Internal Medicine · Year 3 · from Internal Medicine

Case 3: Postrenal AKI - Obstructive Uropathy

Patient Presentation

A 72-year-old man with a history of benign prostatic hyperplasia presents with 2 days of lower abdominal discomfort and inability to urinate. He reports feeling the urge to void but producing only small amounts of urine with significant straining. He has also had some nausea and decreased appetite.

Vital Signs

  • Blood Pressure: 152/88 mmHg
  • Heart Rate: 82 bpm
  • Respiratory Rate: 16/min
  • Oxygen Saturation: 98% on room air
  • Temperature: 37.0°C

Physical Examination

  • General: Mild discomfort
  • Cardiovascular: Normal
  • Pulmonary: Clear
  • Abdomen: Distended lower abdomen, palpable bladder to umbilicus, suprapubic tenderness
  • GU: Enlarged prostate on digital rectal exam (non-tender, smooth)

Laboratory Results

  • Creatinine: 5.2 mg/dL (baseline 1.0 from 6 months ago)
  • BUN: 78 mg/dL
  • Potassium: 5.8 mEq/L
  • Bicarbonate: 19 mEq/L

Imaging

  • Bladder scan: Post-void residual 850 mL
  • Renal ultrasound: Bilateral hydronephrosis, distended bladder

Clinical Image

Figure 3: Renal ultrasound demonstrating moderate hydronephrosis with dilated renal pelvis and calyces secondary to bladder outlet obstruction.

Image Source: Educational illustration for teaching purposes.

Questions

  1. What is the first-line intervention for this patient?
  • A) IV fluids
  • B) Hemodialysis
  • C) Urethral catheter placement
  • D) Emergent nephrostomy tubes
  1. What is post-obstructive diuresis and when does it occur?
  • A) Decreased urine output after catheter removal
  • B) Massive diuresis (>200 mL/hr) after relief of obstruction, can lead to volume depletion
  • C) Inability to concentrate urine permanently
  • D) Rare complication of catheter placement
  1. What level of obstruction causes bilateral hydronephrosis?
  • A) Unilateral ureteral stone
  • B) Bladder outlet or urethral obstruction
  • C) Renal pelvis obstruction
  • D) Single kidney obstruction
  1. How should post-obstructive diuresis be managed?
  1. What are the causes of obstructive uropathy at each level?

Answers

  1. C) Urethral catheter placement - The immediate priority is relieving the obstruction. Foley catheter placement will decompress the bladder and restore urine flow. This is both diagnostic and therapeutic.
  1. B) Massive diuresis (>200 mL/hr) after relief of obstruction, can lead to volume depletion - Post-obstructive diuresis occurs after relief of bilateral obstruction or obstruction of a solitary kidney. It results from accumulated urea acting as an osmotic diuretic, impaired concentrating ability, and natriuretic peptide release.
  1. B) Bladder outlet or urethral obstruction - Bilateral hydronephrosis indicates obstruction below the bladder (prostatic urethra, bladder neck) or at the bladder level. Unilateral obstruction (e.g., ureteral stone) causes unilateral hydronephrosis unless there is a solitary kidney.
  1. Management of post-obstructive diuresis:
  • Monitor urine output closely (initially hourly)
  • Replace 50-75% of urine output with 0.45% saline if >200 mL/hour
  • Avoid over-replacement (can perpetuate diuresis)
  • Monitor electrolytes every 6-12 hours
  • Most cases resolve within 24-48 hours
  • Watch for hyponatremia, hypokalemia, hypomagnesemia
  1. Causes of obstructive uropathy by level:
  • Upper tract (kidney/ureter): Nephrolithiasis, malignancy (transitional cell, cervical, bladder), retroperitoneal fibrosis, ureteral stricture, blood clots
  • Lower tract (bladder): Neurogenic bladder, bladder cancer, bladder stones
  • Outlet (prostate/urethra): BPH (most common in older men), prostate cancer, urethral stricture, posterior urethral valves (pediatric)

Learning Points

  1. AKI classification using KDIGO criteria helps stage severity based on creatinine rise and urine output.
  1. FENa helps differentiate prerenal (<1%) from intrinsic (>2%) AKI; however, it can be unreliable with diuretic use (use FEUrea instead).
  1. Urine sediment provides critical diagnostic information: bland sediment (prerenal/postrenal), muddy brown casts (ATN), RBC casts (glomerulonephritis), WBC casts (pyelonephritis/interstitial nephritis).
  1. Postrenal AKI is reversible with prompt relief of obstruction; renal ultrasound is the imaging modality of choice.
  1. Hold nephrotoxic medications (ACE inhibitors, ARBs, NSAIDs, aminoglycosides) in AKI and reassess for resumption after recovery.

All cases for this lecture as Markdown