Differential Diagnosis of Pigmented Palpable Lesions

Introduction

Pigmented (brown, black, or blue) papules and nodules require careful evaluation due to the importance of excluding melanoma. The combination of color and palpability indicates melanocytic origin, melanin deposition, or other pigmented material within the lesion. Dermoscopy is essential for characterization. This chapter systematically covers the differential diagnosis, emphasizing features that distinguish benign from malignant lesions.

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31.1 Melanocytic Lesions

31.1.1 Common Acquired Melanocytic Nevus (Mole)

• Definition: Benign proliferation of melanocytes.
• Types Based on Location:
  • Junctional Nevus: Melanocytes at dermo-epidermal junction. Flat or minimally elevated, uniform brown.
  • Compound Nevus: Junctional + Dermal nevus cells. Slightly elevated, may be dome-shaped.
  • Intradermal Nevus: Melanocytes entirely in dermis. Elevated, dome-shaped, may be flesh-colored or lightly pigmented.
• Clinical Features:
  • Well-circumscribed.
  • Uniform color (tan, brown).
  • Symmetric, Regular borders.
  • Stable over time (though may slowly evolve with age toward dermal type).
• Dermoscopy: Reticular pattern (regular pigment network), Globular pattern (regular dots/globules), Homogeneous pattern.

31.1.2 Dysplastic Nevus (Atypical Nevus, Clark Nevus)

• Definition: Nevus with clinical and histologic atypia.
• Clinical Features:
  • Larger than common nevi (often >5-6mm).
  • Irregular borders ("fuzzy" or "ill-defined").
  • Variegated color (mixture of tan, brown, pink).
  • "Fried egg" appearance (central raised, peripheral flat).
• Significance: Marker of melanoma risk. Dysplastic Nevus Syndrome (many atypical nevi + family history of melanoma) = Very high melanoma risk.
• Management: Baseline photography, Regular surveillance, Biopsy if changing or highly atypical.

31.1.3 Congenital Melanocytic Nevus (CMN)

• Definition: Melanocytic nevus present at birth or appearing within first few weeks of life.
• Classification by Size (Projected Adult Size):
  • Small CMN: <1.5 cm
  • Medium CMN: 1.5-20 cm
  • Large/Giant CMN: >20 cm
• Clinical Features:
  • May have irregular borders, variegated color.
  • Giant CMN may have "bathing trunk" distribution.
  • May be covered with hair.
• Melanoma Risk: Correlates with size.
  • Small CMN: Very low risk.
  • Giant CMN: ~5-10% lifetime melanoma risk (especially if leptomeningeal involvement).
• Management: Surveillance, Consider excision for large CMN (especially with neurocutaneous melanosis risk).

31.1.4 Blue Nevus

• Definition: Dermal collection of dendritic melanocytes.
• Clinical Features:
  • Blue-black color (due to deep dermal location of melanin + Tyndall effect).
  • Well-circumscribed, homogeneous.
  • Typically small (<1cm).
  • Common on dorsum of hands, feet, Head/Neck, Sacrum.
• Dermoscopy: Homogeneous blue pigmentation ("blue veil").
• Variants: Common blue nevus (benign), Cellular blue nevus (larger, can rarely transform to malignant blue nevus).
• Differential: Nodular melanoma (if irregular borders, growth).

31.1.5 Spitz Nevus (Spindle and Epithelioid Cell Nevus)

• Definition: Benign melanocytic lesion composed of spindle and epithelioid cells.
• Clinical Features:
  • Rapidly growing, dome-shaped papule.
  • Pink/Red OR Pigmented (Reed nevus = heavily pigmented Spitz).
  • Common in children and adolescents.
  • Usually <1 cm.
• Dermoscopy: Starburst pattern (radial streaming), Globular pattern, Dotted vessels.
• Histology: Symmetric, spindle and epithelioid melanocytes, Kamino bodies.
• Differential: Melanoma. Atypical Spiz tumors are controversial—some advocate excision.

31.1.6 Melanoma

[!CAUTION] Any pigmented lesion that is new, changing, or symptomatic warrants evaluation for melanoma.

