What is the initial step in the workup and treatment of a patient presenting with hyponatremia?

Hyponatremia Workup

The initial step in evaluating hyponatremia is to assess volume status (hypovolemic, euvolemic, or hypervolemic) and measure serum osmolality, urine osmolality, and urine sodium to determine the underlying cause. 1

Initial Diagnostic Workup

Immediately obtain the following laboratory tests:

• Serum osmolality to classify hyponatremia as hypotonic, isotonic, or hypertonic 1
• Urine osmolality to assess renal water handling 1
• Urine sodium concentration to differentiate causes (spot urine sodium <30 mmol/L suggests hypovolemia with 71-100% positive predictive value for response to saline) 1
• Serum uric acid (levels <4 mg/dL have 73-100% positive predictive value for SIADH or cerebral salt wasting) 1
• Thyroid-stimulating hormone (TSH) to exclude hypothyroidism 1
• Serum creatinine and electrolytes including potassium, calcium, and magnesium 1

Clinical Assessment of Volume Status

Perform a focused physical examination looking for:

• Hypovolemia indicators: orthostatic hypotension, dry mucous membranes, poor skin turgor, tachycardia 1, 2
• Hypervolemia indicators: jugular venous distention, peripheral edema, ascites, orthopnea 1, 2
• Euvolemia: absence of both hypovolemic and hypervolemic signs 1

The distinction between euvolemia and hypovolemia can be clinically challenging but is critical for appropriate management 3. To confirm moderate to severe volume depletion, look for at least four of seven signs: confusion, non-fluent speech, extremity weakness, dry mucous membranes, dry tongue, furrowed tongue, and sunken eyes 1.

Symptom Severity Assessment

Classify symptoms to guide urgency of treatment:

• Severe symptoms (requiring immediate intervention): seizures, coma, altered mental status, cardiorespiratory distress 1, 2
• Moderate symptoms: nausea, vomiting, headache, confusion 1
• Mild or asymptomatic: minimal or no symptoms 1

Even mild hyponatremia (130-135 mmol/L) should not be ignored, as it increases fall risk (21% vs 5% in normonatremic patients) and is associated with 60-fold increased mortality when sodium drops below 130 mmol/L 1, 2.

Determine Acuity

Establish whether hyponatremia is acute (<48 hours) or chronic (>48 hours):

• Acute hyponatremia is typically hospital-acquired, often postoperative or from excessive fluid administration, and carries higher risk of cerebral edema 4
• Chronic hyponatremia usually develops outside the hospital, is better tolerated, but carries risk of osmotic demyelination syndrome if corrected too rapidly 4

This distinction is crucial because acute hyponatremia can be corrected more rapidly without risk of osmotic demyelination, while chronic hyponatremia requires slower correction rates 1, 4.

Common Diagnostic Pitfalls

Avoid these critical errors:

• Failing to assess volume status accurately, which is essential for differentiating SIADH from other causes 1
• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant 1
• In neurosurgical patients, failing to distinguish between SIADH and cerebral salt wasting (CSW), as treatment approaches differ fundamentally—CSW requires volume and sodium replacement, not fluid restriction 1
• Obtaining plasma ADH levels or natriuretic peptide levels for diagnosis, which is not supported by evidence 1

Initial Treatment Considerations Based on Workup

Once the workup is complete, treatment depends on the underlying cause:

• Hypovolemic hyponatremia: Discontinue diuretics and administer isotonic saline (0.9% NaCl) for volume repletion 1
• Euvolemic hyponatremia (SIADH): Fluid restriction to 1 L/day is first-line treatment 1
• Hypervolemic hyponatremia: Fluid restriction to 1-1.5 L/day for sodium <125 mmol/L 1
• Severe symptomatic hyponatremia: Administer 3% hypertonic saline immediately with goal of 6 mmol/L correction over 6 hours, not exceeding 8 mmol/L in 24 hours 1, 2

Maximum correction rate should never exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome, with even more cautious correction (4-6 mmol/L per day) in high-risk patients with advanced liver disease, alcoholism, or malnutrition. 1, 4