From the Guidelines
The workup for severe hyponatremia should include serum osmolality, urine osmolality, urine sodium, thyroid function tests, morning cortisol, and assessment of volume status. Begin with serum osmolality to distinguish between true hyponatremia (low osmolality) and pseudohyponatremia (normal osmolality) 1. Urine osmolality helps determine if the kidneys are appropriately diluting urine in response to low sodium, while urine sodium helps classify the cause as renal or extrarenal sodium loss. Thyroid and adrenal function tests are essential as hypothyroidism and adrenal insufficiency can cause hyponatremia. Additional tests should include:
- Serum glucose (to rule out hyperglycemia-induced hyponatremia)
- Kidney function tests (BUN, creatinine)
- Liver function tests
- Serum potassium In suspected SIADH, measure serum uric acid and phosphate. A medication review is crucial as many drugs can cause hyponatremia. For patients with neurological symptoms, consider brain imaging to evaluate for cerebral edema. This comprehensive approach allows for proper classification of hyponatremia as hypovolemic, euvolemic, or hypervolemic, which guides appropriate treatment strategies 1.
The most recent and highest quality study, published in 2021, provides guidelines on the management of ascites in cirrhosis, which includes recommendations for the management of hyponatremia 1. The study suggests that fluid restriction to 1–1.5 L/day should be reserved for those who are clinically hypervolaemic with severe hyponatraemia (serum sodium <125 mmol/day), and hypertonic sodium chloride (3%) administration should be reserved for those who are severely symptomatic with acute hyponatraemia.
It is also important to note that the treatment of hyponatremia should be tailored to the underlying cause and the patient's volume status. In patients with hypovolemic hyponatremia, plasma volume expansion with saline solution and correction of the causative factor is necessary 1. In patients with hypervolemic hyponatremia, attainment of a negative water balance is the key to management, and non-osmotic fluid restriction may be helpful in preventing a further decrease in serum sodium levels, but it is seldom effective in improving natremia 1.
Overall, the management of severe hyponatremia requires a comprehensive approach that includes a thorough workup to determine the underlying cause, classification of the type of hyponatremia, and tailored treatment to improve morbidity, mortality, and quality of life.
From the Research
Laboratory Tests for Severe Hyponatremia
To evaluate severe hyponatremia, several laboratory tests can be used, including:
- Serum sodium level to confirm hyponatremia
- Serum osmolality to determine the effective serum tonicity 2
- Urine osmolality to determine whether water excretion is normal or impaired 2
- Urine sodium level to differentiate between hypovolemia and the syndrome of inappropriate antidiuretic hormone secretion 2
- Thyroid-stimulating hormone and cortisol levels in difficult cases of hyponatremia 2
- Arterial blood gases to assess the patient's acid-base status 2
Classification of Hyponatremia
Hyponatremia can be classified based on volume status, including:
Diagnostic Approach
The diagnostic approach to hyponatremia involves:
- A detailed history and physical examination to assess volume status and identify underlying causes 5, 6
- Laboratory studies, including serum and urine electrolytes, osmolality, and other case-specific parameters 2, 6
- Point-of-care ultrasonography to estimate volume status 6
Treatment
Treatment of severe hyponatremia depends on the underlying cause and severity of symptoms, and may include: