Multiple Myeloma With Retroperitoneal Extramedullary Plasmacytoma Causing Renal Failure and Obstructive Shock From Inferior Vena Cava Compression: A Case Report

Multiple myeloma (MM) is a hematologic malignancy of plasma cells. It can lead to multiorgan involvement and thus may present with various clinical manifestations, the most common being hypercalcemia, renal impairment, anemia, and bone involvement. Extramedullary multiple myeloma (EM) with soft tissue plasmacytoma (plasma cell tumor of soft tissue outside of the bone marrow) is an uncommon finding in patients with MM, which, when present, is clinically symptomatic based on the location of the tumor. An EM in the retroperitoneum is an infrequent presentation with only a few reported cases. We present a 56-year-old male with a history of retroperitoneal EM near the renal pelvis causing renal failure, presenting with obstructive shock from inferior vena cava (IVC) compression. Perirenal retroperitoneal plasmacytoma causing acute renal failure has been previously reported. A retroperitoneal EM plasmacytoma compressing the IVC precipitating obstructive shock is a unique finding that has yet to be mentioned in the literature. This report hopes to highlight the consideration of EM in patients with MM with obstructive symptoms, particularly in patients such as ours, who had a history of EM in the past. In addition, it shows the utility of bedside point-of-care ultrasound (POCUS) in the intensive care unit. After seeing persistently collapsed IVC on POCUS despite aggressive fluid management with worsening lower extremity edema and ascites, a presumptive diagnosis was made and later confirmed with magnetic resonance imaging (MRI). Although IVC stenting was planned initially, it was deferred due to fear of stent migration after chemotherapy. He was initially stabilized with vasopressors and treated with chemotherapy with cyclophosphamide and dexamethasone, which resolved hypotension. Timely intervention allowed vasopressors to be tapered, and he was subsequently discharged with outpatient chemotherapy.