• Definition: Malignant tumor of melanocytes.
• ABCDE Criteria:
  • Asymmetry
  • Border irregularity
  • Color variegation (black, brown, red, white, blue)
  • Diameter >6mm
  • Evolution (changing)
• "Ugly Duckling Sign": Lesion that looks different from patient's other nevi.
• Dermoscopy Features of Melanoma:
  • Atypical pigment network (irregular, broken).
  • Blue-white veil.
  • Irregular dots and globules.
  • Regression structures (peppering, blue-gray dots).
  • Atypical vessels (polymorphous).
• Subtypes:
  • Superficial Spreading Melanoma (most common).
  • Nodular Melanoma (rapidly growing nodule, often lacks radial growth phase).
  • Lentigo Maligna Melanoma (on sun-damaged skin of elderly).
  • Acral Lentiginous Melanoma (palms, soles, nails—more common in darker skin).
  • Amelanotic Melanoma (non-pigmented—often missed).

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31.2 Non-Melanocytic Pigmented Lesions

31.2.1 Seborrheic Keratosis

• Definition: Benign keratinocytic proliferation (most common benign skin tumor).
• Clinical Features:
  • "Stuck-on" appearance.
  • Waxy, verrucous surface.
  • Color: Tan, Brown, Black.
  • Well-demarcated.
• Dermoscopy: Milia-like cysts, Comedo-like openings, Fissures/Ridges, Sharp demarcation.
• Differential: Melanoma (when heavily pigmented). Dermoscopy resolves.
• Leser-Trélat Sign: Sudden eruption of multiple seborrheic keratoses → Associated with internal malignancy.

31.2.2 Pigmented Basal Cell Carcinoma

• Definition: BCC variant with melanin pigmentation.
• Clinical Features:
  • Pearly, translucent papule or nodule with brown or blue-gray pigmentation.
  • Rolled borders, Central ulceration ("rodent ulcer").
• Dermoscopy:
  • Arborizing (treelike) vessels (classic for BCC).
  • Blue-gray ovoid nests and globules.
  • Spoke-wheel structures.
  • Absence of pigment network (distinguishes from melanoma).
• Management: Excision.

31.2.3 Dermatofibroma (Pigmented Variant)

• Clinical Features:
  • Firm papule, often on legs.
  • May have hyperpigmented periphery with central white area.
  • Dimple sign positive.
• Dermoscopy: Peripheral pigment network, Central white scar-like patch.

31.2.4 Pigmented Actinic Keratosis

• Clinical Features:
  • Rough, scaly macule or papule on sun-exposed skin.
  • Brown pigmentation (reticulated pattern).
• Dermoscopy: Annular granular pattern, Rhomboidal structures.
• Significance: Pre-malignant (risk of SCC).

31.2.5 Venous Lake

• Definition: Dilated venule in dermis.
• Clinical Features:
  • Soft, compressible, blue-purple papule.
  • Sun-exposed areas: Lips, Ears.
  • Elderly patients.
• Test: Blanches with pressure (diascopy).
• Dermoscopy: Homogeneous blue-purple, Lacunae.

31.2.6 Angiokeratoma

• Definition: Vascular lesion with overlying hyperkeratosis.
• Clinical Features:
  • Dark red, Blue, or Black papule.
  • May not blanch due to thrombosis.
• Types: Solitary, Fordyce (scrotum), Mibelli (acral in children), Fabry disease (diffuse).
• Dermoscopy: Dark lacunae (red-black), White veil (hyperkeratosis).

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31.3 Diagnostic Approach to Pigmented Palpable Lesions

31.3.1 Step 1: Is It Melanocytic?

Dermoscopy helps determine if lesion is melanocytic:

• Pigment network: Melanocytic.
• Aggregated globules: Melanocytic.
• Parallel pattern (acral): Melanocytic (potential melanoma if on ridges).
• If none of the above → Non-melanocytic (Consider BCC, SebK, Dermatofibroma, Vascular).

31.3.2 Step 2: If Melanocytic, Is It Benign or Malignant?

• Benign Nevus: Symmetric, Uniform color/pattern, Regular network.
• Melanoma: Asymmetric, Multi-colored, Atypical network, Blue-white veil, Irregular dots/globules, Regression.

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31.4 Diagnostic Algorithm

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31.5 Summary Comparison Table

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31.6 Clinical Pearls

1. New or changing pigmented lesion → Melanoma until proven otherwise. Dermoscopy + Biopsy.
2. "Ugly duckling" (lesion different from others) → High suspicion for melanoma.
3. Blue-white veil + Atypical network + Regression → Melanoma.
4. "Stuck-on" waxy lesion + Milia-like cysts → Seborrheic keratosis. No biopsy needed if classic.
5. Blue-gray + Arborizing vessels → Pigmented BCC. Excise.
6. Homogeneous blue-black + Well-demarcated → Blue nevus. Stable = benign.
7. Child + Rapidly growing pink/pigmented papule → Spitz nevus. Consider excision for histologic confirmation.