A Rare Manifestation of Myeloma in a Kidney

Introduction: Proteinuria and renal insufficiency are among the most common renal manifestations of Multiple Myeloma [MM] with cast nephropathy being the most common pathophysiology. There have been few case reports of plasma cell infiltration into the kidney resulting in renal dysfunction and proteinuria. Here we describe a case of nephrotic syndrome without deranged creatinine in a patient with long standing MM. Case Description: 64-year-old female with history of anal cancer status post chemoradiation and Ig G multiple myeloma, diagnosed in 2016 refractory to numerous lines of treatment, including CAR-T and stem cell boost, currently on Blenrep and Pomalyst was found to have new onset nephrotic range proteinuria. Urine protein to creatinine ratio 3.74 mg/mg and urine albumin to creatinine ratio 16.1 mg/mg. Serum albumin 1 g/dl, creatinine 1.07 mg/dl [Baseline 0.8-1] and hemoglobin 8.2. Serum protein electrophoresis showed discrete abnormal band measuring 1.7g/dl in the gamma region and serum free kappa/lambda ration >665 unchanged from prior. Renal biopsy revealed diffuse and expansile plasma rich interstitial infiltration distorting the kidney architecture suggestive of infiltrative myeloma in the kidney. Unfortunately, due to the refractory nature and overall decline in her health she was made hospice. Discussion: Renal involvement in multiple myeloma varies widely and there is a wide spectrum of pathologies noted on biopsy. Plasma cell infiltration of the kidney is a rare presentation of MM that showed itself as only proteinuria and no renal function impairment. It is, however, one that is usually associated with advanced myeloma and like in other reported cases, our patient succumbed to complications of advanced myeloma. With this case we emphasize the point that despite marked histologic changes one may not have significant renal dysfunction. It is important to note and recognize this advanced presentation that may otherwise disguise itself as a benign kidney involvement of MM.