An elderly patient is admitted with a serum [Na+] of 120mmol/L. She is alert and oriented and appears well.
- What specific initial laboratory tests would you order to further delineate the cause of her hyponatremia?
Hyponatremia is defined as a low serum sodium concentration and reflects an excess of free water as opposed to a loss of sodium.
Specific laboratory tests to order at this time include:
- Serum Osmolality – helps differentiate between hypotonic/isotonic/hypertonic hyponatremia
- Urine Na+ (UNa)
- Urine Osmolality (UOsm)
Hypotonic hyponatremia can be further divided into hypovolemic/isovolemic/hypervolemic. A clinical assessment of the patient’s volume status will assess this.
- What treatment options would you consider if…
- Your physical exam demonstrated she was euvolemic?
- Your physical exam demonstrated she was hypervolemic?
- Your physical exam demonstrated she was hypovolemic?
If the patient does not demonstrate any symptoms of acute hyponatremia (confusion, lethargy, seizure, nausea), and there is no record of a recent sodium concentration, assume that this patient’s hyponatremia is chronic. The main tenets of managing hyponatremia are:
- Fluid restrict <1.5L/day
- Furosemide (if hypervolemic)
- IV NS (if hypovolemic)
Monitor the patient’s serum sodium closely to avoid overly rapid correction, which could lead to osmotic demyelinating syndrome (central pontine myelinolysis). Limit correction of [Na+] to<8mmol/day.
If administering IV NS to correct hypovolemic hyponatremia, use the following formula to determine how much IV NS to administer to avoid raising the serum [Na+] above 8mmol/day:
- A sign of overly rapid correction may be free water diuresis (high output of dilute urine).
- Monitor urine output to ensure it does not exceed 200cc/2hrs
- Measure UOsm to ensure it is not <100mOsm/L
- How would your management change if this patient presented with new onset confusion, lethargy, a single episode of seizure, and a serum [Na+] 115mmol/L?
This patient is experiencing acute symptomatic hyponatremia. This is a medical emergency, and is likely the result of cerebral edema. This requires rapid correction of her serum sodium with hypertonic saline (3% NS). Ensure the following while treating:
- Raise the serum [Na+] by 1-2mmol/L/hour for the first 4-6hrs (can use the formula above to calculate)
- Never give more than 100cc/hr
- Do not increase serum [Na+] by more than 8mmol/L/day to avoid osmotic demyelinating symdrome
- Monitor urine output and Uosm for free water diuresis at least every 4 hours (may require a monitored setting)
- Call nephrology if you have any concerns
If overly rapid correction occurs, can give IV D5W and/or DDAVP 1-2ug IV (patient must be NPO for 8 hours after giving DDAVP to ensure no free water ingestion, aka “locking”)
Adrogué HJ and Madias NE. Hyponatremia. NEJM. 342:1581-1589.