Write a short note on anaphylaxis.

Definition

Anaphylaxis is a severe, life-threatening, generalised or systemic hypersensitivity reaction of rapid onset, characterised by airway, breathing and/or circulatory compromise, usually with skin and mucosal changes. It is a clinical diagnosis and a medical emergency in which treatment must never await investigation.

Mechanism

• Immunological (IgE-mediated, type I hypersensitivity) — the classic mechanism. Prior sensitisation produces IgE bound to mast cells and basophils; re-exposure cross-links IgE, causing degranulation with release of histamine, tryptase, prostaglandins and leukotrienes.
• Non-IgE immunological — IgG or complement mediated (e.g. some drug and blood product reactions).
• Non-immunological (formerly "anaphylactoid") — direct mast cell degranulation without prior sensitisation, so a first exposure can cause a reaction: radiocontrast media, opioids, vancomycin ("red man syndrome"), NSAIDs.

The mediators cause vasodilatation, increased vascular permeability with plasma leak, bronchoconstriction, mucus secretion and myocardial depression — producing distributive shock.

Common Triggers

• Drugs: penicillins and other beta-lactams, NSAIDs, radiocontrast, neuromuscular blockers, antisnake venom.
• Foods: peanut and tree nuts, shellfish, egg, milk, wheat — the commonest triggers in children.
• Insect stings: bee, wasp, hornet.
• Others: latex, vaccines, blood products, exercise-induced (often food-dependent), and idiopathic.

Clinical Features

Onset is typically within minutes of exposure — the faster the onset, the more severe the reaction.

• Skin and mucosa (80–90%): urticaria, flushing, pruritus, angio-oedema of lips, tongue and eyelids. Skin signs may be absent in up to 20%, particularly in the most severe reactions — their absence must never exclude the diagnosis.
• Airway: throat tightness, hoarseness, stridor, laryngeal oedema — a leading cause of death.
• Breathing: dyspnoea, wheeze, bronchospasm, hypoxia; the other leading cause of death, particularly in asthmatics.
• Circulation: tachycardia, hypotension, dizziness, collapse, arrhythmia, cardiac arrest.
• Gastrointestinal: abdominal cramps, vomiting, diarrhoea — prominent in insect sting and food reactions.
• Neurological: anxiety, "sense of impending doom", confusion, loss of consciousness.

Management

1. Immediate

• Remove the trigger — stop the drug infusion, remove the sting.
• Call for help; position the patient supine with legs elevated (or in the left lateral position if pregnant); do not sit or stand the patient up, as this can precipitate fatal circulatory collapse ("empty ventricle syndrome").
• ADRENALINE INTRAMUSCULARLY into the anterolateral thigh — the first-line, life-saving treatment, given immediately and repeated every 5 minutes if there is no improvement. There is no absolute contraindication to adrenaline in anaphylaxis.
• High-flow oxygen; secure the airway early — intubate before oedema makes it impossible, and be prepared for surgical airway.
• Rapid intravenous fluid resuscitation with crystalloid — large volumes may be needed because of massive capillary leak.

2. Second-line (adjuncts, never substitutes for adrenaline)

• Antihistamines (H1 blocker, with or without H2 blocker) — relieve urticaria and itch only; they do not treat airway or circulatory compromise.
• Corticosteroids — slow onset; traditionally given to reduce biphasic reactions, though the evidence for this is now questioned.
• Nebulised salbutamol for persistent bronchospasm; nebulised adrenaline for stridor.
• Glucagon for patients on beta-blockers who are refractory to adrenaline.
• Intravenous adrenaline infusion only in a monitored setting by experienced staff, for refractory shock.

3. After the acute event

• Observe — a biphasic reaction occurs in a minority, typically within 4–12 hours; observation of 6–12 hours is advised for severe reactions, those needing repeated adrenaline, or patients with asthma.
• Serum mast cell tryptase — take samples soon after onset, at 1–2 hours, and a baseline sample after 24 hours; a transient rise confirms mast cell degranulation retrospectively.
• Discharge planning: prescribe two adrenaline auto-injectors with training for patient and family, provide a written emergency plan, a medical alert bracelet, and clear trigger-avoidance advice.
• Refer to an allergy specialist for confirmatory skin prick or specific IgE testing, and consider venom immunotherapy for insect sting anaphylaxis.