How to manage low osmolality (hyposmolality)?

Management of Low Osmolality (Hyposmolality)

The primary management of hyposmolality involves identifying the underlying cause and implementing targeted interventions, with directly measured serum osmolality being the gold standard diagnostic test for accurate assessment. 1

Diagnosis of Hyposmolality

Recommended Diagnostic Tools

• Directly measured serum or plasma osmolality should be used as the primary indicator of hydration status, particularly in older adults 1
• An action threshold of serum osmolality <275 mOsm/kg indicates hyposmolality 1
• Where directly measured osmolality is not available, calculated osmolarity using the equation: osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all measured in mmol/L) can be used 1

Clinical Assessment

• Evaluate for symptoms of hyposmolality which may include general weakness, confusion, headache, and nausea at serum sodium concentrations of 125-130 mEq/L 1
• Life-threatening manifestations may occur when serum sodium levels drop below 120 mEq/L 1
• Assess volume status to differentiate between euvolemic, hypovolemic, and hypervolemic hyposmolality 1

Tests Not Recommended

• Simple signs and tests such as skin turgor, mouth dryness, weight change, urine color or specific gravity should NOT be used to assess hydration status in older adults 1
• Bioelectrical impedance should NOT be used to assess hydration status as it has not been shown to be diagnostically useful 1

Management Based on Underlying Cause

Syndrome of Inappropriate ADH (SIADH)

SIADH manifests as euvolemic hypoosmolar hyponatremia with the following characteristics:

• Hyponatremia (serum sodium <134 mEq/L)
• Hypoosmolality (plasma osmolality <275 mOsm/kg)
• Inappropriately high urine osmolality (>500 mOsm/kg)
• Inappropriately high urinary sodium concentration (>20 mEq/L)
• Absence of hypothyroidism, adrenal insufficiency, or volume depletion 1

Management of SIADH:

• Free water restriction (<1 L/day) is the first-line treatment for asymptomatic mild SIADH 1
• Hypertonic 3% saline IV is indicated for life-threatening or acute symptomatic severe hyponatremia (<120 mEq/L) 1
• Vasopressin receptor antagonists (tolvaptan) may be used to correct hyponatremia, but must be initiated in a hospital setting with close monitoring of serum sodium 2
• CAUTION: Too rapid correction of hyponatremia (>12 mEq/L/24 hours) can cause osmotic demyelination resulting in serious neurological complications including dysarthria, mutism, dysphagia, lethargy, affective changes, spastic quadriparesis, seizures, coma, and death 2

Low-Intake Dehydration (Hypernatremia/Hyperosmolality)

For patients with high serum osmolality (>300 mOsm/kg):

• Encourage increased oral fluid intake with preferred beverages (tea, coffee, fruit juice, water) 1
• Oral rehydration therapy and sports drinks are NOT indicated for low-intake dehydration 1
• For older adults who appear unwell with serum osmolality >300 mOsm/kg, subcutaneous or intravenous fluids should be offered in parallel with encouraging oral fluid intake 1
• Hypotonic fluids should be administered to correct the fluid deficit while diluting the raised osmolality 1

Volume Depletion (Hypovolemic Hyposmolality)

• For mild to moderate dehydration: Oral rehydration solution (ORS) should be administered 1
• For severe dehydration: Isotonic intravenous fluids such as lactated Ringer's and normal saline should be administered 1
• Once rehydrated, maintenance fluids should be provided and ongoing losses replaced 1

Special Considerations

Monitoring and Follow-up

• Hydration status should be reassessed regularly until corrected, then monitored periodically 1
• All patients treated for hyposmolality should be monitored to ensure serum sodium remains within normal limits 2
• If hypernatremia develops during treatment, management may include dose decreases or interruption of treatment, combined with modification of free-water intake or infusion 2

Prevention Strategies

• All older persons should be screened for hydration status when they contact the healthcare system, if clinical condition changes unexpectedly, and periodically when malnourished or at risk of malnutrition 1
• Older persons and their informal carers may use appropriate tools to assess fluid intake, but should also ask healthcare providers for assessment of serum osmolality periodically 1

Pitfalls to Avoid

• Do not rely on clinical signs like skin turgor or dry mouth to assess hydration status, especially in older adults 1
• Avoid too rapid correction of hyponatremia to prevent osmotic demyelination syndrome 2
• Do not use bioelectrical impedance for hydration assessment 1
• Assessment of fluid intake is often highly inaccurate in older adults, particularly in residential care settings 1
• Do not use oral rehydration therapy or sports drinks for low-intake dehydration 1