What is the differential diagnosis (ddx) for Blaschko lines?

Differential Diagnosis for Blaschko Lines

When evaluating linear skin lesions following Blaschko lines, the differential diagnosis includes both congenital/nevoid conditions and acquired inflammatory dermatoses, with the key distinguishing features being age of onset, presence of inflammation, duration of lesions, and histopathologic findings. 1, 2

Primary Categories of Blaschkolinear Dermatoses

Congenital/Nevoid Conditions

• Linear sebaceous nevus: Lifelong condition present from birth or early childhood, characterized by yellowish papules and plaques along Blaschko lines 1
• Epidermal nevi: Warty or verrucous linear lesions typically present at birth or appearing in early childhood 3, 4
• Inflammatory linear verrucous epidermal nevus (ILVEN): Pruritic, erythematous, scaly linear plaques that persist throughout life 3
• Linear atrophoderma of Moulin: Hyperpigmented atrophic bands developing in childhood/adolescence without preceding inflammation or induration 3

Acquired Inflammatory Dermatoses

• Lichen striatus: Self-limited condition (typically 1-2 years duration) presenting as small, flat-topped papules in a linear band, most common in children but can occur in adults 1, 2
• Adult blaschkitis: Interface dermatitis following Blaschko lines in adults, clinically and histologically overlapping with lichen striatus 2, 4
• Linear psoriasis (Blaschko linear psoriasis): Psoriatic plaques distributed along Blaschko lines, can be type I (isolated linear lesions) or type II (linear plus classical psoriasis lesions elsewhere) 5
• Linear lichen planus: Violaceous papules in linear configuration, significant clinical and histologic overlap with lichen striatus and adult blaschkitis 2, 4

Other Acquired Conditions

• Linear lupus erythematosus: Erythematous scaly plaques with atrophy following Blaschko lines 3
• Linear scleroderma (morphea): Indurated, sclerotic linear bands with preceding inflammation 6, 3
• Lichen sclerosus (Blaschkoid variant): Porcelain-white atrophic plaques in linear distribution, most commonly extragenital sites 6
• Fixed drug eruption: Can present in linear pattern along Blaschko lines with history of medication exposure 4
• Chronic graft-versus-host disease: Linear lichenoid eruption in transplant recipients 4

Key Diagnostic Features to Distinguish Entities

Age of Onset

• Childhood/adolescence: Favors lichen striatus, linear atrophoderma of Moulin, epidermal nevi, or linear sebaceous nevus 1, 3
• Adult onset: Consider adult blaschkitis, linear lichen planus, linear psoriasis, or linear lupus erythematosus 5, 2

Duration and Natural History

• Self-limited (1-2 years): Lichen striatus, adult blaschkitis 1, 2
• Lifelong/persistent: Epidermal nevi, linear sebaceous nevus, linear psoriasis, linear lichen planus 5, 1

Presence of Inflammation

• No preceding inflammation: Linear atrophoderma of Moulin, unilateral nevoid telangiectasia 1, 3
• Active inflammation: Lichen striatus, adult blaschkitis, linear psoriasis, linear lichen planus 5, 2

Associated Features

• Nail/scalp involvement: More common in type II Blaschko linear psoriasis 5
• Coexisting classical lesions elsewhere: Type II linear psoriasis, linear lichen planus 5, 2
• Atrophy/induration: Linear scleroderma, lichen sclerosus, linear atrophoderma of Moulin 6, 3
• Family history of psoriasis: Usually negative in Blaschko linear psoriasis 5

Histopathologic Patterns

• Interface dermatitis: Lichen striatus, adult blaschkitis, linear lichen planus (significant overlap between these three entities) 2
• Psoriasiform hyperplasia: Linear psoriasis 5
• Epidermal hyperplasia with papillomatosis: Epidermal nevi, ILVEN 3
• Dermal sclerosis: Linear scleroderma 6
• Lichenoid interface with basement membrane thickening: Lichen sclerosus 6

Critical Diagnostic Approach

Clinical Evaluation

• Laterality: Most Blaschkolinear dermatoses are unilateral with no preference for left or right side 5, 1
• Symptomatology: Asymptomatic or slight pruritus favors nevoid conditions or linear psoriasis; significant pruritus suggests ILVEN 5
• Triggering factors: Identify potential exogenous triggers (trauma, medications, infections) that may unmask latent mosaicism 5, 4
• Distribution pattern: Distinguish Blaschko lines from Langer's lines, Voight's lines, or dermatomal distribution 1

Histopathologic Confirmation

• Biopsy is essential when diagnosis is uncertain, particularly to distinguish between overlapping entities like lichen striatus, adult blaschkitis, and linear lichen planus 2
• Multiple biopsies may be needed if extensive abnormality or suspicion of malignant transformation (particularly in long-standing linear sebaceous nevus) 6

Special Considerations for Segmental Vitiligo

• Segmental vitiligo presents as unilateral depigmentation following Blaschko lines or dermatomes, suggesting neural basis 7, 8, 9
• Wood's light examination helps delineate pigment loss in lighter skin types 8, 9
• Screen for autoimmune thyroid disease (present in 34% of vitiligo patients) even in segmental distribution 7, 8

Common Diagnostic Pitfalls

• Failing to recognize the spectrum: Lichen striatus, adult blaschkitis, and linear lichen planus exist on a clinical and histopathologic spectrum; clinicopathologic correlation is essential for accurate diagnosis 2, 4
• Overlooking mosaicism: The linear pattern reflects cutaneous mosaicism from early embryonic somatic mutations, not vascular, lymphatic, or neural structures 1, 4
• Misdiagnosing linear scleroderma: Must distinguish from linear atrophoderma of Moulin (no induration/scleroderma) and lichen sclerosus Blaschkoid variant (porcelain-white appearance, different histology) 6, 3
• Missing associated systemic disease: In linear psoriasis type II, evaluate for classical psoriasis elsewhere and consider systemic therapy including biologics 5
• Ignoring malignant potential: Long-standing linear sebaceous nevus requires surveillance for secondary neoplasms; lichen sclerosus requires monitoring for squamous cell carcinoma 6