How do you differentiate pityriasis alba from vitiligo?

Differentiating Pityriasis Alba from Vitiligo

Pityriasis alba presents with ill-defined, hypopigmented (not depigmented) patches with fine scaling and preserved melanocytes on histology, while vitiligo shows well-demarcated, completely depigmented (chalk-white) patches with complete loss of melanocytes that enhance dramatically under Wood's light examination.

Clinical Examination Features

Pigmentation Characteristics

• Pityriasis alba shows incomplete hypopigmentation with indistinct, poorly defined borders that blend gradually into surrounding normal skin 1, 2
• Vitiligo demonstrates complete depigmentation appearing chalk-white with sharply demarcated borders, particularly evident in classical symmetrical presentations 3
• The degree of pigment loss is the most critical distinguishing feature: pityriasis alba retains some pigment while vitiligo shows total absence 1, 4

Surface Changes

• Pityriasis alba consistently presents with fine scaling and a slightly rough texture, often with follicular papules in early stages 2, 5
• Vitiligo has smooth surface without scaling, as it represents pure melanocyte loss without epidermal disruption 3, 6

Hair Involvement

• Pityriasis alba shows normally pigmented hairs within the hypopigmented patches 2
• Vitiligo frequently demonstrates leucotrichia (white hairs) within depigmented areas, indicating follicular melanocyte destruction 6

Wood's Light Examination

• Vitiligo lesions enhance dramatically under Wood's light, appearing bright white due to complete absence of melanin, making this examination particularly useful for diagnosis and monitoring 3
• Pityriasis alba shows minimal to no enhancement under Wood's light because melanin is reduced but not absent 4, 2

Distribution Patterns

Pityriasis Alba

• Predominantly affects the face (especially cheeks), arms, and forearms 5
• More commonly detected in children and adolescents with darker skin types 4, 2
• Lesions may be solitary (16% of cases) or multiple 1

Vitiligo

• Classical presentations show symmetrical distribution affecting fingers, wrists, axillae, groins, and body orifices (mouth, eyes, genitalia) 7, 8
• Can present as segmental (unilateral, following dermatomes) or non-segmental (symmetrical) patterns 7, 8
• Diagnosis is straightforward in symmetrical presentations but atypical cases require dermatologist assessment 3

Dermoscopic Features

Pityriasis Alba

• White structureless spots with indistinct borders 2
• Fine scaling consistently present 2
• Normally pigmented hairs within lesions 2
• Faint pigmented network may be visible (27.5% of cases) 2
• Areas of light brown pigmentation (42.5% of cases) 2

Vitiligo

• Reduced or absent pigment network in lesional center 6
• Perifollicular pigmentation patterns indicating disease activity or treatment response 6
• Sharp borders in stable disease; diffuse borders, trichrome pattern, or "comet tail" phenomenon in active disease 6
• Marginal hyperpigmentation common in pigmented lesions 6

Histopathological Differentiation

Pityriasis Alba

• Melanocyte count remains normal between lesional and non-lesional skin—this is the definitive histologic distinction 1
• Markedly reduced melanin pigment in epidermis despite normal melanocyte numbers 1
• Follicular plugging, follicular spongiosis, and atrophic sebaceous glands 5
• Degenerative changes in melanocytes with reduced melanosomes in keratinocytes 1

Vitiligo

• Complete loss of functioning epidermal melanocytes in affected areas 7, 8
• Progressive melanocyte destruction, not just dysfunction 8

Associated Conditions

Pityriasis Alba

• History of atopic dermatitis in 18% of patients 1
• Associated with xerosis and poor cutaneous hydration 4
• No systemic associations 4, 5

Vitiligo

• Autoimmune thyroid disease in approximately 34% of adults—thyroid function testing is specifically recommended by the British Journal of Dermatology guidelines 7
• Other autoimmune conditions frequently associated 7
• Familial trait in about 18% of cases 3

Common Diagnostic Pitfalls

• Failing to use Wood's light examination in patients with lighter skin types where vitiligo may be subtle 3
• Confusing early-stage pityriasis alba (with erythematous papules) with inflammatory conditions 5
• Missing the psychological impact of vitiligo, which significantly affects quality of life and requires assessment 3, 7
• Not screening for thyroid disease in vitiligo patients, missing a treatable autoimmune condition present in one-third of cases 7
• Overlooking the need for dermatologist referral when vitiligo presents atypically 3

Clinical Algorithm for Differentiation

1. Assess pigment loss degree: Complete (vitiligo) vs. incomplete (pityriasis alba) 1, 4
2. Check for scaling: Present (pityriasis alba) vs. absent (vitiligo) 2, 5
3. Perform Wood's light exam: Dramatic enhancement (vitiligo) vs. minimal/none (pityriasis alba) 3, 4
4. Examine hair within lesions: White hairs (vitiligo) vs. normally pigmented (pityriasis alba) 6, 2
5. Evaluate borders: Sharp demarcation (vitiligo) vs. indistinct (pityriasis alba) 1, 2
6. If vitiligo confirmed: Check thyroid function given 34% prevalence of autoimmune thyroid disease 7