Collagenofibrotic Glomerulopathy With Concurrent Ostium Secundum Atrial Septal Defect and Anti-Ku Antibody Positivity: A Rare Case.

Collagenofibrotic glomerulopathy (CG) is an exceptionally rare idiopathic glomerular disease defined by pathological intraglomerular accumulation of type III collagen fibrils within the mesangial matrix and subendothelial space. We report the case of a 35-year-old woman presenting with generalized anasarca, frothy urine, and exertional dyspnea. Laboratory investigations revealed nephrotic syndrome with a 24-hour urinary protein of 3,658 mg/day, serum albumin of 2 g/dL, and progressive renal dysfunction with serum creatinine rising from 3.13 to 5.25 mg/dL. Echocardiography incidentally demonstrated an ostium secundum atrial septal defect with mild pulmonary arterial hypertension, while serology showed a strongly positive anti-Ku antibody (3+). Renal biopsy also showed diffuse mesangial expansion by Congo red-negative eosinophilic material with marked narrowing and focal obliteration of capillary lumina. Immunofluorescence studies demonstrated weak mesangial staining for both kappa and lambda light chains. Immunohistochemistry confirmed faint positive staining for type III collagen in mesangial regions, confirming the diagnosis of CG. Treatment consisted of diuretics, antihypertensive therapy, corticosteroids, and supportive measures. Despite these interventions, renal function continued to deteriorate, with estimated glomerular filtration rate decreasing from 19 to 10 mL/minute/1.73 m² at discharge. The present case highlights the utility of immunohistochemistry in establishing the diagnosis of CG when electron microscopy is unavailable.