From the Guidelines
To start the workup of hyponatremia, first assess the patient's volume status and measure serum osmolality, as this will guide the management approach and help determine the underlying cause of the hyponatremia. The assessment of volume status is crucial, as it can be hypovolemic, euvolemic, or hypervolemic, and each type requires a different management strategy 1.
Initial Evaluation
The initial evaluation should include basic laboratory tests, such as:
- Serum sodium, potassium, chloride, bicarbonate, blood urea nitrogen, creatinine, and glucose
- Urine studies, including sodium, osmolality, and specific gravity A thorough medication history should also be obtained to identify potential causes of hyponatremia, such as thiazide diuretics, SSRIs, or carbamazepine 2.
Underlying Conditions
Underlying conditions, such as heart failure, cirrhosis, SIADH, adrenal insufficiency, or hypothyroidism, should be evaluated by ordering thyroid function tests and morning cortisol if appropriate 3.
Severity of Hyponatremia
The severity of hyponatremia guides management urgency, with severe hyponatremia (sodium <120 mEq/L) or symptomatic patients (confusion, seizures) requiring more urgent intervention 2.
Treatment Strategies
This systematic approach helps determine whether the hyponatremia is dilutional (excess water) or depletional (sodium loss), which directs appropriate treatment strategies, such as fluid restriction, administration of hypertonic saline, or the use of vaptans, which are effective in improving serum sodium concentration in conditions associated with high vasopressin levels, such as SIADH, heart failure, or cirrhosis 3.
From the Research
Initial Evaluation of Hyponatremia
To start the workup of hyponatremia, the following steps can be taken:
- Evaluate the patient's fluid volume status, categorizing them as hypovolemic, euvolemic, or hypervolemic hyponatremia 4, 5, 6
- Measure the effective serum tonicity (serum osmolality less serum urea level) to determine if the patient has a hypo-osmolar state 5
- Assess the patient's symptoms and signs, which can range from mild and nonspecific to severe and life-threatening 4
- Obtain a detailed history and physical examination to contribute to the evaluation and classification of the disorder 7
Laboratory Tests
The following laboratory tests can be useful in the evaluation of hyponatremia:
- Serum sodium level to confirm the diagnosis of hyponatremia 4, 5, 6, 7, 8
- Serum osmolality to determine if the patient has a hypo-osmolar state 5, 6, 8
- Urine osmolality to determine if water excretion is normal or impaired 5, 6, 8
- Urine sodium level to differentiate between hypovolemic and euvolemic hyponatremia 5, 6
- Thyroid-stimulating hormone and cortisol levels in difficult cases of hyponatremia 5
Classification of Hyponatremia
Hyponatremia can be classified according to the measured plasma osmolality as:
- Isotonic hyponatremia, which usually indicates pseudohyponatremia 6
- Hypertonic hyponatremia, which is typically caused by hyperglycaemia 6
- Hypotonic hyponatremia, which can be further classified according to the volume status of the patient as hypovolemic, hypervolemic, or euvolemic hyponatremia 4, 5, 6, 8