cystic disease (nephronophthisis), and obstructive uropathy In nephrogenic DI, the renal tubule is unresponsive to antidiuretic hormone Patients with nephrogenic DI usually have onset of symptoms in infancy and present with recurrent episodes of dehydration, fever, failure to thrive, psychomotor retardation, and marked thirst Such infants will have a history of frequent and heavy wet diapers and often show a preference for cold water over anything else, including food Pharmacologic causes of polyuria and polydipsia include methylxanthines, amphotericin B, and diuretics In addition, chronic lithium therapy may result in nephrogenic DI Primary polydipsia is diagnosed when the ingestion of water is in excess of that needed to maintain water balance It can be caused by an inappropriate psychological thirst drive (psychogenic polydipsia or compulsive water drinking) or by hypothalamic damage that alters thirst but not antidiuretic hormone release (neurogenic polydipsia) Most children with polydipsia have serious but nonacute problems Potential life-threatening conditions may develop in certain circumstances ( Table 64.3 ) Patients with DI or nephrogenic DI may develop severe dehydration if water is withheld for prolonged periods Conversely, urgent management of hypernatremia is usually unnecessary if patients are able to drink and may be harmful if it is of chronic duration Diabetic ketoacidosis may be an initial presentation of patients with DM, and can result in extreme electrolyte and acid– base imbalances Patients with primary polydipsia who overload their kidneys’ ability to excrete free water may present with hyponatremic seizures Many of the brain lesions that cause DI can become life-threatening Patients with severe brain injury often develop DI toward the end of life EVALUATION AND DECISION When evaluating a child with polydipsia, the physician should seek information from the parent regarding the quantity and type of fluid ingested each day, and whether the child has used any unusual methods to satiate thirst A history of nocturnal polydipsia and polyuria is helpful because most children with psychogenic polydipsia not wake in the middle of the night for fluids A medical history should include questions on growth and development, as well as past episodes of severe dehydration Inquiries should be made about known causes of polydipsia such as sickle cell disease, DM, chronic kidney disorders, head trauma, and medications ( Fig 64.1 ) The physical examination should include a careful evaluation for known systemic and intracranial causes of DI, and particularly a full neurologic examination