CUSHING’S DISEASE IN DOGS

Hyperadrenocorticism, or Cushing’s disease in dogs, is a chronic endocrine disorder characterized by the production of excessive amounts of cortisol. Cortisol is a hormone important in regulating metabolism, immune responses, blood pressure regulation and responding to stress. Prolonged elevation of cortisol will have adverse effects on virtually all organ systems.

Most dogs with Cushing’s disease are middle aged or seniors; however, Cushing’s disease can be present at any age. Due to the gradual development of the disease, early clinical signs are frequently attributed to normal aging. When Cushing’s disease is identified and appropriately treated, the majority of affected dogs can experience a good quality of life for many years.

Causes

The primary cause of increased cortisol levels determines the type of Cushing’s disease, which in turn influences the selection of the best treatment option.

Pituitary Dependent Cushing’s Disease

The most common form of hyperadrenocorticism accounts for approximately 80–85 percent of the total number of cases. This form results from a small, typically benign tumor located in the pituitary gland, releasing excessive amounts of adrenocorticotropic hormone (ACTH). Elevated ACTH stimulates the adrenal glands to increase their secretion of cortisol.

Adrenal Dependent Cushing’s Disease

Excess cortisol is produced independently of ACTH stimulation due to a tumor developing within one of the two adrenal glands. Adrenal dependent hyperadrenocorticism may result from either benign or malignant tumors and requires prompt surgical evaluation.

Iatrogenic Cushing’s Disease

Hyperadrenocorticism secondary to iatrogenesis develops in those dogs receiving large quantities of corticosteroids over long durations. Excessive corticosteroid administration mimics normally produced cortisol and induces clinical manifestations indistinguishable from those found in naturally occurring hyperadrenocortism.

Symptoms

• Increased thirst and excessive water intake
• Frequent urination
• Increased appetite
• Pot bellied appearance
• Hair loss, especially on the body
• Thin, fragile skin
• Panting, even while resting
• Muscle weakness
• Reduced exercise tolerance
• Recurrent skin or ear infections
• Slow wound healing
• Lethargy or decreased activity

Risks and Complications

If you do nothing about Cushing’s disease, it could have an impact on many different parts of the body. It can significantly affect a dog’s comfort, mobility, and overall quality of life.

Cortisol levels in excess of normal create a weaker immune system. This means dogs with Cushing’s disease will be at higher risk of developing infections including bacterial skin infections and urinary tract infections. Cushing’s disease also increases the risk for your pet to develop diabetes mellitus, hypertension, pancreatitis and problems with blood coagulation.

As time progresses, this muscle wasting process continues to worsen. Your pet will suffer from a lack of strength and will struggle to exercise. They may also suffer from an enlarged liver, poor wound healing and chronic skin diseases. Severe untreated cases may result in life-threatening complications such as uncontrolled infections, blood clots, diabetes complications, or organ dysfunction.

Diagnosis

The symptoms associated with Cushing’s disease are similar to those of many other diseases. Therefore, diagnosing your pet with Cushing’s disease involves a physical exam, laboratory testing and advanced diagnostic techniques.

Firstly, your veterinarian will review your pet’s health history. Then they will perform a full physical exam. Blood and urinalysis are common first steps in identifying abnormalities that raise suspicions of Cushing’s disease, such as abnormal liver enzyme activity and urine that is diluted.

After the initial testing has been completed, hormone testing may be conducted to confirm the diagnosis. Hormonal tests include the ACTH stimulation test or the low dose dexamethasone suppression test (LDDST), which measures how the adrenal glands react when placed into a controlled environment.

An abdominal ultrasound is typically used to examine the adrenal glands, to detect masses within these areas and to check the size of the liver and all other organs located in the abdomen. Advanced imaging such as CT or MRI may be considered in selected cases, especially when pituitary tumors require further evaluation.

Treatment

Treatment is based upon both the source of the disorder (pituitary dependent vs. Primary adrenal) and the dog’s general state of health and the extent of the dog’s symptoms.

Medical management

The majority of dogs diagnosed with pituitary dependent Cushing’s disease receive medical therapy that decreases the amount of cortisol produced. A common drug prescribed for this condition is trilostane. Dogs treated with this type of drug must have their hormone levels evaluated periodically through blood work so the dosage can be appropriately adjusted. Since most dogs will need to remain on some form of medical therapy for the remainder of their lives, many owners will see their veterinarians regularly for evaluation.

Surgical treatment

Dogs with an adrenal tumor(s) may be eligible for surgical removal of the involved adrenal glands. When successful, this represents the first time in which a cure is possible, primarily when there is no evidence of metastasis to other areas of the body.

Due to the technical difficulties associated with performing adrenal surgery, these procedures are typically limited to veterinary surgeons with significant experience in this area, who often refer their clients to specialty hospitals for this reason.

Managing Iatrogenic Cushing’s disease

When corticosteroids are the causative agent, the veterinarian will gradually decrease the dose instead of discontinuing them abruptly. By doing so, the animal’s adrenal glands will recover from the effects of prolonged exposure to elevated steroid hormones in a relatively safe manner and minimize potential adverse consequences.

Prevention

While naturally occurring Cushing’s disease cannot always be avoided due to its usual spontaneous origin from either a pituitary tumor or spontaneously developing primary adrenal tumors, there are several steps that owners can take to assist in reducing the risk of developing complications and increasing early detection.

Regular veterinary checkups, particularly during middle age and senior years, can assist in identifying gradual changes prior to development into a more advanced state. Owners should consult with their veterinarian regarding any changes in drinking habits, urination frequency, unusual weight changes, or skin problems.