Renal · Acute Kidney Injury
The facts most likely to be tested
Postrenal acute kidney injury (AKI) is caused by bilateral urinary tract obstruction or obstruction of a solitary functioning kidney.
Benign prostatic hyperplasia (BPH) is the most common cause of postrenal AKI in elderly men, typically presenting with urinary hesitancy, nocturia, and a palpable bladder.
Renal ultrasonography is the initial diagnostic test of choice to evaluate for hydronephrosis in patients with suspected postrenal obstruction.
Patients with complete obstruction often present with anuria, whereas partial obstruction may manifest as polyuria due to a post-obstructive diuresis or impaired concentrating ability.
Bladder catheterization (Foley or suprapubic) is the first-line therapeutic intervention to relieve lower urinary tract obstruction and confirm the diagnosis.
Post-obstructive diuresis is a common complication following the relief of obstruction, requiring careful fluid and electrolyte monitoring to prevent hypovolemia and hypokalemia.
Nephrolithiasis causing bilateral ureteral obstruction or obstruction of a single functioning kidney is a classic cause of acute-onset flank pain and azotemia.
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A 72-year-old male with a history of BPH presents to the emergency department with a 2-day history of decreased urine output and suprapubic discomfort. Physical examination reveals a distended, tender bladder on percussion. Laboratory studies show a serum creatinine of 3.2 mg/dL, up from a baseline of 1.0 mg/dL. A bedside bladder scan shows 900 mL of urine.
What is the most appropriate next step in management?
Placement of a Foley catheter
The patient has postrenal AKI due to bladder outlet obstruction from BPH; immediate decompression via catheterization is the definitive diagnostic and therapeutic step.
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High yield triage
Etiology / Epidemiology
Caused by mechanical obstruction of urine flow. BPH and nephrolithiasis are the most common causes.
Clinical Manifestations
Presents with anuria or polyuria and suprapubic pain. Look for palpable bladder on exam.
Diagnosis
Renal ultrasound is the gold standard. Look for hydronephrosis to confirm obstruction.
Treatment
Bladder catheterization is first-line. Avoid nephrotoxic agents during recovery.
Prognosis
Reversible if treated within < 7 days. Chronic obstruction leads to permanent fibrosis.
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Epidemiology & Etiology
Postrenal AKI accounts for 5-10% of all AKI cases. BPH is the leading cause in elderly men, while nephrolithiasis and malignancy (cervical, prostate, bladder) are primary drivers in younger or oncologic populations. Retroperitoneal fibrosis and neurogenic bladder are critical secondary considerations.
Pertinent Anatomy
Obstruction must be bilateral or occur in a solitary functioning kidney to cause AKI. The ureterovesical junction and ureteropelvic junction are common sites for stone impaction. The bladder neck is the primary site for outflow obstruction in BPH.
Pathophysiology
Increased intraluminal pressure leads to hydronephrosis and decreased glomerular filtration rate. Initially, the kidney maintains flow via prostaglandins, but prolonged pressure causes tubulointerstitial fibrosis. Post-obstructive diuresis often occurs after relief, risking severe volume depletion.
Clinical Manifestations
Patients often present with flank pain or suprapubic tenderness. Anuria suggests complete obstruction, while polyuria may indicate a concentrating defect. Red flags include fever (suggesting infected hydronephrosis) and altered mental status from uremia.
Diagnosis
Renal ultrasound is the initial diagnostic test of choice to visualize hydronephrosis. If ultrasound is nondiagnostic but suspicion remains high, non-contrast CT is the preferred modality for stone detection. A post-void residual volume > 500 mL is highly suggestive of bladder outlet obstruction.
Treatment
Immediate bladder catheterization (Foley) is the first-line intervention for bladder outlet obstruction. For proximal ureteral obstruction, percutaneous nephrostomy or ureteral stenting is required. Avoid NSAIDs in the acute phase to prevent further reduction in renal perfusion.
Prognosis
Recovery of renal function is typically rapid following decompression. Post-obstructive diuresis requires careful monitoring of electrolytes and fluid status to prevent hypovolemic shock. Failure to relieve obstruction within 1-2 weeks significantly increases the risk of irreversible renal damage.
Differential Diagnosis
Prerenal Azotemia: BUN/Cr ratio > 20:1
Acute Tubular Necrosis: Muddy brown casts
Acute Interstitial Nephritis: Eosinophiluria
Glomerulonephritis: RBC casts
Nephrolithiasis: Hematuria without AKI