Differential Diagnosis of Red Palpable Lesions (Papules and Plaques)

Introduction

Red papules and plaques represent an exceptionally broad differential encompassing inflammatory, infectious, neoplastic, and reactive conditions. The morphology (flat-topped vs. rounded, scaly vs. smooth), distribution, surface characteristics, and associated symptoms guide differentiation. This chapter systematically addresses the major categories of erythematous palpable lesions.

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34.1 Inflammatory Papules and Plaques

34.1.1 Lichenoid Papules

Lichen Planus (LP)

• Definition: Chronic inflammatory disorder of unknown etiology affecting skin and mucous membranes.
• Clinical Features (5 Ps):
  • Pruritic
  • Planar (flat-topped)
  • Polygonal
  • Purple (violaceous)
  • Papules
• Wickham's Striae: Fine white lines or lace-like reticulations on surface (best seen with dermoscopy or oil immersion).
• Distribution: Wrists (volar), Ankles, Lumbar region, Oral mucosa, Genitalia.
• Koebner Phenomenon: Positive (lesions at sites of trauma).
• Variants:
  • Hypertrophic LP: Verrucous, hyperkeratotic plaques on shins.
  • Oral LP: Reticular (white Wickham's striae on buccal mucosa), Erosive (painful ulcers), Atrophic.
  • Lichen Planopilaris (LPP): Scarring alopecia of scalp.
  • Erosive LP: Painful erosions on mucous membranes, may affect vulva, vagina, gingiva.
• Histology: Interface dermatitis with saw-tooth acanthosis, Civatte bodies (apoptotic keratinocytes), Band-like lymphocytic infiltrate.
• Course: Chronic relapsing. Cutaneous LP may resolve in 1-2 years; Oral LP is more persistent.
• Malignancy Risk: Oral LP has ~1-2% risk of SCC transformation.

34.1.2 Psoriasiform Papules and Plaques

Psoriasis Vulgaris

• Clinical Features:
  • Well-demarcated, erythematous plaques with thick, silvery, micaceous scale.
  • Auspitz Sign: Pinpoint bleeding when scale removed.
  • Koebner Phenomenon: Positive.
• Distribution: Extensor surfaces (elbows, knees), Scalp, Sacrum, Nails.
• Nail Changes: Pitting, Oil spots, Onycholysis, Subungual hyperkeratosis.
• Dermoscopy: Regular dotted vessels, White scale.

Pityriasis Rubra Pilaris (PRP)

• Clinical Features:
  • Orange-red follicular papules coalescing into plaques.
  • "Islands of sparing": Areas of normal skin within erythroderma.
  • Waxy palmoplantar keratoderma (orange hue).
  • Follicular plugging.
• Course: Type I (classic adult) may self-resolve in 3 years. Type II-V more variable.

34.1.3 Drug Eruptions

Fixed Drug Eruption (FDE)

• Clinical Features:
  • Round, well-demarcated, erythematous-to-violaceous plaque that recurs at the SAME site with rechallenge.
  • May blister acutely.
  • Heals with post-inflammatory hyperpigmentation.
• Common Culprits: Sulfonamides, NSAIDs, Tetracyclines, Phenytoin.
• Distribution: Lips, Genitals, Acral areas.
• Diagnosis: Clinical history (temporal association with drug + recurrence at same site).

Morbilliform Drug Eruption

• Covered in Chapter 19 (Exanthemas).

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34.2 Granulomatous Red Papules and Plaques

34.2.1 Granuloma Annulare (GA)

• Definition: Benign granulomatous dermatosis of unknown etiology.
• Clinical Features:
  • Annular (ring-shaped) plaques with raised beaded border and central clearing.
  • NON-scaly (key differentiator from Tinea).
  • Skin-colored to erythematous.
• Distribution: Dorsal hands and feet, Elbows, Knees.
• Variants:
  • Localized GA: Most common. One or few lesions. Self-limited.
  • Generalized GA: >10 lesions, widespread. Associated with diabetes (controversial), Hyperlipidemia.
  • Subcutaneous GA: Deep nodules, common in children.
  • Perforating GA: Papules with central umbilication or scale.
• Histology: Palisading granuloma with central necrobiotic collagen surrounded by histiocytes. Mucin deposition.
• Course: Spontaneous resolution over months to years is common.

