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Images in Pathology

A A 59-year-old man presented with edema, microscopic hematuria, hypertension, and mild renal insufficiency. Laboratory data included nephrotic range proteinuria (4.8 g/24 hr), elevated creatinine (3.2 mg/dL), and negative serologies. Light microscopy of the renal biopsy showed diffuse mesangial expansion by relatively hypocellular PAS + matrix material with focal/moderate capillary wall extension (A). Congo red stain for amyloid was negative. Immunofluorescence for polyclonal immunoglobulin G (IgG) (B), IgM, C3, and kappa and lambda light chains revealed 3+ granular staining mainly within the mesangium and focally along capillary walls. On electron microscopy (EM), aggregates of fibrillar deposits composed of randomly arranged, nonbranching fibrils with a solid B core, measuring 18-22 nm in diameter, were observed in the same distribution (C). EM was diagnostic for this immune complex mediated Fibrillary glomerulonephritis (FGN). FGN is a primary glomerulopathy, seen in less than 1% of all native renal biopsies. The characteristic deposits in FGN are extracellular, associated with diverse clinical and glomerular pathologic features with varied prognosis. The presence of larger deposits with diameters of 30-50 nm composed of organized microtubular structure is termed "immunotactoid glomerulopathy." Fibrillar amyloid deposits can be differentiated from FGN by virtue of Congo-red positivity and fibril measurement of 8-11 nm.