Water metabolism and homeostasis are regulated by a fine balance between intake and output. [26] [27] This regulation involves the sensing of plasma osmolality by the osmoreceptors of the anterior hypothalamus, which have projections to thirst and vasopressin centres in the hypothalamus. [28]
Arginine vasopressin (AVP), also known as antidiuretic hormone (ADH), is produced in the hypothalamus and delivered via the pituitary stalk to the posterior pituitary gland, where it is stored for release into the systemic circulation. AVP is released in response to osmoreceptor sensing of hyperosmolality and also baroregulated mechanisms. [28]
AVP acts on AVP2 receptors in the kidneys, to cause an increase in the water permeability of the renal collecting ducts; this occurs through the generation of a water channel (aquaporin-2) into the apical membrane of collecting ducts, resulting in urinary concentration and the re-absorption of water back into the circulation.
Central DI results from any condition that impairs the production, transportation, or release of AVP.
Nephrogenic DI results from conditions that impair the renal collecting ducts' ability to respond to AVP.
Both central and nephrogenic DI cause defective water re-absorption in the kidneys, resulting in the excessive production of dilute, hypotonic urine (polyuria), in volumes ranging from 3 litres to >20 litres per day. This may be accompanied by significant thirst and by volume depletion of varying severity, depending on the water intake. If untreated, hypernatraemia (serum sodium >145 mmol/L [>145 mEq/L]) may develop; this may be a medical emergency.
Patients with DI due to a non-traumatic aetiology generally have an insidious onset of symptoms. In patients with central DI following traumatic brain injury or pituitary surgery, a transient or permanent DI may develop. Some patients may develop a triple-phase response: [6]
Polyuric phase: decrease in AVP, leading to immediate increase in urine output with reduced urine osmolality. Typically lasts 4 to 5 days
Antidiuretic phase: axonal necrosis of AVP-secreting neurons, leading to uncontrolled AVP release with reduction in urinary output and increase in urine osmolality (may be associated with hyponatraemia secondary to the syndrome of inappropriate antidiuretic hormone hypersecretion (SIADH). Typically lasts 5 to 6 days
Permanent central DI: neuronal death, with cessation of AVP production and depletion of AVP reserve.
Pregnancy is associated with a number of changes in salt and water regulation. [25] A transient central DI may develop as a consequence of a decreased osmotic threshold for thirst and AVP release, and a decrease in plasma osmolality. There is also a 4-fold increase in metabolic clearance of AVP due to placental production of vasopressinase/oxytocinase. In addition, pregnancy may aggravate the severity of any existing nephrogenic or central DI. [1] [3] [4]