Differential Diagnosis: Hypopigmented Patch in Children

1. Pityriasis Alba

• Slightly scaly, round-to-oval, ill-defined macules/patches with mild-to-moderate hypopigmentation
• Predominantly affects the face (malar region), but also neck, trunk, extremities
• Most apparent during summer when surrounding skin tans
• Lesion size: 0.5-3 cm (occasionally larger)
• Earliest lesions may be pink; later become whitish, dry, powdery with a faintly raised pinkish border
• More noticeable in dark-skinned children
• Wood's lamp: decrease but NOT absence of pigment
• Usually clears spontaneously after puberty
• Histology: reduced active melanocytes; decreased melanosome number and size
• Dermatology 2-Volume Set 5e, p. 1324

2. Tinea Versicolor (Pityriasis Versicolor)

• Superficial fungal infection by Malassezia species
• Hypopigmented (or hyperpigmented/pink) macules and patches with fine scale
• Primarily chest and trunk; may extend to head and limbs
• Hypopigmentation in dark-skinned patients is due to abnormally small, poorly melanized melanosomes that fail to transfer to keratinocytes
• Hypopigmentation may persist for weeks to months even after treatment
• KOH prep: "spaghetti and meatballs" hyphae and spores
• Textbook of Family Medicine 9e

3. Vitiligo

• Acquired disorder with circumscribed depigmented macules and patches due to loss of epidermal melanocytes
• Affects ~0.5-1% of the population; can appear any time from shortly after birth onward
• Complete absence of pigment on Wood's lamp (differentiates from pityriasis alba which shows decrease only)
• Leukotrichia (white hairs within lesions) reflects loss of follicular melanocytes
• Autoimmune pathogenesis: CXCL9/10-driven CD8+ T cell recruitment destroys melanocytes
• Strong genetic component; associated with other autoimmune diseases (thyroiditis, diabetes)
• Childhood vitiligo is well-recognized, representing a distinct subgroup
• Dermatology 2-Volume Set 5e, p. 1307

4. Nevus Depigmentosus

• Well-demarcated hypopigmented patch present from birth (congenital)
• May become more visible in the first year of life as background skin pigmentation increases
• Reflects cutaneous mosaicism
• Classic presentation: central patch that breaks apart into smaller macules at the periphery - resembles a "splash of paint"
• Generally lacks extracutaneous associations when isolated
• 3 or more lesions should prompt evaluation for tuberous sclerosis complex
• Diascopy: border remains crisp (distinguishes it from nevus anemicus)
• Stable and non-progressive
• Fitzpatrick's Dermatology, p. 1763; Dermatology 2-Volume Set 5e, p. 1227

5. Nevus Anemicus

• Hypopigmented patch due to focal vasoconstriction (not true hypopigmentation)
• Caused by local vascular hypersensitivity to catecholamines
• Diascopy (glass slide pressure): border BLURS (surrounding normal skin blanches and matches the lesion) - key distinguishing test
• Present from birth; usually on trunk
• No melanocyte abnormality - melanocytes are normal in number
• Fitzpatrick's Dermatology, p. 1763

6. Ash-Leaf Macule (Tuberous Sclerosis Complex)

• Hypomelanotic macule that is a cardinal skin feature of tuberous sclerosis complex (TSC)
• Often elliptical/lance-ovate shape (ash-leaf), but can be confetti-like or polygonal
• Usually present at birth or in early infancy; may be the first sign of TSC
• Associated findings: facial angiofibromas (adenoma sebaceum), shagreen patch, periungual fibromas, seizures, intellectual disability
• Wood's lamp examination highlights lesions in fair-skinned children
• TSC is caused by mutations in TSC1 (hamartin) or TSC2 (tuberin)
• Goldman-Cecil Medicine; Dermatology 2-Volume Set 5e; Harrison's 22e

7. Leprosy (Hansen's Disease) - Endemic Areas

• Indeterminate leprosy (IL): one or a few hypopigmented or faintly erythematous, ill-defined macular lesions (1-5 cm); external aspects of limbs, buttocks, face; mild sensory impairment (touch/thermal)
• Tuberculoid (TT) leprosy: well-defined hypopigmented macule or raised plaque; complete loss of fine touch and temperature sensation over the lesion surface; dry, hairless, anesthetic skin surface
• Sensory loss, anhidrosis, and loss of hair within the patch are hallmarks
• Thickened peripheral nerves near lesion
• Always exclude in endemic regions (South Asia, Brazil, sub-Saharan Africa)
• Harrison's Principles of Internal Medicine 22e, p. 1450

8. Post-Inflammatory Hypopigmentation (PIH)

• Follows prior skin inflammation (atopic dermatitis, psoriasis, pityriasis lichenoides, infections, burns, contact dermatitis)
• Corresponds to the site of prior lesion
• History of preceding skin disease is key
• Pigmentation typically recovers spontaneously over months

9. Hypopigmented Mycosis Fungoides

• Rare but important in children/adolescents, especially in darkly pigmented skin
• Accounts for 25-50% of all MF in children (much higher proportion than in adults)
• Hypopigmented patches on trunk or extremities; may be mildly pruritic
• No obvious scaling or induration early on
• Diagnosis requires skin biopsy: typical MF histology (epidermotropic T-cell lymphoma)
• Electron microscopy: defect in melanosomal transfer to keratinocytes
• Responds to phototherapy/topical nitrogen mustard with repigmentation
• Dermatology 2-Volume Set 5e, p. 1325

10. Other Less Common Causes

Distinguishing Features - Quick Reference