Addison's disease is characterised by a deficiency of corticosteroid hormones.
As Dee was a known scavenger, her owner didn't think much of the fact that she left most of her breakfast. She appeared a little quiet that morning and passed some loose stools.
When her owner returned from work that evening, she still hadn't eaten. She was fast asleep on her bed.
Her owner started to be concerned, as she was normally a very bouncy dog, and called the vet.
On examination, her heart rate was very low and she appeared clinically dehydrated. A blood screen revealed abnormally high blood Potassium levels and very low Sodium levels.
Addison's disease was suspected, and an ACTH Stimulation test performed immediately, while emergency treatment was started.
Addison's disease is characterised by a deficiency of corticosteroid hormones, which are produced in the adrenal gland.
The adrenal gland is so named because it is located just forward of the kidney (renal means kidney). The center of the gland is called the medulla and the outer area is called the cortex. While both areas produce hormones, Addison's disease concerns the hormones produced by the cortex; these hormones are called corticosteroids.
Corticosteroids are the hormones that enable us to adapt physiologically to stress. The glucocorticoids act on the mechanics of sugar, fat, and protein metabolism. They gear the metabolism towards the preparation of burning rather than storing fuels so as to be ready for a fight or flight situation.
The mineralocorticoids influence the electrolytes: sodium and potassium. As a general biological rule, where there's sodium or salt, there's water. When the mineralocorticoids circulate as part of the fight or flight preparation, sodium is conserved in anticipation of blood loss so that there will be extra fluid in the vascular compartment (spare blood). When sodium is conserved, potassium is lost as part of the biological balance. This whole picture of fat mobilisation, sodium conservation etc. that is part of the fight or flight preparation is far more complex than can be reviewed here but the bottom line is:
Corticosteroid hormones are needed to adapt to stressful situations and without these hormones, even small stresses could lead to physiologic disaster.
In animals with Addison's disease, there is a deficiency of the corticosteroid hormones. It is unusual to discover the direct cause of this deficiency unless the patient is taking medications that disrupt adrenal balance but, fortunately, the disease can be managed with the administration of corticosteroid hormones even if the cause of the deficiency is unknown.
Patients are usually young dogs but any age dog can be affected. (This disease occurs in cats but is very rare.) There is a genetic predisposition for Addison's disease in the standard poodle and bearded collie. Female dogs are affected twice as often as males.
At first signs are vague - listlessness, possibly some vomiting or diarrhea. The pet just does not seem to feel right but not in an obvious way and may seem more or less normal most of the time as symptoms wax and wane with stress. Ultimately, the disease results in a phenomenon known as the Addisonian crisis. The animal collapses in shock due to his inability to adapt to the caloric and circulatory requirements in stress. Blood sugar may drop dangerously low. Potassium levels soar and disrupt the heart rhythm because there is not enough conserved sodium to exchange for potassium. Heart rate slows, arrhythmias result. The patient may not survive this episode.
Making the Diagnosis
Because of the numerous symptoms that can be seen with Addison's disease, Addison's disease has earned the medical nickname The Great Imitator. You would think that you could simply look for an increase in potassium and/or drop in sodium on a basic laboratory blood panel, but it turns out spot checks of electrolyte values like this are not reliable enough for a diagnosis of Addison's disease.
Vets are typically presented with a young animal in shock. There is usually no history of trauma or toxic exposure so general treatment for shock is initiated. By coincidence, this also happens to be similar to the specific treatment for Addison's disease so that often the patient simply recovers without the veterinarian really knowing why.
The only definitive test for Addison's disease is the ACTH stimulation test. The patient receives a dose of ACTH, the pituitary hormone responsible for the release of corticosteroids in times of stress. A normal animal will show an elevation in cortisol in response to ACTH while an Addisonian has no corticosteroids to respond with.
Dee is now on regular treatment for Addison's disease, and back to her bouncy old self. She has to take tablets daily and her Potassium and Sodium levels are checked regularly.
Most importantly, her owners know not to delay contacting us whenever they feel Dee "is not quite right".