Renal lesions in multiple myeloma: their relationship to associated protein abnormalities.

Renal biopsy and autopsy specimens were studied in 43 patients with renal complications of multiple myeloma and correlated with immunoelectrophoresis (IEP) and other clinical data at the time of biopsy. Lesions specifically related to multiple myeloma fell into two categories, with different patterns of protein excretion. (1) Myeloma Cast Formation: When other lesions which might contribute to renal insufficiency (RI) were excluded, there was a good correlation between the extent of myeloma cast formation and severity of RI. Sixteen of 19 patients excreted free light chains (LCs) in the urine, in seven as the predominant or sole urinary protein. (2) Tissue Deposition of Paraproteins: Nine cases had generalized glomerular, tubular basement membrane and vascular deposits of presumed kappa-chains (one with associated alpha-heavy chains). Four patients, all with myelomas secreting lambda LCs, had diffuse amyloid deposits in similar distribution. All patients (save two who were anuric) had diffuse, nonselective proteinuria by IEP, most within the nephrotic range. Four patients had free LCs in the urine, but in none was this the predominant component. Cast nephropathy and LC tissue deposition tended to occur in mutually exclusive fashion. Cases with diffuse tissue deposits of LCs showed few or no myeloma casts. Cases with cast nephropathy had only occasional mild mesangial lesions and focal interstitial and vascular deposits of amyloid. Evidence indicates that these lesions represent incidental LC deposition in cases whose basic lesion is longstanding and/or severe cast nephropathy, and that their contribution to RI is minor in comparison to that of the myeloma casts.