What causes hyponatremia in patients with head injury and how is it corrected?

Causes and Correction of Hyponatremia in Head Injury

Hyponatremia in traumatic brain injury should be managed based on the underlying mechanism, with careful correction to avoid osmotic demyelination syndrome, targeting a maximum correction rate of 8 mEq/L in 24 hours. 1

Causes of Hyponatremia in Head Injury

Hyponatremia (serum sodium <135 mEq/L) in traumatic brain injury (TBI) can occur through several mechanisms:

1. Syndrome of Inappropriate Antidiuretic Hormone (SIADH):

   • Most common cause in TBI patients
   • Characterized by inappropriate ADH secretion leading to water retention
   • Results in euvolemic hyponatremia with inappropriately elevated urine osmolality and urine sodium

2. Cerebral Salt Wasting (CSW):

   • Direct natriuresis from brain injury
   • Results in hypovolemic hyponatremia with elevated urine sodium
   • Distinguished from SIADH by volume status assessment

3. Iatrogenic Causes:

   • Excessive administration of hypotonic fluids
   • Medications that stimulate ADH release or potentiate its effects

Diagnostic Approach

Assessment should focus on:

1. Volume status evaluation:

   • Hypovolemic: suggests CSW
   • Euvolemic: suggests SIADH
   • Hypervolemic: suggests excessive fluid administration

2. Laboratory tests:

   • Serum sodium, osmolality
   • Urine sodium and osmolality
   • Assessment of volume status

Management of Hyponatremia in TBI (Sodium 120 mEq/L)

General Principles:

• Monitor serum sodium levels every 4-6 hours during active correction 1
• Avoid overly rapid correction to prevent osmotic demyelination syndrome 1
• Maximum correction rate: 8 mEq/L in 24 hours 1

Specific Management Based on Etiology:

1. For SIADH (Euvolemic Hyponatremia):

   • Fluid restriction (<1000 mL/day) 1
   • If symptomatic or severe (Na <120 mEq/L):
     • Hypertonic 3% saline with careful monitoring 1
     • Initial bolus of 2 mL/kg if severely symptomatic 1
     • Target correction of 4-6 mEq/L in first hour for severe symptoms 1
   • Consider tolvaptan for persistent hyponatremia despite fluid restriction 2
     • Must be initiated in hospital setting
     • Starting dose: 15 mg once daily
     • May increase to 30 mg after 24 hours if needed
     • Maximum dose: 60 mg daily
     • Do not use for more than 30 days due to risk of liver injury

2. For CSW (Hypovolemic Hyponatremia):

   • Volume replacement with isotonic saline 1
   • Salt supplementation may be needed
   • Treat underlying cause

3. For Iatrogenic Hyponatremia:

   • Discontinue hypotonic fluids
   • Restrict free water intake

Important Cautions

1. Risk of Osmotic Demyelination:

   • Patients with TBI, especially those with alcoholism, malnutrition, or advanced liver disease are at higher risk 1
   • Symptoms include dysarthria, mutism, dysphagia, lethargy, affective changes, spastic quadriparesis, seizures, coma, and death 2

2. Tolvaptan Considerations:

   • Contraindicated in hypovolemic hyponatremia 2
   • Must be initiated in hospital setting with close monitoring 2
   • Do not use with strong CYP3A inhibitors 2
   • Limited to 30 days of use due to risk of liver injury 2

3. Hypertonic Saline Administration:

   • Should be administered via central line when possible
   • Requires close monitoring of serum sodium levels
   • Discontinue when target sodium level is reached or symptoms improve

Monitoring During Correction

• Check serum sodium every 4-6 hours during active correction 1
• Monitor neurological status closely
• If correction is too rapid (>8 mEq/L in 24 hours), consider administering hypotonic fluids or desmopressin to slow correction 1

For your patient with sodium of 120 mEq/L, the approach should focus on identifying the underlying mechanism while initiating appropriate therapy based on volume status and symptom severity, with careful monitoring to avoid overly rapid correction.