What are the investigations for hyponatremia (low sodium levels)?

From the Guidelines

Investigating hyponatraemia requires a systematic approach, starting with confirming true hyponatraemia through repeat serum sodium measurement and serum osmolality, as recommended by the most recent guidelines 1. The initial steps in investigating hyponatraemia should include assessment of volume status, which can be achieved through clinical examination, blood pressure, and heart rate measurements.

• Urine sodium and osmolality tests should be conducted to determine the type of hyponatraemia, with urine sodium levels >30 mmol/L indicating SIADH and <30 mmol/L suggesting hypovolaemia.
• Additional tests, such as thyroid function tests, morning cortisol, liver function tests, and kidney function tests, should be performed to identify potential underlying causes.
• Plasma glucose should be measured to exclude hyperglycaemia-induced hyponatraemia, and serum uric acid, lipids, and proteins should be assessed to exclude pseudohyponatraemia.
• A medication review focusing on diuretics, antidepressants, antipsychotics, and anticonvulsants is also essential, as these can contribute to hyponatraemia. The investigation pathway should be guided by the clinical presentation, with different approaches for acute (<48 hours) versus chronic hyponatraemia, and symptomatic versus asymptomatic patients, as outlined in the guidelines 1. The most critical aspect of investigating hyponatraemia is to identify the underlying cause, which is essential for appropriate management, and this should be done in accordance with the most recent and highest quality guidelines available 1.

From the Research

Hyponatraemia Investigations

• Hyponatremia is defined as a serum sodium level of less than 135 mEq/L and most commonly results from water retention 2.
• The approach to managing hyponatremia should consist of treating the underlying cause, and patients should be categorized according to their fluid volume status (hypovolemic hyponatremia, euvolemic hyponatremia, or hypervolemic hyponatremia) 2, 3.
• Diagnostic evaluation of patients with hyponatremia is directed toward identifying the extracellular fluid volume status, the neurological symptoms and signs, the severity and duration of hyponatremia, and the rate at which hyponatremia developed 4.
• The first step to determine the probable cause of hyponatremia is the differentiation of the hypervasopressinemic and non-hypervasopressinemic hyponatremias with measurement of plasma osmolality, glucose, lipids, and proteins 4.
• Further differential diagnosis of hyponatremia can be achieved by determining urine osmolality, clinically assessing extracellular fluid volume status, and measuring urine sodium concentration 4.

Treatment of Hyponatremia

• Normal saline forms the mainstay of treatment for hypovolemic hyponatremia, while 3% NaCl and fluid restriction are important for euvolemic hyponatremia 3.
• Hypervolemic hyponatremia responds well to fluid restriction and diuretics 3.
• For acute or severely symptomatic hyponatremia, a bolus of hypertonic saline is often given 2, 5.
• Fluid restriction remains the first-line treatment for most forms of chronic hyponatremia, but therapy to increase renal free water excretion is often necessary 5.
• Vasopressin receptor antagonists, urea, and loop diuretics can be used to increase renal free water excretion, but their use may vary depending on the guidelines and patient-specific factors 2, 5.

Clinical Practice Guidelines

• The European Society of Intensive Care Medicine (ESICM), the European Society of Endocrinology (ESE), and the European Renal Association-European Dialysis and Transplant Association (ERA-EDTA) have developed clinical practice guidelines on the diagnostic approach and treatment of hyponatremia 6.
• The guidelines focus on patient-positive outcomes and provide a useful tool for clinicians involved in everyday practice 6.
• The development of guidelines has been important in advancing the field of hyponatremia diagnosis and treatment 5.