What is Cushing’s disease?
Cushing’s disease occurs as a result of excessive production of cortisol in the animal’s body. The pituitary gland has control over cortisol production by negative feedback. The levels of cortisol are frequently monitored and when more cortisol is needed in stressful situations, the pituitary gland will increase the production of it from the adrenal glands. If the negative feedback between the pituitary and adrenal glands fails, the production of cortisol in the body will increase and lead to Cushing disease.
Why is cortisol so important?
Cortisol is produced in the adrenal glands that are located adjacent to the kidneys. When stressful situations occur it mobilises fats, sugars and increases blood pressure in order to provide the body with extra boost to fight the unexpected event.
What causes Cushing’s disease?
There are three reasons why Cushing’s disease occurs.
1. A pituitary gland tumour is the most common reason and occurs in 85% of the cases. Most of these tumours grow slowly and are benign. The tumour releases additional quantities of stimulating hormone called ACTH and causes an overproduction of cortisol in the adrenal glands. Due to chronic over-stimulation the adrenal glands are often both enlarged.
2. Adrenal gland tumour occurs in 15% of the cases, and it may be malignant and often causes enlargement of one adrenal gland. ACTH levels released by the pituitary are either very low or non existent.
3. Iatrogenic Cushing’s is caused when cortisone type drugs are used for long periods of time.
It is crucial to determine the type of Cushing’s disease because the treatment may be different.
Who can get Cushing’s disease?
The great majority of cases are middle-aged dogs. Gender does not play any significant role but some breeds like Poodles, Boston Terriers, Boxers, Dachshunds and Beagles seem to have more issues with Cushing’s disease. However, it can happen in any breed. Cats do get Cushing’s disease but it is uncommon.
What are the symptoms?
Dog suffering from Cushing’s disease often drink, urinate and pant excessively, have a potbelly and suffer from hair loss. Hair loss is not associated with itching. Thinning of hair with “blackheads” is often seen. Other symptoms such as calcium deposits in the skin, increase in skin pigmentation, facial nerve paralysis and muscle wasting can also be noticed.
Are the symptoms enough to diagnose Cushing’s?
Many other diseases have similar symptoms but also not every single case of Cushing’s disease shows all the symptoms. Sometimes only one or two can be noticeable which makes diagnosing Cushing’s disease without blood tests very risky.
Most common tests used are:
1. A low dose dexmethasone suppression test can detect up to 90% of Cushinoid dogs. Dexmethasone is very similar to cortisol. When injected into the blood stream in healthy animals, it suppresses the production of ACTH in the pituitary for 8 hours. As a result of the suppression cortisol levels stay low. However if there is a tumour, the suppression will be absent and the cortisol level will stay high.
2. The ACTH stimulation test can be used to diagnose specific types of Cushing’s disease but is often used for monitoring patients. ACTH analogue (Synacthen) is injected into the blood stream and cortisol levels are measured in an hour. A Cushinoid dog will have more cortisol stored in the adrenal glands. As a result of this, additional stimulation with the drug causes higher than expected increase in cortisol levels.
3. Urine cortisol levels can be used to rule out Cushing’s disease. If cortisol levels are low to absent then Cushing’s disease is unlikely to be a problem but if the levels are high more investigation is needed.
The treatment depends on the type of Cushing’s disease. Adrenal gland tumours are treated surgically. Pituitary gland tumours are usually treated medically. If left untreated Cushing’s disease is progressive and affects most of the body. If diagnosed early survival time is between 2-4 years. Small benign tumours in the pituitary and the adrenals have better prognosis then large benign or malignant tumours.
Mitotane (Lysodren) is used to destroy steroid secreting cells in the adrenal gland. This drug is used in management of the pituitary and adrenal dependent Cushing’s disease. If a dog is overdosed or too sensitive to it, it develops the condition called Addison’s disease. Addison’s disease or hypoadrenocorticism is the condition opposite to Cushing’s.
Trilostane (Vetoryl) works well for pituitary gland dependant cases of Cushing’s with its effectiveness being comparable to mitotane. The side effects are similar to mitotane but less severe and usually resolve within 72 hours.
Other medications such as L deprenyl and ketoconazole can also be used. Radiation therapy of large pituitary tumours has been successful.
It is very important to follow safety precautions when handling mitotane and trilostane. Both medications can be harmful to humans - mitotane in particular. Mitotane must be handled with gloves and cutting of tablets should be avoided as much as possible. Before the treatment commences you will be supplied with a handout in which all issues are discussed in detail.
Monitoring of Cushinoid dogs is crucial. If overdosed your dog can develop Addison’s disease and it can be life threatening.
Before the treatment starts it is important to measure water intake for 2-3 days because if the water intake is up it will influence the initial dose of the drug. It is useful to decrease the amount of food by 20 % in order to keep the dog hungry. If the dog is hungry and eating well, it is unlikely that there is side effect to medication. Our staff will give you a regular call during the induction phase that usually lasts 5-7 days to make sure your dog is doing well.
Your dog needs a blood test after the induction phase. The ACTH stimulation test is done to see if the initial treatment was successful. On this occasion the dose will be re-evaluated and adjusted accordingly. Most dogs will do well on medications and only 6 monthly check ups will be required. However, unlucky dogs may need more work or can develop diabetes mellitus which complicates the treatment. Our vets will advise you in detail on the best treatment available for your dog.