Characteristics of Syndrome of Inappropriate Antidiuretic Hormone (SIADH)
Increased urine sodium concentration (>20 mEq/L) is consistent with SIADH, as this reflects inappropriate renal sodium excretion despite hyponatremia. 1, 2
Diagnostic Criteria for SIADH
SIADH is characterized by the following features:
- Hyponatremia (serum sodium <134 mEq/L) 1, 2
- Hypoosmolality (plasma osmolality <275 mosm/kg) 1, 2
- Inappropriately high urine osmolality (>500 mosm/kg) relative to plasma osmolality 1, 2
- Inappropriately high urinary sodium concentration (>20 mEq/L) 1, 2
- Euvolemic status (absence of volume depletion or expansion) 2, 3
- Absence of hypothyroidism, adrenal insufficiency, or other causes of hyponatremia 1, 2
Key Distinguishing Features of SIADH
Volume Status
- SIADH is characterized by clinical euvolemia, not hypovolemia 2, 3
- Patients with SIADH maintain normal extracellular fluid volume despite water retention 1
- The absence of edema and volume depletion is a cardinal diagnostic criterion 3
Sodium Levels and Osmolality
- SIADH presents with hypotonic hyponatremia, not hypernatremia 1, 2
- Plasma osmolality is reduced (<275 mosm/kg), not increased 1, 2
- Urine osmolality is inappropriately high relative to the low plasma osmolality 2, 3
Urinary Findings
- Increased urinary sodium concentration (>20 mEq/L) is a hallmark finding 1, 2
- A urinary sodium level >30 mmol/L has a positive predictive value of 71-100% for distinguishing SIADH from hypovolemic causes of hyponatremia 1
- Serum uric acid <4 mg/dL in the presence of hyponatremia has a positive predictive value for SIADH of 73-100% 1, 2
Pathophysiology
- SIADH results from persistent detectable or elevated plasma arginine vasopressin (AVP) concentrations despite hypoosmolality 3, 4
- This leads to water retention, dilutional hyponatremia, and increased urinary sodium excretion 3, 4
- Normal osmoregulated inhibition of ADH secretion is impaired, leading to continued water reabsorption despite hyponatremia 5, 3
Common Pitfalls in Diagnosis
- Failing to distinguish between SIADH (euvolemic) and cerebral salt wasting (hypovolemic) 2
- Misinterpreting hyponatremia in the setting of volume depletion as SIADH 1, 2
- Not recognizing that hypernatremia and plasma hyperosmolality are inconsistent with SIADH 1, 2
- Overlooking the importance of assessing volume status in the diagnostic workup 1, 2
In conclusion, among the options presented, increased urine sodium (>20 mEq/L) is consistent with SIADH, while hypovolemia, hypernatremia, and plasma hyperosmolality are all inconsistent with this syndrome.