Introduction:
Function tests challenge an organ or structure to
determine its response. In the urinary tract, the most commonly
used function tests are used to detect the kidney tubules' ability to
reabsorb water.
Failure to properly reabsorb water is called diabetes
insipidus (DI). It is caused by either failure to produce
anti-diuretic hormone (ADH) or inability of the renal tubules to
properly respond to ADH.
Antidiuretic hormone is produced in the
hypothalamus and released from the posterior pituitary. Its
function is water conservation: it interacts with the ADH receptor on
the tubule to facilitate the reabsorption of water. If an animal
is dehyrated, for example, increased ADH will be released so that water
is conserved. Inadequate ADH or response to ADH results in
polyuria.
Central diabetes inspidus is caused by decreased
secretion of ADH. It may be a congenital condition, idiopathic,
traumatic or due to a pituitary tumor. Nephrogenic diabetes
insipidus results from renal insensitivity to ADH, caused by a
congenital malformation or secondary to another problem, such as
hyperadrenocorticism, hypokalemia or pyometra.
Regardless of the cause, diabetes insipidus is
characterized by polyuria and polydipsia and hyposthenuria (the
urine specific gravity may be 1.001 - 1.006), and may result in a fatal
dehydration.
Laboratory Diagnosis of Diabetes Insipidus:
The initial in-clinic test performed if diabetes
insipidus is suspected is the modified abrupt water deprivation test,
a function test.
There are important contraindications for
performing this test--performing the test if these conditions are
present may be life-threatening! If an animal is dehydrated, it is
already releasing the maximum amount of ADH. If an animal is known
to have renal disease (and testing should be done to ensure that it does
not!), the renal tubules will be unable to respond appropriately to ADH
during the test.
The test begins with a 12 hour fast of the animal.
All food and water should be withheld during the test. Start by
completely emptying the urinary bladder, then weight the animal and
determine its urine specific gravity and blood urea nitrogen (BUN)
levels every two hours.
The test should continue until one of the following
occurs:
-
Adequate urine concentration, as determined by a urine
specific gravity of greater than 1.025 in the dog and 1.035 in the
cat.
-
Two consecutive identical urine specific gravity
readings below 1.025 or 1.035.
-
Weight loss exceeds 5% of the animal's starting weight
(which reflects water lost in urine)
-
The BUN exceeds the reference range of 10 - 30 mg/dl.
The only animal that "passes" the test, is the one with
the first result: adequate urine concentration. The other three
test results indicate the presence of diabetes insipidus, or the
inability of the kidneys to properly conserve water, especially if the
specific gravity is less than 1.010.
If an animal fails the water deprivation test or the
test is contraindicated by pre-existing renal disease or dehydration,
the vasopressin (ADH) response test can be performed. ADH
is injected and urine specific gravity is measured at 30 minute
intervals. The test is continued until two consecutive identical
specific gravity readings are obtained.
In central diabetes insipidus (inadequate production),
the urine specific gravity will increase 50% or more in response to the
injection of ADH. In nephrogenic diabetes insipidus, no change
will occur; the tubules are unable to respond.
Uroliths are macroscopic mineral structures in
the urinary tract: in other words, they're stones or calculi.
They are formed of minerals and a matrix, the organic part that "glues"
the mineral together to make the stone. The matrix generally makes
up less than 5% of a canine urolith.
Most uroliths are located in the bladder and urethra,
and can affect any age dog, although they are most common in dogs that
are 3 - 7 years old. Females are more commonly affected, and most
stones are primarily struvite. Breeds such as the miniature
Schnauzer, corgi, Lhasa apso, and dalmation are prone to develop stones.
Clinical signs vary with the size, type, number and
location of stones. They can include pollakiuria, hematuria,
stranguria, anuria, distended bladder, tenesmus and incontinence.
Urinary tract infections may predispose a dog to urolithiasis or may be
secondary to urolithiasis.
Canine urolithiasis is diagnosed based on signalment,
signs, urinalysis, and radiographs or ultrasound. Uroliths can be
sent to referral labs to determine their exact composition. The
most common uroliths are made of struvite, especially in female and
young male dogs, making up 60 - 70% of all canine stones.
FLUTD is a group of idiopathic lower urinary tract
diseases that are characterized by hematuria and dysuria. Urethral
plugs may form; unlike the canine urolith, these structures are
predominately matrix and are typically soft, paste-like and
compressible. The primary mineral is usually struvite or calcium
oxalate.
Cats with FLUTD exhibit signs similar to dogs with
urolithiasis, plus genital grooming. Clients often mistaken think
that their cat is constipated.
Approximately 0.5 - 1.0% of the total cat population is
affected by FLUTD, but the disease makes up approximately 5 - 10% of the
cat visits to a veterinary clinic. The disease is most frequently
seen in cats that are 2 - 6 years old and is more common in Persians and
uncommon in Siamese.
There is no correlation between sex and unobstructed
FLUTD, but males are more likely to be obstructed due to their narrower
urethra (and this is an emergency...death can occur in 46 - 72 hours
after obstruction).