Differential Diagnosis of Papulosquamous Dermatoses

Introduction

Papulosquamous dermatoses are characterized by scaly papules and plaques—lesions that are both raised and covered with scale. This morphological pattern encompasses a wide differential from common conditions like psoriasis and eczema to infectious (tinea), inflammatory (lichen planus), and neoplastic (mycosis fungoides) etiologies. The quality of the scale, distribution, and associated features are key to differentiation.

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36.1 Scale Quality and Characteristics

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36.2 Major Papulosquamous Conditions

36.2.1 Psoriasis Vulgaris

• Definition: Chronic immune-mediated inflammatory disease.
• Clinical Features:
  • Well-demarcated, erythematous plaques with thick, silvery, micaceous (layered) scale.
  • Auspitz Sign: Pinpoint bleeding when scale is removed (exposed dilated capillaries).
  • Koebner Phenomenon: New lesions at sites of trauma.
  • Candle Wax Sign: Silvery scale resembles wax drippings.
• Distribution: Extensor surfaces (Elbows, Knees), Scalp, Sacrum, Nails.
• Nail Changes: Pitting, Oil spots (salmon patches), Onycholysis, Subungual hyperkeratosis, Nail dystrophy.
• Dermoscopy: Regular dotted vessels, White scale.
• Subtypes: Plaque (most common), Guttate (drop-like, post-streptococcal), Inverse (flexural), Pustular, Erythrodermic.

36.2.2 Lichen Planus (Covered in Ch. 34)

• Scale: Fine, whitish (Wickham's striae on surface).
• Morphology: Flat-topped, Polygonal, Purple, Pruritic Papules (5 Ps).
• Distribution: Wrists, Ankles, Oral mucosa, Genital.

36.2.3 Pityriasis Rosea

• Definition: Self-limited papulosquamous eruption, possibly viral etiology (HHV-6/7).
• Clinical Features:
  • Herald Patch: Large (2-5cm), oval, salmon-pink plaque with collarette scale (fine peripheral scale with central clearing). Precedes generalized eruption by 1-2 weeks.
  • Secondary Eruption: Multiple smaller oval patches on trunk.
  • "Christmas Tree" Distribution: Long axes of lesions follow Langer's lines on back.
  • Often preceded by viral prodrome.
• Differential: Secondary Syphilis (must exclude—RPR/VDRL).
• Course: Self-limited, resolves in 6-8 weeks.

36.2.4 Tinea Corporis (Ringworm)

• Definition: Dermatophyte infection of glabrous skin.
• Clinical Features:
  • Annular, erythematous plaques with active, raised, scaly border and central clearing.
  • Scale is prominent at the advancing edge.
  • Pruritic.
• Diagnosis: KOH preparation shows septate, branching hyphae.
• Treatment: Topical antifungals (mild). Systemic (widespread, Tinea capitis, failed topical).

36.2.5 Secondary Syphilis

[!WARNING] "The Great Imitator" - Always consider in papulosquamous differential.

• Clinical Features:
  • Widespread, non-pruritic, ham-colored (copper-red) papules and plaques.
  • Collarette scale (fine scale at periphery).
  • PALMOPLANTAR INVOLVEMENT: Scaling papules on palms and soles—HIGHLY SUGGESTIVE.
  • May have Condyloma lata (moist, wart-like plaques on genitals/perineum).
  • "Moth-eaten" (patchy) alopecia.
  • Generalized lymphadenopathy.
• Diagnosis: RPR/VDRL (screening), FTA-ABS or TP-PA (confirmatory).
• Treatment: Penicillin.

36.2.6 Pityriasis Rubra Pilaris (PRP)

• Clinical Features:
  • Orange-red colored follicular papules coalescing into plaques.
  • "Islands of Sparing": Patches of normal skin within erythroderma (distinctive).
  • Waxy palmoplantar keratoderma (orange hue).
  • Follicular plugging.
• Types: Type I (Classic Adult)—may self-resolve over 3 years.

36.2.7 Seborrheic Dermatitis

• Clinical Features:
  • Erythematous patches with greasy, yellowish scale.
  • Distribution: Seborrheic areas—Scalp (dandruff), Eyebrows, Glabella, Nasolabial folds, Ears (retroauricular), Presternal.
  • Pruritus variable.
• Associations: Parkinson's disease, HIV (severe, refractory seborrheic dermatitis).

36.2.8 Mycosis Fungoides (Cutaneous T-Cell Lymphoma)

• Definition: Low-grade cutaneous T-cell lymphoma.
• Clinical Features:
  • "Patches and Plaques" that are often misdiagnosed as eczema or psoriasis for years.
  • Predilection for sun-protected ("bathing trunk") areas—Buttocks, Inner thighs, Breasts.
  • Poikilodermatous variant: Atrophy, telangiectasia, pigmentary changes.
  • Pruritus variable.
  • Fixed, persistent lesions that don't respond to standard treatments.
• Histology: Epidermotropism of atypical lymphocytes (cerebriform nuclei), Pautrier microabscesses.
• Immunophenotype: CD4+ T-cells with loss of CD7 or CD26.
• Clinical Pearl: Suspect MF in any persistent "eczematous" patch on buttocks or other non-sun-exposed areas.

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36.3 Diagnostic Algorithm

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36.4 Summary Comparison Table

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36.5 Clinical Pearls

1. Silvery scale + Extensor surfaces + Nail pitting → Psoriasis.
2. Herald patch followed by Christmas tree pattern → Pityriasis rosea. Self-limited 6-8 weeks.
3. Scaling papules on PALMS AND SOLES → Secondary Syphilis until proven otherwise. Order RPR.
4. Annular plaque with active scaly border + Central clearing → Tinea. KOH scraping.
5. Greasy scale on scalp and face → Seborrheic dermatitis. If severe, consider HIV.
6. Orange-red plaques + Islands of sparing → PRP.
7. Fixed patches on buttocks/sun-protected areas for years → Mycosis Fungoides. BIOPSY.
8. Always exclude syphilis in any papulosquamous eruption → RPR/VDRL is simple and inexpensive.