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Erythema multiforme
Acute hypersensitivity. EM minor = skin only; EM major = mucosal. Distinct from SJS/TEN.
Risk factors: HSV most common trigger; also Mycoplasma, drugs (penicillins, sulfonamides, carbamazepine, allopurinol, NSAIDs, OCP).
Features: Classic three-zone target lesions; starts acrally then spreads proximally.
Management: Stop trigger; symptomatic care. Mucosal/ocular or systemically unwell: same-day hospital. Recurrent HSV-EM: consider aciclovir prophylaxis.
Related notesAllopurinol HLA-B*5801 and ciclosporin pearls are a dedicated pharmacology card.Pharmacology · allopurinol
Related notesAcute gout treatment and urate-lowering sit in the MSK crystal-arthritis section.MSK · gout
Erythema nodosum
Tender subcutaneous nodules on shins (thighs/forearms too); bruise-like colour change; no ulceration. Self-limits about 6 weeks.
Risk factors: Causes mnemonic MISFIT: Meds (penicillins, sulfonamides, OCP); Infections (strep, TB, Mycoplasma, EBV); Sarcoidosis; Inflammatory bowel disease; Idiopathic; Timing including pregnancy.
Management: Targeted work-up: throat/resp/GI history; FBC/ESR/CRP; ASO/throat swab if sore throat; CXR; stool tests if GI symptoms; pregnancy test if relevant. Treat cause; rest, NSAIDs if suitable. Severe cases may need specialist input after excluding infection/TB.
Acanthosis nigricans
Velvety brown plaques (neck, axilla, groin). Often insulin resistance (obesity, T2DM, PCOS).
Management: Work-up: BMI, HbA1c/glucose, lipids; review meds. Treat underlying cause.
Referral: Sudden extensive disease, mucosal involvement, tripe palms, weight loss, age over 40: 2WW for GI malignancy.
Pyoderma gangrenosum
Very painful pustule evolving to rapid violaceous undermined ulcer (often legs); pathergy. Links: IBD, inflammatory arthritis, haematological disease (about 50% idiopathic).
Management: Exclude infection/vasculitis. Urgent dermatology: prednisolone with or without potent topical/intralesional; ciclosporin or anti-TNF if refractory. Good wound care; avoid debridement.
Polymorphic light eruption and juvenile spring eruption
PLE: commonest photodermatosis; young women; spring/summer; UVA more than UVB. Juvenile spring eruption: PLE variant on boys' ear tops after spring sun; manage as PLE.
Features: Itchy papules/vesicles/plaques on sun-exposed skin; resolves without scars; hardening with continued exposure.
Management: SPF50+ (UVA), clothing/shade, emollient, antihistamine, short topical steroid. Refer severe/recurrent for pre-season NB-UVB. Screen SLE if atypical.
Rashes in pregnancy
Four pregnancy rashes: atopic eruption, PEP/PUPPP, ICP, and pemphigoid gestationis.
Features: Atopic eruption of pregnancy: first to second trimester; more in atopic women; pruritic eczematous patches (flexures) with or without prurigo. Polymorphic eruption of pregnancy (PEP / PUPPP): third trimester, often primigravida; itchy urticarial papules/plaques starting in abdominal striae; spares umbilicus; benign for mother and baby. Intrahepatic cholestasis of pregnancy (ICP): nocturnal generalised itch (palms/soles) without primary rash; raised bile acids (with or without LFTs). Pemphigoid gestationis: second to third trimester or postpartum; periumbilical urticarial plaques progressing to tense bullae; umbilicus involved (unlike PEP).
Management: Atopic eruption: emollients, menthol, short mild to moderate steroid, antihistamines; resolves after delivery. PEP: emollients plus short moderate to potent steroid. ICP: urgent bloods and obstetric-led care; UDCA; vitamin K if coagulopathy; avoid oestrogens. Pemphigoid gestationis: derm referral; DIF linear C3 with or without IgG at BMZ; potent topical steroid plus antihistamine; oral prednisolone if extensive; obstetric co-management. Risks: preterm/SGA; may recur worse next pregnancy; avoid COCP postpartum.
AKT pearl: PEP spares the umbilicus; pemphigoid gestationis involves it and forms blisters.
