Diabetes insipidus (DI)comes in two forms. One is an aberration of the central nervous system (CNS) which results in poor body control of water balance. The other is caused by the kidneys’ inability to respond to the hormone produced by the CNS, resulting in identical clinical signs. The former condition is more common.
Diabetes insipidus is seen more commonly in dogs than cats and also occurs in humans.
Patients present with a complaint of constant thirst and excessive urination. Dogs will repeatedly ask to go outside, or even have accidents indoors. Cats will fill a litterbox with urine in short order.
Knowledgeable clients may ask whether their pets might be diabetic, suffering from diabetes mellitus (DM) or “sugar diabetes.” The similarity of signs has led some to refer to diabetes insipidus as “the other diabetes.”
“Insipid” means “dull, flat, lacking in qualities that interest, tasteless.” Taste might seem like an odd way to refer to a disease that involves urine, but diabetes mellitus is so-named because the earliest health-care workers in ancient Greece, China and Egypt discovered that those patients’ urine tasted sweet. Therefore, the contrast of “dull” or “tasteless” urine follows naturally.
As stated above, DI and DM patients drink and urinate a lot. Drinking and urinating in large volumes describes a condition called polyuria/polydipsia, or pu/pd.
Poly is a Latin prefix meaning “many.” Dipsia is a Latin root meaning “thirst.” Therefore, the polydipsic patient drinks often, or in large quantities, resulting in a 24-hour intake being higher than normal.
Of course, if intake is excessive, output, in the form of urine, will also be excessive, resulting in polyuria.
TESTING AND DIAGNOSIS
Routine screening tests, complete blood count (CBC), chemistry profile and urinalysis begin the process of diagnosis. In the DI patient urine specific gravity (SG) will be “fixed,” at or close to 1.010, meaning the dog or cat lacks the ability to concentrate his urine. This finding is nonspecific, and is observed in a number of disease conditions.
Your pet’s doctor will narrow down the differential diagnosis list by eliminating other causes of pu/pd. The next step is a water deprivation (WD) test. In this procedure water is withheld from the pet for 12 hours, the bladder is emptied, and the next urine formed is tested to determine whether the kidneys have produced concentrated urine (high specific gravity). If not, the test is continued to a predetermined (but safe) level of dehydration. The end point is either urine concentrated to an acceptable level, or failure to concentrate.
If the kidneys can concentrate without any medication being administered, DI is ruled out.
However, in order to definitively diagnose diabetes insipidus, a vasopressin test must be performed.
TYPES OF DIABETES INSIPIDUS EXPLAINED
Central diabetes insipidus is caused by failure of the pituitary gland, a structure in the hypothalamus of the brain, to produce the hormone vasopressin, also called antidiuretic hormone or ADH. Through its effect on the kidneys, it plays a part in determining specific gravity, determining thirst satiety and controlling water loss from the body.
Secondary diabetes insipidus or renal diabetes insipidus occurs when the pituitary gland produces appropriate quantities of vasopressin, but the kidneys lack the ability to respond to the hormone.
In either case, the result is polyuria and polydipsia. And a frustrated pet owner.
Vasopressin is available pharmacologically, is used to test for a definitive diagnosis and can be used for treatment of the condition. Genetic engineering has made available a very pure form of the hormone.
For another, more-common cause of pu/pd in dogs, and a major rule-out in the differential diagnosis list when investigating DI, click here to read about psychogenic polydipsia.
Click to read more about dog diabetes
See you tomorrow, Dr. Randolph.