Cushing’s syndrome, technically called hyperadrenocorticism, is a chronic progressive disease and one of the most common endocrine disorders in older dogs. It is named after Dr. Harvey Cushing, who first described the disease in people with pituitary tumors.
Cushing’s syndrome develops following prolonged exposure to abnormally high levels of circulating steroid hormones. These hormones are produced by the adrenal glands under the influence of the pituitary (pituitary) gland.
The most common signs of this disease include an increase in appetite and a sense of thirst, symmetrical loss of hair on both sides of the dog’s body, muscle atrophy, poor tendency to heal wounds and abdominal distension (bellied aspect). Fortunately, Cushing’s syndrome is largely treatable and usually manageable. For this reason, it is important that the dog owner becomes familiar with the signs of this disease.
Cushing’s syndrome is caused by an excess of freely circulating corticosteroid hormones. These hormones are produced by the adrenal glands, two small structures located in the abdomen above each kidney. The outer layer of these glands, called the adrenal cortex, produces corticosteroid hormones which, in turn, are responsible for regulating the concentration of electrolytes, managing inflammatory reactions and suppressing the immune system. The production and release of steroid hormones are regulated by the pituitary or pituitary gland located in the brain. The pituitary gland produces the adrenocorticotropic hormone (ACTH) which, when released, stimulates the production of steroids by the adrenal glands.
The correct term for Cushing’s syndrome is hyperadrenocorticism.
- This condition is most commonly caused by excessive production and secretion of the adrenocorticotropic hormone (ACTH) by the pituitary gland, following functional pituitary tumors or excessive release of the corticotropin-releasing factor (CRF) by the hypothalamus.
- Cushing’s syndrome can also be caused by functional tumors of the adrenal gland, which promote excessive corticosteroid secretion even in the absence of ACTH’s pituitary stimulation.
- Finally, Cushing’s syndrome can be caused by excessive or prolonged use of steroid drugs, which stimulate or simulate an excessive production of adrenal hormones (iatrogenic Cushing’s syndrome).
Since hyperadrenocorticism affects middle-aged and elderly dogs more frequently, the symptoms of the disease can mislead the animal owner causing him to consider them as natural effects of dog aging. Cushing’s syndrome is caused by an excess of the circulating cortisol hormone, which can be due to functional tumors of the adrenal glands and / or pituitary gland (pituitary gland). This ailment can also be caused by medications given to treat other ailments that stimulate excessive adrenal hormone production. This form of Cushing is called “iatrogenic”. However, if diagnosed correctly and in a timely manner, Cushing’s syndrome can usually be managed to eliminate it or mitigate its severity and significantly improve the dog’s quality of life. Since Cushing’s syndrome is largely treatable, recognizing clinical signs is critical to the dog’s prognosis.
Cushing’s syndrome occurs most commonly in older animals and is able to mimic the so-called “normal” signs of aging. The owner of the affected dog may observe one or more of the following signs of disease:
- Increased sense of thirst and water intake (polydpsia)
- Increased urine output (polyuria)
- Inappropriate urine output (the dog urinates at home or in other unusual places)
- Increased appetite and food intake
- Weight gain, obesity
- Symmetrical patchy hair loss (alopecia)
- Thin or fragile skin that tends to tear easily
- Exercise intolerance
- Swollen (hypertrophic) or atrophied external genitalia
- Lack of coordination (ataxia)
- Low tendency to heal wounds
Cortisol increases appetite and thirst, so it is possible that the owner of the affected dog notes the need for more frequent administration of food and water than usual. Often, in fact, in these cases the owner defines the appetite of the voracious dog. Similarly, he may notice and report an anomalous and symmetrical hair loss, which affects both sides of the dog’s body, combined with a loss of muscle mass, particularly observable in the legs. Muscle atrophy and the corresponding redistribution of weight often give the dog suffering from Cushing’s syndrome a “pot-bellied” appearance. Usually, the animal also has a low tendency to heal wounds. Dogs with hyperadrenocorticism are predisposed to developing other problems, including heart failure, diabetes mellitus, infections and high blood pressure. Typically, many of these signs appear simultaneously. As the disease progresses, the signs manifested by the affected dog worsen and increase.
