Orbital and Paraspinal Plasmacytomas with Pseudohyponatremia in Multiple Myeloma: A Case Report

The presence of extramedullary disease at the time of diagnosis in multiple myeloma is rare and is associated with a poor prognosis. Pseudohyponatremia, a unique laboratory abnormality, is observed in patients with elevated circulating protein or lipid levels. A 71-year-old female presented with multiple soft tissue masses in her orbit, paraspinal space, and lung, a urinary tract infection, an acute kidney injury, and hyponatremia. Despite fluid resuscitation, free water restriction, and salt tablets, her hyponatremia persisted. A sodium whole blood measurement yielded 140 mmol/L, indicating pseudohyponatremia due to increased circulating monoclonal immunoglobulins. Paraspinal and orbital mass biopsies revealed extramedullary plasmacytomas, and a bone marrow aspirate showed 70% plasma cells. The patient was discharged on day 13 of admission with outpatient oncology follow-up for chemotherapy and radiation. Orbital lesions occur in less than 1% of patients with extramedullary disease, including those with and without multiple myeloma. A rare laboratory phenomenon, compounded by an even rarer presentation of multiple myeloma with extramedullary disease, resulted in a delayed diagnosis of a hematologic malignancy and unnecessary treatment. Although multiple myeloma is a prevalent and well-researched hematologic malignancy, this case aims to emphasize the defining characteristics of its less frequent, atypical manifestations. Orbital and paraspinal plasmacytomas with pseudohyponatremia in multiple myeloma present a unique diagnostic challenge from multiple perspectives, emphasizing the importance of bridging laboratory and clinical medicine.