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 obtain serum and urine osmolality, urine sodium, and urine electrolytes to determine the underlying cause. 1

Initial Diagnostic Workup

Essential laboratory tests include:

• Serum osmolality to confirm hypotonic hyponatremia 1, 2
• Urine osmolality and urine sodium concentration 1, 3
• Serum electrolytes, blood urea nitrogen, and creatinine 1
• Thyroid-stimulating hormone (TSH) to exclude hypothyroidism 1
• Serum glucose (adjust sodium by 1.6 mEq/L for each 100 mg/dL glucose >100 mg/dL) 1
• Uric acid (serum uric acid <4 mg/dL has 73-100% positive predictive value for SIADH) 1

Clinical volume status assessment is critical:

• Look for orthostatic hypotension, dry mucous membranes, poor skin turgor (hypovolemia) 1
• Check for jugular venous distention, peripheral edema, ascites (hypervolemia) 1, 4
• Euvolemia: absence of both hypovolemic and hypervolemic signs 1

Interpretation Algorithm

Urine sodium interpretation guides diagnosis:

• Urine sodium <30 mmol/L suggests hypovolemic hyponatremia (71-100% positive predictive value for response to saline) 1
• Urine sodium >20-40 mmol/L with high urine osmolality (>300-500 mOsm/kg) suggests SIADH 1, 3

Volume status determines treatment approach:

Hypovolemic Hyponatremia

• Discontinue diuretics immediately 1
• Administer isotonic (0.9%) saline for volume repletion 1, 3
• Urine sodium typically <20 mmol/L 1

Euvolemic Hyponatremia (SIADH)

• Fluid restriction to 1 L/day is cornerstone of treatment 1, 5
• Rule out hypothyroidism and adrenal insufficiency 1
• Consider medications as cause 3

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)

• Fluid restriction to 1-1.5 L/day for sodium <125 mmol/L 1, 3
• Consider albumin infusion in cirrhotic patients 1
• Avoid hypertonic saline unless life-threatening symptoms present 1

Critical Safety Parameters

Correction rate limits to prevent osmotic demyelination syndrome:

• Maximum 8 mmol/L increase in 24 hours for most patients 1, 6
• More cautious correction (4-6 mmol/L per day) for high-risk patients: advanced liver disease, alcoholism, malnutrition, severe hyponatremia 1, 6
• For severely symptomatic patients: correct 6 mmol/L over 6 hours or until symptoms resolve, then limit remaining correction to 2 mmol/L over next 18 hours 1, 5

Severity-Based Treatment

Severe symptomatic hyponatremia (seizures, coma, altered mental status):

• Medical emergency requiring immediate 3% hypertonic saline 1, 2, 3
• Target 4-6 mmol/L increase within 1-2 hours to reverse encephalopathy 2
• Monitor sodium every 2 hours during initial correction 1

Mild to moderate asymptomatic hyponatremia:

• Treat underlying cause 2, 4
• Implement appropriate fluid restriction based on volume status 1
• Monitor sodium levels daily initially 1

Common Pitfalls to Avoid

• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant—even mild hyponatremia increases fall risk (21% vs 5%) and mortality 1, 2
• Overly rapid correction exceeding 8 mmol/L in 24 hours risks osmotic demyelination syndrome 1, 6
• Using fluid restriction in cerebral salt wasting (neurosurgical patients) worsens outcomes 1
• Failing to distinguish SIADH from cerebral salt wasting in neurosurgical patients—treatment approaches differ fundamentally 1
• Inadequate monitoring during active correction 1