Case of Crescentic Fibrillary Glomerulonephritis with Linear IgG Mimicking Anti-glomerular Basement Membrane Nephritis with Good Response to Treatment

Introduction: Fibrillary glomerulonephritis (FGN) is an uncommon glomerular disease whose diagnosis depends on electron microscopy (EM) showing fibrillary immune deposits and immunohistochemistry (IH) with DNAJB9 deposits in glomeruli. Light microscopy of FGN can have variable histological patterns, and can rarely present with diffuse crescentic disease which has a very poor prognosis. The deposits on immunofluorescence (IF) are typically described as “smudgy” but can sometimes be pseudo-linear mimicking anti GBM disease. Here we describe a patient with FGN mimicking anti- GBM disease who had a good response to immunosuppressive therapy. Case Description: The patient is a 31-year-old female with obesity who presented with three weeks of foamy urine, fatigue, and poor appetite. On presentation serum creatinine was 3.0 mg/dL (was 0.5 mg/dl three months prior). Urinalysis showed hematuria and proteinuria and a urine protein/creatinine ratio was 17.3. A kidney biopsy showed diffusely crescentic proliferative glomerulonephritis with fibrinoid necrosis on light microscopy and IF was reported as linear IgG along the glomerular capillaries suspicious for anti-GBM disease. Anti- GBM antibody was negative and the diagnosis of atypical anti- GBM disease was considered. Subsequently, EM revealed diffuse organized fibrillary deposits and later IH showed DNAJB9 positivity in glomeruli. The patient was treated with steroids and intravenous cyclophosphamide. Her creatinine was 1.0 at 6 months follow-up. Discussion: This case touches on clinical challenges related to crescentic FGN. There can be a challenge in distinguishing crescentic FGN and anti-GBM disease if EM and IH are not immediately available. In our case, atypical anti-GBM disease was considered when her anti-GBM antibody was negative, though this is usually a more indolent disease than typical anti-GBM with either no or only focal crescents. Once a diagnosis was established by EM and IH the optimal treatment regimen remained unclear as there is no clear therapy for crescentic FGN and it has been observed that the prognosis of diffusely crescentic FGN is very poor despite immunosuppression. However, this patient responded well to a course of steroids and cyclophosphamide with a significantly improved creatinine.