What is the initial step in managing a patient with hyponatremia?

Hyponatremia Workup

The initial step in managing hyponatremia is to assess volume status (hypovolemic, euvolemic, or hypervolemic) and obtain serum and urine osmolality, urine sodium, and uric acid to determine the underlying cause. 1

Initial Diagnostic Workup

The workup should include the following laboratory tests:

• Serum osmolality to confirm hypotonic hyponatremia (most common) versus hypertonic (hyperglycemia) or isotonic (pseudohyponatremia) 1, 2
• Urine osmolality and urine sodium concentration to differentiate between causes 1, 3
• Serum uric acid (levels <4 mg/dL suggest SIADH with 73-100% positive predictive value) 1
• Assessment of extracellular fluid volume status through physical examination looking for orthostatic hypotension, dry mucous membranes, skin turgor, jugular venous distention, peripheral edema, and ascites 1, 2

Volume Status Classification

Hypovolemic Hyponatremia

• Urine sodium <30 mmol/L has a positive predictive value of 71-100% for response to 0.9% saline infusion 1
• Causes include gastrointestinal losses, excessive diuretic use, severe burns 4
• Treatment: discontinue diuretics and administer isotonic saline for volume repletion 1

Euvolemic Hyponatremia (SIADH)

• Urine sodium >20-40 mmol/L with urine osmolality >300-500 mOsm/kg suggests SIADH 1, 3
• Serum uric acid <4 mg/dL supports SIADH diagnosis 1
• Requires ruling out hypothyroidism (TSH) and adrenal insufficiency (cortisol) 1
• Treatment: fluid restriction to <1 L/day as first-line 1, 2

Hypervolemic Hyponatremia

• Seen in heart failure, cirrhosis, renal disease 4
• Characterized by edema, ascites, jugular venous distention 1
• Treatment: fluid restriction to 1-1.5 L/day for sodium <125 mmol/L 1

Symptom Severity Assessment

Determine if hyponatremia is acute (<48 hours) versus chronic (>48 hours) and assess symptom severity 1, 5:

• Severe symptoms (seizures, coma, altered mental status): require immediate 3% hypertonic saline with target correction of 6 mmol/L over 6 hours 1, 5
• Mild symptoms (nausea, headache, weakness): can be managed more conservatively based on volume status 5
• Asymptomatic: treatment based on underlying cause and volume status 1

Critical Safety Consideration

Total sodium correction must not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome, particularly in high-risk patients with advanced liver disease, alcoholism, malnutrition, or severe hyponatremia 1, 6, 2. Patients with these risk factors require even more cautious correction at 4-6 mmol/L per day 1.

Common Pitfalls to Avoid

• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant—even mild hyponatremia increases fall risk (21% vs 5% in normonatremic patients) and mortality 1, 2
• Using fluid restriction in cerebral salt wasting (common in neurosurgical patients)—this worsens outcomes; CSW requires volume and sodium replacement 1
• Failing to distinguish SIADH from cerebral salt wasting in neurosurgical patients, as treatment approaches are fundamentally opposite 1
• Inadequate monitoring during active correction—check sodium every 2 hours initially for severe symptoms, every 4 hours after symptom resolution 1
• Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms—this worsens edema and ascites 1