Hereditary angioedema | Recurrent swelling without urticaria or itch; family history; low C4 |
| Septic shock | Fever, infection source; slower onset |
| Panic attack | Hyperventilation, tingling, normal blood pressure, no rash |
Hereditary angioedema (HAE)
- Autosomal dominant deficiency (or dysfunction) of C1-esterase inhibitor; about a quarter are new mutations.
- Recurrent deep swelling of face, limbs, genitals and gut (abdominal pain), often after trauma; laryngeal oedema can be fatal.
- Urticaria is rare; adrenaline and antihistamines help little.
- Screen with C4 (low); then measure C1-inhibitor level and function.
- ACE inhibitors can also cause angioedema.
Angioedema without urticaria or itch, recurring, with a family history, points to hereditary angioedema, not allergy.
Emergency management
Act at once. Stop the trigger (for example stop the drug infusion, remove a bee sting) and call for help.
Step 1: ABC and adrenaline
- Airway, breathing, circulation; start CPR if there is no pulse.
- Intramuscular adrenaline is the first and most important drug:
- 0.01 mL/kg of 1:1000 (that is 0.01 mg/kg), maximum 0.5 mL (0.5 mg) per dose (Akbar).
- Repeat if there is no improvement (Akbar repeats about every 15 minutes as needed).
- Do not use the subcutaneous route; absorption is less reliable.
- If signs persist despite repeated IM doses, an adrenaline infusion (0.1-1 microgram/kg/minute) under monitoring is needed.
A 20 kg child with anaphylaxis
Dose = 0.01 mg/kg × 20 kg = 0.2 mg
Volume of 1:1000 (1 mg/mL) = 0.2 mL IM
A 60 kg adolescent: 0.6 mg by weight, so give the maximum 0.5 mL (0.5 mg)
1:1000 (1 mg/mL) is for intramuscular use; 1:10,000 (0.1 mg/mL) is the strength used intravenously in cardiac arrest. Mixing them up causes serious overdose or underdose.
Step 2: supportive measures
- High-flow oxygen by mask.
- Position: keep the child lying down (supine as tolerated); if hypotensive, tilt head down about 30 degrees with legs raised (Akbar).
- Intravenous fluid for hypotension: 0.9% saline 20 mL/kg bolus, repeated in 10-20 mL/kg boluses until blood pressure recovers.
- Bronchospasm: nebulised salbutamol (continuous if needed).
- Upper airway oedema: nebulised adrenaline may help mild to moderate swelling, but early intubation may be needed; in extreme cases a surgical airway.
- Mechanical ventilation if needed.
Step 3: second-line drugs (after adrenaline)
| Drug | Role |
|---|---|
| H1 antihistamine | Relieves itch and urticaria; does not treat airway obstruction or shock |
| H2 antihistamine | Adjunct |
| Corticosteroid | Akbar: IV methylprednisolone 2 mg/kg, then 2 mg/kg/day in divided doses (or oral prednisolone 2 mg/kg once daily); may reduce prolonged symptoms; slow to act |
| Aminophylline | For prolonged bronchospasm (Akbar) |
| Glucagon | For a child on beta-blockers whose shock does not respond to adrenaline |
Antihistamines and steroids never replace adrenaline. Giving them first and delaying adrenaline is a classic, fatal error.
"A before everything": Adrenaline IM, Airway, then fluids and the rest.
After the acute episode
- Observe at least 12 hours; longer after severe reactions.
- Record the reaction clearly in the notes, especially drug and vaccine reactions.
- Refer to an allergy specialist to confirm the trigger.
- Teach the family the signs of anaphylaxis and how to use an adrenaline autoinjector; give a written emergency action plan.
- Advise a medical alert bracelet.
- Stop medicines that worsen anaphylaxis or block its treatment: beta-blockers, ACE inhibitors, MAO inhibitors.
Prevention
- Avoid known triggers (foods, drugs, latex).
- Always ask about drug and food allergy before giving drugs or vaccines.
- Keep adrenaline 1:1000 and resuscitation equipment ready wherever injections and vaccines are given, and observe after vaccination.
- Venom immunotherapy for severe insect sting allergy and desensitisation to essential drugs (for example beta-lactams) are specialist options.
- Children with cold urticaria must not swim alone and should carry adrenaline.
Expect: definition and type I IgE mechanism; anaphylactoid reactions (opiates, radiocontrast); common triggers (antibiotics, peanuts, eggs, milk, insect stings, latex); skin signs common but not essential; IM adrenaline 1:1000 at 0.01 mL/kg (maximum 0.5 mL) as first-line, not subcutaneous; 1:1000 versus 1:10,000; saline 20 mL/kg for shock; salbutamol for wheeze; antihistamines and steroids as adjuncts; glucagon in beta-blocked patients; biphasic reactions and observation for at least 12 hours; tryptase; hereditary angioedema with low C4 and no urticaria; and prevention with autoinjectors and avoidance.
Quick revision
- Type I, IgE, mast cell release of histamine and tryptase.
- Triggers: foods, antibiotics, stings, latex, vaccines, contrast.
- Skin (commonest), airway, breathing, shock, gut, impending doom.
- IM adrenaline 1:1000, 0.01 mL/kg, maximum 0.5 mL; repeat if needed.
- Oxygen; lie flat; saline 20 mL/kg; salbutamol; airway support.
- Antihistamine and steroid only after adrenaline; glucagon if on beta-blocker.
- Biphasic in up to 20% (8-72 hours); observe at least 12 hours.
- HAE: C1-inhibitor deficiency, no urticaria, low C4.
- Discharge with avoidance advice, autoinjector, action plan, alert bracelet.
Written by The House Job from standard references (Basis of Paediatrics by Pervez Akbar; Nelson Essentials of Paediatrics).
FAQ
What is the dose of adrenaline for anaphylaxis in a child?
Intramuscular adrenaline 1:1000 at 0.01 mL/kg (0.01 mg/kg), up to 0.5 mL per dose. Repeat if the child does not improve.
Why not give adrenaline subcutaneously?
Absorption from under the skin is slower and less reliable, especially in shock. The intramuscular route gives faster, more predictable levels.
Are antihistamines enough for a mild reaction?
For isolated hives they help, but once there are airway, breathing or circulation signs it is anaphylaxis and adrenaline is needed first. Antihistamines do not reverse airway swelling or shock.
Why keep a child in hospital after recovery?
Up to one in five children have a second wave of symptoms hours later (biphasic reaction), so observation for at least 12 hours is advised.