- Definition: serum sodium concentration < 135 mEq/L
- Types: true (hypotonic) hyponatremia and pseudohyponatremia (falsely low sodium reading)
Types and comparison
| True (hypotonic) hyponatremia | Pseudohyponatremia | |
|---|---|---|
| Measured plasma osmolality | Low (< 275 mOsm/kg H2O) | High (> 295 mOsm/kg H2O) |
| Subtypes / Causes | - Hypovolemic - Hypervolemic - Euvolemic | - Severe hyperglycemia - Hyperproteinemia (multiple myeloma) - Hypertriglyceridemia - Administration of mannitol |
Causes of True Hyponatremia
| Hypovolemic hyponatremia | Euvolemic hyponatremia | Hypervolemic hyponatremia |
|---|---|---|
| Vomiting Diarrhea Third spacing: burns, pancreatitis Diuretics use | Hypothyroidism Adrenal insufficiency SIADH Accidental/intentional water intoxication (psychogenic polydipsia) | Heart failure Liver cirrhosis Renal failure |
Causes of SIADH
- Pulmonary disorders:
- cancer (especially small cell),
- pneumonia,
- tuberculosis,
- abscess
- CNS disorders:
- infection (meningitis, brain abscess),
- mass (subdural, postoperative, cerebrovascular accident),
- psychosis (with psychogenic polydipsia)
- Drugs:
- thiazide diuretics,
- narcotics,
- oral hypoglycemic agents,
- barbiturates,
- antineoplastics
Clinical Features
- Severity correlates with absolute sodium level and speed of decline (acute drop → more symptoms).
- Nonspecific: anorexia, nausea, vomiting, generalized weakness.
- Acute severe hyponatremia (Na < 120 mEq/L over 24–48 h):
- confusion,
- seizures,
- coma,
- respiratory arrest due to brainstem herniation.
Clinical exam by volume status
| Hypovolemic hyponatremia | Euvolemic hyponatremia | Hypervolemic hyponatremia |
|---|---|---|
| Diminished skin turgor Increased capillary refill Dry mucous membranes Orthostatic hypotension | No edema Normal skin turgor | Jugular venous distention Peripheral / lower limb edema Pulmonary congestion / crackles |
Evaluation
Useful laboratory tests:
- Serum electrolytes
- Serum osmolality
- Urine osmolality
- Urine sodium
Treatment
Severe hyponatremia (neurologic symptoms or Na ≤ 120 mEq/L)
- Indication: severe neurologic symptoms (seizures, reduced consciousness, focal findings)
- Use 3% hypertonic saline.
- Initial bolus: 100 mL of 3% hypertonic saline (≈ 2 mL/kg) over 10 minutes.
- If needed, a second bolus of 100 mL may be given over the next 50 minutes. Sequence can be repeated up to twice more and stopped with clinical improvement or when target sodium achieved.
Management by type
| Hypovolemic hyponatremia | Euvolemic hyponatremia | Hypervolemic hyponatremia |
|---|---|---|
| Isotonic (0.9%) sodium chloride boluses Stop diuretics Add potassium chloride if hypokalemia present Treat underlying cause | Water restriction (< 1 L/day) Treat underlying cause | Water restriction (< 1 L/day) Diuresis in heart failure and cirrhosis Treat underlying cause |
Correction rates and complications
- Typical correction target: 10–12 mEq/24 hours.
- For chronic hyponatremia, do not exceed 6–8 mEq/24 hours; typically corrected over 2–3 days.
- Rapid correction can cause osmotic demyelination syndrome (previously central pontine myelinolysis).
Osmotic Demyelination Syndrome
Clinical features
- Flaccid paralysis / paraparesis or quadriparesis
- Locked-in syndrome (paralysis with sparing of eye blinking)
- Dysarthria, dysphagia, seizures, coma, hypotension
Treatment
- Stop all sodium-containing fluids
- Administer 5% dextrose in water at 3 mL/kg/h