34.2.2 Sarcoidosis

• Definition: Multisystem granulomatous disease of unknown etiology.
• Cutaneous Features:
  • Papules, Plaques, Nodules: Red-brown to violaceous. Firm.
  • Apple jelly nodules on diascopy (translucent yellow-brown due to granulomas).
  • Lupus Pernio: Violaceous, indurated plaques on nose, cheeks, ears. Indicates chronic disease with pulmonary involvement.
  • Scar Sarcoid: Infiltration of old scars.
  • Erythema Nodosum (may be presenting feature of acute sarcoidosis/Löfgren syndrome but is NOT granulomatous).
• Distribution: Face (Lupus Pernio), Trunk, Extremities, Scars.
• Histology: "Naked" non-caseating granulomas (minimal surrounding lymphocytes). Multinucleated giant cells with asteroid bodies, Schaumann bodies.
• Systemic Involvement: Lungs (hilar lymphadenopathy, ILD), Eyes (uveitis), Heart, CNS.
• Diagnosis: Biopsy + Systemic workup (CXR, ACE level, Calcium, Eye exam).

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34.3 Infectious Red Papules

34.3.1 Viral

• Molluscum Contagiosum (Covered in Ch. 30): Dome-shaped, umbilicated.
• Verruca (Warts): Hyperkeratotic papules.

34.3.2 Bacterial

• Folliculitis: Follicle-centered pustules and papules.
• Erysipeloid: Due to Erysipelothrix rhusiopathiae. Violaceous plaque on hand in fish/meat handlers.
• Bacillary Angiomatosis: Vascular papules in HIV (HHV-8 related).

34.3.3 Fungal

• Tinea Corporis: Annular, scaly, active border (key differentiator from GA). KOH+.

34.3.4 Mycobacterial

• Lupus Vulgaris (Cutaneous TB): "Apple jelly" nodules on diascopy. Slowly progressive. Scarring.

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34.4 Neoplastic Red Papules and Nodules

34.4.1 Vascular

• Pyogenic Granuloma (Lobular Capillary Hemangioma):

  • Rapidly growing, friable, bright red nodule that bleeds easily.
  • History of trauma often present.
  • Common in children, Pregnancy.
  • Treatment: Shave excision with curettage/electrodesiccation of base.

• Cherry Angioma (Campbell de Morgan Spots):

  • Small, bright red, dome-shaped papules.
  • Common in adults, increase with age.
  • Benign. No treatment needed.

34.4.2 Malignant

• Kaposi Sarcoma:

  • Violaceous to red-purple macules, papules, plaques, nodules.
  • HHV-8 associated.
  • Types: Classic (elderly Mediterranean men), Endemic (African), Iatrogenic (immunosuppression), AIDS-related.
  • Distribution: Legs (classic), Widespread (AIDS).

• Amelanotic Melanoma:

  • Pink/Red nodule lacking typical melanoma pigmentation.
  • Often diagnosed late due to atypical appearance.
  • HIGH INDEX OF SUSPICION for any new, growing, pink nodule.

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34.5 Diagnostic Algorithm

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34.6 Summary Comparison Table

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34.7 Clinical Pearls

1. Flat-topped violaceous papules + Wickham's striae → Lichen Planus.
2. Annular plaque WITHOUT scale → Granuloma Annulare. With scale → Tinea (KOH).
3. Apple jelly nodules on diascopy + Facial plaques → Sarcoidosis. Get CXR.
4. Orange-red erythroderma + Islands of sparing → PRP.
5. Violaceous plaques + HIV → Kaposi Sarcoma.
6. Friable, rapidly growing, bleeding nodule → Pyogenic granuloma. But also rule out amelanotic melanoma if atypical.
7. Same site recurring after drug → Fixed Drug Eruption.