What is the appropriate treatment for a patient with a serum sodium concentration of 131 mmol/L?

Treatment of Serum Sodium 131 mmol/L

For a patient with serum sodium of 131 mmol/L, treatment should be guided by volume status assessment and symptom severity, with most patients requiring investigation and management even in the absence of severe symptoms. 1

Initial Assessment

Hyponatremia at 131 mmol/L warrants a complete diagnostic workup including serum and urine osmolality, urine sodium concentration, urine electrolytes, serum uric acid, and careful assessment of extracellular fluid volume status through physical examination. 1 This threshold is clinically significant because even mild hyponatremia increases fall risk (21% vs 5% in normonatremic patients) and is associated with a 60-fold increase in mortality when sodium drops below 130 mmol/L. 1, 2

Physical examination should specifically assess for:

• Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1
• Hypervolemic signs: peripheral edema, ascites, jugular venous distention, pulmonary congestion 1
• Euvolemic state: absence of both volume depletion and overload signs 1

Treatment Based on Volume Status

Hypovolemic Hyponatremia

Discontinue diuretics immediately and administer isotonic saline (0.9% NaCl) for volume repletion. 1 Initial infusion rates should be 15-20 mL/kg/h, then 4-14 mL/kg/h based on clinical response. 1 A urine sodium <30 mmol/L predicts good response to saline infusion with 71-100% positive predictive value. 1

Euvolemic Hyponatremia (SIADH)

Implement fluid restriction to 1000 mL/day as first-line therapy. 1 If sodium fails to improve after 24-48 hours of fluid restriction, add oral sodium chloride 100 mEq three times daily. 1 For resistant cases, consider vasopressin receptor antagonists such as tolvaptan starting at 15 mg once daily, though this should be reserved for persistent symptomatic cases. 1, 3

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)

Implement fluid restriction to 1000-1500 mL/day and temporarily discontinue diuretics if sodium remains <125 mmol/L. 1 For cirrhotic patients specifically, consider albumin infusion (6-8 g per liter of ascites drained) alongside fluid restriction. 1 Avoid hypertonic saline unless life-threatening symptoms develop, as it may worsen ascites and edema. 1

Critical Correction Rate Guidelines

The maximum correction rate must not exceed 8 mmol/L in any 24-hour period to prevent osmotic demyelination syndrome. 1, 3, 2 For high-risk patients—including those with advanced liver disease, chronic alcoholism, malnutrition, or prior hepatic encephalopathy—use even more conservative correction rates of 4-6 mmol/L per day. 1

During active correction, monitor serum sodium:

• Every 2 hours for severe symptomatic patients 1
• Every 4 hours after symptom resolution 1
• Every 24 hours for asymptomatic or mildly symptomatic patients 1

Special Considerations for Neurosurgical Patients

In patients with CNS pathology, distinguish between SIADH and cerebral salt wasting (CSW), as they require opposite treatments. 1, 4 CSW is characterized by true hypovolemia (CVP <6 cm H₂O, orthostatic hypotension, tachycardia) with inappropriately high urinary sodium (>20 mmol/L), whereas SIADH presents with euvolemia. 4

For CSW, never use fluid restriction—this worsens outcomes and increases cerebral ischemia risk. 1, 4 Instead, provide aggressive volume and sodium replacement with isotonic or hypertonic saline at 60-100 mL/h, and consider fludrocortisone 0.1-0.2 mg daily for severe cases. 4

When to Use Hypertonic Saline

Reserve 3% hypertonic saline exclusively for severe symptomatic hyponatremia with neurological manifestations such as seizures, altered mental status, or coma. 1, 5 The goal is to increase sodium by 6 mmol/L over 6 hours or until symptoms resolve, but still respecting the 8 mmol/L/24-hour maximum. 1

Patients requiring urgent correction to prevent serious neurological symptoms should NOT be treated with tolvaptan, as it has not been established to provide symptomatic benefit and carries risk of overly rapid correction. 3

Common Pitfalls to Avoid

• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant, when it actually increases fall risk and mortality 1, 2
• Using fluid restriction in cerebral salt wasting, which can precipitate cerebral ischemia 1, 4
• Correcting chronic hyponatremia faster than 8 mmol/L in 24 hours, risking osmotic demyelination syndrome with devastating neurological sequelae (dysarthria, dysphagia, quadriparesis) 1, 3
• Failing to assess volume status accurately, leading to inappropriate treatment (physical exam alone has only 41% sensitivity and 80% specificity) 1
• Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms, which worsens fluid overload 1