Melasma (chloasma)
Symmetric brown facial macules; women, darker skin, pregnancy/OCP/HRT.
Management: Core: high-SPF UVA/UVB plus hats. Pregnancy: azelaic acid plus strict photoprotection only. Outside pregnancy: azelaic, hydroquinone, retinoids/triple combos, cautious procedures.
Granuloma annulare
Smooth annular plaques on dorsal hands/feet; often asymptomatic. About 50% resolve in 1-2 years.
Management: Localised: observation or topical/intralesional steroid. Generalised: derm input; associated with diabetes/hyperlipidaemia.
Vascular birthmarks and childhood spots
Childhood spots: milia, port-wine stain, salmon patch, infantile haemangioma, Mongolian spot, cafe-au-lait.
Features: Milia: tiny white facial keratin cysts; reassure in newborns. Port-wine stain: unilateral flat red-purple from birth; darkens/thickens. V1/eyelid: ophthalmology (and neuro for Sturge-Weber if indicated); limb overgrowth: Klippel-Trenaunay. Salmon patch (naevus simplex): midline pink macules (glabella, eyelids, nape); face fades by 1-2 years; nape may persist; no treatment. Infantile (strawberry) haemangioma: appears weeks after birth, grows then involutes (about 95% before age 10). Mnemonic: grow then go. Mongolian spot: blue-grey sacrum/buttocks; fades by 3-5 years. Cafe-au-lait: flat light-brown; six or more large macules: assess for NF1.
Management: Port-wine stain: early pulsed-dye laser. Haemangioma: treat if ulcerating, obstructing, segmental facial, airway, or visual risk: oral propranolol first-line (monitor; contraindicated in asthma, heart block); topical timolol for small superficial lesions.
Alopecia
Scarring (permanent) versus non-scarring hair loss.
Features: Scarring (permanent): lichen planopilaris, discoid lupus, severe scalp ringworm, trauma/radiation. Non-scarring: androgenetic alopecia, alopecia areata, telogen effluvium, traction, trichotillomania, drugs, iron/zinc deficiency, endocrine disease. Alopecia areata: sudden bald patches; exclamation-mark hairs; nail pitting. About half regrow within a year.
Management: Alopecia areata primary care: reassure/camouflage; potent topical steroid once daily up to 12 weeks (not face); children topical only.
Referral: Scarring: urgent dermatology. Areata: refer rapid/extensive loss, brow/lash loss, scarring, or no improvement after 3-6 months.
Bullous pemphigoid versus pemphigus vulgaris
Two autoimmune blistering diseases distinguished by age, blister type, mucosa, Nikolsky sign, and immunofluorescence.
Features: Bullous pemphigoid: older adults; antibodies to BP180/BP230. Itchy tense blisters on trunk/flexures; mucosa usually spared; Nikolsky negative. DIF: linear IgG/C3 at basement membrane. Often heals without scarring. Pemphigus vulgaris: middle-aged; desmoglein antibodies. Painful flaccid blisters/erosions with prominent mucosa; Nikolsky positive. DIF: intercellular fish-net IgG.
Management: Pemphigoid: derm-led very potent topical clobetasol for limited disease or oral prednisolone if widespread. Pemphigus: urgent specialist care: systemic steroids plus immunosuppressant (rituximab common adjunct).
AKT pearl: Pemphigus = painful flaccid mucosal blisters, positive Nikolsky, intercellular IgG. Pemphigoid = older, tense blisters, basement-membrane linear IgG/C3.
SJS / TEN (emergency)
Painful erythema progressing to flaccid blisters/erosions, mucosal involvement, positive Nikolsky. SJS under 10% BSA; TEN over 30%.
Risk factors: Culprits: sulfonamides, allopurinol, antiepileptics (lamotrigine, carbamazepine, phenytoin), some NSAIDs, penicillins.
Management: Stop the drug immediately; urgent specialist/ICU admission; SCORTEN; meticulous supportive care.
Red flags: This is a dermatological emergency. Do not delay admission.
