What are the criteria for hyponatremia due to Syndrome of Inappropriate Antidiuretic Hormone (SIADH)?

From the Guidelines

The diagnosis of SIADH requires meeting specific criteria, including hyponatremia (serum sodium < 134 mEq/L), hypoosmolality (plasma osmolality < 275 mosm/kg), inappropriately high urine osmolality (> 500 mosm/kg), and inappropriately high urinary sodium concentration (> 20 mEq/L), as well as the absence of hypothyroidism, adrenal insufficiency, or volume depletion. The criteria for hyponatremia due to SIADH can be broken down into several key components:

• Hyponatremia, defined as a serum sodium level of less than 134 mEq/L 1
• Hypoosmolality, characterized by a plasma osmolality of less than 275 mosm/kg 1
• Inappropriately high urine osmolality, typically greater than 500 mosm/kg 1
• Inappropriately high urinary sodium concentration, usually greater than 20 mEq/L 1
• Absence of underlying conditions such as hypothyroidism, adrenal insufficiency, or volume depletion 1 It is essential to assess the patient's intravascular volume status and biochemical measurements in blood and urine to further investigate hyponatremia 1. The accuracy of a diagnostic algorithm for SIADH can approach 95% by assessing the effective arterial blood volume with the fractional excretion of urate 1. Laboratory findings in SIADH include urine osmolality of > 300 mosm/kg, urinary sodium level of > 40 mEq/L, serum osmolality of < 275 mosm/kg, and serum uric acid concentration of < 4 mg/dL 1. Paraneoplastic hyponatremia secondary to elevated atrial natriuretic peptide should be distinguished from SIADH in the differential diagnosis of hyponatremia 1. Management of SIADH is based on expert opinion and may include free water restriction, hypertonic 3% saline IV, demeclocycline, lithium, and vasopressin 2 receptor antagonists 1.

From the FDA Drug Label

Tolvaptan tablets are indicated for the treatment of clinically significant hypervolemic and euvolemic hyponatremia (serum sodium <125 mEq/L or less marked hyponatremia that is symptomatic and has resisted correction with fluid restriction), including patients with heart failure and Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

The criteria for hyponatremia due to SIADH include:

• Serum sodium <125 mEq/L
• Symptomatic hyponatremia that has resisted correction with fluid restriction
• Euvolemic hyponatremia, which is a characteristic of SIADH 2 The main goal is to correct serum sodium levels while avoiding too rapid correction, which can cause osmotic demyelination. Key considerations for treatment include:
• Initiating and re-initiating treatment in a hospital setting to monitor serum sodium levels closely
• Avoiding fluid restriction during the first 24 hours of therapy
• Frequently monitoring for changes in serum electrolytes and volume during initiation and titration of treatment 2

From the Research

Criteria for Hyponatremia due to SIADH

The criteria for diagnosing hyponatremia due to Syndrome of Inappropriate Antidiuretic Hormone (SIADH) include:

• Hypotonic hyponatremia
• Natriuresis
• Urine osmolality in excess of plasma osmolality
• Absence of edema and volume depletion
• Normal renal and adrenal function 3 These criteria are essential in determining the presence of SIADH, which is a common cause of normovolaemic or dilutional hyponatraemia.

Diagnostic Considerations

In diagnosing SIADH, it is crucial to consider the following:

• The patient's clinical presentation, including neuro-muscular and gastro-intestinal symptoms
• The severity of hyponatremia, which is related to both the absolute serum sodium concentration and its rate of fall
• The presence of underlying conditions, such as neoplasia, neurological diseases, lung diseases, or the use of certain drugs 3

Treatment Options

Treatment options for hyponatremia due to SIADH include:

• Fluid restriction, which is the mainstay of therapy for symptomatic patients with chronic SIADH 3
• Hypertonic saline, which can be used to correct severe hyponatremia 4, 5
• Tolvaptan, a vasopressin receptor antagonist that has been shown to be effective in correcting hyponatremia in patients with SIADH 6, 7, 5
• Demeclocycline, which can be used to treat SIADH, although its use is less common 4

Important Considerations in Treatment

When treating hyponatremia due to SIADH, it is essential to:

• Limit the daily increase of serum sodium to less than 8-10 mmol/liter to avoid osmotic demyelination 4
• Monitor serum sodium levels closely, especially during the first 24 hours of treatment with vaptans 4
• Be aware of the potential side effects of treatment, such as thirst, polydipsia, and frequency of urination with vaptans 4, and dehydration, headache, and cramps with fluid restriction 6