Lambda light chain - restricted non - crystalline proximal tubulopathy with cast nephropathy in multiple myeloma: a case report and literature review.

Background: Multiple myeloma (MM) often causes renal tubular damage, such as the light chain cast nephropathy (LCCN) and the light chain proximal tubulopathy (LCPT). The excessive light chains deposited in the proximal and distal tubules usually manifest with different characteristics, leading to a rare coexistence of the two pathological conditions. Here we report a unique case of a patient with multiple myeloma (MM) who presented with acute kidney injury (AKI) due to dual conditions of λ light chain-restricted non-crystalline LCPT and LCCN. This report reviews the clinical presentation and histological findings, comparing them with previously published cases.

Case presentation: A 49-year-old male patient was admitted with a chief complaint of "fatigue, loss of appetite for 40 days and elevated blood creatinine for 10 days." In serum and urine, the λ light chain level and the ratio of κ to λ free light chain were 1235 mg/dl and 93.25 mg/dl, 0.0022 and 0.0316, respectively. Additionally, serum protein electrophoresis showed an M-spike with monoclonal IgD-λ. Bone marrow puncture revealed 30.5% primitive naive plasma cells, indicative of IgD-λ MM. Light microscopy of kidney biopsy specimen showed periodic acid-Schiff (PAS)-negative cytoplasm in some proximal tubules and PAS-negative casts with a rigid appearance in some distal tubule lumens. On immunofluorescence, these proximal tubular epithelial cells cytoplasm and casts stained exclusively with λ-light chains. Electron microscopy did not reveal any crystalline inclusions. Given the clinical and bone marrow puncture findings, the overall pathological presentation was LCPT with LCCN secondary to IgD-λ MM. After chemotherapy and dialysis, the patient's condition was improved and he was tracked in follow-ups.

Conclusion: In some tubular renal injuries caused by MM, the morphological changes are subtle and often overlooked. In this paper, we present a rare case of LCPT with LCCN showing λ restriction in patient with MM. Through the clinicopathological analysis of patients, the understanding of the disease can be deepened and the diagnosis rate improved.