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Cushing's Disease In Pets
Cushing's disease (hyperadrenocorticism) is caused by excess cortisol production.
It is a common condition of older dogs but is very rare in cats. Symptoms, diagnosis and treatment are very similar for both species.
Diagnosing the condition can be difficult and may require your veterinarian to carry out several tests.
Treatment is usually quite successful, but it is crucially important to make sure the condition has been correctly diagnosed in the first place.
What is Cushing's Disease
Hyperadrenocorticism occurs when the paired adrenal glands, located just in front of each kidney, produce excessive amounts of the steroid hormone cortisol. Cortisol is sometimes referred to as the 'stress' hormone, as its main function in normal animals is to help deal with stressful situations in the short term. However, in the longer term, elevated levels of cortisol can be very damaging, with adverse effects on many organ systems including the skin, cardiovascular system, immune system and kidneys to name a few.
Cortisol levels are normally regulated by feedback mechanisms between the adrenal glands and the pituitary gland in the brain. The pituitary gland is responsible for 'telling' the adrenal glands how much cortisol to produce in normal circumstances. Hyperadrenocorticism occurs when this feedback mechanism fails.
The majority of cases of Cushing's disease in dogs and cats are caused by a benign tumour of the pituitary gland which then loses the ability to 'switch off' cortisol production. A minority of cases are caused by adrenal tumours. While surgery may be attempted for adrenal tumours, dogs with pituitary tumours are usually managed with medication, as the risks associated with surgery are considerable. Rather than the tumour itself, it is the effects of cortisol that are the real problem in these patients.
The disease can vary in severity depending on the level of excess cortisol. However, the following are the signs 'classically' associated with Cushing's disease:
- Excessive thirst (may be the only sign in many cases)
- Increased appetite
- Bloated abdomen
- Hair loss; often symmetrical hair loss on flanks
- Skin appears thin, blood vessels more visible, blackheads
- Weakness and muscle wastage
Normal Anatomy of The Adrenal Glands
Diagnosis of Cushing's Disease
Confirming a diagnosis may be difficult. Your veterinarian may be highly suspicious of the disease based on patient history and findings on examination; however, several further investigations are usually required.
Blood screens will be required to look for suggestive changes in liver enzymes as well as altered white blood cell ratios. Your pet's cholesterol level may be elevated.
Urine testing is part of the investigation of any patient with increased thirst and will usually show the production of very dilute urine. Animals with Cushing's disease will also often have crystals or stones in their urine, and bacterial urinary tract infections are very common.
Diagnostic imaging: Xrays and ultrasound may be required to look for any growths (tumours) that may be causing the symptoms, as well as to rule out any other concurrent illnesses.
Having carried out some or all of these procedures it will be necessary to then perform more specific testing. The two most commonly performed tests are an ACTH stimulation test, and a dexamethasone suppression test. Both involve assessing the response of the adrenal gland to a hormone administered by the veterinarian. There are advantages and disadvantages to each test, your veterinarian will advise which is the more suitable in the case of your pet.
Why is it difficult to diagnose Cushing's disease in dogs?
Like many diseases of the endocrine system such as hypothyroidism and diabetes mellitus, it is not always a simple matter to be sure of a diagnosis in canine hyperdrenocorticism. There are a number of reasons for this. In a normal animal, cortisol levels fluctuate throughout the day and so a single cortisol reading cannot be interpreted in isolation. Cortisol is not secreted autonomously, but depends on several hormones for its secretion, and any disorder affecting any part of this hormone axis can yield abnormal laboratory readings.
Probably the most significant problem in diagnosing Cushing's disease is the effect of stress on the hypothalamo-pituitary-adrenal axis. Stress of any sort, mental or physical, will affect all of the available tests of your dog's adrenal function. Most dogs with diabetes mellitus for example would test positive for hyperadrenocorticism unless their diabetes is well controlled. It is therefore vitally important for your veterinarian to rule out any other possible concurrent problems before making a diagnosis of Cushing's disease.
Treatment and Monitoring
Because tumour removal is fraught with risks and associated with high mortality rates, most cases of Cushing's in veterinary patients are treated medically. The medications used have become much safer in recent years; however it is still necessary to be cautious when starting treatment; overdosage can be fatal.
For this reason, your veterinarian will probably begin treatment at the lower end of the safety range and it may be necessary to adjust the dose one or more times. Your pet will need to have blood and urine tests carried out regularly to ensure that he/she remains stable but the long term outlook with treatment is very good for most animals.
Many older pets suffer from age-related problems such as arthritis and dental disease. Because cortisol acts as an anti-inflammatory agent and a painkiller in the body, treatment of Cushing's disease will sometimes 'unmask' underlying problems. It is not unusual for patients to develop lameness or to be more reluctant to eat once their adrenal problem is under control. However, your veterinarian should be able to help you manage these other conditions successfully- there are far lass damaging treatments available rather than enormous doses of cortisol.