What is the recommended evaluation and management for hyponatremia, including assessment of volume status and appropriate treatment strategies?

Hyponatremia: Evaluation and Management

Initial Diagnostic Approach

Hyponatremia (serum sodium <135 mmol/L) requires immediate assessment of volume status, symptom severity, and serum osmolality to guide treatment. 1

Essential Laboratory Workup

• Obtain serum osmolality, urine osmolality, urine sodium, and urine electrolytes to determine the underlying cause and guide therapy 1, 2
• Measure serum uric acid (levels <4 mg/dL have 73-100% positive predictive value for SIADH) 1
• Check thyroid function (TSH) and assess adrenal function to exclude hypothyroidism and adrenal insufficiency 1
• Assess extracellular fluid volume status through physical examination looking for orthostatic hypotension, dry mucous membranes, skin turgor (hypovolemia), or peripheral edema, ascites, and jugular venous distention (hypervolemia) 1, 2

Volume Status Classification

• Hypovolemic hyponatremia: Urine sodium <30 mmol/L suggests extrarenal losses (GI losses, third-spacing); urine sodium >20 mmol/L suggests renal losses (diuretics) 1
• Euvolemic hyponatremia: Urine sodium >20-40 mmol/L with urine osmolality >300 mOsm/kg suggests SIADH 1
• Hypervolemic hyponatremia: Seen in heart failure, cirrhosis, or nephrotic syndrome with total body sodium excess despite low serum sodium 1

Treatment Based on Symptom Severity

Severe Symptomatic Hyponatremia (Seizures, Coma, Altered Mental Status)

Administer 3% hypertonic saline immediately with a goal to increase sodium by 6 mmol/L over 6 hours or until symptoms resolve. 1, 2

• Give 100 mL boluses of 3% saline over 10 minutes, repeatable up to three times at 10-minute intervals 1
• Monitor serum sodium every 2 hours during initial correction 1
• Never exceed 8 mmol/L correction in any 24-hour period to prevent osmotic demyelination syndrome 1, 3
• Consider ICU admission for continuous monitoring during active correction 1

Mild to Moderate Symptomatic Hyponatremia

Treatment depends on volume status and underlying etiology rather than symptom severity alone 1, 2

Treatment Based on Volume Status

Hypovolemic Hyponatremia

Discontinue diuretics immediately and administer isotonic saline (0.9% NaCl) for volume repletion. 1

• Initial infusion rate: 15-20 mL/kg/h, then 4-14 mL/kg/h based on response 1
• Urine sodium <30 mmol/L predicts good response to saline (positive predictive value 71-100%) 1
• Once euvolemic, switch to maintenance isotonic fluids at 30 mL/kg/day 1

Euvolemic Hyponatremia (SIADH)

Fluid restriction to 1 L/day is the cornerstone of treatment for SIADH. 1, 2

• If no response to fluid restriction, add oral sodium chloride 100 mEq three times daily 1
• For resistant cases, consider vasopressin receptor antagonists (tolvaptan 15 mg once daily, titrate to 30-60 mg) 1, 2
• Alternative pharmacologic options include urea, demeclocycline, or loop diuretics 1, 2

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)

Implement fluid restriction to 1-1.5 L/day for serum sodium <125 mmol/L. 1

• Temporarily discontinue diuretics if sodium <125 mmol/L 1
• For cirrhotic patients, consider albumin infusion (6-8 g per liter of ascites drained) alongside fluid restriction 1
• Avoid hypertonic saline unless life-threatening symptoms are present, as it may worsen ascites and edema 1
• Sodium restriction (2-2.5 g/day) is more effective than fluid restriction alone for weight loss in cirrhosis, as fluid follows sodium 1

Critical Correction Rate Guidelines

Standard Correction Limits

The maximum correction rate is 8 mmol/L in any 24-hour period for all patients. 1, 3, 2

• Target correction rate: 4-8 mmol/L per day, not exceeding 10-12 mmol/L in 24 hours 1
• For severe symptomatic hyponatremia, correct by 6 mmol/L over first 6 hours, then limit additional correction to 2 mmol/L over remaining 18 hours 1

High-Risk Populations Requiring Slower Correction

Patients with advanced liver disease, chronic alcoholism, malnutrition, or prior encephalopathy require correction of only 4-6 mmol/L per day, maximum 8 mmol/L in 24 hours. 1

• These patients have 0.5-1.5% risk of osmotic demyelination syndrome even with careful correction 1
• Monitor for signs of osmotic demyelination (dysarthria, dysphagia, oculomotor dysfunction, quadriparesis) typically occurring 2-7 days after rapid correction 1

Special Considerations: Neurosurgical Patients

Distinguishing Cerebral Salt Wasting (CSW) from SIADH

In neurosurgical patients, cerebral salt wasting is more common than SIADH and requires opposite treatment. 1

Cerebral Salt Wasting Characteristics:

• True hypovolemia with CVP <6 cm H₂O 1
• Urine sodium >20 mmol/L despite volume depletion 1
• Clinical signs: orthostatic hypotension, tachycardia, dry mucous membranes 1

CSW Treatment:

Volume and sodium replacement with isotonic or hypertonic saline—never fluid restriction. 1

• For severe symptoms: 3% hypertonic saline plus fludrocortisone 0.1-0.2 mg daily in ICU 1
• Hydrocortisone may prevent natriuresis in subarachnoid hemorrhage patients 1
• Fluid restriction in CSW worsens outcomes and can precipitate cerebral ischemia 1

SIADH in Neurosurgical Patients:

• Euvolemic state with normal CVP 1
• Treat with fluid restriction to 1 L/day 1

Subarachnoid Hemorrhage Patients

Hyponatremia in subarachnoid hemorrhage patients at risk of vasospasm should NOT be treated with fluid restriction. 1

• Consider fludrocortisone to prevent vasospasm 1
• Volume expansion is preferred to prevent cerebral ischemia 1

Management of Overcorrection

If sodium correction exceeds 8 mmol/L in 24 hours, immediately discontinue current fluids and switch to D5W (5% dextrose in water). 1

• Consider administering desmopressin to slow or reverse the rapid rise 1
• Goal is to bring total 24-hour correction back to ≤8 mmol/L from baseline 1

Common Pitfalls to Avoid

• Overly rapid correction exceeding 8 mmol/L in 24 hours causes osmotic demyelination syndrome 1
• Inadequate monitoring during active correction 1
• Using fluid restriction in cerebral salt wasting worsens outcomes 1
• Failing to recognize and treat the underlying cause 1
• Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms 1
• Ignoring mild hyponatremia (130-135 mmol/L), which increases fall risk (21% vs 5%) and mortality (60-fold increase with sodium <130 mmol/L) 1

Monitoring During Treatment

Severe Symptoms:

• Check serum sodium every 2 hours during initial correction 1
• After symptom resolution, check every 4 hours 1

Mild Symptoms:

• Check serum sodium every 4-6 hours initially 1
• Once stable, transition to daily monitoring 1