Question

Describe the pathophysiologic mechanism of syndrome of inappropriate vasopressin secretion, and explain why plasma levels of vasopressin can be normal in these patients.

Answer 1

Syndrome of inappropriate vasopressin secretion (SIADH) is a result of inappropriate continuous secretion or action of antidiuretic hormone arginine vasopressin (ADH or AVP) despite normal plasma volume. This continuous secretion or action of ADH causes hyponatremia (low sodium levels in blood; serum Na+ <135 mmol/L) and hypo-osmolarity (<280 mOsm/kg) due to impaired water excretion.

Cell bodies of neurons in supraoptic and paraventricular nuclei of the anterior hypothalamus synthesize a precursor vasopressin-neurophysin 2–copeptin preprotein. This preprotein migrates into the posterior pituitary where it is cleaved to ADH, neurophysin 2 and co-peptin. These proteins are stored in secretory granules along with another carrier protein, neurophysin.

ADH is the anti-diuretic hormone in human body. ADH in involved in hydro-osmotic effect wherein it promotes the reabsorption of water from the tubular fluid in the collecting duct of the kidneys. ADH has not much effect on rate of Na+ reabsorption. ADH also cause vasoconstriction in the arterioles and increases the arterial blood pressure.

Normally, ADH is secreted by hypothalamus when there is increased osmolarity in serum and then transported to the posterior pituitary gland. Hyperosmolarity and depletion of circulating volume, sensed by Osmoreceptors and baroreceptors respectively trigger AVP secretion. Osmoreceptors are present in hypothalamus while baroreceptors are present in the carotid sinus, aortic arch, and left atrium.ADH travels via the circulatory system to kidneys and binds vasopressin receptors on the distal convoluted tubule. As a result, aquaporin channels move from cytoplasm to apical membrane of the tubules. Water is reabsorbed out of the collecting ducts through these channels into the bloodstream. As s result, there is increased osmolarity of urine excreted. Decreased serum osmolarity cause reduction of ADH secretion.

In SIADH patients, hyponatremia is a result of water absorption and not due to sodium deficiency. The excess AVP causes increase in water reabsorption, resulting in dilutional hyponatremia. If most of this water is intracellular, the extracellular fraction causes volume expansion. Activation of volume receptors causes secretion of natriuretic peptides. There is increased potassium excretion (kaliuresis). Finally, the amount of Na⁺ excreted in the urine is equivalent to Na intake, causing hyponatremia. If water intake is restricted, hyponatremia will not occur. Patients with SIADH also have inappropriate thirst sensation. Hence, the patient drinks more water that the free water excreted out, causing hyponatremia.

In the brain, hyponatremia and hyper-osmolarity increases the water content by 5-10%. This results in fatal cerebral edema and herniation. Brain extracellular fluid moves in to the cerebrospinal fluid, causing loss of potassium and intracellular organic osmolytes and creatinine from brain cells to prevent brain swelling.

Plasma levels of ADH is normal is SIADH. Plasma osmolarity is less than the urine osmolarity in these patients. Hence, in order to excrete the excess water, ADH secretion is suppressed, thereby allowing the plasma osmolarity to be normal. If ADH secretion is completely shut down, urine osmolarity will decrease beyond 100 mOsm. ADH has a short life and therefore unstable. Hence, plasma levels will not changed despite inappropriate secretion of ADH.