What are the differential diagnoses for a persistent unilateral erythematous scaly patch on one cheek in a 4-year-old child with a history of atopic dermatitis?

Differential Diagnoses for Persistent Unilateral Cheek Lesion in a 4-Year-Old with Atopic Dermatitis History

The persistent unilateral erythematous scaly patch on one cheek in this child warrants consideration of several important differential diagnoses beyond chronic atopic dermatitis, most notably tinea faciei (fungal infection), allergic contact dermatitis, and less commonly, cutaneous T-cell lymphoma or other rare conditions.

Key Clinical Red Flags

The unilateral distribution is atypical for atopic dermatitis, which characteristically presents with symmetrical lesions 1. This asymmetry should immediately prompt consideration of alternative diagnoses 1.

Primary Differential Diagnoses to Consider

Tinea Faciei (Fungal Infection)

• Annular papulosquamous lesions without eczematous change are characteristic 1
• Unilateral facial involvement is common with dermatophyte infections
• Perform skin scraping for microscopy and culture to confirm diagnosis 1
• This is particularly important as fungal infections can mimic eczematous conditions

Allergic Contact Dermatitis

• Atypical or localized distribution is a hallmark feature 1
• Common allergens in children include nickel, fragrances, preservatives, and topical medications 1
• The unilateral cheek pattern may reflect contact with a specific item (e.g., phone, pillow, hand-touching behavior) 1
• Consider patch testing if history or physical examination suggests contact dermatitis, especially in recalcitrant cases 1
• Contact dermatitis can coexist with atopic dermatitis and may be both an alternative diagnosis and an exacerbating factor 1

Seborrheic Dermatitis

• Can overlap with atopic dermatitis, particularly in young children 1
• Typically presents with greasy scale and lacks significant pruritus 1
• Usually affects scalp and folds, but facial involvement can occur 1

Nummular Eczema

• Can present as coin-shaped patches that may be unilateral 2
• Represents a distinct morphologic variant that can occur in atopic individuals

Less Common but Important Considerations

Scabies Infestation

• Look for burrows, particularly in interdigital spaces 1, 3
• May present with inguinal, axillary, and genital papules 1
• Can cause localized persistent lesions

Immunodeficiency Disorders

• Consider if there are sinopulmonary infections and failure to thrive 1
• Conditions like Wiskott-Aldrich syndrome or hyper-IgE syndrome can present with eczematous lesions 1

Nutritional Deficiencies

• Zinc deficiency presents with erosive plaques on face and groin with irritability 1
• Should be considered if lesions are treatment-resistant

Diagnostic Approach

Immediate Steps

• Obtain skin scraping for KOH preparation and fungal culture from the unilateral lesion 1
• Document the exact distribution and morphology of the lesion
• Review all topical products, medications, and potential contact exposures 1

If Initial Testing is Negative

• Consider patch testing for contact allergens, particularly if there is an unusual distribution, later onset, or persistent disease not responding to standard therapy 1
• Evaluate for signs of systemic disease or immunodeficiency if constitutional symptoms are present 1

Critical Clinical Pearls

• Atopic dermatitis diagnosis should be reconsidered when lesions do not respond to appropriate therapy 1
• The symmetrical distribution is a core diagnostic feature of atopic dermatitis 1, making this unilateral presentation highly suspicious for an alternative or concurrent diagnosis
• Allergic contact dermatitis occurs in 6-60% of patients with atopic dermatitis and is often clinically indistinguishable 1
• Fungal infections can be easily missed and should always be ruled out with appropriate testing in atypical presentations 1

Treatment Implications

• Do not intensify atopic dermatitis therapy until alternative diagnoses are excluded 1
• If tinea is confirmed, antifungal therapy is required rather than continued topical corticosteroids
• If contact dermatitis is identified, avoidance of the specific allergen is essential for resolution 1
• Continued inappropriate treatment of a misdiagnosed condition will lead to treatment failure and potential complications