What is the difference between Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) and Cerebral Salt Wasting (CSW)?

From the Guidelines

Syndrome of Inappropriate Antidiuretic Hormone (SIADH) and Cerebral Salt Wasting (CSW) are distinct conditions requiring different treatments, with SIADH involving excessive ADH secretion and CSW involving renal sodium loss, and accurate diagnosis is crucial to prevent worsening of hyponatremia or hypovolemia. The key distinction between SIADH and CSW is volume status, with SIADH patients being euvolemic or hypervolemic with concentrated urine, and CSW patients being hypovolemic with high urine sodium 1.

Diagnosis and Treatment

• SIADH involves excessive ADH secretion leading to water retention and dilutional hyponatremia with euvolemia or hypervolemia, and treatment includes fluid restriction (800-1000 mL/day), salt tablets, and in severe cases, hypertonic saline or vasopressin receptor antagonists like tolvaptan 1.
• CSW, conversely, involves renal sodium loss causing hypovolemia and hyponatremia, and treatment requires aggressive volume replacement with normal or hypertonic saline and sometimes fludrocortisone (0.1-0.4 mg daily) 1.
• Physical examination findings like orthostatic hypotension, dry mucous membranes, and decreased skin turgor suggest CSW, while their absence points to SIADH 1.
• Accurate diagnosis is crucial as treating SIADH with fluids worsens hyponatremia, while fluid restriction in CSW can worsen hypovolemia 1.

Recent Guidelines

• The 2023 guideline for the management of patients with aneurysmal subarachnoid hemorrhage recommends fludrocortisone to reduce excess sodium excretion, urine volume, hyponatremia, and intravenous fluid use during acute aSAH, but notes that it did not consistently reduce DCI or affect outcome 1.
• The guideline also notes that other agents, including high-dose hydrocortisone, have been studied in RCTs and demonstrated similar effects on serum sodium, urinary sodium excretion, and natriuresis, but reported more medical complications 1.

From the Research

SIADH vs CSW

• The syndrome of inappropriate antidiuretic hormone secretion (SIADH) and cerebral salt wasting (CSW) are two potential causes of hyponatremia in patients with central nervous system disease 2.
• The primary distinction between SIADH and CSW lies in the assessment of the effective arterial blood volume (EABV), with SIADH being a volume-expanded state and CSW being characterized by a contracted EABV 2.
• SIADH is treated with fluid restriction, whereas CSW requires vigorous salt replacement 2.
• Distinguishing between SIADH and CSW is crucial, as therapy indicated for one disorder but used in the other can result in negative clinical consequences 2.
• The diagnosis of SIADH involves ascertaining the euvolemic state of extracellular fluid volume, both clinically and by laboratory measurements 3.
• Treatment of SIADH includes nonspecific measures such as fluid restriction, hypertonic saline, urea, and demeclocycline, as well as specific therapy with vasopressin receptor antagonists like vaptans 3.
• Vaptans have been shown to be effective in correcting hyponatremia in patients with SIADH, with a rapid and efficient correction of both clinical symptoms and plasma sodium levels 4.
• However, caution is advised when using vaptans, as they can cause overcorrection of hyponatremia, particularly in patients with low baseline serum sodium and urea concentrations 5, 6.
• A very low dose of tolvaptan (7.5 mg) has been shown to be effective in correcting hyponatremia in patients with SIADH, with a decreased rate of overcorrection compared to higher doses 6.