Dermatology and Clinical Allergy
This domain tests anaphylaxis management, severe cutaneous adverse reactions, chronic urticaria and the common inflammatory dermatoses.
Anaphylaxis
Diagnose clinically: acute onset with skin or mucosal involvement plus respiratory compromise or hypotension, or hypotension after exposure to a known allergen. Give adrenaline 0.5 mg intramuscularly (1 mg/mL solution) into the anterolateral thigh, repeated every 5–15 minutes as needed. Antihistamines and corticosteroids are adjuncts only and must never delay adrenaline. Intravenous adrenaline is reserved for refractory cases with cardiac monitoring.
Severe cutaneous reactions
Stevens-Johnson syndrome involves less than 10% body surface area, toxic epidermal necrolysis more than 30%. Stop the culprit drug immediately and manage in a burns or intensive care setting. DRESS presents with eosinophilia, fever and organ involvement, often weeks after starting an anticonvulsant or allopurinol, and responds to systemic corticosteroids.
Urticaria
Second-generation H1 antihistamines are first line and may be upcast up to fourfold before adding a biologic; long-term systemic steroids are avoided.
Common dermatoses
Psoriasis shows well-demarcated plaques with silvery scale on extensor surfaces; eczema is ill-defined, flexural and intensely pruritic. Patch testing identifies allergic contact dermatitis.
Worked example
A patient develops stridor, urticaria and hypotension minutes after a contrast infusion. Give intramuscular adrenaline immediately, lie the patient flat with legs raised, give high-flow oxygen and fluids, and observe for at least 4–6 hours because biphasic reactions occur.
Traps that cost marks
- Giving antihistamines before adrenaline.
- Failing to stop the culprit drug in SJS/TEN.
- Using intramuscular adrenaline in the deltoid with a 1:10,000 dilution.