Dogs most at risk
Cushing’s syndrome occurs most frequently in small dogs weighing less than 20.4 pounds. It is commonly observed in dogs over the age of 6 years, with a slight predisposition for female dogs, although it has also been diagnosed in dogs aged 1 year. Beagle, Boxer, Dachshund, German Shepherd, Labrador Retriever, Poodle and some Terrier breeds (especially Boston Terriers) seem to be overrepresented.
Any dog suspected of having hyperadrenocorticism will undergo a careful physical examination and its clinical history will be assessed. The initial data collection will include a complete blood count (CBC), a biochemical profile of the serum and a urine analysis, with or without bacterial culture. Other routine diagnostic tests include abdominal radiographs and / or ultrasound and blood pressure assessment.
To confirm the diagnosis, the veterinarian may perform more advanced laboratory tests, including tests to evaluate cortisol: creatinine in the urine, suppression test with low and high dose dexamethasone, stimulation test with ACTH and / or a evaluation of endogenous ACTH concentrations in the blood. Magnetic resonance imaging (MRI) and computed tomography (CT or CT) can also be helpful in the diagnostic process, as they can help visualize any tumors of the pituitary gland or adrenal glands.
One of the simplest tests among those mentioned above is the evaluation test of the cortisol ratio: creatinine in the urine. As a rule, the owner is asked to collect a urine sample from the dog in the early morning, which must be taken to the vet to measure cortisol and creatinine levels. Essentially, normal test outcomes rule out the diagnosis of hyperadrenocorticism. However, the so-called “false positive” outcomes can occur for up to 75% of dogs that do not have Cushing’s syndrome, making further diagnostic tests necessary.
The suppression test with low and high dose dexamethasone and the stimulation test with ACTH, performed together or separately, can lead to a definitive diagnosis of hyperadrenocorticism. These tests initially involve taking a blood sample, administering the dog with dexamethasone or ACTH by means of an injection, and then taking blood samples at appropriate intervals. All blood samples are sent to a laboratory for careful analysis. The veterinarian is the most suitable professional figure to decide which tests to perform according to each specific case.
The treatment protocol will depend on which of the three forms of the disease is present. Iatrogenic Cushing’s syndrome is caused by excessive or prolonged administration of corticosteroid drugs. This form of the disease can be treated, and usually cured, by gradually decreasing and unaccustoming the animal to these drugs, if possible.
Cushing’s syndrome caused by functional tumors of the adrenal glands or pituitary gland can only be treated by surgery or by the administration of life-long medications. Adrenal tumors usually require surgical removal, which is highly effective in the absence of metastasization. Unfortunately, few cases of natural hyperadrenocorticism (i.e. not due to the action of drugs) are caused by adrenal tumors. Most dogs with Cushing’s syndrome have pituitary gland tumors. Currently, pituitary tumors are not normally surgically removable.
Medical treatments are also available, although protocols are complex. Certain medications, including Mitotane (Lysodren), Ketoconazole, L-Deprenyl (Anipryl) and Trilostane (Vetoryl), have been used to treat Cushing’s syndrome with varying success to suppress corticosteroid production. Each of these drugs can have serious adverse side effects and most of them can only be administered under close veterinarian supervision. Blood tests should be performed periodically to make sure that any chemotherapy does not cause or contribute to other medical problems. Despite the risk of side effects, supervised medical treatment options can improve and prolong the quality of life of dogs suffering from this disease. In some cases, radiation therapy can also be an option.
The prognosis for dogs with Cushing’s syndrome varies. If the underlying cause is an adrenal tumor that can be successfully removed, the prognosis is quite good. However, if a pituitary tumor is involved and the only treatment is chemotherapy, the prognosis is confidential.
Unfortunately, in addition to managing the medical use of corticosteroids, there is no way to prevent hyperadrenocorticism in the dog. Functional tumors of the pituitary gland and / or adrenal glands arise for unknown reasons and until the cause of these malignancies is discovered the prevention of Cushing’s syndrome can be defined as unrealistic.