What is the algorithm for evaluating hyponatremia (low sodium levels in the blood)?

From the Guidelines

The evaluation of hyponatremia should begin with confirming a true low serum sodium (<135 mEq/L) and assessing the patient's volume status (hypovolemic, euvolemic, or hypervolemic), with a focus on distinguishing between hypotonic hyponatremia, isotonic pseudohyponatremia, and hypertonic hyponatremia, as recommended by the most recent guidelines 1. The initial step in evaluating hyponatremia involves checking serum osmolality to differentiate between these types.

• For hypotonic hyponatremia (<280 mOsm/kg), further evaluation includes measuring urine osmolality and urine sodium.
• If urine osmolality is <100 mOsm/kg, primary polydipsia or beer potomania should be considered.
• In cases of hypovolemia with urine sodium <20 mEq/L, GI losses or sweating are suspected; if urine sodium >20 mEq/L, diuretics, salt-wasting nephropathy, or adrenal insufficiency should be considered. In euvolemic patients with urine sodium >20 mEq/L, evaluation for SIADH, hypothyroidism, or adrenal insufficiency is necessary, as indicated by recent practice guidance 1. For hypervolemic patients, checking urine sodium is crucial, with <20 mEq/L suggesting heart failure or cirrhosis, and >20 mEq/L indicating renal failure. Additional tests may include thyroid function, cortisol levels, medication review, and assessment of renal, liver, and cardiac function, as these can significantly impact the management of hyponatremia, particularly in patients with cirrhosis, where the management of hyponatremia is critical to prevent complications such as hepatorenal syndrome 1. The management of hyponatremia, especially in cirrhosis, should follow the guidance that mild hyponatremia (Na 126-135 mEq/L) does not require specific management apart from monitoring and water restriction, while moderate to severe hyponatremia requires more aggressive management, including water restriction, cessation of diuretics, and potentially the use of vasopressin receptor antagonists or hypertonic saline, with careful monitoring to avoid overcorrection and the risk of osmotic demyelination syndrome (ODS) 1.

From the Research

Evaluation of Hyponatremia

The evaluation of hyponatremia involves several steps to determine the underlying cause and severity of the condition. The following are key components of the evaluation process:

• Determination of the extracellular fluid volume status, which helps to identify the most likely cause of hyponatremia 2, 3, 4
• Measurement of plasma osmolality, glucose, lipids, and proteins to differentiate between hypervasopressinemic and non-hypervasopressinemic hyponatremias 2
• Assessment of neurological symptoms and signs, as well as the severity and duration of hyponatremia 2, 5
• Evaluation of urine osmolality and urine sodium concentration to provide additional information for differential diagnosis 2, 3

Classification of Hyponatremia

Hyponatremia can be classified based on volume status, including:

• Hypovolemic hyponatremia
• Euvolemic hyponatremia
• Hypervolemic hyponatremia 2, 3, 4 It can also be classified based on the presence or absence of vasopressin, including:
• Hyponatremia due to non-osmotic hypersecretion of vasopressin (hypovolemic, hypervolemic, euvolemic) 2
• Hyponatremia of non-hypervasopressinemic origin (pseudohyponatremia, water intoxication, cerebral salt wasting syndrome) 2

Treatment of Hyponatremia

The treatment of hyponatremia depends on the underlying cause and severity of symptoms. The following are general treatment approaches:

• Fluid restriction for euvolemic hyponatremia 3, 6
• Isotonic saline for hypovolemic hyponatremia 3
• Diuresis for hypervolemic hyponatremia 3
• Hypertonic saline for severe symptomatic hyponatremia 3, 5
• Vasopressin receptor antagonists, such as vaptans, for euvolemic and hypervolemic hyponatremia 2, 4