Psoriasis vulgaris developing in healed pemphigus vulgaris: A rare case of epitope spread or isotopic response?

Although psoriasis and autoimmune blistering diseases are considered to be disorders with completely different etiopathogenesis, literature has documented a few cases of psoriasis associated with bullous diseases, particularly bullous pemphigoid. Here, we report the case of a 30-year-old male presenting with multiple flaccid blisters and erosions, clinically and histopathologically consistent with the diagnosis of pemphigus vulgaris. Although all these lesions resolved after two doses of dexamethasone cyclophosphamide pulse therapy, he returned 3 weeks later with multiple erythematous scaly plaques developing over the postinflammatory areas, compatible with the diagnosis of psoriasis vulgaris, which necessitated a modification in the treatment protocol. This rare case highlights the diagnostic and therapeutic challenges accompanying this unique scenario and attempts to elucidate the probable pathogenic mechanisms underlying the co-existence (simultaneous or sequential) of these two apparently unrelated dermatoses.