What are the types of hyponatremia?

Types of Hyponatremia

Hyponatremia (serum sodium <135 mmol/L) is classified based on volume status and serum osmolality into three main categories: hypovolemic, euvolemic, and hypervolemic hyponatremia. 1

Classification Based on Volume Status

1. Hypovolemic Hyponatremia

• Characterized by decreased extracellular fluid volume and total body sodium deficit 1, 2
• Common causes include:
  • Gastrointestinal losses (vomiting, diarrhea) 2, 3
  • Excessive diuretic use 1, 3
  • Severe burns 3
  • Cerebral salt wasting (in neurosurgical patients) 1, 4
  • Adrenal insufficiency 5
  • Salt-losing nephropathy 5
• Laboratory findings typically include:
  • Urinary sodium <30 mmol/L (unless renal sodium wasting) 4
  • Signs of volume depletion on physical exam 5

2. Euvolemic Hyponatremia

• Normal extracellular fluid volume with excess total body water 1, 6
• Most common causes include:
  • Syndrome of Inappropriate ADH secretion (SIADH) 1, 5
  • Hypothyroidism 1, 5
  • Glucocorticoid deficiency 5
  • Primary polydipsia 1, 6
  • Reset osmostat syndrome 3
• Laboratory findings typically include:
  • Elevated urinary sodium (>20-40 mEq/L) 5
  • Elevated urinary osmolality (>300 mOsm/kg) 5
  • Serum uric acid <4 mg/dL has high positive predictive value for SIADH 5, 4

3. Hypervolemic Hyponatremia

• Characterized by increased extracellular fluid volume and excess total body sodium and water 1, 2
• Common causes include:
  • Congestive heart failure 1, 2
  • Liver cirrhosis 1, 2
  • Advanced renal failure 5
  • Nephrotic syndrome 3, 7
• Laboratory findings typically include:
  • Low urinary sodium (<20 mEq/L) in heart failure and cirrhosis 3
  • Elevated urinary sodium (>20 mEq/L) in renal failure 5
  • Clinical signs of volume overload (edema, ascites) 7

Classification Based on Serum Osmolality

1. Hypotonic Hyponatremia (Low Osmolality)

• Most common form of hyponatremia 6, 7
• Plasma osmolality <275 mOsm/kg 5
• Includes all three volume status categories above 7

2. Isotonic Hyponatremia (Normal Osmolality)

• Also called "pseudohyponatremia" 2, 3
• Caused by:
  • Hyperproteinemia 2
  • Hyperlipidemia 2, 3
• Normal plasma osmolality (275-290 mOsm/kg) 5

3. Hypertonic Hyponatremia (High Osmolality)

• Caused by presence of osmotically active substances in plasma 2, 7
• Most commonly due to hyperglycemia 2, 3
• Plasma osmolality >290 mOsm/kg 5

Special Types of Hyponatremia

Cerebral Salt Wasting vs. SIADH

• Both present with elevated urinary sodium and are common in neurosurgical patients 1, 4
• Distinguishing features:
  • CSW: hypovolemic state with signs of dehydration 4
  • SIADH: euvolemic state 5, 4
• Critical distinction as treatment approaches differ significantly:
  • CSW requires volume and sodium replacement 1, 4
  • SIADH requires fluid restriction 1, 4

Drug-Induced Hyponatremia

• Common medications causing hyponatremia include:
  • Diuretics 1, 3
  • Antidepressants 6
  • Antipsychotics 6
  • Anticonvulsants 6

Common Pitfalls in Diagnosis

• Relying solely on physical examination to determine volume status (sensitivity only 41.1%, specificity 80%) 5, 4
• Misdiagnosing CSW as SIADH in neurosurgical patients, leading to inappropriate fluid restriction 1, 4
• Failing to recognize pseudohyponatremia or hyperglycemia-induced hyponatremia 5, 2
• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant, despite its association with increased morbidity and mortality 1, 6

Understanding the type of hyponatremia is crucial for appropriate management, as treatment approaches differ significantly based on the underlying cause and can significantly impact patient outcomes 1, 6.