Page 34 - Demo

Regardless of the mechanism involved, adre- nal insufficiency produces a wide variety of effects. Mineralocorticoid deficiency leads to loss of electrolyte regulation by the dog’s kidneys. The kidney can no longer exchange sodium for potassium. Consequently, there is massive loss of sodium and water into the urine. Blood sodium levels drop as serum potassium levels rise. The large loss of water can lead to a drop in blood pressure. If the dog is also glucocorticoid deficient, then there can be widespread effects on the other organs of the body. If the condition gets out of hand, then the body can literally shut down with frightening speed. The high blood potassium levels, along with shock and other organ dysfunction, can rapidly lead to cardiac arrest and death.
Diagnosis
Canine Addison’s disease is usually seen in young female dogs of any breed (including mixes). A genetic basis for the disease has been suggested due to reports of high rates of Addison’s disease in families of standard and miniature poodles, Labrador retrievers, Rottweilers, German shepherds and Great Danes. Currently, the Canine Genetics Analysis Project at UC Davis is inves- tigating the genetic modes of inheritance of Canine Addison’s in several other breeds (see references for link).
The history and physical findings seen in dogs affected with Addison’s disease can be very non-spe- cific. Affected dogs may have a history of intermittent vomiting, diarrhea, appetite loss and weakness. Often the signs respond to symptomatic care, only to reap- pear within days to weeks. Sometimes the signs come and go on their own leading to a “waxing and waning” type of pattern. The symptoms become more severe as the disease progresses, with the animal showing signs like hypothermia, bloody diarrhea, shaking, increased thirst, increased urination and collapse.
Surprisingly, many dogs with Addison’s disease have few abnormal findings on physical examination. If signs are present, they closely resemble those found in several more common diseases, such as kidney disease, parasites and inflammatory bowel disease. Typically, the most dramatic findings occur if a dog presents in an adrenal crisis. These dogs may be dehydrated and have a slow heart rate and weak pulses. Some may even have signs of sudden circulatory shock. In fact one of the most important clues to the possibility of Addison’s disease is the presence of a normal to slow heart rate in a dog with shock. The problem is that Addison’s hardly ever presents in a “classical manner.” Therefore, it is important for the clinician to suspect Addison’s in any dog who is ADR (“Ain’t doing right”) without any obvi- ous cause.
Routine laboratory work is often unrevealing in dogs with Addison’s disease. The “classic” blood work pattern described in textbooks is a low blood sodium level com- bined with a high blood potassium level. Unfortunately, this pattern does not happen often. Nor is it particularly specific for Addison’s. Usually, there are non-specific
Figure 1: Normal Adrenal Gland Feedback Mechanism
HYPOTHALAMUS CRH
ACTH
ADRENAL GLAND
Negative Feedback
PITUITARY
STEROID HORMONES
changes on routine blood panels. Increases may be seen in the dog’s kidney values (BUN/creatinine). But this can also occur in a wide variety of diseases ranging from dehydration to kidney failure. Approximately 30 percent of affected dogs may have high blood calcium levels. Dogs with atypical Addison’s may only show lower blood glucose, because low glucocorticoid levels impair blood sugar regulation. For all of these reasons, it is impossible to diagnose or rule out Addison’s disease based on routine testing.
The only way to definitively diagnose Addison’s dis- ease in the dog is by performing an ACTH stimulation test. This test is quick and can even be performed on a dog in an adrenal crisis. First, blood is drawn for deter- mination of baseline blood cortisol levels. Next, a set amount of synthetic ACTH (Cortrosyn ®) is injected. Blood is then redrawn at one hour later to measure post- injection cortisol levels. Normal dogs will have normal baseline cortisol levels and will show a good response to ACTH administration, much as they would in respond to their own ACTH. Conversely, dogs with Addison’s disease will have low to undetectable basal levels, with no response to ACTH administration. However, even this test can have gray areas. Therefore, it may be neces- sary to run an endogenous ACTH test. This test is used to determine if the dog could have atypical Addison’s. Dogs with primary Addison’s would be expected to have high ACTH levels in their bloodstream because their normal negative feedback loop has failed (Fig. 1). By contrast, dogs with atypical Addison’s disease will have low blood ACTH levels because of pituitary dysfunc- tion.
32 The Australian Shepherd Journal May/June 2005