What is hyponatremia and how is it managed?

Hyponatremia: Definition and Management

Definition and Classification

Hyponatremia is defined as a serum sodium concentration below 135 mEq/L and represents the most common electrolyte disorder in clinical practice, affecting approximately 5% of adults and 35% of hospitalized patients 1, 2.

The condition is classified by severity 1, 3:

• Mild: 130-135 mEq/L (or 126-135 mEq/L by some definitions)
• Moderate: 120-129 mEq/L (or 125-129 mEq/L)
• Severe: <120 mEq/L (or <125 mEq/L)

Hyponatremia primarily results from water retention rather than sodium depletion, with only two general mechanisms: defective water excretion (usually from elevated vasopressin) or excessive fluid intake 2, 4.

Clinical Significance and Symptoms

Even mild hyponatremia carries significant morbidity 1, 2:

• Mortality risk: Sodium <130 mmol/L is associated with a 60-fold increase in hospital mortality (11.2% vs 0.19%) 1
• Fall risk: 21% of hyponatremic patients experience falls compared to 5% of normonatremic patients 1
• Cognitive effects: Mild chronic hyponatremia causes cognitive impairment, gait disturbances, and increased fracture rates 2

Symptom severity depends on rapidity of onset, duration, and severity 2, 3:

• Mild symptoms: Nausea, vomiting, weakness, headache, mild neurocognitive deficits 3
• Severe symptoms: Seizures, coma, delirium, confusion, impaired consciousness, ataxia, brain herniation 2, 3

Diagnostic Approach

Initial Workup

When serum sodium is <135 mmol/L (and particularly <131 mmol/L), obtain 1:

• Serum and urine osmolality
• Urine sodium and electrolytes
• Serum uric acid (levels <4 mg/dL suggest SIADH with 73-100% positive predictive value) 1
• Assessment of extracellular fluid volume status 1

Volume Status Classification

The cornerstone of diagnosis is determining volume status 1, 3, 5:

Hypovolemic hyponatremia (ECF contraction) 1:

• Clinical signs: Orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins
• Urine sodium <30 mmol/L suggests extrarenal losses (GI losses, burns, dehydration)
• Urine sodium >20 mmol/L suggests renal losses (diuretics, salt-wasting nephropathy)

Euvolemic hyponatremia (normal ECF) 1:

• No edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes
• Most commonly SIADH
• Urine sodium >20-40 mmol/L with urine osmolality >300 mOsm/kg supports SIADH

Hypervolemic hyponatremia (ECF expansion) 1, 5:

• Clinical signs: Peripheral edema, ascites, jugular venous distention, pulmonary congestion
• Causes: Heart failure, cirrhosis, renal disease

Critical caveat: Physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%) for volume assessment 1.

Management Based on Symptom Severity

Severe Symptomatic Hyponatremia (Medical Emergency)

For patients with seizures, coma, or severe neurological symptoms, immediately administer 3% hypertonic saline 1, 2, 3:

• Target: Increase sodium by 6 mmol/L over 6 hours or until symptoms resolve 1
• Bolus dosing: 100 mL of 3% saline over 10 minutes, can repeat up to 3 times at 10-minute intervals 1
• Maximum correction: Never exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 2
• Monitoring: Check serum sodium every 2 hours during initial correction 1

Asymptomatic or Mildly Symptomatic Hyponatremia

Treatment is based on volume status 1, 3, 5:

Hypovolemic hyponatremia 1:

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

Euvolemic hyponatremia (SIADH) 1, 2:

• First-line: Fluid restriction to 1 L/day
• If no response: Add oral sodium chloride 100 mEq three times daily
• Alternative options: Urea, demeclocycline, lithium, loop diuretics (for resistant cases)
• Vaptans (tolvaptan 15 mg once daily, titrate to 30-60 mg): Consider for persistent hyponatremia despite fluid restriction 1, 2

Hypervolemic hyponatremia (heart failure, cirrhosis) 1, 6:

• Fluid restriction: 1-1.5 L/day for sodium <125 mmol/L
• Discontinue diuretics temporarily if sodium <125 mmol/L
• For cirrhosis: Consider albumin infusion alongside fluid restriction
• Avoid hypertonic saline unless life-threatening symptoms present (worsens ascites and edema)

Critical Correction Rate Guidelines

The single most important safety principle: Never exceed 8 mmol/L correction in 24 hours 1, 2, 6:

Standard correction rates 1:

• Average risk patients: 4-8 mmol/L per day, maximum 10-12 mmol/L in 24 hours
• High-risk patients: 4-6 mmol/L per day, maximum 8 mmol/L in 24 hours

High-risk populations requiring slower correction (4-6 mmol/L per day) 1, 6:

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

Rationale: Overly rapid correction causes osmotic demyelination syndrome, a devastating neurological complication with parkinsonism, quadriparesis, or death 2, 4.

Special Populations and Considerations

Neurosurgical Patients

Critical distinction: SIADH vs. Cerebral Salt Wasting (CSW) 1:

SIADH characteristics 1:

• Euvolemic state
• Urine sodium >20-40 mmol/L
• Urine osmolality >300 mOsm/kg
• Treatment: Fluid restriction to 1 L/day

Cerebral Salt Wasting characteristics 1:

• True hypovolemia (CVP <6 cm H₂O)
• Urine sodium >20 mmol/L despite volume depletion
• Evidence of extracellular volume depletion
• Treatment: Volume and sodium replacement with isotonic or hypertonic saline
• For severe symptoms: 3% hypertonic saline plus fludrocortisone (0.1-0.2 mg daily)
• Never use fluid restriction in CSW—this worsens outcomes 1

For subarachnoid hemorrhage patients at risk of vasospasm 1:

• Never use fluid restriction
• Consider fludrocortisone to prevent vasospasm
• Hydrocortisone may prevent natriuresis

Cirrhotic Patients

Hyponatremia in cirrhosis increases risk of 1, 6:

• Spontaneous bacterial peritonitis (OR 3.40)
• Hepatorenal syndrome (OR 3.45)
• Hepatic encephalopathy (OR 2.36)

Management principles 1, 6:

• Fluid restriction to 1-1.5 L/day for sodium <125 mmol/L
• Sodium restriction (not fluid restriction) results in weight loss as fluid follows sodium
• Albumin infusion alongside fluid restriction
• Correction rate: 4-6 mmol/L per day maximum
• Tolvaptan carries higher risk of GI bleeding in cirrhosis (10% vs 2% placebo)

Management of Overcorrection

If sodium correction exceeds 8 mmol/L in 24 hours 1:

• Immediately discontinue current fluids
• Switch to D5W (5% dextrose in water)
• Consider administering desmopressin to slow or reverse the rapid rise
• Target: Bring total 24-hour correction to no more than 8 mmol/L from starting point

Common Pitfalls to Avoid

Critical errors that worsen outcomes 1, 4:

• Overly rapid correction exceeding 8 mmol/L in 24 hours (causes osmotic demyelination syndrome)
• Using fluid restriction in cerebral salt wasting (worsens outcomes)
• Inadequate monitoring during active correction
• Failing to recognize and treat the underlying cause
• Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms
• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant
• Stopping diuretics prematurely in volume-overloaded heart failure patients due to mild hyponatremia