What are the types of hyponatremia?

Types of Hyponatremia

Hyponatremia is classified into three main types based on volume status: hypovolemic, euvolemic, and hypervolemic hyponatremia. 1

Classification by Volume Status

1. Hypovolemic Hyponatremia

• Characterized by decreased extracellular fluid volume and signs of dehydration 1, 2
• Urinary sodium <30 mmol/L suggests extrarenal losses (e.g., vomiting, diarrhea, burns) 2, 3
• Urinary sodium >20 mmol/L suggests renal losses (e.g., diuretic use, adrenal insufficiency, salt-losing nephropathy) 2, 3
• Treatment focuses on volume repletion with isotonic saline 1, 4

2. Euvolemic Hyponatremia

• Characterized by normal extracellular fluid volume (no edema, no orthostatic hypotension) 1, 2
• Most commonly caused by Syndrome of Inappropriate ADH (SIADH) 1, 3
• Diagnostic criteria include:
  • Elevated urinary osmolality (>500 mosm/kg) relative to serum osmolality
  • Elevated urinary sodium (>20-40 mEq/L)
  • Absence of hypothyroidism, adrenal insufficiency, or volume depletion 2
• Other causes include:
  • Hypothyroidism
  • Adrenal insufficiency
  • Psychogenic polydipsia
  • Medications (antidepressants, antipsychotics, chemotherapeutics)
  • Reset osmostat syndrome 5
• Treatment primarily involves fluid restriction to 1L/day 1

3. Hypervolemic Hyponatremia

• Characterized by increased extracellular fluid volume with edema, ascites, or other signs of volume overload 1, 3
• Common causes include:
  • Congestive heart failure
  • Liver cirrhosis
  • Advanced renal failure 1, 3
• Urinary sodium is typically <20 mEq/L except in renal failure 2
• Treatment focuses on fluid restriction (1-1.5 L/day) and addressing the underlying cause 1

Classification by Serum Osmolality

1. Hypotonic Hyponatremia (Low Osmolality)

• Most common form of hyponatremia 6
• Serum osmolality <275 mOsm/kg 2
• Further classified by volume status as described above 6

2. Isotonic Hyponatremia (Normal Osmolality)

• Also called pseudohyponatremia 6, 4
• Caused by laboratory error, hyperproteinemia, or hyperlipidemia 6, 4
• No true water excess or sodium deficit 6

3. Hypertonic Hyponatremia (High Osmolality)

• Typically caused by hyperglycemia 6, 3
• Osmotically active substances draw water from intracellular to extracellular space, diluting sodium 6

Classification by Severity

• Mild: 126-135 mEq/L 1
• Moderate: 120-125 mEq/L 1
• Severe: <120 mEq/L 1, 7

Classification by Onset

• Acute: <48 hours 1, 6
• Chronic: >48 hours or unknown duration 1, 6

Special Types

Cerebral Salt Wasting (CSW)

• Common in neurosurgical patients 1, 2
• Characterized by hypovolemia with elevated urinary sodium 1, 2
• Treatment involves sodium and volume replacement, not fluid restriction 1, 2
• May require fludrocortisone in severe cases 1

Diagnostic Approach

1. Measure serum osmolality to rule out pseudohyponatremia and hyperglycemia 1, 2
2. Assess volume status (though physical examination alone has limited accuracy - sensitivity 41.1%, specificity 80%) 2
3. Measure urine osmolality and sodium 1, 2
4. Check for other contributing factors: medications, thyroid function, adrenal function 1, 2

Common Pitfalls to Avoid

• Misdiagnosing volume status, leading to inappropriate therapy 2
• Using fluid restriction in cerebral salt wasting (can worsen outcomes) 1
• Failing to recognize and treat the underlying cause 1
• Ignoring mild hyponatremia (135 mmol/L) as clinically insignificant 1
• Overly rapid correction leading to osmotic demyelination syndrome 1, 6

Hyponatremia is the most common electrolyte disorder encountered in clinical medicine, affecting approximately 5% of adults and up to 35% of hospitalized patients 1, 7. Proper classification is essential for appropriate management and prevention of complications.