Anaphylactic reaction

Anaphylactic reaction is acute, life-threatening IgE-mediated hypersensitivity reaction, provoked by variety of injected, inhaled or ingested foreign substances. Note that the term anaphylactic reaction denotes clinically similar reactions to certain agents e.g. radid-contrast dyes, which are not immunologically mediated.

Causes

Common precipitating agents for anaphylactic reaction are shown on many sites online, though the cause remains unidentified in many cases.

Pathogenesis

Anaphylactic reaction is type I hypersensitivity reactions, mediated by specific IgE antibodies, formed in response to previous sensitization.

These antibodies are present on surface of basophils and mast cells and subsequent exposure to the same allergen leads to degranulation of mast cells and release of numerous chemical mediators.

Final effect of these mediators may manifest immediately with few minutes (acute phase reaction), or after many hours (late phase reaction), with three essential components of anaphylactic reaction — acute bronchospasm, increased vascular permeability and increased secretory activity of mucous glands.

anaphylactic reaction

Clinically, these reactions usually begin within 20-30 minutes of exposure to offending allergen, presenting with —

• Pen-oral tingling sensation, with flushing of face and extreme diaphoresis (earliest)

• Exudative manifestations e.g. urticaria, conjunctival congestion, sneezing, abdominal cramps with diarrhea,

• Stridor or bronchospasm with labored breathing,

• Sudden hypotension, bradycardia or shock, and

• A general feeling of sinking or impending doom, with rapidly developing unconsciousness.

Many cases die rapidly due to airway obstruction or cardiorespiratory arrest, unless treated immediately. Some patients after initial recovery develop recurrence (Biphasic reaction) after 1-8 hours due to late-phase reactions.

DID: Although history of exposure to an offending agent and catastrophic clinical picture is characteristic, anaphylactic reaction needs to be differentiated from

a) vasovagal attacks,

b) cardiac arrhythmia,

c) foreign body aspiration,

d) hypoglycemia and

e) acute poisoning.

Elevated IgE levels and eosinophilia differentiates anaphylaxis from anaphylactic reaction.

Management of anaphylactic reaction

Anaphylaxis is a life-threatening emergency, which needs immediate resuscitative measures and subsequent hospitalization for at least 24 hours to watch for late-phase reactions. Immediate resuscitative measures include —

a) Subcutaneous Adrenaline 1:1000 aqueous preparation (0.1 ml/kg; max 0.5 ml), followed by repeat doses at 15 minutes interval or continuous IV infusion (0.1 mg/kg/mm). In superficial venom injections e.g. insect stings, half of the SC dose (diluted in 2 ml saline) may be given locally, at the site of injection.

b) Respiratory support with airway maintenance, suction, oxygen/ventilatory support and tracheostomy, if needed.

c) Cardiovascular support with IV fluids and inotropes e.g. dopamine.

d) Nebulized salbutamol or IV theophylline, to control bronchospasm.

e) Antihistaminics e.g. Diphenhydramine (1 mg/kg 8- hourly) or Chlorphenarmine (0.3-0.5 mg/kg 8-hourly) for next 24-48 hours, to prevent late-reactions.

1) Systemic steroids e.g. IV hydrocortisone (5 mg/kg 6- hourly), although controversial in acute management, may help to decrease the duration and severity of manifestations.

g) If a precipitating agent e.g. drug is identified, a clear warning should be noted on medical records to prevent further episodes of anaphylactic reactions.