Overview

Hyponatremia occurs when the sodium concentration in blood falls below normal levels, disrupting the delicate balance of water and electrolytes in the body. Sodium is crucial for maintaining blood pressure, supporting nerve and muscle function, and regulating fluid balance. When sodium levels drop, water moves into cells, causing them to swell. This cellular swelling is particularly dangerous in the brain, where it can lead to increased intracranial pressure and severe neurological complications.

The condition affects millions of people worldwide and is particularly common in hospitalized patients, where it occurs in up to 30% of cases. Hyponatremia can develop gradually over days or weeks (chronic) or rapidly within 48 hours (acute). The speed of onset significantly influences both the severity of symptoms and the approach to treatment. Acute hyponatremia is more likely to cause severe symptoms due to the brain's limited time to adapt to the osmotic changes.

The complexity of hyponatremia lies in its multiple potential causes and the careful balance required in treatment. Correcting sodium levels too rapidly can lead to osmotic demyelination syndrome, a serious neurological condition. Therefore, understanding the underlying cause, assessing the patient's volume status, and determining the chronicity of the condition are essential for appropriate management.

Symptoms

The symptoms of hyponatremia vary widely depending on the severity of sodium depletion, the rate of decline, and individual patient factors. Many patients with mild chronic hyponatremia may be asymptomatic.

Common Symptoms

Mild Hyponatremia (130-134 mEq/L)

  • Often asymptomatic
  • Mild cognitive impairment
  • Subtle gait disturbances
  • Increased risk of falls
  • Attention deficits

Moderate Hyponatremia (125-129 mEq/L)

  • Headache
  • Confusion
  • Muscle cramps
  • Malaise
  • Anorexia
  • Restlessness

Severe Hyponatremia (<125 mEq/L)

  • Altered mental status
  • Hallucinations
  • Decreased consciousness
  • Coma
  • Respiratory arrest
  • Brain herniation
  • Death

Causes

Hyponatremia can result from various mechanisms that either increase water retention, cause sodium loss, or both. Understanding the underlying cause is crucial for appropriate treatment.

Excessive Water Retention

  • SIADH (Syndrome of Inappropriate Antidiuretic Hormone) - Most common cause in hospitalized patients
  • Psychogenic polydipsia - Compulsive water drinking
  • Exercise-associated hyponatremia - Excessive water intake during endurance events
  • Iatrogenic - Excessive hypotonic fluid administration
  • Reset osmostat - Altered sodium setpoint

Sodium Loss

  • Gastrointestinal losses - Vomiting, diarrhea, nasogastric suction
  • Renal losses - Diuretic use, salt-wasting nephropathy
  • Skin losses - Excessive sweating, burns
  • Cerebral salt wasting - Following brain injury
  • Third spacing - Pancreatitis, bowel obstruction

Medical Conditions

  • Heart failure - Reduced cardiac output triggers water retention
  • Cirrhosis - Portal hypertension and decreased effective circulation
  • Kidney disease - Impaired water excretion
  • Hypothyroidism - Decreased cardiac output and GFR
  • Adrenal insufficiency - Cortisol deficiency

Medications

  • Diuretics - Especially thiazides
  • Antidepressants - SSRIs, tricyclics, MAOIs
  • Antipsychotics - Phenothiazines, butyrophenones
  • Anticonvulsants - Carbamazepine, oxcarbazepine
  • NSAIDs - Enhance ADH effect
  • Proton pump inhibitors - May cause hyponatremia

Risk Factors

Several factors increase the risk of developing hyponatremia. Identifying high-risk individuals allows for preventive measures and closer monitoring.

Age-Related Factors

  • Advanced age (>65 years) - Decreased renal function and thirst response
  • Elderly in care facilities - Polypharmacy and comorbidities
  • Postmenopausal women - Increased sensitivity to thiazides
  • Infants - Immature kidneys, inappropriate formula dilution

Medical Conditions

  • Heart failure - Neurohormonal activation
  • Liver cirrhosis - Splanchnic vasodilation
  • Chronic kidney disease - Impaired water excretion
  • Cancer - Particularly small cell lung cancer (SIADH)
  • CNS disorders - Head trauma, infections, tumors
  • Pulmonary diseases - Pneumonia, tuberculosis

Lifestyle and Environmental Factors

  • Endurance athletes - Marathon runners, triathletes
  • Hot climate exposure - Excessive sweating
  • Low sodium diet - Combined with high water intake
  • Alcohol abuse - Beer potomania
  • Recreational drug use - MDMA (ecstasy)

Iatrogenic Factors

  • Multiple medications - Drug interactions
  • Postoperative state - Pain, nausea, stress response
  • Hypotonic fluid administration - D5W, 0.45% saline
  • Colonoscopy preparation - Excessive water intake

Diagnosis

Diagnosing hyponatremia requires confirming low serum sodium and determining the underlying cause through systematic evaluation of volume status, osmolality, and urine studies.

