by Gary D. Norsworthy, DVM, Diplomate, ABVP
What is hypertension?
Hypertension is a medical term that means high blood pressure.
I thought hypertension was caused by a stressful lifestyle. My cat certainly does not have that.
Hypertension in people is related to several factors, including a lifestyle that constantly produces stress. Not all of the causes of hypertension in cats have been identified. However, it does not appear that psychological stress plays a role in development of this disease as it does in humans.
How do I know if my cat has hypertension?
The most common clinical presentation of feline hypertension is sudden blindness. This occurs because high pressure in the blood vessels of the retina causes the retina to detach. Affected cats have widely dilated pupils that do not constrict when exposed to bright light. These cats run into objects in their path because most of them have no vision at all.
What causes hypertension in cats
We have identified two major causes of feline hypertension. The first is kidney failure. The second is heart disease.
How does kidney failure lead to the development of hypertension?
The kidneys are filters that remove waste products from the blood. As a cat gets older, the kidneys undergo normal aging changes, including the slow accumulation of scar tissue. This scar tissue causes the kidneys to shrink in size. With every heartbeat, about 20% of the blood pumped out is delivered to the kidneys. When the kidney shrinks due to the accumulated scar tissue, it is harder for the blood to filter through. There is a backup of blood into the arteries and an increase in blood pressure.
One study found that about 60% of cats in old-age kidney failure had hypertension. Elderly cats not in actual kidney failure may also have hypertension by the same mechanism.
How does heart failure cause hypertension?
Not all forms of heart failure cause hypertension. However, there are at least two forms that have been shown to cause hypertension in cats. Hypertrophic cardiomyopathy is a disease of the heart muscle with an unknown, cause. It makes the heart muscle thicken, resulting in blood being pumped more forcefully. This increase in cardiac output of blood results in hypertension. Hyperthyroidism is another cause of heart disease that may cause hypertension.
What is hyperthyroidism, and how does it cause hypertension?
The thyroid is a gland located in the neck. It plays a very important role in regulating the body's rate of metabolism. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone.
When excessive amounts of thyroid hormone are in the circulation, the body's metabolism speeds up greatly. Many organs are affected by this disease, including the heart. The heart is stimulated to pump faster and more forcefully, much like hypertrophic cardiomyopathy. This results in a greater output of blood and high blood pressure. About 80% of cats with hyperthyroidism have high blood pressure, although most of them do not have blood pressures high enough to cause blindness.
Hyperthyroidism is a fairly common disease of older cats. Although the thyroid gland enlarges, it is usually a non-malignant change (benign). Less than 2% of hyperthyroid cases involve a malignant change in the gland.
How is hypertension diagnosed?
Hypertension is suspected in any cat with a sudden onset of blindness, kidney failure, heart disease (especially hypertrophic cardiomyopathy), or hyperthyroidism. If any of these are diagnosed, the cat's blood pressure will be checked, if the equipment is available. If a sudden onset of blindness occurs and the blood pressure is found to be abnormally high, the other conditions are considered as possible underlying causes. This means that radiographs (x-rays) of the chest are taken and blood tests are performed for kidney and thyroid function.
Why don't all veterinarians have blood pressure testing equipment?
Blood pressure is determined with a device that can detect blood flow in arteries. However, the cat has very small arteries compared to humans. Therefore, the standard blood pressure equipment used on humans will not work on cats. Only two blood pressure machines have been found reliable in cats. One costs several hundred dollars and the other several thousand. Thus, the expense in purchasing this equipment, coupled with the relative infrequency of hypertension, makes ownership of blood pressure equipment prohibitive for many veterinarians.
ls there a way to treat it?
There are several drugs that are very effective in treating hypertension in humans. However, none of these are approved for feline hypertension. Veterinarians have found that some of these drugs are effective in cats. Although research is still ongoing to determine the most effective drugs, several have been found that will lower the cat's blood pressure.
How successful is treatment?
If the cat has blindness due to detached retinas, a medical emergency exists. If the retinas remain detached for more than a day or two, they probably will not return to their proper position and become functional again. Therefore, the key to successful treatment is rapid diagnosis and early administration of the proper medications to lower blood pressure.
If the cat has kidney, heart, or thyroid disease, it is also important to treat those aggressively. Hyperthyroidism is curable, but hypertrophic cardiomyopathy and old-age kidney failure are not. However, even those can be managed successfully in many cats.
What is likely to happen in the long term?
If blood pressure can be lowered quickly, some blind cats regain their sight. If blood pressure can be maintained in the normal range, these cats will retain their sight. However, the underlying disease that caused hypertension must also be cured or controlled. Long-term success depends on whether or not this is possible.