Cushing's Disease (Hyperadrenocorticism)
What is Cushing's Disease?
Cushing's disease is a disorder caused by the harmful effects of high levels circulating cortisol on multiple organ systems. The adrenal gland, which is located near the kidney, is the gland largely responsible for the production of cortisol.
How Does Cushing's Disease Occur?
There are three ways in which Cushing's disease can be caused:
(i) Iatrogenic - Cushing's disease in this case is caused as a result of excessive administration of synthetic cortisols. This can occur from the administration, oral or injectable medication containing cortisol over a long period.
(ii) Adrenal gland tumour - This occurs in 10 - 15% of cushingoid patients. It occurs as a result of either a benign or malignant adrenal gland tumour. These tumours result in excessive levels of cortisol being produced. If the tumour is benign surgical removal results in a cure of the disease. However, surgery helps the patient for a while only when the tumour is malignant and the long term prognosis is poorer than that of benign tumours.
(iii) Pituitary gland tumour - This is the cause of 85% of cushingoid cases. The tumour can also be benign or malignant, and causes the pituitry gland, which is located in the brain, to overproduce a hormone which acts on the adrenal gland and stimulates the adrenal gland to secrete excessive cortisol. The larger the pituitary tumour the greater the signs of Cushing's disease the dog shows.
What Signs do Affected Dogs Show?
The signs seen are due to excessive cortisol levels. They include increased drinking and urination, fat abdomen, enlarged liver, hair loss, lethargy, muscle weakness, obesity and paper thin skin.
How is Cushing's Disease Diagnosed?
Clinical signs may give an indication that the dog is suffering from Cushing's disease. Various tests are required to confirm the presence of disease and find out the primary cause of the disease. The tests include:
(i) Full blood profile - Often shows a stress picture with high levels of liver enzymes, cholesterol and glucose.
(ii) Urine analysis - Shows poor concentration of urine.
(iii) Various cortisone tests - The purpose is to measure cortisol levels in the blood under various conditions.
(iv) Ultrasound of adrenal gland - The aim is to look for tumour in adrenal gland.
Can Cushing's Disease be Treated?
Treatment options differ in accordance with the form of Cushing's disease present.
(i) Iatrogenic Cushing's disease - Treatment includes discontinuing of the cortisone being given. Unfortunately the condition for which the cortisone was being used will often recur.
(ii) Adrenal gland tumour - Surgery is often required to remove the tumour. There is usually a good chance for the patient to regain normal health if the tumour is successfully removed and not malignant in nature.
(iii) Pituitary gland tumour - Mitotane is the drug of choice for the medical management in most cases. It works by destroying the abnormal adrenal tissue. However, care must be given to ensure enough and not too much of the drug is given. If too little is given the abnormal tissue persists and the disease continues, whereas if too much is given most or all of the adrenal tissue is destroyed and this can be life threatening.
Overall What are the Dog's Chances?
Depending on the cause of the problem the cushingoid signs normally resolve within several days to months following appropriate treatment. Untreated cases generally progress and have a poor outcome.
Treated pituitary cases usually have a good progress with the average survival time being 2 years. 10% of cases live 4 years.
Adrenal cases have an excellent chance of survival where the tumour removed is small and benign. Generally those with large or malignant tumours have a poor to fair outcomes.