How do you treat low osmolality (hypotonicity)?

Treatment of Low Osmolality (Hypotonicity)

For symptomatic or severe hyponatremia (serum sodium <120 mEq/L), administer hypertonic 3% saline as a 100-150 mL intravenous bolus immediately to prevent life-threatening neurological complications. 1, 2

Initial Assessment and Diagnosis

Before initiating treatment, confirm true hypotonic hyposmolality and assess the clinical context:

• Measure serum osmolality directly (action threshold <275 mOsm/kg indicates hyposmolality) to confirm the diagnosis 1
• Exclude pseudohyponatremia (isotonic hyponatremia from hyperlipidemia or hyperproteinemia) and hypertonic hyponatremia (from hyperglycemia or mannitol) 3, 4
• Assess volume status to categorize as hypovolemic, euvolemic, or hypervolemic hyposmolality, as this determines treatment approach 3, 4
• Check urine osmolality and urine sodium: In SIADH (the most common cause of euvolemic hyposmolality), urine osmolality is >500 mOsm/kg with serum osmolality <275 mOsm/kg, and urinary sodium >40 mEq/L 5

Treatment Based on Severity and Symptoms

Acute Symptomatic Hyponatremia (<48 hours duration)

This is a medical emergency requiring rapid correction to prevent worsening cerebral edema:

• Administer 3% hypertonic saline as 100-150 mL IV bolus over 10-20 minutes 1, 2
• Repeat boluses every 10-20 minutes until symptoms improve or serum sodium increases by 4-6 mEq/L 2
• Target initial correction rate: 1-2 mEq/L/hour for the first 3-4 hours until symptoms resolve 2
• Monitor serum sodium every 2-4 hours during active correction 2

Chronic Symptomatic Hyponatremia (>48 hours or unknown duration)

Slower correction is mandatory to prevent osmotic demyelination syndrome:

• Administer 3% hypertonic saline as 100-150 mL IV bolus if moderately symptomatic 1, 2
• Target correction rate: 0.5 mEq/L/hour, not exceeding 8 mEq/L in 24 hours or 18 mEq/L in 48 hours 2, 3
• Monitor serum sodium every 4-6 hours initially, then every 6-8 hours once stable 2

Critical pitfall: Too rapid correction (>12 mEq/L/24 hours) can cause osmotic demyelination resulting in dysarthria, dysphagia, quadriparesis, seizures, coma, and death 6, 3

Asymptomatic Mild Hyponatremia

For patients without neurological symptoms:

• Initiate fluid restriction to <1000 mL/day as first-line therapy 7, 2
• Ensure adequate solute intake (salt and protein) to promote free water excretion 2
• Monitor serum sodium every 24-48 hours until stable 2

Treatment Based on Underlying Etiology

SIADH (Euvolemic Hyposmolality)

First-line treatment: Free water restriction <1 L/day for asymptomatic mild cases 1, 5

However, nearly half of SIADH patients do not respond to fluid restriction alone 2. For refractory cases:

• Oral urea is considered very effective and safe as second-line therapy 2
• Tolvaptan (vasopressin V2-receptor antagonist) is FDA-approved for clinically significant euvolemic hyponatremia 6
  • Starting dose: 15 mg once daily, may increase to 30 mg then 60 mg daily at ≥24-hour intervals 6
  • Must initiate in hospital setting with close serum sodium monitoring 6
  • Limit duration to 30 days due to hepatotoxicity risk 6
  • Contraindicated with strong CYP3A inhibitors (ketoconazole, clarithromycin, ritonavir) 6

Hypovolemic Hyposmolality (Volume Depletion)

This occurs with sodium and water loss (vomiting, diarrhea, diuretics):

• Administer isotonic saline (0.9% NaCl) intravenously for severe dehydration 1
• Oral rehydration solution for mild-moderate dehydration 1
• Avoid hypotonic fluids which worsen hyposmolality 7

Hypervolemic Hyposmolality (Heart Failure, Cirrhosis, Nephrotic Syndrome)

These conditions involve total body sodium excess with even greater water excess:

• Fluid restriction <1000 mL/day 7
• Sodium restriction (typically <2 g/day) 4
• Loop diuretics to promote free water excretion 4
• Tolvaptan may be considered in heart failure patients with persistent hyponatremia despite standard therapy 6

Fluid Selection for Concurrent Dehydration

When intravenous fluids are required:

• Use isotonic crystalloids (0.9% saline, lactated Ringer's, or balanced crystalloids like PlasmaLyte) 7, 8
• Avoid hypotonic solutions (5% dextrose, 0.45% saline) as they distribute into intracellular spaces and worsen cerebral edema 7
• Exception: In nephrogenic diabetes insipidus with hypernatremia, 5% dextrose in water is appropriate 8

Monitoring and Prevention of Overcorrection

Monitor serum sodium frequently during active treatment:

• Every 2-4 hours during hypertonic saline administration 2
• Every 4-6 hours for the first 24 hours in chronic hyponatremia 2
• Continue monitoring until stable within normal range 6

If overcorrection occurs (>8 mEq/L in 24 hours):

• Administer hypotonic fluids (5% dextrose in water) or free water orally 2
• Consider desmopressin (2-4 mcg IV/SC) to induce water retention and lower serum sodium 2

Special Populations

Patients with cirrhosis: Exercise particular caution as they have increased risk of gastrointestinal bleeding with tolvaptan (10% vs 2% placebo) 6

Older adults: Directly measured serum osmolality should be used for diagnosis, as clinical signs (skin turgor, dry mouth) are unreliable 7, 1

Patients unable to sense or respond to thirst: Tolvaptan is contraindicated 6