Dermatology: Morphology Before Diagnosis
What you must know
Describe by morphology (macule, papule, plaque, vesicle, bulla, pustule, wheal, nodule), colour, distribution, arrangement and secondary change such as scale, crust, ulceration or lichenification. Then match the pattern: an annular scaly lesion with central clearing suggests tinea; flexural itching suggests eczema; grouped vesicles in a dermatome suggest herpes zoster; honey-crusted lesions suggest impetigo.
Classic traps
- Not every scaly itchy plaque is fungal. Confirm with potassium hydroxide microscopy before starting a steroid, because steroids make tinea unrecognisable.
- Urticaria and anaphylaxis differ: anaphylaxis involves airway, breathing or circulation.
- Severe drug reactions such as Stevens-Johnson syndrome and toxic epidermal necrolysis begin with fever, mucosal involvement and skin pain; stop the culprit drug immediately.
- In HIV, dermatoses are often atypical and extensive; consider Kaposi sarcoma, molluscum and severe seborrhoeic dermatitis.
Rule of thumb
Any rash with mucosal involvement, blistering or skin peeling is a medical emergency: stop the suspected drug and resuscitate. For impetigo, choose topical or oral antibiotics according to extent. For scabies, treat the entire household simultaneously and wash bedding and clothing in hot water.