What are the causes and symptoms of hyponatremia (low sodium levels)?

Causes and Symptoms of Hyponatremia (Low Sodium)

Hyponatremia (serum sodium <135 mEq/L) is the most common electrolyte disorder, affecting approximately 5% of adults and 35% of hospitalized patients, with significant implications for morbidity and mortality when left untreated. 1, 2

Classification by Severity

• Mild: 126-135 mEq/L - Often asymptomatic
• Moderate: 120-125 mEq/L - Nausea, headache, confusion
• Severe: <120 mEq/L - Risk of seizures, coma, respiratory arrest 1

Causes Based on Volume Status

1. Hypovolemic Hyponatremia (Low Blood Volume)

• Gastrointestinal losses: Vomiting, diarrhea 3
• Diuretic use (especially thiazides) 4
• Adrenal insufficiency 1
• Cerebral salt wasting 1
• Third-space losses: Burns, pancreatitis 3

2. Euvolemic Hyponatremia (Normal Blood Volume)

• SIADH (Syndrome of Inappropriate ADH secretion) 1
• Medications: Antidepressants, antipsychotics, anticonvulsants 2
• Hypothyroidism 1
• Excessive water intake (psychogenic polydipsia) 4
• Reset osmostat syndrome 3
• Alcohol consumption 4

3. Hypervolemic Hyponatremia (Excess Blood Volume)

• Heart failure - Reduced cardiac output activates neurohormonal systems 5
• Liver cirrhosis - Reduced effective arterial blood volume 6
• Kidney disease - Impaired water excretion 3

Symptoms

Mild Symptoms (Sodium 126-135 mEq/L)

• Often asymptomatic
• Subtle cognitive changes
• Mild headache
• Fatigue

Moderate Symptoms (Sodium 120-125 mEq/L)

• Nausea and vomiting
• Headache
• Confusion
• Weakness
• Muscle cramps
• Gait disturbances and increased fall risk 2

Severe Symptoms (Sodium <120 mEq/L)

• Delirium
• Seizures
• Coma
• Respiratory arrest
• Brain herniation (rare)
• Death 1, 4

Special Considerations

Cirrhotic Patients

• Cirrhotic patients with hyponatremia have poorer prognosis and higher risk of:
  • Refractory ascites requiring frequent paracentesis
  • Spontaneous bacterial peritonitis
  • Hepatorenal syndrome 6
• Cirrhotic patients do not usually have symptoms until sodium falls below 110 mmol/L or unless the decline is very rapid 6

Chronic vs. Acute Hyponatremia

• Acute hyponatremia (developing within 48 hours) causes more severe symptoms than chronic hyponatremia due to insufficient time for brain adaptation 7
• Chronic hyponatremia (>48 hours) is associated with cognitive impairment, gait disturbances, and increased rates of falls and fractures 2

Clinical Implications

• Hyponatremia is associated with increased hospital stay and mortality 2
• Patients with sodium levels <120 mEq/L have a mortality rate of 25% compared to 9.3% in patients with sodium >120 mEq/L 1
• Even mild chronic hyponatremia increases risk of falls and fractures and is a secondary cause of osteoporosis 2

Diagnostic Approach

1. Measure plasma osmolality:

   • High osmolality - Hyperglycemia
   • Normal osmolality - Pseudohyponatremia
   • Low osmolality - True hyponatremia 3

2. Assess volume status (hypovolemic, euvolemic, hypervolemic)

3. Measure urinary sodium:

   • High urinary sodium (>20-40 mEq/L) - Renal losses, SIADH, endocrine deficiencies
   • Low urinary sodium (<20 mEq/L) - Extrarenal losses, heart failure, cirrhosis 1, 3

Understanding the specific cause and severity of hyponatremia is essential for appropriate management and preventing complications such as osmotic demyelination syndrome from overly rapid correction.