Syndrome of inappropriate antidiuretic hormone secretion

Overview

The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a disorder characterized by excessive release of antidiuretic hormone (ADH), also known as vasopressin, leading to water retention and dilutional hyponatremia (low sodium levels in the blood). This hormonal imbalance disrupts the body's fluid regulation, causing water to be retained despite low plasma osmolality.

SIADH can occur in various clinical settings and is an important cause of hyponatremia in hospitalized patients. The condition can range from mild to severe and may cause significant neurological complications if untreated.

Causes

SIADH can be triggered by multiple factors, including:

• Central nervous system disorders: Head trauma, infections (meningitis, encephalitis), stroke, and tumors.

• Malignancies: Especially small cell lung carcinoma and other cancers producing ectopic ADH.

• Medications: Drugs such as carbamazepine, selective serotonin reuptake inhibitors (SSRIs), antipsychotics, and chemotherapeutic agents.

• Pulmonary diseases: Pneumonia, tuberculosis, and acute respiratory failure.

• Other causes: Postoperative states, HIV infection, and hereditary disorders.

Symptoms

Symptoms of SIADH are primarily related to hyponatremia and its effects on the nervous system and may include:

• Mild symptoms: Nausea, headache, lethargy, and muscle cramps.

• Moderate symptoms: Confusion, irritability, vomiting, and weakness.

• Severe symptoms: Seizures, coma, respiratory arrest, and brain herniation in extreme cases.

• Symptoms often develop gradually but can be acute in rapid-onset cases.

Diagnosis

Diagnosis of SIADH involves laboratory and clinical criteria:

• Hyponatremia with low plasma osmolality (<275 mOsm/kg)

• Inappropriately concentrated urine (urine osmolality >100 mOsm/kg) despite low plasma osmolality

• Euvolemia: absence of signs of dehydration or fluid overload

• Normal renal, adrenal, and thyroid function tests to exclude other causes of hyponatremia

• Clinical evaluation to identify underlying causes or associated conditions

Treatment

Treatment of SIADH focuses on correcting hyponatremia and addressing the underlying cause:

• Fluid restriction: Limiting water intake is the primary treatment to reduce water retention.

• Sodium supplementation: In moderate to severe cases, careful administration of hypertonic saline may be required under close monitoring.

• Medications: Vasopressin receptor antagonists (vaptans), demeclocycline, or loop diuretics may be used in refractory cases.

• Treat underlying cause: Management of infections, malignancies, or medication-induced SIADH.

• Monitoring: Regular assessment of electrolytes and clinical status to avoid rapid correction which can cause osmotic demyelination syndrome.

Prognosis

The prognosis of SIADH depends on the severity of hyponatremia and the underlying cause. Mild cases with timely treatment generally have a good outcome.

Severe or prolonged hyponatremia can lead to serious neurological damage and increased mortality risk. Effective management of the primary condition and careful correction of sodium levels are key to improving prognosis.