What does SIADH (Syndrome of Inappropriate Antidiuretic Hormone) stand for?

SIADH: Definition and Clinical Significance

SIADH stands for Syndrome of Inappropriate Antidiuretic Hormone secretion, which refers to the abnormal release of antidiuretic hormone (ADH, also known as vasopressin) that leads to water retention and hyponatremia 1.

Pathophysiology

SIADH occurs when there is persistent release of ADH despite low plasma osmolality, causing:

• Increased water reabsorption in renal tubules
• Impaired free water clearance
• Development of hypotonic hyponatremia
• Normal or increased extracellular fluid volume (euvolemic state)

Diagnostic Criteria

The diagnosis of SIADH requires all of the following cardinal features 1:

• Hyponatremia (serum sodium < 134 mEq/L)
• Hypoosmolality (plasma osmolality < 275 mosm/kg)
• Inappropriately high urine osmolality (> 500 mosm/kg)
• Inappropriately high urinary sodium concentration (> 20 mEq/L)
• Absence of hypothyroidism, adrenal insufficiency, or volume depletion

Common Causes

SIADH can result from various conditions 1:

1. Malignancies:

   • Small cell lung cancer (most common, occurring in 10-45% of cases)
   • Other lung cancers (squamous cell, adenocarcinoma)

2. Medications:

   • Chemotherapeutic agents (platinum-based, vinca alkaloids)
   • Antidepressants
   • Anticonvulsants
   • Opioids
   • NSAIDs

3. Pulmonary disorders

4. Neurological disorders

Clinical Manifestations

Symptoms depend on the severity and rate of development of hyponatremia 1:

• Mild to moderate (Na+ 125-130 mEq/L):

  • General weakness
  • Confusion
  • Headache
  • Nausea

• Severe (Na+ < 120 mEq/L):

  • Seizures
  • Coma
  • Potentially death

Management

Treatment approaches for confirmed SIADH 1:

1. First-line treatment:

   • Fluid restriction (< 1 L/day)
   • Discontinuation of implicated medications

2. For severe or symptomatic cases:

   • Hypertonic 3% saline for acute, severe hyponatremia
   • Careful monitoring to prevent overly rapid correction

3. For refractory cases:

   • Vasopressin receptor antagonists (vaptans)
   • Demeclocycline
   • Urea

Important Clinical Considerations

• Hyponatremia in SCLC patients is associated with shortened survival 1
• Early detection and appropriate management can prevent severe complications
• The rate of sodium correction should be limited to prevent osmotic demyelination
• In patients with short prognosis, strict fluid restriction may not be appropriate 1

SIADH represents a significant clinical challenge that requires careful diagnosis and management to prevent serious neurological complications while addressing the underlying cause.