How is Diabetes Insipidus Diagnosed?
There are three other conditions that need to be considered when diagnosing cranial diabetes insipidus:
- Compulsive water drinking (psychogenic polydipsia) - this is a psychiatric disturbance characterised by excessive intake of water. The plasma sodium levels will fall and there will be large volumes of dilute urine. This diagnosis is excluded using a water deprivation test (see below).
- Nephrogenic diabetes insipidus - this is a condition where the kidney itself is unresponsive to the vasopressin.
- Neurogenic polydipsia - this is a disorder where there is direct stimulation of the thirst centres in the hypothalamus (usually from a tumour or surgical damage). This is an important condition to recognise as the administration of vasopressin can lead to acute fluid overload.
A water deprivation test is used in the diagnosis of cranial diabetes insipidus. The test usually starts at about 08.00 in the morning. The patient is then dehydrated for the next 8 hours with only dry food and limited water being allowed. The plasma/urine osmolality, body weight and urine volumes are then measured hourly. After 8 hours 2µg injection of desmopressin (synthetic vasopressin analogue) is given and the recordings continued.
The results of the test are interpreted as follows:
- A normal person will maintain a normal plasma osmolality while concentrating their urine during the dehydration phase. The urine osmolality will not change after the injection
- A patient with cranial diabetes insipidus will not be able to concentrate their urine and so their plasma osmolality will rise. On administration of the injection the plasma osmolality will correct and the urine osmolality increase
- A patient with nephrogenic diabetes insipidus will behave the same as a patient with the cranial form except that the injection will have no effect
- A patient with psychogenic polydipsia will generally behave normally.