To report the case of a patient diagnosed with Multiple Myeloma, with myelomatous ascites. Methodology: Assessment of patient's information kept on HSPE computer-based patient records. Case report: 53 year-old female, diagnosed with IgG/Kappa Multiple Myeloma (MM), diagnosed on 10/2018, with a clinical picture of medullary plasmacytosis, anemia, pathological fracture of the right femur and end stage renal disease in dialysis. She started treatment with only Bortezomib/Dexamethasone (VD) due to poor clinical condition. Cyclophosphamide was added (VCD) as soon as the patient's condition improved. She remained stable for 1 year and 4 months, after which time she developed plasmacytomas in the ribs and hip; Local radiotherapy and 8 cycles of VD were then prescribed (due to the lack of other available therapies at that time). Three months later (12/2022), she presented with increasing abdominal volume due to ascites. Diagnostic and therapeutic abdominal paracentesis was performed. The serum ascites albumin gradient (SAAG) was < 1.1. Bacterial analysis of ascitic effusion samples (AES) was also negative. Cytology of the AES revealed some plasma cells. Immunofixation and Flow cytometry of AES came back positive for the lambda free light chain and for CD45/CD38/CD138/CD56/CD28/CD117 respectively, in addition to the presence of 0.16% of monoclonal plasma cell. AES cultures were negative. Daratumumab and Dexamethasone (Dd) were then prescribed, but no current improvement has been seen so far. New treatment with Carfilzomib is now being considered.

Overall, extramedullary MM is seen in advanced and aggressive disease and occurs due to plasma cell infiltration into sites other than the bone marrow. Myelomatous ascites or pleural effusion is seen in less than 1% of cases and can be differentiated from infectious etiologies based on fluid cytology and predicts poor prognosis in relapsing/refractory MM. Its pathophysiology is not well understood. The diagnosis of plasmacytic effusion can be confirmed based on detection of atypical plasma cells on fluid cytological analysis derived via paracentesis supplemented by immunofixation of proteins and identification of cytoplasmic immunoglobulin, kappa, or lambda light chains, and high CD38 and/or CD138 expression on their immunophenotypic analysis. It's important to rule out other conditions that may present similar findings, such as renal insufficiency, heart failure, peritonitis and other possible causes.

Relapsed/refractory MM can evolve to aggressive forms of infiltrative disease. Such cases are represented by dissemination of plasma cells to peritoneal and pleural cavities. When that occurs, it is usually related to advanced disease, when patients have already been exposed to several lines of chemotherapy, thereby reducing treatment success rates.