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Summary
Hyperadrenocorticism or Cushing's disease, also known as Cushing's syndrome, is a common endocrine disease of middle-aged dogs responsible for a wide variety of symptoms associated with increased secretion of the hormone cortisol from the adrenal glands.
With Cushing's disease, dogs' adrenal glands will produce too much cortisol either 1) because of a tumor located in the brain's pituitary gland (which stimulates the adrenal gland's cortisol-producing tissue) or, 2) less commonly, because of a tumor in the adrenal gland itself.
In the former case, the disease may go by the name of "pituitary gland adenoma," a nod to the benign type of tumor most commonly responsible for Cushings or as adrenal gland hyperplasia by way of describing the brain tumor's effect on the pituitary. In the latter case, Cushing's can be referred to by the kind of tumor less commonly responsible for the disease: "adrenocortical tumor."
A third type of Cushing's disease exists and is known as "iatrogenic Cushing's disease" or "iatrogenic hyperadrenocorticism." This occurs when dogs or cats have been exposed to quantities of cortisol-like steroids over time, therefore eliciting the same symptoms as naturally occurring Cushing's. This third form will not be treated in the discussion to follow.
Though a direct connection to inheritance has not been established, it's widely suspected that a genetic predisposition to Cushing's disease exists due to the prevalence of the disease in certain breeds.
Symptoms and Identification
As previously mentioned, a wide array of symptoms can be attributed to the excess of cortisol in the body: excessive drinking, urination, muscle wasting, a pot-bellied appearance, a thinning of the skin, the prevalence of typically minor infections.
Unfortunately, most symptoms of Cushing's are non-specifically related to abnormal fat and protein metabolism, the immune system and inflammatory processes. This often makes for an insidious onset, difficult identification and delayed diagnosis relative to other diseases.
Diagnosis is usually made based on screening tests that suggest the possibility of excess cortisol secretion. Confirmation is achieved through one or two challenge tests that measure the secretion of cortisol once injectable hormones are supplied. Measuring cortisol is not an effective enough approach given this hormone's fluctuating presence in the bloodstream. That's why the ACTH stimulation and low-dose Dexamethasone challenge tests are employed. These tests may also be used to discriminate between the two types of Cushing's.
Ultrasonography is also strongly recommended to help ensure that no tumor is visible (generally on one of the two adrenal gland), thus altering the choice of treatments available.
Affected Breeds
Cushing's is more prevalent in these breeds:
Treatment
Treatment can be divided into two camps: Medical and surgical. For both basic types of Cushing's disease (brain tumor or adrenal tumor), both medical and surgical options are available.
For "pituitary dependent" hyperadrenocorticism, medical treatment is typically undertaken with one of two common medications. Hypophysectomy, however, is a surgical option that's becoming more widely undertaken as a one-time procedure that may trump side-effect fraught medical therapy.
For the adrenal tumor variety, medical therapy is available but a complete cure may be affected by removing the affected adrenal gland surgically (adrenalectomy). The contralateral adrenal is generally perfectly normal in these cases.
Veterinary Cost
The cost of diagnosis can be inexpensive or not, depending on the degree to which the dog's symptoms confuse the clinician or on the number of affiliated problems a dog may be experiencing. $500 to $1,500 is considered typical for a complete diagnosis (though the low end of this estimate would not include an ultrasound).
Medical treatment can be as low as $50 a month or as high as $200, depending on the dog's response to treatment and the drug selected. Frequent bloodwork must also be factored in to ensure that patients are responding appropriately.
The cost of surgical options like adrenalectomy or hypophysectomy can be very high due to the need to see a board-certified specialist for these complex surgeries. A range of $2,500 to $10,000 is estimated for these procedures (the higher end for the more atypical approach of hypophysectomy).
Prevention
There is no known means of prevention save genetic counseling which would serve to eliminate affected dogs and their first degree relatives from the breeding pool.
References
Merck Veterinary Manual
Feldman, E.C. 1995. Hyperadrenocorticism. In S.J. Ettinger and E.C. Feldman (eds.) Textbook of Veterinary Internal Medicine p. 1538-1578. W.B. Saunders Co., Toronto.