Acute Allergic Reactions

Also written as Immediate hypersensitivity, Anaphylaxis (severe), Urticaria with systemic symptoms.

Acute allergic reactions are rapid hypersensitivity responses that can progress to life‑threatening anaphylaxis. Prompt recognition, severity grading, and targeted treatment are essential, especially where resources are limited.

Definition and diagnosis

An acute allergic reaction is a sudden onset (usually ≤4 h) immune‑mediated response to an allergen, ranging from localized urticaria to systemic anaphylaxis.

• Diagnosis is clinical: exposure history, rapid symptom onset, and involvement of skin, respiratory, cardiovascular, or gastrointestinal systems.

Severity and staging

Severity is graded by the World Allergy Organization (WAO) system: mild (skin only), moderate (skin plus respiratory or GI symptoms), severe (airway compromise, hypotension, or shock).

• Mild: limited urticaria, itching.
• Moderate: wheeze, abdominal pain, vomiting, or angio‑edema without airway obstruction.
• Severe: stridor, hypoxia, hypotension, loss of consciousness.

Causes, most likely first

In low‑resource East African settings, the most common triggers are insect stings, foods (peanuts, shellfish), drugs (penicillins, sulfonamides), and traditional herbal preparations.

• Insect venom (bees, wasps, ants)
• Food allergens (peanuts, shellfish, milk)
• Beta‑lactam antibiotics
• Sulfonamide drugs
• Herbal remedies containing unknown botanicals

How it presents

Patients develop sudden itching, flushing, or hives, followed by possible swelling of lips/eyes, wheezing, abdominal cramps, or collapse.

• Skin: urticaria, angio‑edema, flushing
• Respiratory: wheeze, stridor, dyspnoea
• Cardiovascular: tachycardia, hypotension, syncope
• GI: nausea, vomiting, diarrhoea

Investigations

Investigations support severity assessment; many may be unavailable, so rely on bedside findings.

• Pulse oximetry: Is there hypoxia? – SpO₂ < 94 % indicates need for supplemental O₂ and possible airway support.
• Blood pressure measurement: Is the patient hypotensive? – Systolic < 90 mmHg or MAP < 65 mmHg signals severe reaction.
• Serum tryptase (if available): Was anaphylaxis present? – Levels > 11.4 µg/L within 1–2 h support mast‑cell activation.
• Basic metabolic panel: Is there renal or electrolyte disturbance from shock? – Look for rising creatinine or metabolic acidosis.

Red flags and complications

Rapid progression to airway obstruction or circulatory collapse demands immediate emergency care.

• Stridor or inability to speak
• Rapidly falling blood pressure
• Altered mental status
• Persistent tachycardia > 120 bpm despite fluids

Management

First step is to secure the airway, then give intramuscular adrenaline, followed by supportive measures and identification of the trigger.

• High‑flow O₂ via mask – maintain SpO₂ ≥ 94 %.
• IV crystalloid bolus 20 mL/kg if hypotensive – restores intravascular volume.
• H1‑antihistamine (e.g., chlorpheniramine) – relieves cutaneous symptoms; avoid sedating antihistamines if airway at risk.
• Observe for at least 4 h after symptom resolution; longer if severe.

Follow‑up and prevention

Patients need allergen identification, education, and an emergency action plan.

• Prescribe an adrenaline auto‑injector if available; teach self‑administration.
• Referral to allergy clinic for skin‑prick or specific IgE testing when feasible.
• Avoid known triggers; wear medical alert identification.
• Review all recent medications and herbal products.

Often confused with

Conditions that mimic acute allergic reactions must be distinguished to avoid mis‑management.

• Vasovagal syncope – prodrome of light‑headedness without skin changes.
• Acute asthma exacerbation – wheeze without urticaria or angio‑edema.
• Septic shock – fever, purulent source, and gradual onset.

Where results can mislead

Certain bedside findings may be falsely reassuring.

• Normal blood pressure early in anaphylaxis – can fall rapidly; repeat measurements.
• Absence of skin signs – up to 10 % of anaphylaxis present without cutaneous involvement.

Questions an examiner may ask

• A 22‑year‑old farmer presents with sudden wheeze and hypotension after a bee sting – what is your immediate management and why? — Give IM adrenaline, high‑flow O₂, IV fluids; explain adrenaline’s actions on airway, vasculature, and heart.
• How would you differentiate anaphylaxis from acute severe asthma in a patient with wheeze and no rash? — Look for skin or GI symptoms, hypotension, and rapid onset after exposure; asthma lacks systemic signs.
• A patient on penicillin develops urticaria and mild hypotension; why is epinephrine preferred over antihistamines as first‑line? — Antihistamines relieve itching but do not reverse airway edema or vasodilation; epinephrine addresses life‑threatening physiology.
• In a low‑resource clinic without serum tryptase, how do you confirm anaphylaxis? — Rely on clinical criteria: exposure, rapid multisystem involvement, and response to adrenaline.
• What are the contraindications to using high‑dose corticosteroids in acute allergic reactions? — None for anaphylaxis; avoid delaying adrenaline; be aware of infection risk in immunocompromised patients.
• A child receives an intramuscular adrenaline dose but remains hypotensive; what are the next steps? — Repeat IM dose after 5 min, start IV crystalloid bolus, consider IV adrenaline infusion if monitored.
• Explain why a patient with known sickle cell disease may have a more severe reaction to a malaria drug allergy. — Baseline endothelial dysfunction and chronic inflammation increase risk of shock; careful monitoring is required.
• What follow‑up plan would you give a patient discharged after a moderate reaction to a herbal remedy? — Advise avoidance, prescribe an adrenaline auto‑injector if possible, arrange allergy testing, and provide written emergency action plan.

When to seek help

• Airway obstruction (stridor, inability to speak)
• Systolic blood pressure < 90 mmHg or MAP < 65 mmHg
• Rapid loss of consciousness

Quick questions

Can anaphylaxis occur without a rash?

Yes; up to 10 % present without skin involvement, so treat based on respiratory or circulatory signs.

Why is adrenaline given intramuscularly, not intravenously?

IM provides rapid absorption with lower risk of arrhythmia; IV is reserved for refractory shock under cardiac monitoring.