Annular plaques in a biologic-treated psoriasis patient
A 46-year-old man with chronic plaque psoriasis, controlled on adalimumab 40 mg subcutaneously every other week for 22 months, presents with a 6-week history of expanding annular plaques on the trunk and proximal limbs. He had a 5-day course of oral prednisolone 30 mg daily for acute bronchitis 5 weeks before onset. The plaques measure 3–9 cm, have a raised erythematous scaly edge and central clearing, and several show tiny follicular pustules. Itch is moderate. Interdigital maceration and scaling suggest tinea pedis. A junior resident scraped the centre of one plaque for potassium hydroxide microscopy and reported it as negative. Full blood count, liver and renal function are normal; HIV serology is negative. He asks whether the psoriasis has become resistant and whether the adalimumab dose should be increased.
1. How do you interpret the negative KOH result and the clinical morphology? What bedside test would you perform next, and what would you expect to see?
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The negative KOH is not reassuring. The junior resident scraped the centre of a annular plaque, where there is central clearing and little scale; dermatophytes are concentrated at the active, scaly edge. A false-negative result is therefore likely. The morphology is more suggestive of tinea incognito than of a psoriasis flare: annular plaques with central clearing, a raised scaly edge, follicular pustules, coexisting tinea pedis, and onset 5 weeks after systemic prednisolone. Psoriasis can annular, but follicular pustules and interdigital scaling point elsewhere.
I would scrape the active edge and examine with KOH, ideally with calcofluor white or a fungal culture. Dermatoscopy of the active edge would be expected to show broken or comma hairs, follicular pustules, and scaling; psoriasis would show regularly distributed dotted vessels and white scale without broken hairs. If repeat microscopy is negative, a punch biopsy from the edge with PAS staining is reasonable. I would not increase adalimumab on the assumption of resistant psoriasis until infection has been excluded.
2. Outline your management, including whether adalimumab should be continued, and what you would tell him about the prednisolone.
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Management should be directed at confirmed dermatophyte infection. For extensive tinea corporis, I would use oral terbinafine 250 mg daily for 2 weeks (4 weeks if extensive or slow to respond) plus a topical azole, and treat the tinea pedis to reduce reinfection. Liver function should be checked before and after terbinafine. His adalimumab can usually be continued while treating uncomplicated dermatophyte infection, because there is no evidence of disseminated or invasive fungal disease; however, he should be reviewed at 2–4 weeks and advised to report fever, systemic symptoms, or rapidly spreading lesions.
The key explanation is that the recent prednisolone course probably converted a dermatophyte infection into tinea incognito by suppressing inflammation and altering the clinical appearance. Systemic steroids should be avoided for this episode. If the lesions fail to improve despite correct treatment, I would consider poor adherence, terbinafine resistance, or an alternative diagnosis such as a psoriasis flare after steroid withdrawal. Increasing the biologic dose is not the correct first response to presumed “resistance” when infection has not been excluded.
3. If repeat KOH and culture are negative and the plaques persist, what is your next step?
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I would perform a punch or incisional biopsy from the active edge, including skin and superficial subcutaneous fat, and request histology plus PAS and, if available, fungal immunostaining. Histology may show hyphae in the stratum corneum even when surface sampling is negative, or it may show psoriasiform hyperplasia with neutrophils and no fungi. I would also review whether the plaques could represent a drug eruption, annular erythema, or a psoriasis flare precipitated by steroid withdrawal.
If histology confirms fungus, I would extend antifungal treatment and look for reasons for failure, such as inadequate duration, reinfection from the feet, or drug interactions. If histology shows psoriasis, I would assess adherence to adalimumab, check for anti-drug antibodies if available, and consider switching therapy rather than simply increasing the dose. The important principle is to establish a tissue diagnosis before escalating immunosuppression in a patient whose eruption began after systemic steroids and who has a coexisting fungal infection.