From the Guidelines
The workup for hyponatremia should begin with a thorough clinical assessment, including history, physical examination, and basic laboratory tests to determine the cause, with a focus on classifying hyponatremia based on volume status and osmolality, as recommended by recent guidelines 1. The initial evaluation should include serum sodium, potassium, glucose, blood urea nitrogen, creatinine, and serum osmolality, as well as urine studies, particularly urine sodium and osmolality, to help distinguish between different causes of hyponatremia. Key considerations in the workup include:
- Classifying hyponatremia based on volume status (hypovolemic, euvolemic, or hypervolemic) and osmolality (hypotonic, isotonic, or hypertonic) 1
- Assessing urine osmolality to distinguish between appropriate and inappropriate ADH secretion in cases of hypotonic hyponatremia
- Using urine sodium concentration to differentiate between extrarenal losses (low urine sodium) and renal losses or SIADH (high urine sodium) Additional tests, such as thyroid function tests, cortisol levels, and medication review, may also be necessary to identify the underlying cause of hyponatremia. In severe or symptomatic cases, particularly with sodium <125 mEq/L or neurological symptoms, immediate intervention may be necessary while the workup continues, with a goal of correcting serum sodium by 4-6 mEq/L per 24-hour period, not to exceed 8 mEq/L per 24-hour period, as recommended by the American Association for the Study of Liver Diseases 1.
From the FDA Drug Label
The primary endpoint for these studies was the average daily AUC for change in serum sodium from baseline to Day 4 and baseline to Day 30 in patients with a serum sodium less than 135 mEq/L. Symptomatic patients, patients likely to require saline therapy during the course of therapy, patients with acute and transient hyponatremia associated with head trauma or postoperative state and patients with hyponatremia due to primary polydipsia, uncontrolled adrenal insufficiency or uncontrolled hypothyroidism were excluded Fluid restriction was to be avoided if possible during the first 24 hours of therapy to avoid overly rapid correction of serum sodium, and during the first 24 hours of therapy 87% of patients had no fluid restriction.
The work up for hyponatremia is not directly addressed in the provided drug label. However, it can be inferred that the following key points should be considered:
- Exclude certain conditions such as primary polydipsia, uncontrolled adrenal insufficiency, or uncontrolled hypothyroidism
- Assess the need for fluid restriction
- Monitor serum sodium concentrations The FDA drug label does not answer the question.
From the Research
Evaluation of Hyponatremia
The evaluation of hyponatremia begins with a detailed history and physical examination, including an assessment of extracellular volume status 2. This information can provide useful clues as to the pathogenesis of hyponatremia.
Laboratory Diagnosis
The laboratory evaluation of hyponatremia involves several steps, including:
- Measurement of the effective serum tonicity (serum osmolality less serum urea level) 2
- Determination of urine osmolality to assess water excretion 2
- Measurement of urine sodium level to assess volume status 2
- Determination of hormone levels (thyroid-stimulating hormone and cortisol) and arterial blood gases in difficult cases 2
Classification of Hyponatremia
Hyponatremia can be classified into three categories based on fluid volume status:
Clinical Evaluation and Workup
The clinical evaluation and workup of hyponatremia involve:
- A detailed history and physical examination 5
- Appropriate urine and serum studies to contribute to the evaluation and classification of the disorder 5
- Estimation of serum sodium, urine electrolytes, and serum and urine osmolality in addition to other case-specific laboratory parameters 4
- Point-of-care ultrasonography as an important adjunct to physical assessment in estimation of volume status 4
Treatment Approach
The approach to managing hyponatremia should consist of treating the underlying cause 3. Treatment decisions are based on the underlying cause and severity of symptoms 5. Severely symptomatic hyponatremia is a medical emergency and requires prompt treatment with bolus hypertonic saline 3.