Disorders of water balance

Water homeostasis is critical to all mammals and maintenance of the tonicity of extracellular fluids is crucial for cell function. The maintenance of water balance in healthy humans is achieved by thirst, vasopressin and kidney function. Renin-angiotensin-aldosterone and natriuretic peptides also regulate water and electrolyte balance. In diabetes insipidus, large volumes of dilute urine (polyuria) are excreted due to vasopressin deficiency (Central Diabetes Insipidus (CDI)), vasopressin resistance (nephrogenic diabetes insipidus) or excessive water intake (primary polydipsia). Polyuria must be evaluated by means of a water deprivation test, with measurement of plasma and urine osmolality, and a desmopressin challenge should be performed if diabetes insipidus has been diagnosed. Patients with CDI and adipsia have a significantly higher risk of morbidity and mortality than in non-adipsic diabetes insipidus. Nephrogenic diabetes insipidus is secondary to arginine vasopressin receptor-2 mutations, abnormalities of aquaporin 2 water channel or other intrinsic kidney disorders.