What is the diagnosis for erosions and scales on interdigital webs of toes, vesicles on palms and sides of fingers, and a pruritic painful rash on hands?

Diagnosis: Tinea Pedis with Secondary Dermatophytid Reaction (Id Reaction)

The clinical presentation of erosions and scales in the interdigital toe webs combined with vesicles on the palms and sides of fingers represents tinea pedis (fungal infection) with a secondary id reaction (dermatophytid reaction), which is an immunologic response to the fungal infection at a distant site. 1

Primary Pathology: Interdigital Tinea Pedis

The erosions and scales in the interdigital toe webs are the hallmark of tinea pedis, particularly the interdigital variant. 1

• Macerated or fissured interdigital toe spaces serve as the reservoir for pathogens, including dermatophytes, and are a well-documented source of streptococcal colonization that can lead to cellulitis. 1
• The interdigital involvement with scaling and fissuring is specifically mentioned as requiring treatment to eradicate pathogen colonization. 1
• Careful examination and treatment of toe web abnormalities (fissuring, scaling, or maceration) is essential to eradicate colonization and prevent recurrent infections. 1

Secondary Manifestation: Dermatophytid (Id) Reaction

The vesicles on palms and sides of fingers represent a dermatophytid reaction (also called an "id reaction"), which is an immunologic response to the primary fungal infection. 2, 3, 4

• Vesicles on the sides of fingers and palms that are pruritic and painful are characteristic of dyshidrotic eczema/pompholyx, which can be triggered by fungal infections elsewhere on the body. 2, 3, 5
• These vesicles are "imbedded in the epidermis below the thick stratum corneum" and are "strongly distended and painful when they grow in size." 2
• The id reaction occurs as a hypersensitivity response to fungal antigens from the primary infection site (the feet), manifesting as vesicular eruptions on the hands. 3, 6

Diagnostic Approach

Obtain 10% KOH preparation from the toe web scales to confirm dermatophyte infection. 1

• Scrape scales from the interdigital spaces and examine under microscopy with KOH to visualize fungal hyphae. 1
• The hand vesicles (id reaction) will be KOH-negative as they represent an immunologic response rather than direct fungal infection. 3, 6
• Culture is not routinely necessary but can be performed if diagnosis is uncertain. 1

Treatment Algorithm

Step 1: Treat the Primary Fungal Infection (Toe Webs)

Topical antifungal therapy is first-line for interdigital tinea pedis: 1

• 0.77% ciclopirox cream or gel twice daily for 4 weeks, OR 1
• 1% terbinafine gel once daily for 1 week, OR 1
• Naftifine ointment twice daily for 4 weeks 1

Step 2: Address the Id Reaction (Hand Vesicles)

The id reaction will resolve once the primary fungal infection is treated, but symptomatic management may be needed: 2, 3, 5

• Topical corticosteroids (moderate to high potency) for the vesicular hand eruption to reduce inflammation and pruritus. 5
• Topical calcineurin inhibitors (tacrolimus or pimecrolimus) are alternative options if corticosteroids are insufficient. 5
• Avoid treating the hands with antifungals alone, as this is an immunologic reaction, not a direct infection. 3, 6

Step 3: Prevent Recurrence

Address predisposing factors to prevent recurrent tinea pedis: 1

• Daily change of socks and careful drying between toes after showers 1
• Wear rubber-soled flip-flops or sandals in communal showers 1
• Apply foot powder after bathing 1
• Wear socks before undershorts if infected to prevent autoinoculation to groin 1

Critical Pitfalls to Avoid

Do not mistake this for primary hand dermatitis or pompholyx without examining the feet. The key diagnostic clue is the interdigital toe web involvement, which points to tinea pedis as the primary pathology. 1

Do not treat only the hands while ignoring the feet. The hand vesicles will persist or recur if the primary fungal infection in the toe webs is not eradicated. 2, 3

Do not confuse this with cellulitis. While fissured toe webs can serve as a portal for streptococcal cellulitis, the presence of vesicles on the hands (rather than erythema and edema) points to an id reaction rather than bacterial infection. 1

Alternative Diagnoses to Consider

If KOH preparation is negative, consider:

• Primary dyshidrotic eczema (pompholyx) without fungal trigger, though this would not explain the toe web findings. 2, 3, 5, 4
• Hand-foot-mouth disease if the patient is a young child with oral lesions, though this typically presents with fever and oral involvement. 7
• Contact dermatitis with secondary bacterial infection of toe webs, though this is less likely given the classic distribution. 1