Cushing's Disease in Dogs

Overview of Canine Hyperadrenocorticism

Hyperadrenocorticism, commonly referred to as Cushing’s disease or Cushing’s syndrome, refers to a disease state in which an overactive adrenal tissue produces excessive amounts of cortisone. Cortisone and related substances are essential hormones of the body, but when produced in excessive amounts these substances may cause systemic illness.

A small tumor in the pituitary gland (located at the base of the brain) is the cause of Cushing’s syndrome in 80 to 85 percent of dogs with hyperadrenocorticism. The tumor produces a hormone called adrenocorticotropic hormone or ACTH that stimulates the adrenal glands to grow larger (become hyperplastic) and produce excessive amounts of cortisone. This type of Cushing’s syndrome is called pituitary-dependent hyperadrenocorticism because it originates from the pituitary gland.

In the remaining 15 to 20 percent of dogs with Cushing’s syndrome, the cause is a tumor of the adrenal gland. This form is called adrenal-dependent hyperadrenocorticism because it originates from the adrenal gland itself.

Occasionally, a dog might have a diagnosis of iatrogenic Cushing’s disease. This is not an adrenal disorder, but rather it is caused by the administration of steroids (given to treat other diseases) to dogs. Long-term administration of steroids can cause a dog to exhibit all the classic signs of Cushing’s disease. In this case, excessive steroids are not being produced in the body; they are being provided as a form of medication to your pet.

Canine Cushing’s syndrome usually occurs in middle-aged to older dogs with most affected dogs being over 9 years of age at presentation. The syndrome does not have a strong gender bias, but it may occur slightly more often in female dogs than in males. Dogs of any breed can develop Cushing’s syndrome, but it is most common in Poodles, Dachshunds, Miniature Schnauzers, and German Shepherds. Boxers and Boston Terriers are prone to the development of Cushing’s syndrome caused by pituitary tumors.

Hyperadrenocorticism can be difficult to recognize due to its variable clinical symptoms and very gradual onset. For example, many owners mistakenly assume that the changes they see in their dog are simply a result of the aging process.

Hyperadrenocorticism Symptoms in Dogs

Abnormal high blood concentration of cortisone results in the clinical symptoms of Cushing’s syndrome. These include:

• Increased water consumption (polydipsia)
• Increased urinations (polyuria)
• Increased appetite (polyphagia)
• Abdominal distension (pot-bellied appearance)
• Loss of hair on the trunk (alopecia)
• Chronic skin or urinary tract infections, excessive panting, lethargy, muscle weakness, and calcium deposits in the skin (calcinosis cutis) are other symptoms of Cushing’s syndrome.

Diagnosis of Hyperadrenocorticism in Dogs

No single laboratory test definitively identifies Cushing’s syndrome, and the disorder should not be diagnosed solely on the basis of laboratory tests. Your veterinarian should also consider the medical history and physical examination findings when establishing a diagnosis and determining the appropriate laboratory tests to perform.

Several of the following diagnostic tests may be required for the diagnosis of Cushing’s syndrome:

• Complete blood count (CBC)
• Biochemical profile
• Urinalysis and bacterial culture of the urine
• Blood pressure measurement
• Radiographs (X-rays) of the chest and abdomen
• Urine cortisol-to-creatinine ratio
• Abdominal ultrasound examination
• ACTH stimulation test
• Low dose dexamethasone suppression test
• Measurement of blood ACTH concentration
• High dose dexamethasone suppression test
• CT (computed tomography) or MRI (magnetic resonance imaging) scans of the brain or abdomen

Treatment of Hyperadrenocorticism (Cushing’s Syndrome) in Dogs

• Several approaches may be used to treat dogs with Cushing’s syndrome. Treatment options depend primarily on whether the Cushing’s syndrome is pituitary-dependent or adrenal-dependent.

Pituitary-dependent Hyperadrenocorticism in Dogs

• Medical therapy with o,p’-DDD (mitotane or Lysodren®) causes selective destruction of the cortisone-producing portions of the adrenal gland and limits the ability of the gland to produce cortisone.
• Therapy with Trilostane (Vetoryl) is also a very good option. It inhibits cortisol synthesis. It is a registered treatment in Europe but has had limited availability in the United States until recently.
• Ketoconazole (Nizoral®) is an anti-fungal drug that also blocks the synthesis of cortisone in the adrenal gland. It is sometimes used to treat hyperadrenocorticism.
• L-Deprenyl increases dopamine concentration in the brain. Increased dopamine concentration may decrease cortisone production in the adrenal glands. The use of L-deprenyl to treat Cushing’s syndrome in dogs is very controversial. Some veterinarians believe it has a beneficial clinical effect whereas others have found no evidence of an effect despite extensive laboratory testing of treated dogs.

Adrenal-dependent Hyperadrenocorticism in Dogs

• Surgical removal of an adrenal tumor is a difficult, but potentially curative surgery. Surgery is not indicated if the adrenal tumor has invaded adjacent vessels and organs or spread distantly (metastasized).
• Adrenal tumors can also be treated medically with mitotane or ketoconazole. Medical therapy may be the only choice if the tumor has invaded locally or spread distantly.

Home Care

Follow your veterinarian’s instructions very closely when administrating medications, especially in the induction phase when using mitotane. Observe your dog for weakness, vomiting, diarrhea, loss of appetite, or change in attitude. You should also observe your dog for improvement or worsening of clinical signs.

Follow up with your veterinarian for routine re-evaluation of blood tests so as to maximize the chance for successful treatment.

Preventative Care

There is no known way to prevent Cushing’s syndrome. However, some preventative measures may lead to earlier diagnosis and potentially more effective treatment:

• As your dog gets older, more frequent routine visits to the veterinarian may identify early symptoms of the disease.
• Routine performance of blood tests (complete blood count, biochemical profile, urinalysis) in older dogs may identify laboratory abnormalities associated with hyperadrenocorticism.
• Monitor your dog for any changes in behavior or attitude, especially increased water consumption, increased urination, and increased appetite.