Postoperative acute kidney injury associated with cast nephropathy and marked renal interstitial plasma cell infiltration in multiple myeloma: a case report.

Multiple myeloma (MM) is frequently associated with renal impairment, most commonly due to cast nephropathy. We report a case of severe and persistent postoperative acute kidney injury (AKI) in an 84-year-old man with smoldering MM who underwent posterior lumbar fixation for a lumbar burst fracture. Twelve hours after surgery, he developed profound hypotension and severe anemia due to surgical-site bleeding, followed by progressive renal dysfunction despite prompt transfusion and fluid resuscitation. Urinalysis revealed heavy proteinuria, and kidney biopsy demonstrated marked accumulation of eosinophilic casts within the tubules, consistent with cast nephropathy. In addition, marked renal interstitial infiltration by λ light-chain-restricted monoclonal plasma cells and focal peritubular capillary (PTC) occlusion were observed. Congo red staining was negative, and no significant glomerular immune deposits were detected. Renal function partially improved with supportive care but did not return to the preoperative baseline. This case suggests that postoperative AKI in MM may be multifactorial, with perioperative circulatory failure precipitating clinically overt cast nephropathy, while coexisting interstitial myeloma cell infiltration and focal PTC occlusion may increase susceptibility to ischemic injury and contribute to the severity and persistence of renal dysfunction. In patients with MM, latent myeloma-related kidney injury should be considered a potential contributor to perioperative AKI even when serum creatinine is preserved, particularly in those with marked proteinuria or elevated free light chain levels.