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Cushing's disease (hyperadrenocorticism) is a common hormonal disorder of older dogs which may have a significant impact on their quality of life. This article provides you with information about the causes and management of the disease and how to recognise if your dog could be affected by it.
Cushing's disease may occur either spontaneously or as a consequence of long-term use of steroid medication.
In spontaneous Cushing's disease, there is excessive production of the steroid hormone cortisol by the adrenal glands (small glands located next to each kidney). In the normal dog, cortisol is released in response to stress and has the following actions:
Release of cortisol from the adrenals is, in turn, stimulated by release of the hormone ACTH from the pituitary gland in the brain. In a dog with spontaneous Cushing's disease, the excessive cortisol causes continual stimulation of these processes even in the absence of stress, to the point that they are detrimental to the animal's health. There are two causes of spontaneous Cushing's disease: a) Pituitary gland-dependent - 85% of spontaneous cases are caused by a tumour of the pituitary gland, which overproduces ACTH and results in excessive cortisol production by the adrenal glands. Although a tumour sounds very concerning, try not to worry - unlike an aggressive cancer, these generally grow extremely slowly and it is rare for them to cause any problems apart from Cushing's disease. b) Adrenal gland-dependent - the remaining 15% of spontaneous cases are caused by tumours of the adrenal glands themselves. Approximately 50% of these tumours are benign, whilst the remaining 50% are aggressive tumours which can spread to other organs such as the lungs. Unfortunately, there is no reliable diagnostic technique to distinguish between benign and aggressive tumours.
Cushing's disease may also be caused by long-term administration of high doses of certain steroid tablets (e.g. prednisolone) and injections, or eye/ear drops containing steroid, as these work in the same way as natural cortisol. Therefore use of these medications is best restricted to short courses and/or low doses, unless there is no alternative.
Breeds that are at increased risk of Cushing's disease are:
However, any breed may potentially be affected. The average age of onset is 10 years of age but Cushing's can affect dogs of a much younger age. Symptoms may include:
Not all dogs will display all of these symptoms, but the vast majority will have noticeable changes in their drinking and urination soon after the onset of disease. Changes in the skin and muscles tend to occur a little later on.
If your dog is displaying any of these (or indeed any other) symptoms, it is best to book an appointment with your vet as soon as you can. Left untreated, Cushing's disease can cause further complications such as diabetes. If you think your dog is drinking more, it can be helpful to measure approximately how much water he/she is drinking per day and let your vet know (although admittedly this can be challenging if you have more than one pet). At the beginning of the day, use a measuring jug to record exactly how much water you are providing in the dog's bowl (s), then measure how much is left at the end of the day and work out how much your dog has consumed in total. If he/she is very thirsty then you may well have to refill the bowl (s). Water consumption should not normally exceed 100ml per kg of bodyweight over a 24 hour period - if it does, it is too high. Do remember that your pet will start drinking more anyway if you change him/her from a wet to a dry diet - but measuring water intake will still help to confirm if this is abnormally high. It is usually helpful if you can collect a urine sample from your dog on the morning of the day of appointment so that this can be analysed by your vet. If you are not used to doing this, the video in this link has some good advice: view video Unfortunately, there is no single diagnostic test that will 100% reliably diagnose Cushing's disease, so your vet will need to use the urine test in conjunction with several other tests to assess whether your dog is likely to have the disorder. Blood tests are most commonly used and can help to rule out other illnesses that may be causing the symptoms. An abdominal ultrasound may be recommended by your vet to help distinguish pituitary-dependent from adrenal-dependent Cushing's disease; however, the adrenal glands are difficult organs to ultrasound so your vet may advise referral to a specialist in diagnostic imaging. Not all owners can afford this, so you may choose to manage presumptively for a pituitary-dependent disorder as this is the most common form of Cushing's disease and is most easily treated. Where Cushing's disease is caused by a benign adrenal tumour, there is often a good response to this treatment anyway. However, with this approach you have to accept that there is a very small chance of your dog getting worse due to an undiagnosed aggressive adrenal tumour. Your vet may advise x-rays to look for secondary abnormalities such as liver enlargement; adrenal tumours can occasionally be seen on these.
In the UK, there is only one licensed medical treatment for pituitary-dependent Cushing's disease. This is a capsule called Vetoryl, manufactured by Dechra Veterinary Products (the active ingredient is the drug trilostane). This acts to inhibit cortisol production. The medication is short-acting so should be given daily, and is normally required for the rest of the dog's life. Symptoms generally improve quickly, often within the first 2 weeks of starting treatment. Your dog will need regular check-ups and blood tests with your vet to make sure that the medication is working and that there are no side-effects or complications. He/she will need a blood test at 10 days, 4 weeks and 12 weeks after starting Vetoryl, and every 3 months thereafter. The prognosis is generally good with these dogs. Most dogs respond very well to Vetoryl, but in rare cases that do not respond an alternative, unlicensed drug called mitotane may be used. This destroys the cells in the adrenal glands that are over-producing cortisol. The pituitary gland tumour itself is not normally treated directly unless it is causing neurological symptoms, such as incoordination, or behavioural problems.
Management of dogs with adrenal-dependent Cushing's disease is more complex, because of the chance that the tumour is aggressive. Often these cases are best managed by a specialist vet, at least in the initial stages of treatment. Essentially there are three options: surgery to remove the adrenal tumour, treatment with mitotane to destroy the overactive adrenal cells, or treatment with Vetoryl to control the symptoms. Prognosis is quite variable and is influenced by whether there are any signs of spread on diagnostic imaging, so is best discussed with your vet on an individual case basis. Surgery should never be performed if there are already signs of spread. Your vet will be able to talk about these treatment options and their associated pros and cons in more detail with you.
In most cases, symptoms improve simply by withdrawing the treatment, but it is very important that this is done by slowly tapering the dose - suddenly stopping the treatment can be very dangerous as it does not give the body time to compensate. Never stop medications without prior discussion with your vet.
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