Diagnose This (July 6, 2020)

What is your diagnosis based on the EM image provided?

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The electron microscopy image provided is that of a proximal tubule and demonstrates confluent “powdery” or “pellet-like” deposits along the tubular basement membrane consistent with a diagnosis of light chain deposition disease (LCDD), the most common subtype of monoclonal immunoglobulin deposition disease (MIDD). By immunofluorescence, the case revealed diffuse, linear, 3+ staining for lambda light chain along tubular basement membranes as well as glomerular basement membranes while kappa light chain was completely negative. By light microscopy, glomeruli demonstrated a spectrum of mild to severe, early nodular, PAS-positive mesangial matrix expansion with a somewhat chunky appearance while tubules showed evidence of acute tubular injury.

As the name implies, MIDD is caused by the systemic deposition of monoclonal immunoglobulin along tubular and glomerular basement membranes and within the glomerular mesangium. The monoclonal immunoglobulin is typical of the light chain type but can also be due to the deposition of an abnormal heavy chain (HCDD) or even both, light and heavy chains (LHCDD). Of note, while most patients will demonstrate an abnormal SPEP/UPEP at the time of diagnosis, approximately 15-20% will have an undetectable paraprotein at that time. Regarding prognosis, renal survival is poor with a mean of around 5 years.