Acromegaly is not a very common disease in cats, but under certain circumstances veterinarians and owners need to be more aware of it than we currently are.
The condition is caused by a benign tumor within the pituitary gland that secretes excess amounts of growth hormone. The abnormally high circulating levels of growth hormone have effects throughout the body. Physically, cats develop a broad face, big feet, increased body mass, and often their lower jaw will protrude past their upper jaw, which makes their bottom teeth line up in front of their top teeth. Keep in mind that these are changes that occur in an adult cat, not traits that become evident as a kitten matures. Acromegaly most commonly affects middle-aged and older, neutered, male cats.
More important than outward appearances are the changes that are going on internally. The soft tissues at the back of the cat’s mouth can increase in size, making it hard for them to breath. Growth hormone has an effect on heart muscle, which can lead to hypertrophic cardiomyopathy and heart failure. In cases where the pituitary tumor becomes especially large, it can press on surrounding brain tissue, leading to neurologic abnormalities.
One of the most distinctive characteristics of acromegaly is that it is diagnosed almost exclusively in cats with diabetes mellitus. This is because growth hormone antagonizes the effect of insulin, leading to high blood sugar levels. To be clear, cats with diabetes are not developing acromegaly; acromegaly is a relatively rare cause of diabetes … and the diabetes that does develop tends to be relatively unresponsive to treatment with normal doses of insulin.
Acromegaly is usually diagnosed in an ass-backwards manner. A veterinarian will begin treating a newly diagnosed diabetic and it’s not until the cat’s insulin dose reaches shockingly high levels and the disease is still not well-regulated that we pause and think, “Hmm, what’s going on here?”
In a perfect world, we should be evaluating cats for acromegaly at the time they are diagnosed with diabetes. A quick and dirty way to do this is to simply pay more attention to the cat’s overall condition. If he’s a big guy with an underbite, our index of suspicion should go up. Otherwise, acromegaly is rare enough that we can probably continue to ignore the possibility until it reaches out and slaps us.
Confirming a tentative diagnosis of acromegaly is not simple. A blood test called IGF-1 is most commonly used. IGF-1 levels rise with chronically high growth hormone levels, but insulin treatment can do the same thing (which is problematic since cats with acromegaly often are already being treated for diabetes) and untreated diabetics can have falsely low IGF-1 levels. A MRI or CT scan can identify a pituitary mass, but they don’t indicate whether or not it is secreting growth hormone. (Cushing’s disease can also cause poor diabetic regulation and a pituitary mass.)
Treatment is also not easy. Most cats are managed symptomatically. They receive large doses of insulin to control their diabetes (rebound hypoglycemia is a concern, though) and if necessary, therapy for heart disease and any other secondary conditions they might have. Surgery and radiation therapy to remove or shrink pituitary tumors are options worthy of consideration for owners who can afford them, but these advanced treatment modalities are relatively new and only available at veterinary specialty centers.
Dr. Jennifer Coates