Dd for psoriasis
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differential diagnosis psoriasis distinguishing features
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psoriasis vs eczema seborrheic dermatitis lichen planus tinea clinical differentiation
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Here is a comprehensive differential diagnosis for psoriasis:
Differential Diagnosis of Psoriasis
Key Distinguishing Features of Psoriasis (Anchor)
Before differentiating, the classic psoriasis picture:
- Well-demarcated, erythematous plaques with silvery-white scale
- Auspitz sign: pinpoint bleeding on scale removal
- Koebner phenomenon: lesions at sites of trauma
- Predilection for extensor surfaces (elbows, knees), scalp, lumbosacral area, umbilicus, intergluteal cleft
- Nail changes: pitting, onycholysis, oil-drop sign, subungual hyperkeratosis
- Associated psoriatic arthritis (~30%)
Main Differentials by Clinical Variant
Plaque Psoriasis
| Condition | Key Features | How to Distinguish from Psoriasis |
|---|---|---|
| Seborrheic Dermatitis | Greasy, yellowish scales on scalp, nasolabial folds, eyebrows, ears | Less induration; greasier (not dry silvery) scale; distribution favors central face/scalp folds; no Auspitz sign. Overlap exists ("sebopsoriasis") |
| Atopic Dermatitis (Eczema) | Pruritic, ill-defined, lichenified patches | Flexural distribution (antecubital, popliteal fossae); worse pruritus; personal/family history of atopy; no silvery scale |
| Lichen Planus | Violaceous, flat-topped, polygonal papules | 4 Ps: Pruritic, Purple, Polygonal, Planar; Wickham's striae; favors flexor wrists, mucous membranes; different scale character |
| Tinea Corporis | Annular, scaly plaque with central clearing | Ring-shaped with active advancing border; KOH prep positive; no Auspitz sign |
| Nummular Eczema | Coin-shaped, crusted, oozing plaques | Less well-defined borders; more exudative/crusted; responds differently to treatment |
| Pityriasis Rubra Pilaris (PRP) | Salmon-orange plaques, follicular keratosis | "Islands of sparing" within erythroderma; follicular plugging; palmoplantar keratoderma |
Guttate Psoriasis
| Condition | Distinguishing Points |
|---|---|
| Pityriasis Rosea | Herald patch; lesions follow skin cleavage lines ("Christmas tree" pattern on back); self-limiting; scale collarette inside lesion border |
| Secondary Syphilis | Involves palms and soles; painless mucous membrane lesions; positive RPR/VDRL; systemic symptoms (fever, lymphadenopathy) |
| Viral exanthem | History of viral prodrome; non-scaly typically; resolves quickly |
(Harrison's, p. 1534)
Scalp Psoriasis
| Condition | Distinguishing Points |
|---|---|
| Seborrheic Dermatitis | Greasy dandruff-like scale; less thick plaques; no sharp demarcation at hairline (Harrison's, p. 1531) |
| Tinea Capitis | Hair loss (alopecia); positive fungal culture/KOH; more common in children |
| Discoid Lupus | Scarring alopecia; follicular plugging; photosensitive; ANA positive |
Nail Psoriasis
| Condition | Distinguishing Points |
|---|---|
| Onychomycosis | KOH/culture positive; no pitting; may co-exist with psoriasis |
| Lichen Planus of Nails | Pterygium formation; thinning, not thickening |
| Alopecia Areata | Associated alopecia; finer "geometric" pitting pattern |
Palmoplantar Psoriasis
| Condition | Distinguishing Points |
|---|---|
| Palmoplantar Eczema (Dyshidrosis) | Vesicular/bullous component; intense pruritus; deeper vesicles on lateral fingers |
| Tinea Pedis/Manuum | KOH positive; typically asymmetric; interdigital maceration |
| Reactive Arthritis (Keratoderma Blennorrhagica) | History of urethritis/conjunctivitis/arthritis triad; pustular lesions on soles |
Erythrodermic Psoriasis
| Condition | Distinguishing Points |
|---|---|
| Sézary Syndrome (Cutaneous T-Cell Lymphoma) | Intense pruritus; lymphadenopathy; circulating Sézary cells on blood smear |
| Drug reaction (DRESS) | Drug history; eosinophilia; internal organ involvement |
| PRP | Orange-red hue; follicular keratosis; "islands of sparing" |
Investigations to Confirm
- KOH prep / fungal culture — rule out tinea
- Skin biopsy — when diagnosis uncertain (psoriasiform hyperplasia with parakeratosis, elongated rete ridges, Munro microabscesses)
- RPR/VDRL — rule out secondary syphilis in guttate pattern
- ASO titer / throat swab — guttate psoriasis post-streptococcal trigger
- ANA, dsDNA — if lupus suspected
Memory Tip
"SELFTT-P" for psoriasis DD: Seborrheic dermatitis, Eczema/atopic, Lichen planus, Fungal (tinea), Tinea versicolor, Tinea capitis, Pityriasis rosea/PRP
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