What is the appropriate workup algorithm for Hyponatremia (low sodium levels in the blood)?

From the Guidelines

The hyponatremia workup algorithm should begin with an assessment of volume status and measurement of serum and urine osmolality to guide further evaluation and management, as recommended by expert opinion 1. When evaluating a patient with hyponatremia, it is essential to first confirm true hyponatremia by checking serum osmolality. If osmolality is normal or high, consider pseudohyponatremia or hyperosmolar hyponatremia. For true hypoosmolar hyponatremia, assess volume status to determine the underlying cause.

• For hypovolemic patients, check urine sodium: if <20 mmol/L, consider extrarenal losses; if >20 mmol/L, consider renal losses.
• For euvolemic patients, measure urine osmolality: if >100 mOsm/kg with urine sodium >20 mmol/L, consider SIADH, hypothyroidism, or adrenal insufficiency; if <100 mOsm/kg, consider primary polydipsia.
• For hypervolemic patients, check urine sodium: if <20 mmol/L, consider heart failure, cirrhosis, or nephrotic syndrome; if >20 mmol/L, consider renal failure. Additional testing should include thyroid function tests, cortisol levels, and medication review, as certain conditions such as hypothyroidism and adrenal insufficiency can cause hyponatremia 1. Treatment depends on severity, chronicity, and underlying cause, ranging from fluid restriction for mild cases to 3% hypertonic saline for severe symptomatic hyponatremia, with correction rates not exceeding 8-10 mEq/L in 24 hours to prevent osmotic demyelination syndrome, although there are no evidence-based guidelines for managing SIADH, and recommended management is based on expert opinion 1.

From the FDA Drug Label

14 CLINICAL STUDIES 14. 1 Hyponatremia

In two double-blind, placebo-controlled, multi-center studies (SALT-1 and SALT-2), a total of 424 patients with euvolemic or hypervolemic hyponatremia (serum sodium <135 mEq/L) resulting from a variety of underlying causes (heart failure, liver cirrhosis, syndrome of inappropriate antidiuretic hormone [SIADH] and others) were treated for 30 days with tolvaptan or placebo, then followed for an additional 7 days after withdrawal.

The FDA drug label does not answer the question.

From the Research

Hyponatremia Workup Algorithm

The approach to managing hyponatremia should consist of treating the underlying cause and categorizing patients according to their fluid volume status (hypovolemic hyponatremia, euvolemic hyponatremia, or hypervolemic hyponatremia) 2.

• Evaluation:
  • Clinical assessment
  • Estimation of serum sodium, urine electrolytes, and serum and urine osmolality
  • Other case-specific laboratory parameters
  • Point-of-care ultrasonography to estimate volume status 3
• Classification:
  • Hypovolemic hyponatremia
  • Euvolemic hyponatremia
  • Hypervolemic hyponatremia
• Treatment:
  • Treat the underlying cause
  • Use of vasopressin receptor antagonists (vaptans) for euvolemic or hypervolemic hyponatremia 4, 5, 6
  • Hypertonic saline for severely symptomatic hyponatremia 2

Special Considerations

• Severely Symptomatic Hyponatremia:
  • Treat with bolus hypertonic saline to reverse hyponatremic encephalopathy
  • Increase serum sodium level by 4-6 mEq/L within 1-2 hours, but no more than 10 mEq/L within the first 24 hours 2
• Chronic Hyponatremia:
  • Avoid overly rapid correction to prevent osmotic demyelination
  • Use urea and vaptans with caution due to potential adverse effects 2, 4, 6