More than meets the naked eye: an unusual psoriatic arthritis mimicry and the important role of dermoscopic examination

BACKGROUND: Psoriatic arthritis (PsA) can manifest in various forms. This includes mimicry of other diseases. We describe an unusual mimicry of PsA. CASE PRESENTATION: We report a case of a middle-aged lady who presented with severe pain and morning stiffness over the small joints of the left hand for 3 months and painless deformity of the affected joints 1 year before. She was under treatment for pruritic rash over her ankles and knees for the past 1 year as well. Physical examination revealed a fixed flexion deformity, swelling and tenderness of the left ring and little fingers’ distal interphalangeal (DIP) joints. Left hand radiograph showed sclerotic joint margin, narrowed joint space and marginal osteophytes of the affected DIP joints. Dermoscopic examination showed red- violaceous, flat-topped papules and plaques with minimal scales on both ankles; hyperpigmented scaly plaques over both knees and vertical fingernail ridges. Serum autoimmune screening and inflammatory markers were unremarkable. Left ankle skin biopsy showed features consistent of psoriasis. PsA was diagnosed. Weekly titrated oral methotrexate and topical steroid were started. The patient showed significant improvement after 1 month of treatment. CONCLUSION: PsA is a great mimicker. Dermoscopy is an accessible and valuable tool to assess skin lesions in greater detail. Clinicians should be aware of coexisting diseases or misdiagnosis when patients do not respond to treatment.