Hypertension is the medical term for high blood pressure. In humans, hypertension is related to several factors, including a stressful lifestyle. Although not all causes of feline hypertension have been identified, stress does not appear to play a role in the development of this disorder in cats. However, kidney, thyroid, and heart disease are known to cause feline hypertension.
Hypertension is a "silent" disease with few real symptoms that can be detected by owners or on a physical exam. Visual abnormalities are the most common findings. This may include dilated, unresponsive pupils, blood within the chamber of the eye, and blindness. Blindness is generally secondary to detachment of the retina.
In some cases, hypertension is suspected because of a heart murmur or kidney-related signs, such as increased water intake or urination. Kidney failure and hyperthyroidism have been identified as the two most common predisposing factors for development of feline hypertension. Certain heart diseases can also cause hypertension.
1. Kidney disease
It appears that several different mechanisms may lead to development of hypertension in cats with kidney disease. For various reasons the kidneys become damaged with age and it is harder for the blood to filter through. The effect is like trying to pour spaghetti sauce through a very fine strainer; the blood flow through the kidneys slows down.
The kidneys normally receive about 1/5 of the blood volume of each heartbeat (and that is a lot). If the blood flow through the kidneys gets sluggish, the blood backs up into the arteries and leads to an increase in blood pressure. To imagine this, think of the Lions Gate Bridge at rush hour. Traffic slows on the bridge and then starts to back up all the feeder routes. Although the problem really is only on the bridge, the effects are seen as far away as the highway. This is what happens when the kidneys are not working the blood gets "backed up" into the rest of the body, resulting in high blood pressure.
One study found that about 60% of cats in old-age kidney failure have hypertension. Even cats in the very early stages of kidney disease may have hypertension.
The thyroid gland is located in the neck and plays a very important role in regulating the body's rate of metabolism. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in the metabolic rate. This 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.
Many organs are affected by this disease, including the heart. The heart is stimulated to pump faster and more forcefully, and eventually, the heart enlarges to meet these increased demands for blood flow. The increased pumping pressure leads to a greater output of blood and high blood pressure. About 25% of cats with hyperthyroidism have high blood pressure, although most of them do not have blood pressures high enough to cause blindness.
Diagnosis and treatment of hypertension
Suspected hypertension in :
an older cat with kidney disease (chronic renal insufficiency)
an older cat with hyperthyroidism
kidney problems other than chronic renal insufficiency
Blood pressure is determined with a device (Doppler) that can detect blood flow in arteries. We have special cuffs and instruments for reading feline blood pressure.
Although there are several drugs available for the treatment of hypertension. Studies are still being done in an effort to determine which drugs are the safest and most reliable.
Long-term success depends on whether it is possible to control the underlying health problems. Kidney, heart, or thyroid disease must be treated. Hyperthyroidism is curable; hypertrophic cardiomyopathy and old-age kidney failure are manageable but not curable.
Blindness due to detached retinas is a medical emergency. Blood pressure must be lowered immediately if there is any hope of preserving vision. Retinal detachment for more than 24-48 hours carried an extremely poor prognosis for return of vision.