How do you differentiate and manage hyponatremia?

Differentiating Hyponatremia: A Systematic Approach

Initial Assessment and Classification

Begin by confirming true hyponatremia (serum sodium <135 mEq/L) and immediately assess volume status through physical examination, as this determines the underlying mechanism and guides treatment. 1

Step 1: Confirm True Hyponatremia

• Measure serum osmolality to rule out pseudohyponatremia or hypertonic hyponatremia 1, 2
• Normal serum osmolality (275-290 mOsm/kg) suggests pseudohyponatremia from laboratory error, hyperglycemia, or hypertriglyceridemia 2
• High plasma osmolality indicates hyperglycemia-induced hyponatremia 3
• Proceed with workup only if plasma osmolality is low (<275 mOsm/kg) 2

Step 2: Determine Volume Status

Physical examination is the starting point, though it has limited accuracy (sensitivity 41.1%, specificity 80%) 2

Hypovolemic Signs:

• Orthostatic hypotension, dry mucous membranes, decreased skin turgor 2
• Confusion, non-fluent speech, extremity weakness, furrowed tongue, sunken eyes 1
• Postural pulse change or severe postural dizziness preventing standing 1

Euvolemic Signs:

• No edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes 1

Hypervolemic Signs:

• Jugular venous distention, peripheral edema, ascites, orthopnea, dyspnea 1, 2

Step 3: Measure Urine Studies

Obtain urine sodium and urine osmolality simultaneously with serum studies 1, 2

Diagnostic Algorithm Based on Volume Status and Urine Studies

Hypovolemic Hyponatremia

Urine Sodium <30 mEq/L:

• Extrarenal losses: Gastrointestinal losses (vomiting, diarrhea), burns, severe dehydration 1, 2
• Positive predictive value of 71-100% for response to 0.9% saline 1, 2

Urine Sodium >20 mEq/L:

• Renal losses: Diuretic use, cerebral salt wasting (CSW), adrenal insufficiency, salt-losing nephropathy 2
• In neurosurgical patients, CSW is more common than SIADH 1, 2

Key distinguishing feature for CSW: Evidence of volume depletion (hypotension, tachycardia, dry mucous membranes) with inappropriately high urinary sodium (>20 mmol/L) and high urine osmolality relative to serum 1

Euvolemic Hyponatremia

Urine Osmolality >100 mOsm/kg AND Urine Sodium >40 mEq/L:

• Strongly suggests SIADH 2
• Serum uric acid <4 mg/dL has positive predictive value of 73-100% for SIADH 1, 2
• Must rule out: Hypothyroidism (check TSH), adrenal insufficiency (check cortisol), medications 1, 2

SIADH diagnostic criteria:

• Hypotonic hyponatremia (serum sodium <134 mEq/L) 2
• Plasma osmolality <275 mOsm/kg 2
• Inappropriately elevated urine osmolality (>500 mOsm/kg) 2
• Elevated urine sodium (>20-40 mEq/L) 2
• Euvolemia on examination 2
• Normal thyroid, adrenal, and renal function 1

Urine Osmolality <100 mOsm/kg:

• Primary polydipsia (excessive water intake) 2

Hypervolemic Hyponatremia

Characterized by volume overload with dilutional hyponatremia 1

Common Causes:

• Heart failure: Jugular venous distention, orthopnea, dyspnea, peripheral edema 1
• Cirrhosis: Ascites, portal hypertension, occurs in ~60% of cirrhotic patients 1
• Advanced renal failure with elevated urine sodium (>20 mEq/L) 2

Pathophysiology: Non-osmotic hypersecretion of vasopressin, enhanced proximal nephron sodium reabsorption, impaired free water clearance 1

Critical Distinguishing Features: SIADH vs. CSW

This distinction is crucial in neurosurgical patients as treatment approaches are opposite 1, 2

SIADH:

• Euvolemic on examination 2
• Central venous pressure (CVP) 6-10 cm H₂O 2
• Treatment: Fluid restriction 1

Cerebral Salt Wasting:

• Hypovolemic on examination 1
• CVP <6 cm H₂O 2
• More common with poor clinical grade, ruptured anterior communicating artery aneurysms, hydrocephalus 1
• Treatment: Volume and sodium replacement, NOT fluid restriction 1

Additional Diagnostic Tests

Complete the workup with these studies 1:

• Complete blood count, urinalysis 1
• Serum electrolytes, BUN, creatinine, glucose 1
• Liver function tests (if cirrhosis suspected) 1
• TSH (rule out hypothyroidism) 1, 2
• Morning cortisol (rule out adrenal insufficiency) 1
• Lipid profile 1

Do NOT routinely obtain: ADH levels or natriuretic peptide levels (not supported by evidence, class III) 2

Common Diagnostic Pitfalls to Avoid

• Ignoring mild hyponatremia (130-135 mEq/L): Associated with increased falls (21% vs 5%), fractures, and mortality 1, 4
• Relying solely on physical examination for volume status: Sensitivity only 41.1%, specificity 80% 2
• Misdiagnosing SIADH in neurosurgical patients: CSW is more common and requires opposite treatment 1, 2
• Using fluid restriction in CSW: Worsens outcomes and increases cerebral ischemia risk 1
• Failing to check urine studies before initiating treatment: Essential for accurate diagnosis 1, 2