Anuria is clinically defined as a urine output of less than 100 mL in 24 hours. It is a critical sign often indicating either severe urinary tract obstruction or acute renal failure (ARF) due to various underlying mechanisms. Despite its severity, anuria is relatively rare, as the kidneys typically maintain a minimum urine output of approximately 75 mL daily even in the setting of renal failure.

Pathophysiology and Major Causes

Anuria results from either a failure of urine formation by the kidneys or an obstruction preventing urine excretion. The main categories include:

• Pre-renal causes: Severe hypoperfusion leading to acute tubular necrosis (ATN).

• Renal causes: Acute tubular necrosis, cortical necrosis, acute glomerulonephritis, vasculitis, hemolytic-uremic syndrome.

• Post-renal causes: Urinary tract obstruction at any level, including bladder outlet obstruction, ureteral blockage, or bilateral ureteral obstruction.

Medical causes of Anuria – Summary table 1

Clinical Presentation

Patients with anuria may present with:

• Complete absence or severe reduction in urine output.

• Symptoms of urinary tract obstruction: lower abdominal or flank pain, bladder distention, burning micturition, overflow incontinence.

• Signs of systemic illness or renal failure: edema, hypertension, uremic symptoms (nausea, vomiting, confusion), electrolyte imbalances.

In acute renal failure cases, anuria may be accompanied by:

• Hyperkalemia symptoms: muscle weakness, arrhythmias.

• Uremia manifestations: anorexia, pruritus, lethargy, seizures.

• Signs of volume overload and heart failure: edema, jugular venous distension, crackles on lung auscultation.

Initial assessment and history

A comprehensive history should focus on:

• Voiding patterns and recent changes.

• Fluid intake and last urine output timing.

• Past medical history: kidney disease, urinary tract infections, prostate issues, stones, congenital abnormalities.

• Medication history: nephrotoxic drugs or agents that affect bladder function.

• Surgical history involving the abdomen, urinary tract, or kidneys.

Physical examination should include:

• Vital signs monitoring.

• Abdominal inspection and palpation for bladder distension or masses.

• Flank examination and percussion over the costovertebral angles.

• Auscultation for renal artery bruits.

• Neurological examination if neurogenic bladder is suspected.

Diagnostic approach

• Bladder catheterization: Attempt immediate catheterization to relieve obstruction and quantify urine output.

  • Urine volume >75 mL suggests lower urinary tract obstruction.

  • Urine volume <75 mL may indicate renal parenchymal disease or higher urinary tract obstruction.

• Laboratory tests: Serum electrolytes, renal function tests (BUN, creatinine), urinalysis, and markers of infection or hemolysis.

• Imaging: Ultrasonography is the first-line to detect obstruction or kidney abnormalities.

• If obstruction suspected but unclear, consider cystoscopy, retrograde pyelography, and renal scanning.

• Additional renal function studies may be warranted if no obstruction is identified.

Emergency interventions

• Relieve obstruction promptly via catheterization or surgical intervention if catheter insertion is not possible.

• Initiate fluid management with daily fluid allowance restricted to 600 mL above previous day’s urine output to avoid fluid overload.

• Restrict dietary potassium and sodium; provide balanced nutrition with controlled protein intake.

• Monitor vital signs, fluid balance, and daily weights closely.

• Prepare for nephrostomy or ureterostomy if surgical drainage is needed.

• Treat underlying causes such as infection, vasculitis, or thrombotic events accordingly.

Special populations

• Neonates: Anuria is absence of urine for 24 hours; may be primary (renal agenesis/dysplasia) or secondary (renal ischemia, thrombosis).

• Elderly: Anuria may develop gradually due to underlying pathology, and immobility can reduce ability to void despite urine production.

Prognosis and Follow-up

Anuria is a medical emergency requiring rapid diagnosis and intervention to prevent life-threatening complications such as uremia, electrolyte disturbances, and volume overload. Early identification and treatment of underlying causes can improve outcomes. Long-term follow-up is essential for monitoring renal function recovery and preventing recurrence.

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