Overview

Hyponatremia is a common electrolyte imbalance characterized by abnormally low levels of sodium in the blood, typically defined as a serum sodium concentration below 135 mEq/L. Sodium plays a crucial role in regulating fluid balance, nerve function, and muscle contractions. When sodium levels drop too low, it can cause cellular dysfunction, particularly affecting the brain. Hyponatremia can range from mild and asymptomatic to severe and life-threatening, depending on the speed of onset and the degree of sodium depletion.

Causes

Hyponatremia occurs due to either excessive loss of sodium, excessive water intake, or impaired water excretion. Common causes include:

• Excessive Water Intake: Drinking excessive amounts of water, especially during endurance sports, can dilute sodium levels (exercise-associated hyponatremia).
• Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): Increased antidiuretic hormone (ADH) leads to water retention and dilutional hyponatremia.
• Heart Failure: Reduced cardiac output triggers hormonal responses that cause water retention and low sodium levels.
• Liver Cirrhosis: Advanced liver disease can lead to fluid overload and dilutional hyponatremia.
• Kidney Disorders: Conditions like nephrotic syndrome impair the kidney's ability to regulate sodium and water balance.
• Diuretics: Thiazide diuretics especially promote sodium loss through urine.
• Adrenal Insufficiency: Addison’s disease or adrenal crisis can lead to sodium loss and water retention.
• Vomiting and Diarrhea: Gastrointestinal losses of sodium and fluids can cause hyponatremia, especially if fluids are replaced without electrolytes.
• Hypothyroidism: Low thyroid hormone levels can contribute to low sodium levels.

Symptoms

The symptoms of hyponatremia depend on how quickly sodium levels fall and the severity of the deficiency. Symptoms range from mild to severe:

• Mild Symptoms: Nausea, vomiting, headache, and fatigue.
• Moderate Symptoms: Muscle cramps, weakness, confusion, difficulty concentrating, and balance problems.
• Severe Symptoms: Seizures, decreased consciousness, coma, and in extreme cases, death due to cerebral edema (brain swelling).
• Chronic Hyponatremia: Slowly developing hyponatremia may cause subtle symptoms like forgetfulness, lethargy, or gait disturbances, particularly in the elderly.

Diagnosis

Hyponatremia is diagnosed through a combination of clinical assessment and laboratory testing:

• Blood Tests: Measurement of serum sodium concentration to confirm low sodium levels (less than 135 mEq/L).
• Serum Osmolality: To differentiate between hypotonic, isotonic, and hypertonic hyponatremia.
• Urine Sodium and Osmolality: Helps determine if the kidneys are appropriately excreting sodium and water.
• Physical Examination: Assess for signs of fluid overload (edema), dehydration, or neurological changes.
• Additional Tests: Thyroid function tests, adrenal function tests, and kidney function tests to identify underlying causes.

Treatment

Treatment of hyponatremia depends on its severity, underlying cause, and the rate at which it developed:

General Measures

• Fluid Restriction: Limiting fluid intake is often the first step in treating dilutional hyponatremia.
• Salt Tablets or Sodium Supplementation: Used in cases of sodium depletion to restore balance.

Acute or Severe Hyponatremia

• Hypertonic Saline (3% NaCl): Carefully administered in critical cases to rapidly correct sodium levels while avoiding overcorrection.
• Frequent Monitoring: Close monitoring of sodium levels to prevent complications like osmotic demyelination syndrome (ODS).

Treating Underlying Causes

• Managing Heart, Liver, or Kidney Disease: Addressing these conditions helps control fluid balance.
• Discontinuation of Offending Medications: Stopping diuretics or drugs associated with SIADH when appropriate.
• Hormonal Replacement: Treating adrenal insufficiency or hypothyroidism if diagnosed.

Medications

• Vaptans (Vasopressin Antagonists): Tolvaptan or conivaptan can promote free water excretion in cases of SIADH or heart failure-related hyponatremia.

Prognosis

The prognosis of hyponatremia depends on the underlying cause, severity, and how quickly it is treated. Mild hyponatremia generally has a good outcome with appropriate treatment. Chronic hyponatremia can lead to subtle but significant cognitive and physical impairments if untreated, especially in older adults.

Severe or rapidly developing hyponatremia can be life-threatening due to the risk of brain swelling and requires urgent medical care. With prompt identification and appropriate correction, most patients recover fully. However, improper correction—particularly overly rapid correction—can cause serious neurological damage such as osmotic demyelination syndrome. Close medical monitoring is essential for safe and effective treatment.