Definition:
- < 135 mmol\L
- Low serum osmolality
Causes:
1. Hypotonic hyponatremia: “true hyponatremia”, serum osmol <280 mOsm\kg
- Hypervolemic: intravascular volume depletion -> ↑ ADH
- CHF
- Nephrotic
- Cirrhosis
- Euvolemic:
- SIADH
- Psychogenic polydipsia (15-20 L\d): *no nocturia
- Hypothyroidism
- Post-op
- Hypovolemic
- Low urine Na <10 mEq\L: diarrhea, vomiting, burns, sweating -> kidneys retain Na to compensate
- High urine Na >20 mEq\L: renal loss: diuretics, ACE, Addison’s (↓ aldosterone), cerebral sodium wasting
2. Isotonic hyponatremia: “pseudohyponatremia”
- Lab artifact from high proteins or lipids
3. Hypertonic hyponatremia:
- High glucose, mannitol, sorbitol. Radiocontrast agents -> increases serum osmolality -> water shifts in ECF -> dilutional hyponatremia
Presentation:
- [CNS]: headache, irritability, confusion, high ICP, seizures, coma
- The severity of hyponatremia is defined according to the severity of symptoms, not to sodium levels
Treatment:
- Asymptomatic\mild -> fluid restriction
- Symptomatic -> NS + furosemide
- Refractory -> 3% hypertonic saline + V2 receptor antagonist (tolvaptan)
- Rate of correction:
- Asymptomatic\mild: < 0.5 mEq\L\h
- Symptomatic: initial rapid correction 2 mEq\L\h
- Rate of Na rise shouldn’t exceed 0.5 mEq\L\h or 12 mEq\L\d
- Cerebral salt-wasting disease -> give fludrocortisone
- SIADH -> give demeclocycline or lithium
Complication of treatment:
- If sodium levels are brought up too rapidly -> central pontine myelinolysis
Download the PDF version: here
References:
- Step up to medicine
- Toronto notes
- Master the boards
