Anaphylaxis: Causes, Clinical Features & Emergency Treatment | Medical Learner

🩺 Anaphylaxis

Anaphylaxis is a generalized immunological condition of sudden onset, which develops after exposure to a foreign substance.

The mechanism may:

• Involve an IgE-mediated reaction to a foreign protein (stings, foods, streptokinase), or to a protein–hapten conjugate (antibiotics) to which the patient has previously been exposed.
• Be complement mediated (human proteins eg G-globulin, blood products).
• Be unknown (aspirin, ‘idiopathic’).

Irrespective of the mechanism, mast cells and basophils release mediators (eg histamine, prostaglandins, thromboxanes, platelet activating factors, leukotrienes) producing clinical manifestations.

Angio-oedema caused by ACE inhibitors and hereditary angio-oedema may present in a similar way to anaphylaxis. Hereditary angio-oedema is not usually accompanied by urticaria and is treated with C1 esterase inhibitor.

🔹 Common Causes

• Drugs and vaccines (eg antibiotics, streptokinase, suxamethonium, aspirin, non-steroidal anti-inflammatory drugs (NSAIDs), intravenous (IV) contrast agents).
• Hymenoptera (bee/wasp) stings.
• Foods (nuts, shellfish, strawberries, wheat).
• Latex.

🔹 Clinical Features

The speed of onset and severity vary with the nature and amount of the stimulus, but the onset is usually in minutes/hours. A prodromal aura or a feeling of impending death may be present. Patients on B-blockers or with a history of ischaemic heart disease (IHD) or asthma may have especially severe features. Usually two or more systems are involved:

Respiratory

Swelling of lips, tongue, pharynx, and epiglottis may lead to complete upper airway occlusion. Lower airway involvement is similar to acute severe asthma — dyspnoea, wheeze, chest tightness, hypoxia, and hypercapnia.

Skin

Pruritus, erythema, urticarial, and angio-oedema.

Cardiovascular

Peripheral vasodilation and vascular permeability cause plasma leakage from the circulation, with decreased intravascular volume, hypotension, and shock. Arrhythmias, ischaemic chest pain, and electrocardiogram (ECG) changes may be present.

GI tract

Nausea, vomiting, diarrhoea, abdominal cramps.

🚨 Treatment

• Discontinue further administration of suspected factor (eg drug). Remove stings by scraping them carefully away from skin.
• Give 100 % oxygen (O₂).
• Open and maintain airway. If upper airway oedema is present, get specialist senior help immediately. Emergency intubation or a surgical airway and ventilation may be required.
• In patients with shock, airway swelling, or respiratory difficulty give 0.5mg (0.5mL of 1:1000 solution) adrenaline intramuscular (IM). Repeat after 5min if there is no improvement. In adults treated with an adrenaline auto-injector (eg EpiPen®) the 300mcg dose is usually sufficient, but additional doses may be required. Give only 50% of the usual dose of adrenaline to patients taking tricyclic antidepressants, MAOIs, or B–blockers.
• In profound shock or immediately life-threatening situations, give CPR/ALS as necessary, and consider slow IV adrenaline 1:10,000 or 1:100,000 solution. This is recommended only for experienced clinicians who can also obtain immediate IV access. Note the different strength of adrenaline required for IV use. If there is no response to adrenaline, consider glucagon 1–2mg IM/IV every 5min (especially in patients taking B-blockers).
• Give a B2-agonist (eg salbutamol 5mg) nebulized with O₂ for bronchospasm, possibly with the addition of nebulized ipratropium bromide 500mcg.
• Give IV fluid if hypotension does not rapidly respond to adrenaline. Rapid infusion of 1–2L IV 0.9 % saline may be required, with further infusion according to the clinical state.
• Antihistamine H1 blockers (eg chlorphenamine 10–20mg slow IV) and H2 blockers (eg ranitidine 50mg IV) are commonly given. They are second line drugs that, with hydrocortisone 100–200mg slow IV, may reduce the severity/duration of symptoms.
• Admit/observe after initial treatment: prolonged reactions and biphasic responses may occur. Observe for at least 4–6hr after all symptoms have settled.

Report anaphylactic reactions related to drugs/vaccines to the Committee on Safety of Medicines. Further investigation of the cause (and possibly desensitization) may be indicated. Where identified, the patient and GP must be informed and the hospital records appropriately labelled. Medic-Alert bracelets are useful.

📝 Notes on Treatment Algorithm

• An inhaled B2-agonist such as salbutamol may be used as an adjunctive measure if bronchospasm is severe and does not respond rapidly to other treatment.
• If profound shock judged immediately life-threatening give CPR/ALS if necessary. Consider slow IV adrenaline (epinephrine) 1:10,000 solution. This is hazardous and is recommended only for an experienced practitioner who can also obtain IV access without delay. Note the different strength of adrenaline (epinephrine) that may be required for IV use.
• If adults are treated with an EpiPen® the 300mcg will usually be sufficient. A second dose may be required. Half doses of adrenaline (epinephrine) may be safer for patients on amitriptyline, imipramine, or B-blocker.
• A crystalloid may be safer than a colloid.

❓ FAQ (High-Yield for Exams)

What is the first drug of choice in anaphylaxis?
Adrenaline (epinephrine) IM is the first-line treatment.

What is the dose of adrenaline in adults?
0.5 mg IM (0.5 mL of 1:1000 solution), repeat after 5 minutes if needed.

Which route is preferred for adrenaline?
Intramuscular (IM) route is preferred over IV in most cases.

Can anaphylaxis have delayed reactions?
Yes, biphasic reactions can occur, so observation is required.

Which systems are commonly involved?
Respiratory, skin, cardiovascular, and gastrointestinal systems.

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