What is the differential diagnosis for a child presenting with irregularly shaped hypopigmented lesions on the dorsum of the foot with some hyperpigmented parts below the lateral malleolus?

Differential Diagnosis of Irregular Hypopigmented Lesions on Dorsum of Foot with Hyperpigmented Areas Below Lateral Malleolus in a Child

The most likely diagnoses to consider are pityriasis alba, tinea versicolor (pityriasis versicolor), vitiligo, and nevus depigmentosus, with pityriasis alba and tinea versicolor being the most common hypopigmentation disorders in children. 1

Primary Differential Diagnoses

Pityriasis Alba

• Presents as ill-defined, scaly patches of hypomelanosis, most commonly on the face but can occur on extremities 1
• More visible in darker-skinned children and exacerbated by sun exposure, which increases contrast between normal and affected skin 2
• Associated with atopic diathesis, xerosis, and poor cutaneous hydration 2
• The irregular shape and hypopigmented appearance with scaling fits this diagnosis 1, 2
• Diagnosis is clinical; look for fine scaling, incomplete pigment loss, and ill-defined borders 1

Tinea Versicolor (Pityriasis Versicolor)

• Can present with both hypopigmented and hyperpigmented patches, making this a strong consideration given the mixed pigmentation pattern described 3, 1
• Typically favors the upper trunk in adolescents but can occur on extremities 1
• Dermoscopy shows nonuniform pigmentation (92.68% of hypopigmented lesions) and scales (86.56% of cases), with patchy scaling being most common 4
• Confirm diagnosis with potassium hydroxide (KOH) preparation showing hyphal and yeast forms 1
• The combination of hypopigmented and hyperpigmented areas in close proximity is characteristic 4, 3

Vitiligo

• Presents with complete depigmentation (not hypopigmentation) with well-defined borders, typically in periorificial or acral distribution 5, 1
• The dorsum of the foot is a common site (fingers, wrists, axillae, groins are typical) 5
• Wood's lamp examination enhances visualization of complete pigment loss 5, 6
• Vitiligo shows symmetrical distribution in non-segmental type or unilateral distribution following Blaschko's lines in segmental type 5, 7
• Screen for autoimmune thyroid disease (found in 34% of adults with vitiligo) 5
• The irregular shape could represent active disease with confetti-like depigmentation 6

Nevus Depigmentosus

• Congenital, stable leukoderma present from birth or early infancy, distinguishing it from acquired disorders 1
• Does not progress or change over time 1
• Localized form presents as single hypopigmented patch; systematized form follows Blaschko's lines 1
• Must distinguish from ash leaf spots of tuberous sclerosis 1

Key Diagnostic Features to Assess

Clinical Examination Details

• Assess border definition: ill-defined suggests pityriasis alba; sharp borders suggest vitiligo or nevus depigmentosus 1, 6
• Evaluate degree of pigment loss: incomplete hypopigmentation (pityriasis alba, tinea versicolor) versus complete depigmentation (vitiligo) 1
• Check for scaling: present in pityriasis alba and tinea versicolor; absent in vitiligo and nevus depigmentosus 4, 1
• Document distribution pattern: symmetrical (vitiligo vulgaris), unilateral/dermatomal (segmental vitiligo), or random 5, 6
• Examine for signs of atopy (xerosis, eczema) supporting pityriasis alba 2

Diagnostic Testing

• Perform KOH preparation of scale to identify fungal elements in tinea versicolor 1
• Use Wood's lamp examination to enhance visualization: vitiligo shows bright white fluorescence; tinea versicolor shows yellow-gold fluorescence 5
• Consider dermoscopy showing nonuniform pigmentation and patchy scaling for tinea versicolor 4
• If vitiligo is confirmed, check thyroid function tests and thyroid autoantibodies 5

Critical Pitfalls to Avoid

• Do not assume all hypopigmentation in children is benign without proper examination; always perform KOH preparation when scaling is present 1
• Failing to use Wood's lamp examination can lead to misdiagnosis, particularly in lighter-skinned children where lesions may be subtle 5
• Sun exposure makes pityriasis alba more apparent due to increased contrast; counsel families that this is not disease progression 2
• The presence of both hypopigmented and hyperpigmented areas strongly suggests tinea versicolor rather than other diagnoses 4, 3
• If vitiligo is diagnosed, failure to screen for thyroid disease misses a treatable autoimmune condition present in one-third of patients 5
• Nevus depigmentosus should be stable from birth; any progression rules out this diagnosis 1

Management Approach Based on Diagnosis

If Pityriasis Alba

• Educate on proper skin care and hydration 2
• Prescribe emollients and lubricants for xerosis 2
• Consider mild topical corticosteroids if inflammation present (limited duration to avoid atrophy) 2
• Topical calcineurin inhibitors are alternative anti-inflammatory options 2

If Tinea Versicolor

• Treat with topical antifungals once KOH confirms diagnosis 3
• Counsel that pigmentation changes may persist for months after successful treatment 3

If Vitiligo

• For localized disease, use potent topical steroids (maximum 2-month trials) or calcineurin inhibitors (pimecrolimus or tacrolimus with better safety profiles) 5
• For widespread disease, narrowband UVB phototherapy is preferred over PUVA 5
• Assess disease stability: surgical interventions require 12 months of stability with no new lesions, no Koebner phenomenon, and no extension 5
• Screen for thyroid disease and monitor for other autoimmune conditions 5