How can I differentiate eczema (atopic dermatitis) from tinea corporis (ringworm)?

Differentiating Eczema (Atopic Dermatitis) from Ringworm (Tinea Corporis)

Ringworm presents as a well-demarcated annular plaque with a raised, scaly advancing border and central clearing, while eczema shows poorly demarcated patches with diffuse erythema, excoriations from scratching, and no central clearing. 1

Key Distinguishing Clinical Features

Morphology and Distribution

• Ringworm appears as circular or ring-shaped lesions with a distinct raised border that advances centrifugally, often with clearing in the center 1
• Eczema presents with poorly defined erythematous patches, papules, and plaques that lack the characteristic ring shape and central clearing 2, 3
• Eczema in infants affects the cheeks, scalp, and extensor surfaces (sparing the diaper area), while older children and adults show flexural involvement (antecubital and popliteal fossae, neck) 3, 4
• Ringworm can occur anywhere on the body without the age-specific distribution pattern seen in eczema 1

Surface Characteristics

• Ringworm has fine scaling primarily at the advancing border with relatively smooth central areas 1
• Eczema shows diffuse xerosis (dry skin), erosions, excoriations from scratching, oozing and crusting in acute stages, and lichenification (thickened skin with accentuated markings) in chronic stages 2, 3

Pruritus Pattern

• Eczema features intense, persistent pruritus that is a hallmark of the condition and drives the itch-scratch cycle 2, 4
• Ringworm may be mildly pruritic but lacks the severe, relentless itching characteristic of eczema 1

Historical Features That Distinguish the Conditions

Eczema-Specific History

• Chronic relapsing course with flares and remissions 2, 4
• Personal or family history of atopy (allergic rhinitis, asthma, food allergies) 2, 3
• Onset typically between 3-6 months of age, with 60% developing symptoms in the first year and 90% by age 5 2
• Elevated serum IgE levels (though not required for diagnosis) 2

Ringworm-Specific History

• Recent contact with infected individuals or animals 1
• No association with atopic conditions 1
• Acute onset without chronic relapsing pattern 1

Diagnostic Confirmation

When Ringworm is Suspected

• Perform potassium hydroxide (KOH) preparation of skin scrapings from the active border to visualize fungal hyphae 1
• Fungal culture can confirm dermatophyte infection when KOH is negative but clinical suspicion remains high 1
• Histological examination with periodic acid-Schiff (PAS) staining can identify fungal elements in atypical cases 1

When Eczema is Suspected

• Diagnosis is clinical based on morphology, distribution, chronicity, and pruritus 2, 4
• No laboratory testing is required for typical presentations 2
• Consider patch testing if allergic contact dermatitis coexists (occurs in 6-60% of eczema patients) 2

Critical Pitfalls to Avoid

• Atypical ringworm presentations can mimic eczema when vesicles, pustules, or bullae develop due to severe inflammation, making clinical diagnosis challenging 1, 5
• Tinea incognito occurs when ringworm is misdiagnosed as eczema and treated with topical corticosteroids, which suppress inflammation but allow fungal proliferation, creating bizarre morphologies 1
• Always perform KOH examination when the diagnosis is uncertain or when eczema fails to respond to appropriate therapy 1
• Eczema patients are prone to secondary bacterial infections (particularly Staphylococcus aureus), which can alter the clinical appearance and confuse the diagnosis 3, 4
• Multiple conditions can coexist—a patient with eczema can simultaneously develop ringworm in affected areas 5

Practical Diagnostic Algorithm

1. Assess lesion morphology: Ring-shaped with raised border and central clearing suggests ringworm; diffuse patches with excoriations suggest eczema 1, 2
2. Check distribution: Age-specific flexural pattern favors eczema; random distribution favors ringworm 3, 1
3. Evaluate chronicity: Chronic relapsing course with atopic history indicates eczema; acute onset without atopy suggests ringworm 2, 1
4. Perform KOH preparation when ringworm is suspected or diagnosis is uncertain 1
5. Assess treatment response: Eczema improves with emollients and topical corticosteroids; ringworm worsens or becomes atypical with corticosteroids alone 4, 1