Hyperadrenocorticoidism (Cushing’s Disease)

Hyperadrenocorticoidism (HA) is a disorder caused by excessive secretion of glucocorticoids (Cortisone) from the adrenal glands.  The adrenal glands are small glands that lie in the abdominal cavity, close to the kidneys.  They are responsible for secreting, amongst other things, the hormones Cortisol and Adrenaline.  HA is a common disorder of dogs and is quite rare in cats.
In a normal animal, the secretion of cortisol from the adrenal gland is under the control of the Pituitary gland, situated at the base of the brain.  The pituitary gland secretes a hormone, Adrenocorticotrophic hormone (ACTH), which stimulates the adrenal gland to release cortisol.  This cortisol then “feeds back” on the pituitary gland to inform it that the instructions of the ACTH have been followed and it can cease or reduce the release of ACTH.
This balanced production of cortisol can go wrong in a number of different ways.  HA animals can be divided into 2 distinct groups – Pituitary dependant HA (PDHA), and Adrenal dependant HA (ADHA).

PDHA

Approximately 80% of animals with HA are pituitary dependant.  The majority of these are due to a tumour of the pituitary gland leading to increased secretion of ACTH.  These tumours are usually benign and very slow growing and therefore rarely cause problems apart from the effect of the increased ACTH levels.  Malignant tumours are rare.  A small proportion of PDHA is caused by a failure of the feedback mechanism of cortisol upon ACTH release.  The precise defect responsible for this failure is unknown.  From a clinical viewpoint, the exact type of PDHA an animal has is not significant, providing there are no other signs of neurological disease.

ADHA

Adrenal dependant HA animals have tumours affecting the adrenal glands.  These tumours may affect both adrenal glands, or just one side.  Approximately 50% are malignant and 50% are benign.

COMPLICATIONS of CUSHINGS DISEASE

Dogs with Cushings develop high blood pressure, are prone to blood clots (and therefore may suffer stroke like episodes), often get infections and skin problems and can also develop lower urinary tract problems associated with either infection or the formation of bladder stones.

DIAGNOSIS

HA is generally a disease of older animals.  Clinical symptoms can vary enormously.  The classic “textbook” case of HA has an increased appetite and increased urine production.  This increased urine output leads to a dilute urine and an increased thirst.  Changes can often be seen on the skin and hair, with hair coat thinning and skin being prone to blackhead formation, thinning and infection.  HA can also cause muscle weakness, leading to a “pot-belly” appearance and difficulty exercising or even rising.  The muscles may “waste away” and the pet becomes lethargic and inactive.  Changes can occur in the liver and the heart and a small number of affected pets may have neurological (nerve/brain) symptoms.
However, not all HA patients are “textbook” cases and diagnosis relies upon laboratory tests.  Commonly seen are a dilute urine with increased water intake, a change in a blood enzyme called Alkaline Phosphatase and sometimes changes in types and numbers of white blood cells.  A dynamic test of the “Pituitary-Adrenal” axis called a Dexamethasone Suppression Test is usually required to confirm a suspicion of HA.  Occasionally, despite this test giving a “negative” result for HA, the level of clinical suspicion may be high enough to warrant retesting for HA 4-6 months later.

TREATMENT

Treatment is dependant upon the type of HA and other factors such as patient age and general health and whether there are any other health problems at the same time.  Treatment can be quite a complicated process and is best discussed with your vet in detail.