Differential Diagnosis of Hyperpigmented Lesions

Introduction

Hyperpigmentation, defined as skin darkening due to excess melanin or other pigments, is among the most common dermatological complaints, particularly in skin of color (Fitzpatrick Phototypes III-VI). Unlike hypopigmentation where the differential is relatively narrow, hyperpigmentation encompasses a vast spectrum from benign cosmetic conditions (melasma) to life-threatening malignancies (melanoma) and systemic diseases (Addison's disease). The initial clinical challenge is to correctly classify the pigment:

1. Epidermal Melanin (Brown/Black): Melanin located in keratinocytes.
2. Dermal Melanin (Blue-Gray/Slate): Melanin deposited in dermal melanophages (Ceruleoderma).
3. Non-Melanin Pigments (Yellow, Green, Ochre): Hemosiderin, bilirubin, exogenous substances.

This chapter provides a systematic approach to differentiating these lesions using clinical pattern recognition, dermoscopy, and Wood's lamp examination.

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16.1 Diagnostic Toolkit

16.1.1 Wood's Lamp Examination

The Wood's lamp is invaluable for determining the depth of melanin deposition.

[!IMPORTANT] Melasma Typing: Wood's lamp is essential for classifying melasma into Epidermal (enhanced), Dermal (no change), or Mixed types. This guides treatment selection: Epidermal melasma responds better to topical depigmenting agents, while dermal melasma is recalcitrant.

16.1.2 Dermoscopy of Hyperpigmentation

Dermoscopy allows for assessment of pigment color, distribution, and vascular changes.

• Melasma:
  • Pseudoreticular Pigment Network: An irregular network with accentuation around follicles, creating a "honeycomb" pattern.
  • Perifollicular Sparing: Follicular openings appear pale/white against a darker background. Highly specific.
  • Telangiectasias: Irregular, arborizing vessels are common.
  • Arcuate Structures: Curved pigment lines.
• Lichen Planus Pigmentosus (LPP):
  • Brownish Dots/Globules: Represent superficial dermal melanophages (papillary dermis).
  • Brownish Background: Overall brown hue.
  • Diffuse/Speckled/Reticular Pattern: Variable.
• Ashy Dermatosis (Erythema Dyschromicum Perstans):
  • Gray-Blue Dots: Represent deep dermal melanophages (reticular dermis).
  • Bluish Background: The key differentiator from LPP.
  • Irregular Linear Vessels: May be present in early lesions.
• Post-Inflammatory Hyperpigmentation (PIH):
  • Variable Patterns: Depends on the preceding inflammation.
  • Homogeneous Brown Pigmentation: Can be seen.
  • Vascular Components: Erythema, red dots, glomerular vessels if recent.
• Dermal Melanocytoses (Nevus of Ota/Ito):
  • Homogeneous Blue-Gray Color: Uniform deep blue.
  • Absence of Network: No reticulated pattern.

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16.2 Classification by Color

16.2.1 Brown / Black Pigmentation (Melanoderma)

Implies melanin at the epidermal or dermo-epidermal junction level.

Major Differentials

A. Melasma (Chloasma)

• Clinical Features: Symmetric, macular hyperpigmentation on sun-exposed areas of the face (centrofacial, malar, mandibular patterns).
• Epidemiology: Predominantly affects women (90%), often triggered by pregnancy ("Mask of Pregnancy"), oral contraceptives, and UV exposure.
• Pathogenesis: Upregulation of melanogenesis influenced by estrogen, UV, and vascular factors.
• Dermoscopy: Pseudoreticular network with perifollicular sparing, telangiectasias.

B. Post-Inflammatory Hyperpigmentation (PIH)

• Definition: Acquired hyperpigmentation following cutaneous inflammation or injury.
• Causes: Acne vulgaris (most common), eczema, psoriasis, lichen planus, burns, laser therapy, chemical peels.
• Distribution: Corresponds exactly to the site of the preceding lesion.
• Dermoscopy: Variable; often homogeneous brown with remnants of the primary pathology (e.g., acne scars).

C. Freckles (Ephelides) vs. Solar Lentigines

D. Café-au-Lait Macules (CALMs)

• Morphology: Uniformly pigmented, well-demarcated, oval macules with smooth ("coast of California") or irregular ("coast of Maine") borders.
• Significance:
  • ≥6 CALMs (>5mm pre-pubertal, >15mm post-pubertal): Criteria for Neurofibromatosis Type 1 (NF1).
  • Large, irregular CALM with "coast of Maine" border: Consider McCune-Albright Syndrome.

E. Acanthosis Nigricans

• Clinical Features: Velvety, hyperpigmented, thickened plaques in flexural areas (neck, axillae, groin).
• Associations:
  • Benign: Obesity, Insulin Resistance, PCOS, Type 2 DM.
  • Malignant: Rapidly progressive, involving mucosae → suspect internal malignancy (Gastric Adenocarcinoma).

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16.2.2 Blue-Gray Pigmentation (Ceruleoderma)

Implies melanin located deep in the dermis (Tyndall effect).

