What is the diagnosis for a patient with hypoosmolar hyponatremia and a urine sodium level of less than 20, indicating hyporeninemic (hypoaldosteronism) or hypovolemic hyponatremia?

From the Guidelines

Hypoosmolar hyponatremia with urine sodium less than 20 mEq/L should be treated with volume repletion using isotonic saline (0.9% NaCl) as the primary approach, aiming to correct serum sodium levels without exceeding 8 mEq/L in 24 hours to prevent osmotic demyelination syndrome. This condition typically indicates a volume-depleted state with appropriate renal sodium conservation. The management strategy should focus on addressing the underlying cause of volume depletion, which may include gastrointestinal losses, third-spacing, or excessive sweating, while discontinuing any contributing medications such as diuretics 1.

Key Considerations

• The low urine sodium (<20 mEq/L) reflects the kidney's appropriate response to hypovolemia, with enhanced sodium reabsorption to preserve intravascular volume.
• Once euvolemia is achieved, sodium levels typically normalize.
• In cases of severe hyponatremia (sodium <120 mEq/L) with neurological symptoms, more aggressive initial correction with 3% hypertonic saline may be considered at 1-2 mL/kg/hr, with close monitoring of serum sodium levels to avoid rapid overcorrection 1.
• The use of hypertonic saline should be limited to severely symptomatic cases due to the risk of worsening ascites and edema in patients with cirrhosis, and the potential for central pontine myelinolysis, especially in advanced cirrhosis 1.

Treatment Approach

• Begin with a 1-2 L bolus of normal saline, followed by maintenance fluids at 100-125 mL/hr, adjusting based on clinical response.
• Monitor serum sodium every 4-6 hours initially, aiming for correction rates not exceeding 8 mEq/L in 24 hours.
• Address the underlying cause of volume depletion and discontinue contributing medications.
• Consider albumin infusion as it may improve serum sodium concentration, although more evidence is needed to fully support its use in this context 1.

From the Research

Hypoosmolar Hyponatremia

Hypoosmolar hyponatremia occurs when the serum sodium level is less than 135 mEq/L and the serum osmolality is less than 280 mOsm/kg 2. This condition can be caused by various factors, including:

• Water retention
• Plasma volume deficiency (hypovolemia)
• Liver cirrhosis and cardiac failure (hypervolemic hyponatremia)
• Syndrome of inappropriate antidiuretic hormone secretion (euvolemic hyponatremia) 3

Urine Sodium Less Than 20

A urine sodium level less than 20 mmol/L is indicative of hypovolemia 2. In patients with hypoosmolar hyponatremia and urine sodium less than 20, the approach to management should consist of treating the underlying cause of hypovolemia. This may involve:

• Isotonic saline to correct hypovolemia 3
• Addressing the underlying cause of fluid loss, such as gastrointestinal fluid loss

Management of Hyponatremia

The management of hyponatremia depends on the underlying cause and the severity of symptoms. For most patients, the approach to managing hyponatremia should consist of treating the underlying cause 4. In cases of severely symptomatic hyponatremia, bolus hypertonic saline may be used to reverse hyponatremic encephalopathy 4. Vaptans, a new class of pharmacological agents, have been shown to be effective in the treatment of euvolemic and hypervolemic hyponatremia 3.

Key Considerations

When evaluating patients with hyponatremia, clinicians should:

• Categorize patients according to their fluid volume status (hypovolemic, euvolemic, or hypervolemic) 4
• Measure serum and urine osmolality and sodium levels to determine the underlying cause of hyponatremia 2, 5
• Consider the potential for unmeasured osmoles, such as ethanol, to interfere with the interpretation of serum and urine studies 5