What are the causes, symptoms, diagnostic approach, and management of anuria for the NAC OSCE Canada, presented simply?

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Anuria: Causes, Symptoms, Diagnosis, and Management for NAC OSCE Canada

Definition

Anuria is defined as complete cessation of urine output (<50-100 mL/24h) or urine output <0.3 mL/kg/h for 24 hours, representing the most severe form of acute kidney injury (Stage 3 AKI) 1.

Causes

Categorized by Mechanism (Prerenal, Renal, Postrenal):

Postrenal (Obstructive) - 5-10% of AKI cases 2:

  • Bilateral ureteral obstruction (stones, blood clots, tumors)
  • Bladder outlet obstruction (BPH, urethral stricture, neurogenic bladder)
  • Single functioning kidney with unilateral obstruction
  • Note: Obstruction accounts for <3% of AKI overall but is critical to identify as it's immediately reversible 3

Prerenal (Hypoperfusion):

  • Severe hypotension/shock
  • Hypovolemia (hemorrhage, dehydration)
  • Cardiogenic shock
  • Renal artery occlusion (bilateral or in solitary kidney)

Renal (Intrinsic):

  • Sepsis (most common cause of anuric AKI - 60.5% of cases) 4
  • Acute tubular necrosis (postoperative, nephrotoxins)
  • Rapidly progressive glomerulonephritis
  • Acute cortical necrosis
  • Bilateral renal vein thrombosis

Uncommon but Important:

  • Reflex anuria (functional spasm from unilateral kidney/ureter trauma) 5

Symptoms & Clinical Presentation

Patient Complaints:

  • Complete absence of urine output (key distinguishing feature from oliguria)
  • Lower abdominal discomfort/suprapubic pain (if bladder distension from obstruction)
  • Flank pain (if ureteral obstruction)
  • Symptoms of uremia if prolonged: nausea, vomiting, confusion, altered mental status

Physical Examination Findings:

  • Palpable/percussible bladder (suggests bladder outlet obstruction)
  • Absent bladder distension (suggests prerenal/renal cause)
  • Signs of volume depletion (hypotension, tachycardia, dry mucous membranes)
  • Signs of fluid overload (edema, pulmonary crackles if prolonged)
  • Fever (suggests sepsis or infection)

Associated Features:

  • Younger age, multi-organ dysfunction, and absence of chronic kidney disease are characteristic of anuric AKI 4

Diagnosis

Step 1: Immediate Bedside Assessment

Bladder ultrasound or bladder scan - First diagnostic step to differentiate obstructive from non-obstructive causes 6:

  • Distended bladder = bladder outlet obstruction → immediate catheterization
  • Empty bladder = proceed to further workup

Step 2: Laboratory Evaluation

  • Serum creatinine (AKI Stage 3: ≥354 μmol/L or ≥3× baseline) 1
  • Blood urea nitrogen (BUN)
  • Electrolytes (hyperkalemia is life-threatening)
  • Complete blood count
  • Urinalysis with microscopy (if any urine obtainable):
    • Casts suggest intrinsic renal disease
    • Crystals suggest stones
    • Pyuria suggests infection

Step 3: Imaging

Renal ultrasound - Usually appropriate as first-line imaging 3:

  • Hydronephrosis = obstructive uropathy
    • Grade 1 or less: consider JJ stent
    • Greater than grade 1: may require percutaneous nephrostomy 7
  • Normal collecting system: suggests prerenal or intrinsic renal cause
  • Assess for renal size, echogenicity, and perfusion

CT scan without contrast - If ultrasound inconclusive or to identify stone location 3

Step 4: Determine Etiology

  • History of recent surgery, sepsis, hypotension → ATN/septic AKI
  • Known BPH, neurogenic bladder → bladder outlet obstruction
  • Flank pain, hematuria → ureteral obstruction
  • Single functioning kidney → any unilateral process can cause anuria

Management

Immediate Life-Threatening Issues (ABC Approach):

  1. Hyperkalemia management (if K+ >6.5 mEq/L or ECG changes):

    • Calcium gluconate 10% 10 mL IV (cardiac protection)
    • Insulin 10 units + dextrose 50% 50 mL IV
    • Salbutamol nebulizer
    • Sodium bicarbonate if severe acidosis

  2. Volume status optimization:

    • If hypovolemic: IV fluid resuscitation with crystalloids
    • If euvolemic/hypervolemic: fluid restriction

Definitive Management Based on Cause:

Obstructive (Postrenal):

  • Bladder outlet obstruction:

    • Immediate urethral catheterization (14-16 Fr Foley)
    • If unsuccessful: suprapubic catheter or urology consultation

  • Upper tract obstruction (hydronephrosis):

    • Percutaneous nephrostomy (PCN) for:

      • Hydronephrosis >grade 1 with radiopaque stones in solitary kidney
      • Bilateral ureteric stones requiring bilateral ureterolithotomy
      • Provides tract for subsequent percutaneous nephrolithotomy 7

    • JJ stent (4.8-5 Fr) for:

      • Hydronephrosis ≤grade 1
      • Radiolucent stones regardless of location
      • Can serve dual purpose as drainage and preparation for ESWL 7

Non-Obstructive (Prerenal/Renal):

  • Septic AKI (most common) 4:

    • Source control and broad-spectrum antibiotics
    • Hemodynamic support with vasopressors if needed
    • Early nephrology consultation for RRT consideration

  • Prerenal:

    • Aggressive fluid resuscitation if hypovolemic
    • Treat underlying cause (cardiogenic shock, hemorrhage)

  • Intrinsic renal:

    • Discontinue nephrotoxic medications
    • Treat underlying glomerulonephritis if suspected (may require renal biopsy)

Renal Replacement Therapy Indications:

Urgent dialysis if:

  • Refractory hyperkalemia
  • Severe metabolic acidosis (pH <7.1)
  • Pulmonary edema refractory to diuretics
  • Uremic symptoms (pericarditis, encephalopathy)
  • Anuric AKI patients more frequently require continuous RRT 4

Key Clinical Pitfalls

  1. Don't assume obstruction is rare - While only 3% of AKI, it's the most reversible cause; always check bladder first 3

  2. Creatinine takes 24-48 hours to rise - Patient may be anuric before creatinine reflects severity 1

  3. Post-obstructive diuresis - After relieving obstruction, massive diuresis can occur; replace 50-75% of urine output with IV fluids to prevent hypovolemia

  4. Bilateral process required - Anuria from unilateral pathology only occurs with solitary functioning kidney 2

  5. Reflex anuria exists - Functional cessation from unilateral kidney trauma; diagnosis of exclusion, treat underlying cause 5

  6. Anuric AKI has worse prognosis - Higher in-hospital mortality and long-term dialysis dependence compared to oliguric/non-oliguric AKI 4

References

3
Guideline

acr appropriateness criteria® renal failure.

Journal of the American College of Radiology, 2021

5
Research

Reflex anuria: an old concept with new evidence.

International urology and nephrology, 2014

6
Research

Point-of-Care Ultrasound to Assess Anuria in Children.

Pediatric emergency care, 2016

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