DESCRIPTION

Inappropriate secretion of antidiuretic hormone (ADH) or ADH-like peptide in the presence of low serum sodium, low serum osmolality, and high urine osmolality and in the absence of renal, adrenal, or thyroid pathology

EPIDEMIOLOGY

The syndrome of inappropriate ADH secretion (SIADH) can occur at any age. Incidence depends on etiology.

ETIOLOGY

• Idiopathic
• CNS pathology, causing increased secretion of ADH or ADH-like peptides: meningitis, head trauma, neurosurgical procedures, encephalitis, brain tumor, brain abscess, hydrocephalus, hypoxia, subarachnoid hemorrhage, cerebral venous thrombosis
• Ectopic production of ADH or ADH-like peptides: oat cell carcinoma of the lung, bronchogenic carcinoma, olfactory neuroblastoma, and pancreatic carcinoma
• Pulmonary disease (leading to secondary elevation in ADH secretion or ADH-like peptides): tuberculosis, viral or bacterial pneumonia, asthma, cystic fibrosis, pneumothorax, positive pressure ventilation
• Drugs (which mimic ADH or stimulate its release): vincristine, cyclophosphamide, carbamazepine, chlorpropamide, phenothiazines, clofibrate, nicotine, selective serotonin reuptake inhibitors (SSRIs)
• Iatrogenic exogenous ADH administration: vasopressin infusion for treatment of diabetes insipidus, excess desmopressin (DDAVP^®) in conjunction with fluid intake
• Severe, prolonged nausea
• Postoperative patient (e.g., as part of triple-phase response after hypothalamic-pituitary surgery, after transsphenoidal pituitary surgery)
• Rocky Mountain spotted fever
• Stem cell transplantation

RISK FACTORS

PATHOPHYSIOLOGY

• ADH is synthesized within hypothalamic neurons, transported in conjunction with neurophysin down the supraopticohypophyseal tract and stored in the posterior pituitary.
• Circulating ADH acts on the renal collecting ducts.
• ADH interaction with its receptors forms intracellular cyclic adenosine monophosphate (cAMP), which increases water permeability through insertion of aquaporins (water channels) in renal collecting ducts and consequently enhances free water reabsorption.
• SIADH results when elevated ADH or ADH-like peptide concentrations cause free water retention and hypervolemia, leading to hyponatremia; three possible mechanisms include the following:
  • Increased hypothalamic ADH production (e.g., CNS disorders such as stroke or meningitis)
  • Independent production of ADH or ADH-like substances from ectopic sources (e.g., oat cell carcinoma of the lung or olfactory neuroblastoma)
  • Decreased venous return that stimulates atrial volume receptors and thereby leads to ADH release (e.g., heart failure, cirrhosis; pulmonary and intrathoracic diseases, such as tuberculosis)