Monday, June 18, 2012

Hypertension (High Blood Pressure): A Common Problem in Cats


Hypertension is the medical term for high blood pressure, which is a common problem in older cats. In cats, hypertension is commonly found as a complication of other underlying medical conditions (so-called secondary hypertension). However, primary or essential hypertension (i.e., hypertension that develops without any underlying medical disorder) may also be seen in cats (1).

In contrast to people, where essential hypertension is most common, secondary hypertension is more common in cats. The most common cause of secondary hypertension in cats is chronic kidney disease (CKD). After CKD, the next 3 most common causes for hypertension in cats are all hormonal problems (1). These include the following:
  • Hyperthyroidism (caused by a tumor of the thyroid gland that oversecretes thyroid hormone).
  • Hyperaldosteronism or Conn's syndrome (usually caused by a tumor of the adrenal gland that secretes too much of the hormone aldosterone)
  • Diabetes mellitus (caused by lack of sufficient insulin secretion by the pancreas, or resistance to the action of the body's insulin)
  • Obesity (yes, fat tissue is the bodies largest endocrine gland, so obesity is a common endocrine disease) 
Damaging Effects of Hypertension
Hypertension is damaging to the body. In general, hypertension becomes an issue when the blood pressure becomes too high for the vessels carrying the blood.

Imagine attaching a garden hose to a fire hydrant. The high pressure from the hydrant would cause the garden hose to explode. Hypertension is similar. When a blood vessel is too small for the pressure on it, it can “explode,” causing internal bleeding. Since the affected vessels are small, the bleeding may not be noticeable, but a lot of little bleeds and a lot of blood vessel destruction can create big problems long-term.

The effects are most serious in certain vulnerable organs, including the eye, brain, and kidneys.

EyesThe retina (in the back of the eye) is especially at risk in cats with hypertension—sudden or gradual blindness is often the first sign of latent hypertension. Bleeding into the eye and retinal changes such as swelling and detachment can occur. This may result in damage to the cat's vision which is often permanent. In some cats, bleeding into the front of the eye can be seen without the use of special ophthalmology equipment.

Blindness secondary to bilateral retinal hemorrhage

Brain and central nervous system: If a blood vessel ruptures in the brain, the cat may develop neurological signs such as changes in behavior, a wobbly or drunken gait, seizures, dementia, and even coma. In addition to hemorrhage, high blood pressure also increases the risk of embolism: tiny blood clots that form when blood flow is abnormal. These clots can lodge in dangerous locations, such as the brain.

KidneysThe kidney can also be affected, as it relies on tiny vessels to filter toxins from the bloodstream. Not only is kidney disease the most important cause of hypertension in cats, but CKD also progresses much more rapidly in the presence of high blood pressure.

Even in cats that have hypertension from another cause, high blood pressure damages the kidneys and may increase the risk of kidney failure developing.

Clinical Findings in Feline Hypertension
In many cats, no specific clinical signs of hypertension will be seen until the condition advances to the point when blindness develops from spontaneous bleeding into the eye or retina.

As hypertension is often secondary to another disease, most cats with hypertension will be showing signs attributable to their underlying problem. For example, hyperthyroid cats will generally have weight loss (in spite of an increased appetite) and hyperactivity as the major clinical signs. Cats with CKD or diabetes will generally show an increase in thirst and urination.

Diagnosis of Hypertension in Cats
Early recognition of hypertension is important to minimize the damaging effects of persistently high blood pressure on the eyes and other organs (1,2). Without obvious signs of hypertension, such as blindness, we can diagnose hypertension through screening, as in humans.

If your cat has one of the disorders commonly associated with secondary hypertension, such as renal disease or hyperthyroidism, your veterinarian should check its blood pressure. I recommend that even healthy cats have their blood pressure checked annually, especially if they are over 10 years old. Measuring blood pressure only takes a few minutes, is completely pain-free and is extremely well tolerated by most cats.

A complete eye examination is also essential since ocular disease is common in hypertensive cats. In mildly affected cats, subtle changes to the appearance of the blood vessels at the back of the eye (retina) and to the retina itself may be seen. In more severely affected cats, the changes can be dramatic and include retinal detachment and bleeding into the eye.

Treatment of Feline Hypertension
For any cat diagnosed as having hypertension, our goal of treatment is 3-fold:
  1. To reduce the blood pressure using anti-hypertensive drugs
  2. To search for an underlying disease, such as kidney disease, which has caused the hypertension. In some cases, for example hyperthyroidism, treatment of the underlying disease may also resolve the high blood pressure.
  3. To assess what complications of hypertension are present (such as ocular disease)
Cats vary in their response to anti-hypertensive drugs and some will require dose adjustments to normalize their blood pressure. Once stabilized, hypertensive cats should have their blood pressure monitored every 2 to 4 months to ensure that the pressure remains normal.

References:
  1. Jepson RE. Feline systemic hypertension: Classification and pathogenesis. Journal of Feline Medicine and Surgery 2011;13:25-34.
  2. Stepien RL. Feline systemic hypertension: Diagnosis and management. Journal of Feline Medicine and Surgery 2011;13:35-43.

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