What is a characteristic of hepato-renal syndrome?

Low Urinary Sodium is a Characteristic of Hepatorenal Syndrome

Low urinary sodium concentration (<10 mmol/L) is a characteristic feature of hepatorenal syndrome (HRS). 1 This finding reflects the intense renal sodium retention that occurs as part of the pathophysiological process in HRS.

Diagnostic Criteria for Hepatorenal Syndrome

According to the EASL clinical practice guidelines, the diagnostic criteria for hepatorenal syndrome include:

1. Serum creatinine >1.5 mg/dL (133 μmol/L)
2. Absence of shock
3. No improvement of renal function after at least 2 days of diuretic withdrawal and volume expansion with albumin
4. No current or recent treatment with nephrotoxic drugs
5. Absence of parenchymal renal disease as defined by:
   • Proteinuria <0.5 g/day
   • No microhematuria (<50 red cells/high power field)
   • Normal renal ultrasonography 1

Pathophysiology Explaining Low Urinary Sodium

The low urinary sodium in HRS results from several pathophysiological mechanisms:

1. Splanchnic vasodilation causing a reduction in effective arterial blood volume
2. Activation of the sympathetic nervous system and renin-angiotensin-aldosterone system leading to intense renal vasoconstriction
3. Impaired cardiac function due to cirrhotic cardiomyopathy
4. Increased synthesis of vasoactive mediators affecting renal blood flow 1

These mechanisms lead to intense sodium and water retention by the kidneys in an attempt to restore effective arterial blood volume, resulting in very low urinary sodium excretion.

Types of Hepatorenal Syndrome

HRS is classified into two types:

• Type 1 HRS: Rapidly progressive renal failure with doubling of serum creatinine to >2.5 mg/dL in less than 2 weeks
• Type 2 HRS: More stable, less progressive impairment of renal function 1

Differentiating HRS from Other Causes of Renal Failure

The low urinary sodium (<10 mmol/L) helps differentiate HRS from other causes of renal failure in cirrhotic patients:

• Acute tubular necrosis: Usually has urinary sodium >30 mmol/L
• Pre-renal azotemia: May have low urinary sodium but responds to volume expansion
• Parenchymal renal disease: Often has significant proteinuria (>500 mg/day) and/or hematuria 1

Clinical Implications

The finding of low urinary sodium has important clinical implications:

1. Diagnostic value: Helps confirm HRS diagnosis when other criteria are met
2. Prognostic significance: Associated with poor prognosis and short survival without treatment
3. Treatment guidance: Patients with HRS require specific treatments including vasoconstrictors with albumin, rather than diuretics 1

Treatment Approach

For patients with HRS:

1. Vasoconstrictors with albumin: First-line therapy (terlipressin with albumin is most studied)
2. Liver transplantation: Definitive treatment for suitable candidates
3. Avoidance of nephrotoxic drugs and careful fluid management
4. Hemodialysis: May be used as a bridge to transplantation 1

The low urinary sodium concentration is a key laboratory finding that reflects the underlying pathophysiology and helps guide appropriate management in patients with hepatorenal syndrome.