Case Report: Atypical anti-GBM nephritis coexisting with Henoch-Schönlein purpura nephritis: exploring the pathogenic nexus.

Background: While anti-glomerular basement membrane (GBM) nephritis typically manifests with circulating antibodies targeting the GBM, atypical seronegative variants may occur. Henoch-Schönlein purpura nephritis (HSPN) is characterized by IgA-dominant immune complex deposition. The simultaneous presentation of these two distinct immune-mediated glomerulopathies poses unique diagnostic and therapeutic dilemmas, with limited cases reported in the literature. Case presentation: We describe a 16-year-old female presenting with rapidly progressive glomerulonephritis and cutaneous purpura. Initial serological testing was negative for anti-GBM antibodies. Renal biopsy was performed, with light microscopy showing segmental glomerulosclerosis. Immunofluorescence demonstrated distinctive dual deposition patterns: mesangial IgA consistent with HSPN and linear IgG along with capillary walls and GBM, confirming concurrent atypical anti-GBM nephritis. The patient responded favorably to combination therapy including glucocorticoids, immunosuppressants, and plasmapheresis, with subsequent improvement in renal function and resolution of symptoms. Conclusion: This case illustrates the diagnostic challenges posed by seronegative anti-GBM nephritis with HSPN overlap, emphasizing the critical role of histopathological examination in establishing the diagnosis. Our experience supports the efficacy of early, aggressive immunosuppressive therapy in such complex presentations. These findings warrant further investigation into the possible shared pathogenic mechanisms of these two disease entities.