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(a) What is the most likely diagnosis?
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On examination there is a red rash, in some areas smooth and in others scaly, affecting the dorsum of the feet and the lower legs, especially on the left. There is a clear scaly edge proximally on both legs. There are also some ulcerated areas, presumably from intense scratching. The most likely diagnosis is tinea incognito, a fungal infection that has been treated inappropriately with topical corticosteroids for presumed eczema. The anti-inflammatory effects of the steroid initially result in the rash being less obvious and the itching improves, but dermatophyte fungi thrive in the presence of steroids and so the disease then gets worse.
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(b) Where else would you examine?
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Most dermatophyte infections have spread from the most common ‘landing site’ for fungal infection, the fourth and fifth toe webs. This is a warm relatively moist area, an environment in which bacteria and fungi thrive well. Walking in bare feet, for example in communal changing areas, allows transfer of dermatophytes from scale from someone else’s ‘athlete’s foot’. So it is important to examine the toe webs and the other flexural areas, to ensure that any fungal infection elsewhere is also treated.
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(c) How would you confirm the diagnosis?
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Scrapings should be taken from the scaly edge of the rash and also from other possible involved areas, including the toe webs. The scrapings should be examined under the microscope after being treated with potassium hydroxide (KOH), which dissolves the stratum corneum cell walls, making it easier to see the fungi. The scrapings should also be cultured to confirm the microscopy. Culture also allows determination of the type of fungus: however, knowing the type of dermatophyte does not influence management.
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(d) What treatment would you recommend?
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It is important that any topical corticosteroid should be stopped. The topical therapy of choice is terbinafine cream, which should be applied daily to all affected areas for 2 weeks. Terbinafine is fungicidal and so is very effective provided that all affected areas are treated. If the fungal infection is very widespread or if there are practical difficulties in applying the cream, then oral terbinafine can be used.
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(e)What is the prognosis?
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There is rapid improvement, usually with a cure within 1–2 weeks.
