Paraneoplastic Pemphigus Mimicking Stevens-Johnson Syndrome in a Patient with Multiple Myeloma: A Rare and Clinically Challenging Presentation.

Paraneoplastic pemphigus (PNP) is a rare, severe autoimmune mucocutaneous disorder most commonly associated with lymphoproliferative malignancies. Here, we report the first documented case of PNP as a paraneoplastic manifestation of multiple myeloma (MM). A 61-year-old male with MM developed widespread mucocutaneous ulcerations shortly after his eleventh chemotherapy cycle, initially suspected to represent Stevens-Johnson syndrome. Clinical examination revealed diffuse skin peeling, mucosal involvement of the eyes, oral cavity, and genital region, and a positive Nikolsky sign. Laboratory evaluation demonstrated acute kidney injury requiring hemodialysis. Despite initial treatment with high-dose intravenous immunoglobulin, lesions persisted. Skin biopsy revealed lichenoid lymphocytic infiltration, basal vacuolar changes, subcorneal and suprabasal acantholysis, and keratinocyte dyskeratosis, confirming PNP. Viral serologies were negative, supporting the autoimmune etiology. The patient was subsequently treated with rituximab, resulting in significant improvement of cutaneous lesions over three months, with residual post-inflammatory hyperpigmentation. This case emphasizes the importance of early recognition and accurate differentiation of PNP from other blistering disorders in patients with underlying hematologic malignancies. Importantly, this represents the first reported instance of PNP presenting as a paraneoplastic manifestation of MM, highlighting the need for awareness of atypical autoimmune syndromes in this population.