What are the different types of hyponatremia?

Types of Hyponatremia

Hyponatremia is classified primarily by volume status into hypovolemic, euvolemic, and hypervolemic types, with each category requiring different management approaches based on the underlying pathophysiology. 1

Classification by Severity

Hyponatremia is stratified by serum sodium levels:

• Mild: 130-135 mEq/L
• Moderate: 125-129 mEq/L
• Severe: <125 mEq/L 1

Classification by Volume Status

1. Hypovolemic Hyponatremia

This occurs when both sodium and water are lost, but sodium loss exceeds water loss.

Clinical features:

• Signs of dehydration
• Orthostatic hypotension
• Tachycardia
• Decreased skin turgor
• Dry mucous membranes 1

Common causes:

• Gastrointestinal losses (vomiting, diarrhea)
• Excessive sweating
• Diuretic use
• Renal salt wasting
• Third-space losses (burns, pancreatitis)

Laboratory findings:

• Urinary sodium <20 mEq/L (if extrarenal losses)
• Urinary sodium >20 mEq/L (if renal losses)
• High urine osmolality (>500 mosm/kg)

Management: Isotonic fluid resuscitation with 0.9% saline is the first-line treatment 1

2. Euvolemic Hyponatremia

This occurs with normal total body sodium but excess total body water.

Clinical features:

• No edema
• No signs of volume depletion
• May have neurological symptoms depending on severity and acuity

Common causes:

• Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH)
• Hypothyroidism
• Adrenal insufficiency
• Reset osmostat syndrome
• Medication-induced

SIADH diagnostic criteria:

• Hyponatremia (serum sodium <134 mEq/L)
• Hypoosmolality (plasma osmolality <275 mosm/kg)
• Inappropriately high urine osmolality (>500 mosm/kg)
• Inappropriately high urinary sodium concentration (>20 mEq/L)
• Absence of hypothyroidism, adrenal insufficiency, or volume depletion 2

Management:

• Fluid restriction (<1 L/day) is first-line treatment
• Vasopressin receptor antagonists (vaptans) for persistent cases 1, 3

3. Hypervolemic Hyponatremia

This occurs with increased total body sodium and even greater increases in total body water.

Clinical features:

• Edema
• Ascites
• Jugular venous distension
• Weight gain

Common causes:

• Heart failure
• Liver cirrhosis
• Nephrotic syndrome
• Renal failure

Laboratory findings:

• Urinary sodium <20 mEq/L (in heart failure, cirrhosis)
• Urinary sodium >20 mEq/L (in renal failure)
• High urine osmolality

Management:

• Fluid restriction (<1 L/day)
• Diuretics
• Treatment of underlying condition
• Vasopressin receptor antagonists in selected cases 1

4. Pseudohyponatremia

This is not true hyponatremia but rather a laboratory artifact.

Causes:

• Hyperlipidemia
• Hyperproteinemia
• Hyperglycemia (each 100 mg/dL increase in glucose decreases sodium by approximately 1.6 mEq/L) 4

Laboratory findings:

• Normal or high serum osmolality
• Normal plasma tonicity

Management: Treat underlying condition; no specific sodium correction needed

Special Considerations in Management

Acute Symptomatic Hyponatremia

For severe symptoms (seizures, coma, altered mental status):

• Administer 3% hypertonic saline
• Aim to increase serum sodium by 1-2 mEq/L per hour until symptoms abate
• Limit correction to 8-10 mEq/L in 24 hours and 18 mEq/L in 48 hours to prevent osmotic demyelination syndrome 1

Chronic Hyponatremia

• More gradual correction is necessary
• For patients with risk factors (alcoholism, malnutrition, liver disease), limit correction to 4-6 mEq/L per 24 hours 1

Medication-Induced Hyponatremia

Up to 30% of hyponatremia cases are medication-induced. Common culprits include:

• Diuretics
• Antidepressants
• Antiepileptic drugs
• Antipsychotics
• Pain medications 1

Monitoring

• For active correction: Check serum sodium every 2-4 hours
• For chronic cases: Regular follow-up within 24-48 hours for outpatients
• Extended monitoring for at least two weeks post-correction 1

Pitfalls to Avoid

1. Overly rapid correction: Can lead to osmotic demyelination syndrome, a potentially fatal neurological condition
2. Misclassification of volume status: Can lead to inappropriate treatment
3. Failure to identify and treat underlying causes: Essential for definitive management
4. Inadequate monitoring: Particularly important during active correction

Remember that even mild hyponatremia is associated with increased morbidity, including cognitive impairment, gait disturbances, falls, and fractures 5. Proper classification and management are essential for improving outcomes.