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Because the most common cause of polyuria and polydipsia is diabetes mellitus (DM), diabetes insipidus (DI) may be overlooked by physicians. An incorrect diagnosis of DM instead of DI can not only cause needless suffering, but it can also lead to severe dehydration and, in rare cases, brain damage or possibly death. A proper diagnosis is also important when considering that DI may be an early sign of another serious underlying condition, such as a brain tumor.

To make a diagnosis of diabetes insipidus, a physician may take a medical history that includes:

• A review of conditions or illnesses that could be causing or contributing to dehydration
  
  
• A review of medications that the patient is taking
  
  
• A review of recent drinking, urinary and bowel habits

A physical examination should include taking vital signs including pulse, blood pressure and an evaluation of any difficulty swallowing. When more water than salt is being lost, the physician may attempt to rule out excessive diuretic use, infections and fever based on the medical history and physical examination.

A diagnosis of DI is also based on blood tests and urine tests. If DI is suspected, most physicians will recommend a water deprivation test. Because of the various reasons for developing DI, an ultrasound or MRI (magnetic resonance imaging) may be used to check for brain injury, tumors or any signs of kidney disease.

Some of the more common diagnostic tests used to screen for diabetes insipidus include:

• Measurements of antidiuretic hormone (ADH, also called vasopressin) and other substances in blood and urine before and during a water deprivation or dehydration test.
  
  
• MRI of the brain.
  
  
• A closely monitored administration of a synthetic antidiuretic hormone known as DDAVP.
  
  
• The water deprivation test. During this test the patient is not permitted to drink any water. The weight of the patient will be checked and urine and blood tests performed. Urine volume and the ability of the kidneys to concentrate urine is checked every hour. Blood concentration is measured every two hours.

However, if the DI appeared in infancy or early childhood or if it affects other members of the patient’s family, a genetic test may be ordered to determine if it is an inherited form of DI.