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Dog: Cushing’s Syndrome

General information

Cushing’s syndrome is a condition in which the dog’s adrenal glands produce excess amounts of corticosteroids (cortisol).

The adrenal glands are small organs located next to the kidneys. The inner portion of the gland is called the medulla and the outer area is called the cortex. While both layers produce different types of hormones, Cushing’s syndrome involves the hormone, cortisol, produced in the cortex.

Cortisol is released in times of stress to help the body prepare for a fight or flight situation by adjusting the metabolism to mobilize carbohydrates, fat and protein into energy. This puts the body in a state of break down so the stored energy resources can be accessed quickly. However, high levels of cortisol for an extended period of time can result in immune system suppression and be debilitating to a dog’s body.

Production of cortisol is controlled by the pituitary gland located at the base of the brain. The pituitary gland produces adrenocorticotropic hormone (ACTH) which stimulates the adrenal cortex to produce cortisol. When cortisol levels are declining in the blood, the pituitary gland releases ACTH stimulating the adrenal glands to produce cortisol. Conversely, when the pituitary gland detects appropriate or high levels of cortisol, ACTH production is stopped and the adrenal glands shut down the excretion of cortisol.

Causes

There are three classifications of Cushing’s syndrome:

  1. Pituitary Dependent Cushing’s
  2. Adrenal Dependent Cushing’s
  3. Iatrogenic Cushing’s

Pituitary Dependent Cushing’s

This is the most common form of Cushing’s accounting for 85% of cases in dogs. With pituitary dependent Cushing’s, a small tumor develops in the pituitary gland. While this tumor is usually benign, it does result in the overproduction of ACTH and thus, the over stimulation and enlargement of the adrenal glands leading to high levels of cortisol.

Adrenal Dependent Cushing’s

This form accounts for about 15% of the cases in dogs and is caused by an adrenal tumor producing excess levels of cortisol. In this case, there is little or no production of ACTH and the opposite adrenal gland is usually atrophied or small. Approximately half of these tumors will be benign and half malignant.

Iatrogenic Cushing’s

This form of Cushing’s occurs when a dog is administered corticosteroid type medications for an extended period of time. The high level of cortisol-like medication in the blood stream causes the pituitary gland to shut down production of ACTH resulting in both adrenals shrinking in size.

Cardinal symptom

  • Increased drinking
  • Increased urination

Symptoms

Cushing’s disease usually affects middle aged (7-12 years) dogs. There are many clinical signs associated with Cushing’s including increased drinking, increased urination, increased appetite, an enlarged, sagging abdomen, obesity, listlessness, muscle weakness, panting and recurring infections. Skin changes include hair loss, thinning and darkening of the skin. Initially, the clinical signs of increased thirst and urination can mimic diabetes mellitus or chronic renal failure.

Poodles, Dachshunds, Beagles and Boston terriers have an increased incidence of Cushing’s Disease.

Abb. GFTEDOYZ
Abb. GFTEDOYZ: Cushing’s syndrome.
This is a photograph of a dog with typical clinical signs of hair loss, pendulous abdomen and muscle weakness.

Diagnosis

Diagnosis is based on clinical signs and screening tests consisting of a complete blood count (CBC), serum biochemistry and urinalysis tests. Additional discriminating tests will be needed to confirm Cushing’s syndrome and determine which type of disease is present. These tests measure the ability of the pituitary to produce ACTH and the adrenal gland to produce cortisol.

Other tests used to determine the type of Cushing’s include abdominal ultrasonography to check for adrenal tumors. Computed tomography (CT scan) and magnetic resonance imaging (MRI) are used in university and referral hospitals to diagnose adrenal and pituitary tumors.

Treatment

Treatment of Cushing’s syndrome depends on which form of the disease is causing the increase in cortisol levels. Dogs which develop pituitary dependent Cushing’s disease are treated with a medication to reduce the level of cortisol in the blood. There are currently four drugs available in the US to treat this type of Cushing’s in dogs.

If a benign adrenal tumor is diagnosed, surgical removal of the tumor or adrenal gland can be performed. This surgery is difficult and should only be performed by a veterinary specialist. If the cancer is suspected to be malignant or has spread to other parts of the body, surgery is not warranted and medical treatment can be attempted.

Dogs which have symptoms of Cushing’s from being on long term corticosteroids can be weaned off this medication. Given time, the adrenal glands will usually return to normal size and function.

Prognosis

Medical treatment of pituitary dependent Cushing’s can be quite successful returning the symptomatic dog to a healthy pet. However, these medications will need to be given for the life of the dog. They can be costly and associated with different side effects. Frequent recheck examinations will be recommended by your veterinarian. Dogs with malignant adrenal cancer have a poor prognosis.

Prevention

Only iatrogenic Cushing’s can be prevented by avoiding corticosteroid medications. However, many dogs with allergic conditions, immune-mediated disease or debilitating arthritis need these medications long term. If your dog is being treated with a cortisol like medication, you should watch for any signs of Cushing’s syndrome.

CAUTION

Some medications used for treatment of Cushing’s will destroy the cortisol producing cells of the adrenal gland. Because of this, some dogs develop the opposite condition called Addison’s disease where the body does not produce enough cortisol.

Update version: 4/24/2014, © Copyright by www.enpevet.de
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