What is the cause of hyponatremia (low sodium levels in the blood) with a urine sodium of 10 mEq/L (milliequivalents per liter), serum osmolality of 249 mOsm/kg (milliosmoles per kilogram), and urine osmolality of 227 mOsm/kg?

From the Guidelines

The cause of hyponatremia (sodium level of 114 mEq/L) with low urine sodium (10 mEq/L), low serum osmolality (249 mOsm/kg), and low urine osmolality (227 mOsm/kg) is most likely hypovolemic hyponatremia due to extrarenal fluid losses, as suggested by the low urine sodium and low serum osmolality, which is consistent with the body's attempt to conserve sodium in response to volume depletion 1. This pattern suggests that the body is appropriately conserving sodium in response to volume depletion. Common causes include vomiting, diarrhea, excessive sweating, or third-spacing of fluids as in pancreatitis, burns, or peritonitis.

• The low serum osmolality confirms true hyponatremia rather than pseudohyponatremia.
• The urine osmolality being close to the serum osmolality indicates the kidneys are attempting to excrete free water but are limited by the hypovolemic state.
• Once volume status improves, urine osmolality would typically increase as ADH levels normalize in response to restored blood volume. Treatment should focus on addressing the underlying cause of fluid loss and carefully replacing both sodium and volume with isotonic saline (0.9% NaCl) 1.
• Initial correction should not exceed 6-8 mEq/L in 24 hours to avoid osmotic demyelination syndrome, especially with severe, chronic hyponatremia.
• The use of vaptans, such as tolvaptan, may be considered in the treatment of hypervolemic hyponatremia, but its use should be carefully monitored to avoid rapid correction of sodium levels 1.
• Albumin infusion may also be considered to improve serum sodium concentration, but more information is needed 1. It is essential to note that the management of hyponatremia in patients with cirrhosis requires careful consideration of the underlying pathophysiology and the potential risks and benefits of different treatment strategies 1.

From the Research

Causes of Hyponatremia

• The patient's serum sodium level is 114 mmol/L, which is lower than the normal range of 135-145 mmol/L 2, 3, 4, 5, 6.
• The urine sodium level is 10 mmol/L, which is relatively low, and the serum osmolality is 249 mosmol/kg, while the urine osmolality is 227 mosmol/kg 4.
• These values suggest that the patient may have a condition known as the syndrome of inappropriate antidiuretic hormone secretion (SIADH), which is characterized by excessive secretion of antidiuretic hormone (ADH) leading to water retention and hyponatremia 2, 4, 6.

Diagnosis of SIADH

• The diagnosis of SIADH is confirmed by decreased serum osmolality, inappropriately elevated urine osmolality (>100 mosmol/L), and elevated urine sodium (Na) levels 4.
• However, in this case, the urine sodium level is low (10 mmol/L), which may not be consistent with SIADH 4.
• Other causes of hyponatremia, such as hypovolemia, heart failure, or liver disease, should also be considered and ruled out 3, 5, 6.

Treatment Options

• The treatment of hyponatremia depends on the underlying cause and the severity of the condition 3, 4, 5, 6.
• For patients with SIADH, fluid restriction is often the primary treatment, while for patients with hypovolemia, isotonic saline infusion may be necessary 3, 4.
• Tolvaptan, a vasopressin V2-receptor antagonist, may be effective in increasing serum sodium concentrations in patients with euvolemic or hypervolemic hyponatremia 2, 5.