What is the initial workup and management for a patient presenting with hyponatremia?

Hyponatremia Workup

Initial Diagnostic Evaluation

Begin by confirming true hyponatremia (serum sodium <135 mmol/L) and immediately assess symptom severity, as this determines urgency of treatment. 1

Essential First-Line Laboratory Tests

• Serum osmolality to exclude pseudohyponatremia (from hyperlipidemia/hyperproteinemia) and hypertonic hyponatremia (from hyperglycemia—add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL) 1
• Urine osmolality and urine sodium concentration to differentiate causes and assess water excretion capacity 1
• Serum creatinine, BUN, glucose to evaluate renal function and exclude hyperglycemia-induced pseudohyponatremia 1
• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism 1
• Serum uric acid (<4 mg/dL has 73-100% positive predictive value for SIADH) 1

Volume Status Assessment

Physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%), so combine clinical findings with laboratory data. 1

Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1

Euvolemic signs: normal blood pressure, no edema, no orthostasis, moist mucous membranes 1

Hypervolemic signs: peripheral edema, ascites, jugular venous distention, pulmonary congestion 1

Diagnostic Algorithm Based on Urine Studies

If Urine Osmolality <100 mOsm/kg

• Indicates appropriate ADH suppression 1
• Consider primary polydipsia or reset osmostat 1

If Urine Osmolality >100 mOsm/kg with Low Plasma Osmolality

Urine sodium <30 mmol/L (hypovolemic):

• Extrarenal losses: vomiting, diarrhea, burns, third-spacing 1
• Positive predictive value 71-100% for response to 0.9% saline 1

Urine sodium >20-40 mmol/L (euvolemic):

• SIADH: urine osmolality >300 mOsm/kg, normal thyroid/adrenal function 1
• Hypothyroidism or adrenal insufficiency (must exclude before diagnosing SIADH) 1
• Medications: SSRIs, carbamazepine, NSAIDs, opioids, chemotherapy 1

Urine sodium >20 mmol/L (hypervolemic):

• Heart failure, cirrhosis, nephrotic syndrome 1
• Compensatory natriuresis despite total body sodium excess 1

Management Framework by Volume Status

Hypovolemic Hyponatremia

Administer isotonic saline (0.9% NaCl) at 15-20 mL/kg/h initially, then 4-14 mL/kg/h based on response. 1

• Discontinue diuretics immediately if sodium <125 mmol/L 1
• Target correction: 4-8 mmol/L per day, maximum 8 mmol/L in 24 hours 1

Euvolemic Hyponatremia (SIADH)

Fluid restriction to 1 L/day is the cornerstone of treatment. 1

• If no response, add oral sodium chloride 100 mEq three times daily 1
• For persistent cases, consider vasopressin receptor antagonists (tolvaptan 15 mg once daily, titrate to 30-60 mg) 1, 2
• Critical distinction: In neurosurgical patients, cerebral salt wasting requires volume/sodium replacement, NOT fluid restriction 1

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)

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

• Temporarily discontinue diuretics if sodium <125 mmol/L 1
• Consider albumin infusion in cirrhotic patients 1
• Avoid hypertonic saline unless life-threatening symptoms present 1

Severe Symptomatic Hyponatremia (Medical Emergency)

For seizures, coma, altered mental status, or cardiorespiratory distress, immediately administer 3% hypertonic saline. 1, 3

• Target: Correct by 6 mmol/L over first 6 hours or until severe symptoms resolve 1
• Maximum correction limit: 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 3
• Monitoring: Check serum sodium every 2 hours during initial correction 1
• ICU admission required for close monitoring during treatment 1

Critical Safety Considerations

Never exceed 8 mmol/L correction in 24 hours—overly rapid correction causes osmotic demyelination syndrome (dysarthria, dysphagia, quadriparesis, seizures, coma, death). 1, 2

High-risk patients require even slower correction (4-6 mmol/L per day): 1

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

Common Pitfalls to Avoid

• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant—associated with increased falls (21% vs 5%), fractures, and 60-fold increased mortality when <130 mmol/L 1, 3
• Using fluid restriction in cerebral salt wasting—worsens outcomes; requires volume replacement 1
• Administering normal saline for SIADH—worsens hyponatremia; fluid restriction is correct treatment 1
• Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms—worsens edema and ascites 1
• Inadequate monitoring during active correction—risks overcorrection and osmotic demyelination 1