Steroid-modified tinea corporis with pseudo-Koebner phenomenon

A 50-year-old female presented with linearly arranged annular plaques with peripheral activity over the upper abdomen [Figure 1]. She had been applying topical corticosteroids for pruritus and itchy lesions on the chest for 2 months. On clinical examination, similar annular plaques were noted at other sites, including the groin and lower back, consistent with tinea corporis.

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Figure 1:

Linear annular plaques over the abdomen showing pseudo-Koebner phenomenon.

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Dermoscopy of the abdominal lesions revealed scales, black dots, background pigmentation, erythema, and micropustules [Figure 2]. A potassium hydroxide mount from skin scrapings revealed branching, septate hyphae, confirming a dermatophytosis. The altered morphology on the abdomen, including the linear arrangement, suggestive of pseudo-Koebner phenomenon, was attributed to the application of topical steroids, consistent with tinea incognito.

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Figure 2:

Dermoscopy showing black dots (blue arrow), erythema (red arrow), out of focus micropustules (blue star), scales (red star), and pigmentation (DL4, poralized, 10×).

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Tinea incognito often presents with modified clinical features due to inappropriate use of corticosteroids, leading to misdiagnosis and delayed treatment. While both tinea incognito and typical tinea share features such as erythema, scaling, micropustules, and some hair changes, the presence of telangiectasia, atrophy, a higher frequency of certain vellus hair abnormalities like translucent, easily deformable, and Morse code hairs, as well as a less distinct scaling pattern, can help differentiate tinea incognito, particularly when modified with the use of steroids. Recognition of subtle signs on dermoscopy, along with correlation to classical lesions elsewhere, is essential for early identification and appropriate management. This case highlights an uncommon presentation of dermatophytosis with steroid-modified morphology and linear pseudo-Koebner phenomenon, emphasizing the need for high clinical suspicion and dermoscopic evaluation in atypical presentations.