Atypical Presentation of IgG4-Negative Idiopathic Retroperitoneal Fibrosis

Introduction: Retroperitoneal fibrosis (RPF) is a rare condition in which fibro-inflammatory tissue surrounds the iliac arteries and abdominal aorta. Ureteral obstruction is common, resulting in acute or chronic renal failure. Most cases are idiopathic and often develop alongside autoimmune conditions or IgG4-related disease. Clinical manifestations include fever, anorexia, and weight loss with back or flank pain. Case Description: A 53-year-old African American male presented to the emergency room with extreme fatigue, worsening anorexia, and weight loss. Past medical history included hypertension, type 2 diabetes mellitus, obstructive sleep apnea, depression, and medication noncompliance. Physical exam was unremarkable and blood testing revealed serum creatinine of 5.28 (baseline 1.2), eGFR of 12.2 (68.6), and BUN of 35 (17). He denied flank pain, urinary symptoms, and history of kidney disease. Ultrasound showed moderate to severe bilateral hydronephrosis. CT scan of the abdomen and pelvis revealed no stones within the urinary tract but identified a retroperitoneal mass at the level of the aortic bifurcation. Following admission, bilateral upper pole percutaneous nephrostomy tubes and lower pole nephroureteral stents were placed. CT scan of the chest showed no evidence of malignancy. Two weeks later, diuretic renal scintigraphy determined renal function to be 68% (left) and 32% (right). Repeat CT abdomen/pelvis revealed improved hydronephrosis. Biopsy of the retroperitoneal mass showed myofibroblastic proliferation with thick collagen and inflammatory cells but rare IgG4-positive plasma cells. Treatment of idiopathic RPF was initiated with high dose corticosteroids. Serum creatinine has since decreased to 1.89, eGFR increased to 41.9, and BUN decreased to 22. Discussion: This case illustrates the importance of maintaining a broad differential in the setting of AKI and considering rare causes for hydronephrosis. Idiopathic RPF can have a varied clinical presentation as seen in our patient lacking back or flank pain despite extensive fibrosis causing hydronephrosis. Prompt recognition of idiopathic RPF is vital to early intervention and preservation of renal function.