A 52-year-old woman from Mumbai with a 10-year history of diabetes mellitus type 2 presents with progressive bilateral flank pain, nausea, and a serum creatinine of 3.2 mg/dL (baseline 0.9 mg/dL). Urinalysis shows proteinuria and no hematuria. Renal ultrasound demonstrates bilateral hydronephrosis with dilated collecting systems but normal-caliber ureters. Non-contrast CT abdomen shows no stones or masses. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis ### Key Features of This Case - **Bilateral hydronephrosis** with **normal-caliber ureters** (excludes ureteric obstruction) - **No stones, masses, or external compression** on imaging - **Progressive bilateral flank pain** (suggests mechanical obstruction, not just renal disease) - **Acute rise in creatinine** from 0.9 to 3.2 mg/dL - **Proteinuria without hematuria** (atypical for acute obstruction) ## Differential Diagnosis Table | Feature | Retroperitoneal Fibrosis | Diabetic Nephropathy | Bilateral Stones | RAS | |---------|-------------------------|---------------------|------------------|-----| | **Hydronephrosis pattern** | Bilateral, normal ureter caliber | Absent | Bilateral with ureteric dilation | Absent | | **Imaging findings** | Periureteric soft tissue density | Normal renal echogenicity | Hyperdense foci in ureters | Normal | | **Hematuria** | Absent | Rare | Common | Absent | | **Flank pain** | Yes (mechanical) | No (insidious) | Yes (acute colic) | No | | **Proteinuria** | Minimal | Marked | Absent | Minimal | | **CT appearance** | Fibrotic tissue around ureters | Renal atrophy, echogenicity change | Stones visible | Normal | ## Why Retroperitoneal Fibrosis? ```mermaid flowchart TD A[Bilateral hydronephrosis]:::outcome --> B{Ureteric caliber?}:::decision B -->|Dilated| C[Ureteric obstruction]:::action B -->|Normal| D[Extraureteric obstruction]:::action D --> E{Imaging shows mass/stone?}:::decision E -->|Yes| F[Tumor or stone]:::outcome E -->|No| G[Retroperitoneal process]:::action G --> H{Periureteric soft tissue?}:::decision H -->|Yes| I[Retroperitoneal Fibrosis]:::outcome H -->|No| J[Other: vascular, inflammatory]:::action ``` ## High-Yield: **Retroperitoneal fibrosis (RF) classically presents with:** - Bilateral hydronephrosis with **normal or slightly dilated ureteric caliber** (key distinguishing feature) - Periureteric soft tissue density on CT (often subtle) - Progressive renal insufficiency - Flank/abdominal pain - ~70% idiopathic; 30% associated with malignancy, IgG4 disease, or medications (beta-blockers, ergotamines) ## Key Point: **The normal ureteric caliber is the critical clue.** In ureteric obstruction (stones, tumors), the ureter dilates proximal to the block. Normal caliber ureters with hydronephrosis suggests the obstruction is at or above the ureteropelvic junction—pointing to retroperitoneal fibrosis or other extraureteric causes. ## Clinical Pearl: **CT with contrast (if renal function permits) or MRI is essential** to visualize the periureteric fibrotic tissue and rule out malignancy. IgG4 levels and inflammatory markers (ESR, CRP) may support the diagnosis. ## Warning: **Do NOT attribute bilateral hydronephrosis to diabetic nephropathy alone.** Diabetic kidney disease causes glomerular proteinuria and renal atrophy—not mechanical obstruction or hydronephrosis. The acute rise in creatinine with imaging evidence of obstruction demands investigation for a mechanical cause. [cite:Harrison 21e Ch 282]