From the Guidelines
The cause of hyponatremia with low urine sodium (<20 mEq/L) and low urine osmolality (178 mOsm/kg) is most likely hypovolemic hyponatremia due to extra-renal fluid losses. This pattern suggests the body is appropriately conserving sodium in response to volume depletion, while producing dilute urine. Common causes include:
- Gastrointestinal losses (vomiting, diarrhea)
- Third-spacing (pancreatitis, burns, trauma)
- Excessive sweating The low urine sodium indicates intact renal sodium conservation mechanisms, while the low urine osmolality suggests the kidneys are attempting to excrete free water but cannot overcome the hypovolemia. Management should focus on treating the underlying cause and carefully replacing both sodium and volume, typically with isotonic saline (0.9% NaCl) 1. Rapid correction should be avoided to prevent osmotic demyelination syndrome, with sodium correction not exceeding 8 mEq/L in 24 hours 1. Close monitoring of serum sodium levels during treatment is essential. This pattern differs from SIADH, which would typically show inappropriately elevated urine sodium and osmolality despite hyponatremia. It's also important to note that hyponatremia in patients with cirrhosis is associated with increased mortality and morbidity, particularly neurological complications, and reduced survival after liver transplantation 1. Therefore, it's crucial to address the underlying cause of hyponatremia and manage it accordingly to improve patient outcomes. In patients with cirrhosis, the use of vaptans, such as tolvaptan, may be considered to improve serum sodium concentration in hypervolemic, hyponatremic states 1. However, the management of hyponatremia should be individualized based on the patient's clinical presentation and underlying condition.
From the Research
Causes of Hyponatremia
The patient's hyponatremia with a urine osmolality of 178 and urine sodium of less than 20 can be caused by several factors.
- Hypovolemic hyponatremia is accompanied by extracellular fluid (ECF) volume deficit 2.
- The syndrome of inappropriate antidiuretic hormone secretion (SIADH) should be suspected in any patient with euvolaemic hyponatraemia with a urine osmolality above 100 mOsm/kg and urine sodium concentration above 40 mEq/l 2.
- Diuretics are one of the most common causes of severe hyponatremia, particularly thiazide diuretics 3.
Diagnosis and Classification
Hyponatremia can be classified according to the measured plasma osmolality as isotonic, hypertonic or hypotonic 2.
- Hypovolemic hyponatremia is treated with normal saline infusions 4.
- Euvolemic hyponatremia includes restricting free water consumption or using salt tablets or intravenous vaptans 4.
- Hypervolemic hyponatremia is treated primarily by managing the underlying cause (e.g., heart failure, cirrhosis) and free water restriction 4.
Treatment Options
Treatment of hyponatremia depends on the underlying cause and the patient's volume status.
- Fluid restriction and close monitoring alone are sufficient for asymptomatic patients with chronic hyponatraemia 2.
- Hypertonic saline and vasopressin antagonists can be used to correct symptomatic hyponatraemia 2.
- Tolvaptan, a vasopressin V2-receptor antagonist, may be used to treat hyponatremia secondary to SIADH 5, 6.