What is the approach to evaluating and managing hyponatremia?

Hyponatremia Evaluation and Management

Initial Diagnostic Workup

Begin by confirming true hyponatremia (serum sodium <135 mmol/L) and obtain serum osmolality to exclude pseudohyponatremia from hyperglycemia or hyperlipidemia. 1, 2

Essential initial laboratory tests include:

• Serum osmolality to confirm hypotonic hyponatremia (normal: 275-290 mOsm/kg) 1
• Urine osmolality to assess water excretion capacity (<100 mOsm/kg indicates appropriate ADH suppression; >100 mOsm/kg suggests impaired water excretion) 1
• Urine sodium concentration to differentiate causes (<30 mmol/L suggests hypovolemia; >20-40 mmol/L with high urine osmolality suggests SIADH) 1, 2
• Serum and urine electrolytes, blood urea nitrogen, creatinine, glucose, thyroid-stimulating hormone to rule out other causes 1, 3
• Serum uric acid (<4 mg/dL has 73-100% positive predictive value for SIADH) 1

Volume Status Assessment

Categorize patients by extracellular fluid volume status through physical examination, though recognize this has limited accuracy (sensitivity 41.1%, specificity 80%). 1

Hypovolemic Signs:

• Orthostatic hypotension, tachycardia, dry mucous membranes, decreased skin turgor, flat neck veins 1

Euvolemic Signs:

• No edema, normal blood pressure, normal skin turgor, moist mucous membranes 1

Hypervolemic Signs:

• Peripheral edema, ascites, jugular venous distention, pulmonary congestion 1, 4

Treatment Algorithm Based on Symptom Severity

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

Immediately administer 3% hypertonic saline with a target correction of 6 mmol/L over 6 hours or until severe symptoms resolve. 1, 3, 2

• Give 100 mL boluses of 3% saline over 10 minutes, repeatable up to three times at 10-minute intervals 1
• Total correction must not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 3, 5
• Monitor serum sodium every 2 hours during initial correction 1, 3
• Consider ICU admission for close monitoring 1

Asymptomatic or Mildly Symptomatic Hyponatremia

Treatment depends on volume status:

Hypovolemic Hyponatremia:

• Administer isotonic saline (0.9% NaCl) for volume repletion 1, 3
• Initial infusion rate: 15-20 mL/kg/h, then 4-14 mL/kg/h based on response 1
• Discontinue diuretics if sodium <125 mmol/L 1, 4
• Urine sodium <30 mmol/L predicts good response to saline (positive predictive value 71-100%) 1

Euvolemic Hyponatremia (SIADH):

• Fluid restriction to 1 L/day is the cornerstone of treatment 1, 3, 2
• Add oral sodium chloride 100 mEq three times daily if no response to fluid restriction 1, 4
• Consider vasopressin receptor antagonists (tolvaptan 15 mg once daily) for resistant cases 1, 5
• Alternative options include urea, demeclocycline, or lithium for refractory cases 1, 3

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis):

• Implement fluid restriction to 1-1.5 L/day for sodium <125 mmol/L 1, 4, 3
• Temporarily discontinue diuretics if sodium <125 mmol/L 1, 4
• Consider albumin infusion in cirrhotic patients 1, 3
• Avoid hypertonic saline unless life-threatening symptoms are present, as it worsens edema and ascites 1, 3

Critical Correction Rate Guidelines

The maximum correction rate is 8 mmol/L in 24 hours for all patients to prevent osmotic demyelination syndrome. 1, 3, 5, 2

High-Risk Populations Requiring Slower Correction (4-6 mmol/L per day):

• Advanced liver disease 1, 3
• Alcoholism 1, 3
• Malnutrition 1, 3
• Prior encephalopathy 1, 3
• Severe hyponatremia (<120 mmol/L) 1

If Overcorrection Occurs:

• Immediately discontinue current fluids and switch to D5W (5% dextrose in water) 1
• Consider administering desmopressin to slow or reverse the rapid rise 1, 3

Special Considerations for Neurosurgical Patients

Distinguish between SIADH and cerebral salt wasting (CSW), as treatment approaches are fundamentally opposite. 1, 3

SIADH Characteristics:

• Euvolemic state, normal to slightly elevated central venous pressure 1
• Urine sodium >20-40 mmol/L, urine osmolality >300 mOsm/kg 1
• Treatment: Fluid restriction 1, 3

Cerebral Salt Wasting Characteristics:

• True hypovolemia with low central venous pressure (<6 cm H₂O) 1
• Urine sodium >20 mmol/L despite volume depletion 1
• Evidence of extracellular volume depletion (hypotension, tachycardia, dry mucous membranes) 1
• Treatment: Volume and sodium replacement with isotonic or hypertonic saline, NOT fluid restriction 1, 3
• Consider fludrocortisone 0.1-0.2 mg daily for severe symptoms 1, 3
• Never use fluid restriction in subarachnoid hemorrhage patients at risk of vasospasm 1, 3

Common Pitfalls to Avoid

• Overly rapid correction exceeding 8 mmol/L in 24 hours causes osmotic demyelination syndrome 1, 3
• Using fluid restriction in cerebral salt wasting worsens outcomes 1, 3
• Inadequate monitoring during active correction 1, 3
• Failing to recognize and treat the underlying cause 1, 3
• Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms 1, 3
• 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, 2

Monitoring During Treatment

• Severe symptoms: Check serum sodium every 2 hours during initial correction 1, 3
• Mild symptoms: Check serum sodium every 4-6 hours initially 1, 4
• Watch for signs of osmotic demyelination syndrome (dysarthria, dysphagia, oculomotor dysfunction, quadriparesis) typically occurring 2-7 days after rapid correction 1, 5