Dermatology
Core concepts
Describe before diagnosing: site, distribution, symmetry, colour, border, surface and mucosal involvement. Eczema is itchy, ill-defined and flexural in atopic disease; treat with emollients and topical steroids chosen by potency and site (mild for the face, potent for limbs). Psoriasis shows well-demarcated erythematous plaques with silvery scale on extensor surfaces and scalp, nail pitting and onycholysis, the Koebner phenomenon, and association with metabolic syndrome and psoriatic arthritis; guttate psoriasis follows streptococcal infection; pustular and erythrodermic forms are emergencies. Skin cancer: basal cell (pearly rolled edge, telangiectasia, slow-growing, rarely metastasises), squamous cell (keratinised, ulcerated, sun-exposed, may metastasise, arises from actinic keratosis), melanoma (ABCDE; Breslow thickness is the key prognostic factor; excise urgently). Infections: impetigo (golden crust), cellulitis (warm, tender, raised white count - mark the edge), tinea (annular with central clearing), scabies (nocturnal itch, web spaces, treat contacts). Drug eruptions: morbilliform, urticaria, DRESS (eosinophilia with hepatic or renal involvement), Stevens-Johnson syndrome and toxic epidermal necrolysis (mucosal involvement, epidermal detachment - stop the drug, burn-unit care), fixed drug eruption. Bullous disease: pemphigus vulgaris (intraepidermal, flaccid, anti-desmoglein) versus bullous pemphigoid (subepidermal, tense, anti-basement membrane). Vasculitis presents as palpable purpura - check the urine.
Traps
Missing a non-blanching rash (meningococcal sepsis, vasculitis, thrombocytopenia). Treating presumed cellulitis when the diagnosis is DVT or stasis dermatitis. Applying topical steroids to an undiagnosed infected or malignant lesion.
Rule of thumb
A rash with mucosal involvement and skin pain or a positive Nikolsky sign is a dermatological emergency: stop the suspected drug and involve a burns or dermatology team immediately.