How do you approach and evaluate hyponatremia?

Approach and Evaluation of Hyponatremia

The evaluation of hyponatremia should include assessment of volume status, measurement of serum and urine osmolality, urine electrolytes, and uric acid to determine the underlying cause and guide appropriate treatment. 1

Initial Assessment

Definition and Clinical Significance

• Hyponatremia is defined as serum sodium <135 mmol/L, with clinically significant hyponatremia typically considered as <131 mmol/L 2, 1
• Even mild hyponatremia is associated with increased mortality (60-fold increase in fatality with sodium <130 mmol/L) and morbidity (increased falls, fractures, cognitive impairment) 2, 3

Essential Laboratory Evaluation

• Serum sodium and osmolality to differentiate hypotonic from non-hypotonic hyponatremia 1
• Urine osmolality and sodium concentration to help determine etiology 1, 4
• Serum uric acid (level <4 mg/dL has 73-100% positive predictive value for SIADH) 2, 1
• Additional tests based on clinical suspicion: thyroid function, cortisol, renal function, liver function 4

Diagnostic Algorithm

Step 1: Rule Out Pseudohyponatremia

• Check serum osmolality (normal: 275-290 mOsm/kg) 1
• Normal or high serum osmolality may indicate pseudohyponatremia from laboratory error, hyperglycemia, or hypertriglyceridemia 2

Step 2: Assess Volume Status

• Categorize as hypovolemic, euvolemic, or hypervolemic based on clinical assessment 1, 4
• Physical examination alone has limited accuracy (sensitivity 41.1%, specificity 80%) 2

Hypovolemic Signs:

• Orthostatic hypotension (increase in pulse >10%, decrease in systolic BP >10%)
• Dry mucous membranes, decreased skin turgor
• Urinary sodium typically <30 mmol/L (unless renal sodium wasting) 2, 1

Euvolemic Signs:

• No signs of volume depletion or overload
• Urinary sodium typically >40 mEq/L with urine osmolality >100 mOsm/kg in SIADH 1, 4

Hypervolemic Signs:

• Edema, ascites, elevated jugular venous pressure
• Urinary sodium typically <20 mEq/L (except in renal failure) 1, 4

Step 3: Determine Specific Etiology

For Hypovolemic Hyponatremia:

• Extrarenal losses: diarrhea, vomiting, burns, pancreatitis (urine Na <10 mEq/L)
• Renal losses: diuretics, cerebral salt wasting, adrenal insufficiency (urine Na >20 mEq/L) 2, 1

For Euvolemic Hyponatremia:

• Rule out hypothyroidism, hypocortisolism, and polydipsia
• SIADH: elevated urine osmolality (>100 mOsm/kg) with urine sodium >40 mEq/L 1, 4

For Hypervolemic Hyponatremia:

• Cirrhosis, congestive heart failure, nephrotic syndrome, renal failure 2, 4

Treatment Approach

Based on Symptom Severity

Severe Symptoms (seizures, coma):

• Administer 3% hypertonic saline with goal to correct by 6 mmol/L over 6 hours or until symptoms resolve 4
• Do not exceed total correction of 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 4, 5
• For high-risk patients (advanced liver disease, alcoholism, malnutrition), use more cautious correction (4-6 mmol/L per day) 4, 5

Moderate/Mild Symptoms:

• Treatment based on underlying cause and volume status 4

Based on Volume Status

Hypovolemic Hyponatremia:

• Isotonic saline (0.9% NaCl) for volume repletion 4
• Discontinue diuretics if applicable 4

Euvolemic Hyponatremia (SIADH):

• Fluid restriction (<1 L/day) as first-line treatment 1, 4
• For resistant cases, consider:
  • Tolvaptan (vasopressin receptor antagonist) starting at 15 mg once daily, titrating up to 60 mg as needed 5
  • Urea, demeclocycline, or lithium as alternative options 4

Hypervolemic Hyponatremia:

• Fluid restriction to 1-1.5 L/day 4
• Consider albumin infusion for cirrhotic patients 4
• Avoid hypertonic saline unless life-threatening symptoms present 4

Special Considerations

Neurosurgical Patients

• Distinguish between SIADH and cerebral salt wasting (CSW), as treatment approaches differ significantly 2, 1
• For CSW, treatment focuses on volume and sodium replacement, not fluid restriction 2, 1
• Consider fludrocortisone for hyponatremia in subarachnoid hemorrhage patients 4

Monitoring During Treatment

• Monitor serum sodium every 2-4 hours during initial correction for severe symptoms 4
• Calculate sodium deficit using formula: Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg) 4
• Watch for signs of osmotic demyelination syndrome (dysarthria, dysphagia, oculomotor dysfunction, quadriparesis) 4, 5

Common Pitfalls to Avoid

• Overly rapid correction exceeding 8 mmol/L in 24 hours (or 12 mEq/L as per tolvaptan label) 4, 5
• Using fluid restriction in cerebral salt wasting 2, 4
• Inadequate monitoring during active correction 4
• Failing to recognize and treat the underlying cause 4
• Ignoring mild hyponatremia as clinically insignificant 4
• Obtaining levels of hormones such as ADH and natriuretic peptides (not supported by evidence) 2