From the Guidelines
To determine the etiology of hyponatremia, first assess the patient's volume status (hypovolemic, euvolemic, or hypervolemic) and measure serum osmolality, as recommended by the most recent guidelines 1. When evaluating hyponatremia, it is crucial to consider the patient's clinical context, including recent medication changes, fluid intake, medical conditions, and symptoms, as these provide important diagnostic clues.
Key Steps in Evaluation
- For hypovolemic hyponatremia, check urine sodium (>20 mEq/L suggests renal losses, <10 mEq/L suggests extrarenal losses) and manage by expanding plasma volume with normal saline and cessation of diuretics 1.
- For euvolemic hyponatremia, evaluate thyroid and adrenal function, medication use, and measure urine osmolality and sodium to distinguish SIADH from other causes.
- For hypervolemic hyponatremia, assess for heart failure, cirrhosis, or nephrotic syndrome, and consider fluid restriction, reduction or discontinuation of diuretics and laxatives, administration of hyperoncotic albumin, and/or vasopressin receptor antagonists (“vaptans”) 1.
Laboratory Tests
- Specific laboratory tests include serum sodium, osmolality, urine sodium and osmolality, thyroid function tests, morning cortisol, and kidney function tests.
- Additional helpful tests include serum glucose (to rule out pseudohyponatremia), serum uric acid, and BUN/creatinine ratio. Understanding the underlying pathophysiology is essential, as hyponatremia results from water retention relative to sodium, which can occur through various mechanisms including inappropriate ADH secretion, decreased effective arterial blood volume, or impaired water excretion due to kidney dysfunction 1.
From the Research
Determining Etiology of Hyponatremia
To determine the etiology of hyponatremia, several factors must be considered, including:
- The patient's fluid volume status, which can be categorized as hypovolemic, euvolemic, or hypervolemic hyponatremia 2
- The measurement of effective serum tonicity, serum osmolality, and urine osmolality to determine whether water excretion is normal or impaired 3
- The assessment of extracellular volume status on physical examination and the patient's history of concurrent illness and medication use 3
- The classification and causes of hyponatremia, including pseudohyponatremia, the syndrome of inappropriate antidiuretic hormone secretion, and other disorders of water metabolism 4, 5
Evaluation and Management
The evaluation of hyponatremia begins with a detailed history and physical examination, followed by appropriate urine and serum studies to contribute to the evaluation and classification of the disorder 4. Key points to consider in the evaluation and management of hyponatremia include:
- Clinical assessment and estimation of serum sodium, urine electrolytes, and serum and urine osmolality 6
- Point-of-care ultrasonography as an adjunct to physical assessment in estimation of volume status 6
- Understanding the pathophysiology of the underlying process to lead to a timely diagnosis and appropriate management of hyponatremia 6
- Treatment options, including arginine vasopressin receptor antagonists for euvolemic and hypervolemic hyponatremia, and hypertonic saline for severely symptomatic hyponatremia 2, 5
Laboratory Diagnosis
The laboratory diagnosis of hyponatremia involves: