With Rashes, Consider the Source (Or Lack Thereof)

DISCUSSION
This case nicely illustrates a common dilemma in dermatology: “ringworm” that is neither fungal nor any other kind of infection. Presented with this case, the vast majority of primary care providers would do what this man’s clinician did: treat it as fungal. This attempt would fail, for one basic reason: There is an extensive differential for round, scaly lesions, of which dermatophytosis (superficial fungal infection) is but one item.

In this case, there was no source from which our patient could have acquired a fungal infection (animal, child, soil) and no particular susceptibility by virtue of immune suppression. To these facts we add the negative KOH, forcing us to consider other items in the differential, including psoriasis, T-cell lymphoma, erythema annulare centrifugum, chronic cutaneous lupus, and nummular eczema, just to name a few.

One way to settle the issue is to obtain a skin biopsy, which in this case showed results most consistent with eczema and effectively ruled out items such as lupus or cutaneous T-cell lymphoma. This provides extremely useful treatment guidance, as well as reassurance to the patient. Stains for fungal organisms were requested and obtained as part of specimen processing—and, of course, results were negative.

In most derm clinics, skin biopsies are routinely done—but what really sets the specialty apart is the fact that a full differential is considered, not just the first (and often only) diagnostic idea that comes to mind. More often than not, a reasonable diagnosis can be found without biopsy, using corroborative history and physical data. 

This patient responded nicely to the application of a topical steroid lotion (fluocinonide 0.05%) and the use of emollients. In the process of history-taking, it was discovered that the patient had a habit of taking long, hot showers while standing with his back to the nozzle. Since this certainly can contribute to dry skin, he was advised to reduce the temperature to a more moderate level and limit his time spent in the shower.

TAKE-HOME LEARNING POINTS
• Just because a rash presents with round, scaly lesions doesn’t mean it’s fungal.

• The rather extensive differential for such rashes includes lupus, psoriasis, eczema, and a type of skin cancer called cutaneous T-cell lymphoma.

• Biopsy, performed under local anesthetic and with a 4-mm punch (closed with two sutures), can be extremely useful in sorting through the differential.

• A KOH prep is a helpful first step (one that would confirm or rule out fungal origin).

• In the absence of a source or indications of susceptibility, fungal infection is an unlikely diagnosis in these cases.