Cushing's Disease

Although Cushing's disease is a severe disease, the changes it causes can be quite subtle in the early stages. Many owners do not recognize the signs of Cushing's disease in their pet, instead confusing the changes caused by the disease with aging. It is important to get an early diagnosis for this disease because, with treatment, affected animals can lead a normal and full life.

What is Cushing's Disease?

Cushing's disease is caused by prolonged exposure of the body's tissues to high levels of the hormone, cortisol. It is called Cushing's disease because it was named after a famous neurosurgeon, Harvey Cushing who first recognized it. It is also sometimes called "hyperadrenocorticism"or "hypercortisolemia".

Cushing's disease is caused by an excess of the steroid hormone, cortisol. In the normal dog cortisol is produced by the adrenal glands, (which are located just in front of the kidneys). Scientists think that cortisol has hundreds of possible effects in the body. Among its other vital tasks, cortisol helps to:

• maintain blood pressure.
• slow the immune system's inflammatory response.
• balance the effects of insulin in breaking down sugar for energy.
• regulate the use of proteins, carbohydrates and fats in the body.

Because cortisol is so vital to health, the amount of cortisol produced by the adrenal glands is precisely balanced. Cortisol production is regulated by hormones produced in the brain (from the pituitary gland). The hormones produced by this gland stimulate the adrenal glands. When the adrenal glands receive the signal from the pituitary they respond by producing cortisol. In the normal animal cortisol is produced mainly at times of stress - in Cushing's disease the levels of cortisol in the blood are always too high.

What causes Cushing's Disease?

Nearly all cases of Cushing's disease are caused by a tumor in the pituitary gland. Although this is, strictly speaking, a brain tumor, the tumor is usually tiny and benign and causes no effects related to pressure in the brain. A few cases of Cushing's disease are caused by a tumor in the adrenal gland.

The two forms of natural Cushing's disease are:

• Pituitary-dependant Cushing's Disease

• Adrenal-dependant Cushing's Disease

Sometimes signs of Cushing's disease are caused by steroids given by the veterinarian to treat another disease, an allergic skin disease.

How do I know if my dog has Cushing's Disease?

Common signs of Cushing's Disease

The steroid hormones affect almost every tissue in the body and the signs of Cushing's disease can be diverse. The most obvious sign of Cushing's disease is increased thirst. If your dog is drinking more (or suddenly needs to get up in the night to urinate) you should always take them to the veterinarian for a check-up. However, not all dogs with Cushing's disease have increased thirst.

An increased appetite (and weight gain) is also very common in Cushing's disease but owners may not recognize this as a sign of illness.

Cushing's disease causes changes to the skin and hair coat. The hair is lost in patches (particularly over the flanks) but there is no scratching. Other symptoms appear in the skin, which becomes fragile and thin. It bruises easily and heals poorly. Purplish pink stretch marks may appear on the abdomen. Female dogs may stop having heat cycles.

Steroid hormones cause muscle wasting and weak muscles may make it difficult for your pet to exercise. You may notice that your dog is panting a lot or they may develop joint problems. In Cushing's disease, fat is deposited in the liver making it swell and this may be visible causing the abdomen to hang down with a pot-bellied appearance.

High levels of steroid hormone in the blood suppress the immune system and healing process; so animals with Cushing's disease may have repeated infections or wounds that do not heal as quickly as expected.

In most cases of pituitary-dependent disease the tumor in the brain is tiny and causes no physical effects. However, in a few animals the brain tumor is sufficiently large such that it can cause neurological signs of depression, blindness or seizures.

How will my veterinarian diagnose the disease?

Cushing's disease can be very difficult to confirm. Your veterinarian may suspect the disease based on simple blood tests but specific blood tests are needed to confirm the diagnosis. These special tests measure the level of cortisol in the blood. However, because the levels of this hormone vary from hour to hour in a normal animal, the disease cannot be diagnosed on the basis of one blood test. Your veterinarian will need to take a number of blood samples before and after injection of hormones that affect the amount of cortisol produced by your dog. Some of these blood samples have to be handled very carefully and will need to be sent away to veterinary laboratories for analysis.

X-rays may also be needed to show other potential problems caused by the disease.

Why does my veterinarian need a urine sample from my dog?

There are a number of reasons why examination of a urine sample can be useful in a dog with Cushing's disease:

• When levels of hormone in the blood are very high, some hormone may spill over into the urine and this can be measured. If there is no cortisol in the urine it is unlikely that your dog has Cushing's disease. Unfortunately, finding cortisol in the urine does not mean that your dog has Cushing's disease as many other conditions can cause this change.
• If your dog has Cushing's disease it is likely that their immune system will not be working as well as it should. Your veterinarian will want to test a urine sample to see if there is any evidence of an infection in the urine.
• Additionally, the high levels of cortisol in the blood can cause diabetes mellitus and your veterinarian will want to check for sugar in the urine to rule this out.

How can the disease be treated?

There are different kinds of medication that may be used to treat the pituitary form of the disease.

In a few cases, surgical removal of an adrenal tumor is possible. This surgery is difficult and should be performed by a specialist in veterinary surgery. In addition to the risks of surgery itself, it is very important that animals are closely monitored immediately after surgery and they may need to spend time in an intensive care facility. Your veterinarian may want to start medical treatment before surgery to help decrease the side-effects of high cortisol levels during anesthesia and surgery.

If your dog has signs associated with a mass in the brain they will need additional treatment; as the drugs available will not affect the size of this tumor. Radiotherapy is available at some specialty hospitals and this can help shrink the tumor and eventually decrease the levels of ACTH and cortisol in the blood. Unfortunately, because of its location, surgical removal of the tumor in the brain is usually not possible in Cushing's disease.

Is it worth treating my dog?

Most dogs with Cushing's disease are middle-aged or elderly and owners sometimes ask if it is worth treating them. Once an animal with Cushing's disease is stabilized on treatment they will usually live a normal life (simply taking their tablets on a regular basis). The outcome for dogs with pituitary-dependent Cushing's disease with treatment is very good. Some signs will disappear quickly and others more gradually. Appetite and water consumption usually return to normal in a few weeks; whereas full return of the fur may take several months. Many dogs go on to live a normal lifespan. Without treatment the complications can be significant and will seriously affect the quality of your pet's life.