Initial Laboratory Tests

  • Serum sodium - <135 mEq/L confirms hyponatremia
  • Serum osmolality - Differentiates true from pseudohyponatremia
  • Urine osmolality - Assesses kidney's concentrating ability
  • Urine sodium - Helps determine etiology
  • Basic metabolic panel - Glucose, BUN, creatinine

Volume Status Assessment

Hypovolemic:

  • Orthostatic hypotension
  • Tachycardia
  • Dry mucous membranes
  • Poor skin turgor
  • Decreased urine output

Euvolemic:

  • Normal blood pressure
  • No edema
  • Moist mucous membranes
  • Normal skin turgor

Hypervolemic:

  • Edema
  • Ascites
  • Jugular venous distention
  • Pulmonary crackles

Additional Testing

  • Thyroid function tests - TSH, free T4
  • Cortisol level - Morning cortisol or ACTH stimulation test
  • Chest X-ray - Pulmonary pathology
  • CT/MRI brain - If CNS cause suspected
  • Liver function tests - If cirrhosis suspected

Diagnostic Algorithm

  1. Confirm true hyponatremia (normal osmolality)
  2. Assess volume status clinically
  3. Check urine osmolality and sodium
  4. Consider underlying causes based on findings
  5. Order specific tests for suspected etiology

Treatment Options

Treatment of hyponatremia must be individualized based on severity, chronicity, symptoms, and underlying cause. The rate of correction is critical to avoid osmotic demyelination syndrome.

General Principles

  • Correction rate - Maximum 10-12 mEq/L in 24 hours, 18 mEq/L in 48 hours
  • High-risk patients - Limit to 6-8 mEq/L in 24 hours
  • Frequent monitoring - Check sodium every 2-4 hours initially
  • Treat underlying cause - Essential for long-term management

Acute Symptomatic Hyponatremia

  • 3% hypertonic saline - 100-150 mL bolus over 20 minutes
  • Repeat up to 3 times - If symptoms persist
  • Goal - Increase sodium by 4-6 mEq/L
  • ICU monitoring - Neurological checks
  • Treat seizures - Benzodiazepines plus hypertonic saline

Chronic Asymptomatic Hyponatremia

Hypovolemic:

  • Isotonic saline (0.9% NaCl)
  • Treat underlying volume loss
  • Monitor for overcorrection

Euvolemic (SIADH):

  • Fluid restriction (500-1000 mL/day)
  • Salt tablets
  • Loop diuretics with salt supplementation
  • Vasopressin receptor antagonists (tolvaptan)
  • Demeclocycline (in refractory cases)

Hypervolemic:

  • Fluid and sodium restriction
  • Loop diuretics
  • Treat underlying condition (heart failure, cirrhosis)
  • Consider dialysis in renal failure

Prevention of Overcorrection

  • DDAVP (desmopressin) - If correction too rapid
  • D5W infusion - To lower sodium if needed
  • Frequent monitoring - Every 2-4 hours
  • Calculate correction rate - Use formulas to guide therapy

Long-term Management

  • Address underlying cause
  • Medication review and adjustment
  • Patient education on fluid intake
  • Regular monitoring
  • Dietary sodium counseling

Prevention

Preventing hyponatremia involves identifying at-risk individuals, modifying risk factors, and implementing appropriate monitoring strategies.

General Prevention Strategies

  • Medication review - Identify and minimize high-risk drugs
  • Patient education - Appropriate fluid intake guidelines
  • Regular monitoring - For high-risk patients
  • Treat underlying conditions - Optimize management
  • Avoid overhydration - Especially during exercise

Exercise-Related Prevention

  • Drink to thirst, not on schedule
  • Use sports drinks for prolonged exercise
  • Know your sweat rate
  • Avoid excessive water preloading
  • Recognize early symptoms

Hospital-Based Prevention

  • Use isotonic fluids when possible
  • Monitor high-risk patients closely
  • Avoid hypotonic maintenance fluids
  • Daily electrolyte checks for at-risk patients
  • Careful postoperative fluid management

Medication Management

  • Start thiazides at low doses
  • Check sodium within 1-2 weeks of starting high-risk drugs
  • Consider alternatives in elderly patients
  • Educate about symptoms to report
  • Avoid drug combinations that increase risk

When to See a Doctor

Hyponatremia can range from mild to life-threatening. Knowing when to seek medical attention is crucial for preventing serious complications.

Seek Emergency Care For:

  • Seizures or convulsions
  • Severe confusion or altered mental status
  • Loss of consciousness
  • Severe headache with vomiting
  • Difficulty breathing
  • Chest pain or pressure
  • Extreme weakness or inability to move

See a Doctor Promptly For:

  • Persistent nausea and vomiting
  • Confusion or memory problems
  • Muscle cramps or spasms
  • Unusual fatigue or weakness
  • Headaches that don't improve
  • Swelling in legs or abdomen
  • Decreased urine output

Schedule an Appointment If:

  • Taking medications known to cause hyponatremia
  • Have conditions that increase risk
  • Experience mild but persistent symptoms
  • Need monitoring after previous hyponatremia
  • Planning endurance exercise events

Related Conditions

Several conditions are associated with or may mimic hyponatremia:

References

  1. Spasovski G, et al. (2023). Clinical practice guideline on diagnosis and treatment of hyponatraemia. European Journal of Endocrinology.
  2. Verbalis JG, et al. (2022). Diagnosis, Evaluation, and Treatment of Hyponatremia: Expert Panel Recommendations. Am J Med.
  3. Hoorn EJ, Zietse R. (2023). Diagnosis and Treatment of Hyponatremia: Compilation of the Guidelines. J Am Soc Nephrol.
  4. Seay NW, et al. (2022). Diagnosis and Management of Disorders of Body Tonicity. N Engl J Med.
  5. Sterns RH. (2023). Treatment of Hyponatremia. UpToDate.