Pleural Involvement and True Hyponatraemia in Section Multiple Myeloma: A Report of Two Rare Cases

Multiple Myeloma (MM) is a plasma cell proliferative malignancy that produces monoclonal immunoglobulin. Pleural effusion in MM, caused by the direct infiltration of the pleural space by plasmacytes, leading to true myelomatous pleural effusion, is rare. Patients with MM often exhibit pseudohyponatraemia due to high serum paraprotein levels; a far less common occurrence is true hyponatraemia resulting from the Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH). This report presents two such cases. In Case 1, a 45-year-old woman with no known co-morbidities presented with complaints of shortness of breath and pain in both hips. Investigations revealed anaemia, renal impairment, a reversed albumin-to-globulin ratio and the presence of an M band on serum electrophoresis. Imaging showed lytic lesions and minimal pleural effusion. A bone marrow biopsy confirmed plasmacytosis and diagnostic pleural tapping was performed, revealing malignant myelomatous pleural effusion. The pleural effusion progressed to a massive effusion, necessitating therapeutic tapping. Despite treatment with dexamethasone, cyclophosphamide and bortezomib, the patient eventually succumbed. In Case 2, a 56-year-old man, previously treated for MM, presented with complaints of back pain, giddiness and vomiting. Investigations revealed anaemia, hyponatraemia and elevated globulin levels. A Positron Emission Tomography-Computed Tomography (PET-CT) scan indicated the possibility of relapse. The patient exhibited low serum osmolality alongside high urine spot sodium and urine osmolality, while renal, adrenal and thyroid functions were normal, pointing towards SIADH. He responded well to fluid restriction, salt supplementation, tolvaptan and chemotherapy, resulting in the resolution of hyponatraemia. These cases are reported to shed light on the various presentations of MM that may be overlooked during patient evaluation.