Related notesAllopurinol HLA-B*5801 and ciclosporin pearls are a dedicated pharmacology card.Pharmacology · allopurinol
Related notesAcute gout treatment and urate-lowering sit in the MSK crystal-arthritis section.MSK · gout
Erythroderma (dermatology emergency)
Diffuse erythema and scale over 90% BSA. Causes: eczema, psoriasis, drugs; CTCL/Sezary; pityriasis rubra pilaris.
Features: Risks: fluid/heat/protein loss, infection/sepsis, cardiac strain.
Management: Discuss with on-call dermatology; admit if unwell/elderly/high risk. Stop non-essential drugs; warm room; fluids; liberal emollients; cautious topical steroids; treat infection; bloods including film for Sezary; biopsies as advised.
Urticaria and angio-oedema
Airway compromise, voice change, tongue/lip swelling with dyspnoea or hypotension: 999/A&E; IM adrenaline 0.5 mg for anaphylaxis.
Management: First-line: avoid triggers; daily non-sedating H1 antihistamine (cetirizine/loratadine/fexofenadine); up-titrate to four times adult dose if needed (off-label). Short prednisolone 20-40 mg for 3-5 days for severe flares only. Angio-oedema: stop ACE inhibitor permanently; issue adrenaline auto-injector if prior airway symptoms or high risk; review beta-blockers.
Referral: Refer if symptoms for 6 weeks or more despite high-dose antihistamine; bradykinin-mediated angio-oedema; vasculitic or inducible urticaria.
Generalised pruritus without primary rash
Generalised itch without a primary rash: think systemic causes.
Features: Cholestasis: night itch palms/soles before jaundice. Iron deficiency: pallor, koilonychia, angular stomatitis. Polycythaemia vera: aquagenic itch after hot bath. CKD/uraemia: xerosis, restless legs, low eGFR. Lymphoma (especially Hodgkin): B symptoms, nodes. Thyroid disease; diabetes (genital candidiasis); pregnancy ICP; drugs (opioids, antibiotics, antimalarials, statins, ACEIs, NSAIDs). Also consider HIV, elderly asteatotic (crazy paving) eczema, subtle scabies, subtle eczema/psoriasis.
AKT pearl: Brown staining after venous ulcer = haemosiderin from chronic venous insufficiency. Crazy-paving lower legs in winter = asteatotic eczema (craquele): soap substitute plus liberal emollients with or without short mild to moderate steroid.
Miscellaneous high-yield snippets
High-yield snippets: Koebner, vitiligo, necrobiosis lipoidica, pretibial myxoedema, PCT, keloid, hyperhidrosis, hirsutism, HHT, dermatitis artefacta, onycholysis.
Features: Koebner phenomenon: new lesions at injury sites (psoriasis, vitiligo, lichen planus). Vitiligo: well-demarcated depigmentation; autoimmune links. Necrobiosis lipoidica: yellow-brown atrophic shin plaques with telangiectasia; diabetes link; ulcerates in about one third. Pretibial myxoedema: Graves association; non-pitting peau d'orange plaques on shins/feet. Porphyria cutanea tarda: fragile photosensitive blisters on hands/face; triggers alcohol, HCV, oestrogens, iron overload. Keloid: scar beyond wound edges. Hirsutism versus hypertrichosis: hirsutism androgen-dependent (PCOS common); rapid onset/virilisation is a red flag. Hypertrichosis androgen-independent (drugs such as minoxidil, ciclosporin). Hereditary haemorrhagic telangiectasia: autosomal dominant; epistaxis plus mucocutaneous telangiectasia with or without visceral AVMs; Curacao criteria; family screen. Dermatitis artefacta: geometric lesions in accessible sites; diagnosis of exclusion. Onycholysis: trauma, fungus, psoriasis/eczema, photosensitising drugs, thyroid, poor circulation.
Management: Vitiligo: sun protection/camouflage; topical steroid or tacrolimus; NB-UVB if extensive. Necrobiosis: protect from trauma; optimise diabetes. PCT: venesection plus photoprotection. Keloid: silicone plus intralesional triamcinolone first-line; excision alone recurs often. Hyperhidrosis (focal): aluminium chloride hexahydrate 20% first-line; then iontophoresis or botulinum toxin (axillae). Dermatitis artefacta: mental-health involvement; avoid confrontation. Onycholysis: treat cause.
