A Rare Case of Concomitant Fibrillary Glomerulonephritis and Membranous Nephropathy in a Patient With Chronic...

Fibrillary glomerulonephritis (FGN) is a rare glomerular disorder characterized by non-branching fibrillary deposits visible on electron microscopy (EM). Rarely, FGN may present with light microscopy (LM) features resembling membranous nephropathy (MN). Immunofluorescence (IF) and EM can distinguish the two entities. We describe, to our knowledge, the first reported case of dual FGN and MN in association with chronic inflammatory demyelinating polyneuropathy (CIDP). A 68-year-old woman presented with nephrotic syndrome and CIDP. LM demonstrated diffuse mild mesangial expansion. EM revealed mild glomerular basement membrane thickening, subepithelial granular deposits, and diffuse (>90%) podocyte foot process effacement. Additional mesangial and paramesangial intramembranous fibrillary deposits (12-28 nm diameter) were identified. IF demonstrated diffuse granular capillary and mesangial staining for IgG (3+), C3 (2+), IgA (1+), C1q (trace), kappa (4+), and lambda (4+). M-type phospholipase A2 receptor (PLA2R) staining was strongly positive (3+) with a predominantly subepithelial pattern. The patient was treated with furosemide, lisinopril, prednisone, and tacrolimus, resulting in improved proteinuria and edema. She also received intravenous immunoglobulin for CIDP. FGN may rarely have subepithelial immune deposits resembling MN, but strong anti-PLA2R positivity is unusual and supports the coexistence of two distinct glomerulopathies. The concurrent CIDP raises the possibility of shared immune dysregulation contributing to both renal and neurologic disease. Recognition of such dual pathology is essential, as management may require addressing each condition individually. This case highlights the rare coexistence of FGN and PLA2R-positive MN in a patient with CIDP. Accurate diagnosis requires careful correlation of LM, IF, and EM findings. Further research is warranted to clarify the pathogenesis, clinical implications, and optimal management of dual glomerulopathies.