D/D of chronic plaque psoriasis
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differential diagnosis chronic plaque psoriasis
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psoriasis mimics distinguishing features eczema seborrheic dermatitis fungal
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Good initial results. Let me synthesize a comprehensive differential diagnosis table using these sources and my clinical knowledge.
Differential Diagnosis of Chronic Plaque Psoriasis
Chronic plaque psoriasis (psoriasis vulgaris) presents with well-demarcated, erythematous plaques with silvery scale, predilecting the scalp, elbows, knees, and presacral area. Several conditions can mimic it closely.
Key Differentiating Features
| Condition | Morphology | Scale | Distribution | Key Distinguishing Features |
|---|---|---|---|---|
| Chronic plaque psoriasis | Well-demarcated erythematous plaques | Silvery-white, thick, micaceous | Extensor surfaces, scalp, sacrum, nails | Auspitz sign, nail pitting, Koebner phenomenon, PASI scoring |
| Seborrheic dermatitis | Erythematous patches/plaques | Greasy/yellowish, less prominent induration | Scalp, nasolabial folds, central face, chest | Less sharply demarcated; sebopsoriasis overlap exists (Harrison's, p. 1531) |
| Atopic eczema (AD) | Ill-defined erythematous plaques, lichenification | Fine, dry scale | Flexural surfaces (antecubital, popliteal fossae) | Pruritus dominant; personal/family atopy history; no Auspitz sign |
| Lichen simplex chronicus | Lichenified plaque | Fine, variable | Single or few plaques, accessible areas | History of repeated rubbing/scratching; no systemic association |
| Nummular eczema | Coin-shaped plaques | Crusted, oozing possible | Limbs | More vesicular initially; less sharp demarcation; extremely pruritic |
| Tinea corporis / capitis | Annular plaque with central clearing | Peripheral scale | Any area; scalp (capitis) | KOH positive; responds to antifungals; active border |
| Pityriasis rosea | Herald patch + oval plaques | Collarette scale (inner border) | Trunk in "Christmas tree" distribution | Self-limiting; usually clears in 6–12 weeks; no Auspitz sign |
| Pityriasis rubra pilaris (PRP) | Salmon-orange plaques with follicular papules | Fine, adherent | Head-to-toe; islands of sparing characteristic | Keratoderma; follicular keratosis; islands of normal skin |
| Secondary syphilis | Papulosquamous lesions | Variable | Palms and soles involvement classic | Serology (VDRL/RPR) positive; systemic symptoms |
| Mycosis fungoides (CTCL) | Thin, atrophic patches or plaques | Fine | Bathing-suit distribution | Histology: epidermotropism, Pautrier microabscesses; chronic course |
| Lichen planus | Flat-topped, violaceous papules/plaques | Wickham's striae | Wrists, ankles, mucous membranes | 4 Ps: planar, purple, polygonal, pruritic; oral mucosa involvement |
| Discoid lupus erythematosus (DLE) | Scarring plaques with follicular plugging | Adherent "carpet-tack" scale | Face, scalp, ears | ANA may be positive; scarring alopecia; photodistribution |
| Bowen's disease (SCC in situ) | Solitary, well-defined erythematous plaque | Variable scale/crust | Sun-exposed areas, lower limbs | Persistent single lesion; biopsy diagnostic |
| Chronic cutaneous eczema | Ill-defined lichenified plaques | Dry, fine | Flexures, variable | Less well-demarcated; contact history; patch testing helpful |
Anatomical Variants and Their Specific Differentials
| Site | Psoriasis Variant | Key Differentials |
|---|---|---|
| Scalp | Scalp psoriasis | Seborrheic dermatitis, tinea capitis, DLE |
| Flexures / Groins | Inverse psoriasis | Candidiasis, intertrigo, tinea cruris, erythrasma |
| Palms & Soles | Palmoplantar psoriasis | Palmoplantar eczema, tinea manuum/pedis, secondary syphilis, PRP |
| Nails | Nail psoriasis | Onychomycosis, lichen planus of nails |
| Genitalia | Genital psoriasis | Candidiasis, contact dermatitis, extramammary Paget's |
Helpful Clinical Clues
- Auspitz sign (pinpoint bleeding on scale removal): positive in psoriasis, rare in mimics
- Koebner (isomorphic) phenomenon: new lesions at trauma sites — seen in psoriasis, lichen planus, viral warts
- Nail pitting + onycholysis: strongly favors psoriasis over fungal infection
- KOH scraping: mandatory to exclude tinea when diagnosis uncertain
- Biopsy: resolves ambiguity — psoriasis shows regular acanthosis, parakeratosis, Munro microabscesses, absent granular layer, dilated capillaries in dermal papillae
- Serology: VDRL/RPR if palmoplantar involvement or at-risk patient (secondary syphilis)
Severity Classification (for context)
Per the Management and Treatment of Psoriasis guidelines (p. 39):
- Mild: < 3% BSA
- Moderate: 3–10% BSA
- Severe: > 10% BSA — or any extent with emotional/functional impact or involvement of hands, feet, scalp, face, genitalia
The PASI (0–72) incorporates BSA, redness, scaling, and plaque thickness — used in clinical trials and biologic therapy thresholds.
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