Major Differentials

A. Lichen Planus Pigmentosus (LPP)

• Clinical Features: Dark brown to slate-gray macules and patches. Often asymptomatic or mildly pruritic.
• Distribution: Sun-exposed areas (face, neck) OR flexures (LPP inversus).
• Histology: Interface dermatitis with pigment incontinence. Melanophages in upper dermis.
• Dermoscopy: Brownish dots/globules on a brownish background.

B. Ashy Dermatosis (Erythema Dyschromicum Perstans)

• Clinical Features: Asymptomatic, blue-gray macules and patches. Early lesions may have a raised, reddish border.
• Distribution: Trunk and proximal extremities. Face is usually spared.
• Dermoscopy: Gray-blue dots on a bluish background (deeper dermal pigment).
• Differentiation from LPP: Ashy dermatosis has a bluer hue and deeper pigment dermoscopically.

C. Dermal Melanocytoses

• Mongolian Spot: Blue-gray patch on the lumbosacral area. Present at birth. Fades by age 5-7.
• Nevus of Ota: Blue-gray pigmentation in the distribution of V1/V2 (trigeminal nerve). Involves sclera (episcleral melanosis). Risk of glaucoma and melanoma.
• Nevus of Ito: Similar to Ota, but affects the shoulder and upper arm distribution (C3-C4).

D. Drug-Induced Hyperpigmentation

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16.2.3 Yellow / Orange / Green Pigmentation

Implies non-melanin pigment.

• Carotenemia: Yellow-orange discoloration, especially on palms and soles. Sclerae are spared (differentiates from jaundice). Caused by excessive dietary carotene.
• Jaundice (Icterus): Yellow discoloration of skin AND sclerae. Bilirubin deposition.
• Hemosiderin: Brown-ochre discoloration, typically on lower legs. Seen in chronic venous insufficiency, purpura, hemochromatosis.
• Tattoo Pigments: Green (chrome), blue (cobalt), etc.

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16.3 Classification by Distribution & Pattern

The arrangement of hyperpigmentation provides powerful diagnostic clues.

16.3.1 Blaschko Lines

Linear or whorled patterns following embryonic cell migration lines.

• Incontinentia Pigmenti (Stage 3): Whorled hyperpigmentation on trunk.
• Linear Epidermal Nevus: Verrucous hyperpigmented streaks.
• McCune-Albright Syndrome: CALMs following Blaschko lines, respecting midline.

16.3.2 Reticulate (Net-like) Pattern

• Erythema Ab Igne: Reticulated hyperpigmentation from chronic heat exposure (e.g., laptop, hot water bottle).
• Confluent and Reticulate Papillomatosis (Gougerot-Carteaud): Reticulated, verrucous papules on the intermammary trunk. Responds to minocycline.
• Dowling-Degos Disease: Reticulate pigmentation in flexures, comedones, pitted scars.

16.3.3 Flagellate Pattern

Linear, whip-like hyperpigmentation.

• Bleomycin-induced: Classic cause. Appears during chemotherapy.
• Dermatomyositis: "Flagellate erythema" on trunk.
• Shiitake Mushroom Dermatitis: Caused by consumption of raw/undercooked shiitake mushrooms.

16.3.4 Topographical Distribution

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16.4 Systemic Causes of Diffuse Hyperpigmentation

Generalized, poorly-defined hyperpigmentation requires investigation for systemic disease.

[!CAUTION] Do Not Miss: Addison's Disease Primary adrenal insufficiency leads to loss of cortisol feedback, causing elevated ACTH and MSH, which stimulate melanogenesis.

• Key Sites: Sun-exposed areas, flexural creases (palmar), oral mucosa, areolae, recent scars.
• Associated Signs: Fatigue, weight loss, hypotension, salt craving, hypoglycemia.
• Diagnosis: Low morning cortisol, high ACTH, ACTH stimulation test.

Other Systemic Causes

• Hemochromatosis: "Bronze diabetes". Slate-gray or bronze skin due to iron and melanin deposition.
• Chronic Renal Failure: Diffuse sallow/yellow-brown due to urochrome retention.
• Primary Biliary Cholangitis: Generalized hyperpigmentation, often with xanthelasma.
• POEMS Syndrome: Polyneuropathy, Organomegaly, Endocrinopathy, M-protein, Skin changes (hyperpigmentation, hypertrichosis).
• Pellagra: Photosensitive hyperpigmentation (Casal's necklace).

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16.5 Pigmented Tumors: "Do Not Miss"

[!WARNING] Any single, new, or changing pigmented lesion in an adult must be evaluated for Melanoma.

• Melanoma: Asymmetry, Border irregularity, Color variegation (brown, black, red, white, blue), Diameter >6mm, Evolution.
• Seborrheic Keratosis: "Stuck-on" appearance, horn cysts on dermoscopy. Benign but can mimic melanoma.
• Pigmented Basal Cell Carcinoma: Blue-gray ovoid nests, arborizing vessels, spoke-wheel structures.
• Dermatofibroma: "Dimple sign", central white scar-like patch on dermoscopy.

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16.6 Summary Diagnostic Algorithm

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16.7 Common Bedside Differentiations (Pairwise)

Melasma vs. Post-Inflammatory Hyperpigmentation

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Lichen Planus Pigmentosus vs. Ashy Dermatosis

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Table 16.1: Master Differential Diagnosis Matrix