Diabetes insipidus is an interesting pathological condition that is closely related to the actions of the antidiuretic hormone (ADH). The disease can be fatal without proper treatment.
Antidiuretic hormone also known as vasopressin is produced in the brain in hypothalamus. From the brain tissue it gets into the rear lobe of the pituitary gland where the hormone is stored and out of where it is secreted into the blood. Antidiuretic hormone has two main functions. It is able to narrow the blood vessels, thus increasing the blood pressure. In addition it affects the kidneys and allows the kidney tissue to reabsorb the filtered water. ADH acts in microscopic channels of the kidney tissue and opens special transport areas for water molecules (aquaporines). Water reabsorbs and reenters local blood vessels. This mechanism allows concentrating the urine and protects the body from an excessive fluid loss.
Diabetes insipidus has two forms. The central form is easy to understand as it is a hypothalamic damage from any cause (injury, bleeding, infection, brain tumor, etc.), which disrupts the production of ADH. Approximately in one third of the patients with central form we are unable to detect the cause. The hypothalamus seems to be anatomically healthy but it still does not produce the ADH. The second is the peripheral form when the antidiuretic hormone is produced normally but because of a congenital defect the aquaporines do not respond to its effect.
In both cases the result is more or less the same. It is necessary to add, however, that the peripheral form usually manifests less severely.
Symptoms are related to antidiuretic hormone deficiency or dysfunction. The affected person looses large amount of water by urination. In the central form it may be more than ten liters per day. The result is naturally a dehydration that can be fatal. Peripheral form is less severe, although even there is a loss of about four liters of water per day. The excessive fluid loss is often associated with excessive thirst, fatigue, dizziness and fainting. In the central form the problems are aggravated by low blood pressure caused by the missing effect on the blood vessels.
The above mentioned symptoms can be very suspicious. Large volumes of urine and thirst are however typical for untreated diabetes mellitus that should be always excluded in the first place as it is much more common. Urinalysis in diabetes insipidus shows that the urine is not adequately concentrated. To distinguish the central and peripheral form of the disease we can perform a simple test with administration of a substance known as desmopressin (a synthetic analogue of ADH) and later repeat the urinalysis. If the urine concentration increases, it is most probably the central form and vice versa. If we suspect the central form, it is advisable to examine the hypothalamus and surrounding areas by an imaging method such as computed tomography or magnetic resonance imaging to confirm or exclude organic diseases such as brain tumors.
The central form is characterized by the lack of ADH; therefore we have to replace it. We use regular administration of the above mentioned drug desmopressin, a synthetic substitute of ADH. The peripheral form is much more difficult to treat as ADH lacks its effect. Therefore we focus more on symptomatic therapy by increased fluid intake. Acute states with low blood pressure and dehydration must be solved in inpatient department with ensuring intravenous hydration and monitoring the vital functions.