What is the Difference Between SIADH and CSW?

SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion) and CSW (Cerebral Salt Wasting) are both conditions that present with hyponatremia, but they have distinct clinical and laboratory differences. The main difference between the two lies in the extracellular fluid volume (ECFV) status.

SIADH is characterized by a euvolemic state, where the patient's fluid volume is normal or slightly increased. In contrast, CSW is defined by the renal loss of salt with concomitant extracellular fluid loss. The key distinction between SIADH and CSW is the response to fluid therapy.

Laboratory findings for both conditions include:

• Low serum sodium level
• Low serum osmolality
• High urine osmolality
• High urine sodium level
• Decreased urine output

One approach to differentiate between SIADH and CSW is by determining the fractional excretion of uric acid, as it has been shown to consistently and accurately differentiate between the two conditions.

Regardless of the distinction, many researchers argue that the treatment for hyponatremia complicating intracranial disease should be treated with hypertonic saline, regardless of the patient's volume status. In cases where SIADH or CSW are associated with intracranial pathology, prompt recognition and treatment are crucial to avoid increasing the risk of mortality, length of stay in the ICU, and hospital costs.

Comparative Table: SIADH vs CSW

Here is a table comparing the differences between Syndrome of Inappropriate Antidiuretic Hormone (SIADH) and Cerebral Salt Wasting (CSW):

Both SIADH and CSW are associated with hyponatremia and have overlapping clinical findings, which can make differentiation challenging. However, the key difference between the two conditions is the extracellular fluid volume (ECFV) status. SIADH is a euvolemic to mildly hypervolumic state, whereas CSW is a volume-depleted state. Additionally, uric acid excretion may be useful in differentiating between the two conditions, as SIADH is associated with decreased sodium resorption in the proximal tubule, which can lead to less uric acid absorption and increased uric acid loss in the urine. The exact mechanism of uric acid loss in CSW has not been well defined.