What is Addison’s disease?
Hypoadrenocorticism or Addison’s disease occurs when the adrenal glands stop working. Dogs (and humans) have two adrenal glands which lie next to the kidneys, these are about the size of a broad bean and produce steroid hormones.
What does Addison’s disease involve?
There are 2 types of steroid hormones which are essential for life; these are the mineralocorticoid, aldosterone and the glucocorticoid, cortisol. Aldosterone is extremely important in regulating kidney function and without it the kidneys are not able to function efficiently, leading to signs of renal failure. Cortisol is needed at a low level for many processes in the body such as gut function, metabolism of fat and regulation of blood sugar levels. Cortisol is also needed at higher levels at times of ‘stress’, at which point cortisol helps the body to adapt. Stressful situations for our pets include times of illness, strenuous exercise or when their daily routine is changed e.g. being in kennels or hospital.
Addison’s disease occurs when the adrenal gland stops working and steroid hormones are not produced in adequate levels. Most commonly, Addison’s disease is manifest as a deficiency in both mineralocorticoids and glucocorticoids; occasionally however, dog’s with Addison’s disease will just be deficient in glucocorticoids. Addison’s disease usually results from the body’s immune system gradually destroying the adrenal tissue, why this occurs is not known. Occasionally Addison’s disease can also occur as a result of infection, haemorrhage or drugs which have damaged the adrenal glands. A hormone called adrenocorticotrophic hormone (ACTH) is released by the pituitary to stimulate the adrenal glands to produce cortisol; rarely therefore, dysfunction of the pituitary gland can result in Addison’s disease.
Addison’s disease is an uncommon disease in dogs and very rare in cats, with only a handful of cases reported in the veterinary literature. Addison’s is more common in females compared to males and more common in castrated males compared with intact males, this may be because of a protective effect of testosterone. Some breeds are also more commonly affected, these include the Standard Poodle, Leonberger, Great Dane and several others.
What are the signs of Addison’s disease?
The destruction of the adrenal gland is usually a gradual process so most dogs with Addison’s disease will have a waxing and waning episodes of lethargy, stomach upset and inappetence. If left untreated, these signs will worsen over time, and become more frequent and/or more severe. In it’s morst severe form, Addison’s disease can cause dogs to go into shock, with a so called ‘Addisonian crisis’. Often this is preceded by a period of non specific illness or waxing and waning disease.
An Addisonian crisis is precipitated by the combination of kidney effects as a result of mineralocorticoid deficiency (e.g. dehydration as a result of renal dysfunction and the accumulation of renal toxins such as urea and potassium) and the lack of glucocorticoids to cope with this stressful situation. Animals with Addisonian crisis usually present collapsed and can have very marked disturbances to their heart rate and rhythm as a result of high levels of potassium. A variety of other signs are also possible including abdominal pain, intestinal bleeding and occasionally seizures due to low blood sugar.
Because of the lack of mineralocorticoids blood tests usually reveal signs of renal dysfunction, such as high urea, creatinine and phosphate. Blood electrolytes may also be abnormal, often with low sodium levels and high potassium. The lack of glucocorticoids also means there may be a reduced stress response to illness, with lower numbers of white blood cells than expected.
Although the history, clinical examination and initial blood results may be very suggestive of Addison’s, they can sometimes be caused by renal disease and an ACTH stimulation test must be performed to allow a definitive diagnosis. ACTH is a hormone which stimulates the adrenal gland to produce glucocorticoids and is permissive in mineralocorticoid production, thus both normally rise dramatically after injection allowing an assessment of adrenal function. In Addison’s disease this response is usually very low and the lack of response allows us to be sure of the diagnosis.
Animals with glucocorticoid deficiency, but normal mineralocorticoids have very vague signs and as such represent a diagnostic challenge. Normally these signs have been progressing gradually over a long period and include gastrointestinal signs, such as vomiting and diarrhoea, weakness, lethargy and reduced activity. Because of the vague signs associated with ‘atypical’ hypoadrenocorticism an ACTH stimulation test may be performed as part of the investigation for a wide variety of presenting signs such as gastrointestinal disease or lethargy.
How is it treated?
Treatment for Addison’s is based on replacing the missing hormones and in an Addisonian crisis providing aggressive intravenous fluid therapy to help the kidney function and reduce potassium levels. Mineralocorticoids are replaced using a drug called dexoxycorticosterone pivalate (Zycortal), which is usually given once a month via injection. The dose is adjusted to maintain renal function and reduce the body electrolytes into the normal range.
In order to facilitate this blood tests are generally obtained 10 and 25 days following initial administration of Zycortal with subsequent dose adjustments and follow up blood tests as necessary. Dogs will also require additional maintenance glucocorticoids which are usually given in the of prednisolone or hydrocortisone. All dogs will need additional glucocorticoids at stressful times such as increased exercise and illness. Some dogs may also need the addition of salt to their diet to help return sodium levels to the normal range, is this is the case it is important to use sea salt (sodium chloride) rather than a low sodium salt, which contain large amounts of potassium.
The long term prognosis for Addison’s disease is excellent and most animals return to completely normal lives; however this can only be achieved with lifelong medication and careful monitoring.