From the Guidelines
Hyponatremia with low urine sodium is primarily caused by conditions that lead to effective circulating volume depletion, such as gastrointestinal losses, third-spacing of fluids, excessive sweating, and early stages of renal failure, as well as cirrhosis with ascites, as noted in the 2021 practice guidance by the American Association for the Study of Liver Diseases 1. The main causes of hyponatremia with low urine sodium can be broken down into several key categories, including:
- Gastrointestinal losses (vomiting, diarrhea)
- Third-spacing of fluids (pancreatitis, burns, peritonitis)
- Excessive sweating
- Early stages of renal failure
- Cirrhosis with ascites, which can lead to hypovolemic, euvolemic, or hypervolemic hyponatremia, depending on the underlying cause and the patient's volume status, as discussed in the 2021 practice guidance 1. In these conditions, the body attempts to conserve sodium by reducing urinary sodium excretion to less than 20 mEq/L, as the renin-angiotensin-aldosterone system is activated, enhancing sodium reabsorption in the kidneys, and non-osmotic release of antidiuretic hormone (ADH) occurs in response to volume depletion, leading to water retention that dilutes serum sodium. Treatment of hyponatremia with low urine sodium should focus on addressing the underlying cause, such as discontinuation of diuretics and/or laxatives, providing fluid resuscitation with 5% IV albumin or crystalloid solution, and carefully replacing both sodium and volume status, as recommended in the 2021 practice guidance by the American Association for the Study of Liver Diseases 1. It is essential to note that the correction rate of hyponatremia should not exceed 8-10 mEq/L in 24 hours to avoid osmotic demyelination syndrome, and the treatment approach may vary depending on the specific underlying cause and the patient's clinical condition, as discussed in the 2021 practice guidance 1.
From the Research
Causes of Hyponatremia with Low Urine Sodium
- Hyponatremia is defined as a serum sodium level of less than 135 mEq/L and most commonly results from water retention 2.
- The causes of hyponatremia can be classified according to the patient's fluid volume status as hypovolemic, euvolemic, or hypervolemic hyponatremia 2, 3.
- Hypovolemic hyponatremia is accompanied by extracellular fluid (ECF) volume deficit, while hypervolemic hyponatremia manifests with ECF volume expansion 3.
- Euvolemic hyponatremia can be caused by the syndrome of inappropriate antidiuresis (SIADH), which is characterized by a urine osmolality above 100 mOsm/kg and urine sodium concentration above 40 mEq/l 3.
- Low urine sodium can be seen in hypovolemic hyponatremia, where the body tries to conserve sodium and water 3.
- In patients with heart failure, hyponatremia can occur due to fluid overload and increased ADH secretion, leading to water retention and dilutional hyponatremia 4.
Diagnosis and Management
- The diagnosis of hyponatremia involves measuring serum sodium levels and assessing the patient's fluid volume status 2, 3.
- The management of hyponatremia depends on the underlying cause and the patient's volume status 2, 3.
- Restricting free water and hypotonic fluid intake is a common approach to managing hyponatremia, regardless of the patient's volume status 3.
- Hypertonic saline and vasopressin antagonists can be used to correct symptomatic hyponatremia, but the rate of correction should be carefully monitored to avoid overly rapid correction 2, 3.