What is the appropriate workup for hyponatremia (low sodium levels)?

From the Guidelines

The workup for hyponatremia should begin with a thorough history, physical examination, and basic laboratory tests to determine the underlying cause, including assessing the patient's volume status and measuring serum osmolality, urine osmolality, and urine sodium 1. The initial evaluation should focus on determining the patient's volume status, which can be categorized as hypovolemic, euvolemic, or hypervolemic.

• For hypovolemic hyponatremia, check for dehydration signs, measure orthostatic vital signs, and assess for GI losses, diuretic use, or third-spacing.
• In euvolemic hyponatremia, evaluate for SIADH by confirming normal renal, adrenal, and thyroid function while checking medications that may cause SIADH, as SIADH is characterized by low serum osmolality and inappropriately high urine osmolality in the absence of diuretic treatment, adrenal insufficiency, heart failure, cirrhosis, and hypothyroidism 1.
• For hypervolemic hyponatremia, assess for heart failure, cirrhosis, or nephrotic syndrome. Additional tests should include serum glucose (to rule out pseudohyponatremia), cortisol levels (for adrenal insufficiency), and thyroid function tests. The rate of correction depends on symptom severity, with severe symptoms requiring administration of 3% hypertonic saline at 100-150 mL over 10-20 minutes, aiming for a 4-6 mEq/L increase in the first 4-6 hours, while chronic or asymptomatic hyponatremia should be corrected slowly (no more than 8 mEq/L in 24 hours) to prevent osmotic demyelination syndrome 1. Treatment should always address the underlying cause while carefully monitoring serum sodium levels during correction. In patients with cirrhosis, water restriction to 1,000 mL/day and cessation of diuretics is recommended for moderate hyponatremia, and a more severe restriction of water intake with albumin infusion is recommended for severe hyponatremia 1. Vasopressin receptor antagonists can be used with caution for a short term (≤30 days) in cirrhosis, and hypertonic saline is reserved for short-term treatment of patients with symptomatic or severe hyponatremia 1.

From the Research

Hyponatremia Workup

• Hyponatremia is defined by a serum sodium level of less than 135 mEq/L and most commonly results from water retention 2
• The approach to managing hyponatremia should consist of treating the underlying cause, and clinicians should categorize patients according to their fluid volume status (hypovolemic hyponatremia, euvolemic hyponatremia, or hypervolemic hyponatremia) 2
• Evaluation of hyponatremia relies on clinical assessment and estimation of serum sodium, urine electrolytes, and serum and urine osmolality in addition to other case-specific laboratory parameters 3
• Point-of-care ultrasonography is an important adjunct to physical assessment in estimation of volume status 3

Diagnostic Approach

• The initial step is to differentiate hypotonic from nonhypotonic hyponatremia 4
• Hypotonic hyponatremia is further differentiated on the basis of urine osmolality, urine sodium level, and volume status 4
• Recently identified parameters, including fractional uric acid excretion and plasma copeptin concentration, may further improve the diagnostic approach 4

Treatment

• For acute or severely symptomatic hyponatremia, both US and European guidelines adopted the approach of giving a bolus of hypertonic saline 2, 4
• Fluid restriction remains the first-line treatment for most forms of chronic hyponatremia, but therapy to increase renal free water excretion is often necessary 4
• Vasopressin receptor antagonists, urea, and loop diuretics can be effective in managing hyponatremia, but have different recommendations in various guidelines 4, 5, 6