Hypertensive retinopathy is a syndrome that commonly affects older cats causing an abrupt onset of bilateral blindness. This condition is often associated with a cat that suddenly begins to bump into things, appears to be lost, and displays very cautious movements. The loss of vision is associated with dilated pupils that are unresponsive or poorly responsive to incoming light. Affected eyes may appear slightly cloudy, and focal red areas may be visible thorough the pupil, as a result of retinal hemorrhage. This condition is often associated with a systemic hypertension, which is another term for high blood pressure.
The retina is a thin-layered photo sensory nerve tissue, that is closely adhered to the posterior fibrous coat of the eye. When rays of light enter the eye they are focused onto the retina by the cornea and the lens. The retina then transmits a signal to the optic nerve for the brain to interpret as an image. The retina itself contains blood vessels making it vulnerable to damage with an increase in blood pressure. Initially, hypertension can cause vasoconstriction of precapillary retinal arteries, causing hypoxia of the arteriol initiating smooth muscle necrosis. This damage leads to vascular dilation, and leakage of serum and blood between the retina and the fibrous portion of the eye. The leakage of these vessels results in accumulation of fluid between the two layers and is visualized as an out-pouching of the retina, termed serous retinal detachment. When detached, the retina is unable to generate a clear, concise picture from the incoming rays of light causing vision to become distorted and dim. Retinal blood vessels also have the potential to rupture with increased pressure, therefore causing hemorrhage into the vitreous fluid within the posterior chamber of the eye.
In cats the most common causes of systemic hypertension are classified as diseases that either increase cardiac output or cause increased peripheral resistance. Unlike in humans, stress does not seem to play a major role in the cause of hypertension in felines. Instead the two primary diseases feline hypertension is commonly associated with are hyperthyroidism and kidney disease. These two diseases are among some of the most prevalent diseases seen in geriatric felines, making the potential for retinal detachment a valid concern for many patients and clients.
Feline hyperthyroidism is a condition characterized by the overproduction of thyroid hormone resulting in a subsequent increase in metabolic rate. Some studies have shown that eighty-seven percent of feline hypertension cases are attributed to hyperthyroidism. The increase in thyroid hormone initiates the heart to pump faster and as seen with over exertion of any muscle, the contractility of the heart muscle begins to increase with time, resulting in an increased capacity of blood volume to be pumped through the heart. Together the increase in stroke volume and heart rate produces a systemic hypertension.
Kidney disease appears to have several different mechanisms, which may lead to the development of hypertension in cats due to increasing peripheral resistance. Although the exact mechanism is not known it is suggested that as a cat ages, the kidneys slowly accumulate scar tissue. With time, this scar tissue impacts the glomerular capillaries and arterioles, making it more difficult for blood to filter through the kidneys. Because the kidneys normally receive such a substantial amount of blood with every heartbeat, blood backs up into the arteries and leads to an increase in blood pressure. Chronic renal failure is also associated with a decrease in production of renal vasodilatory prostaglandins, increased salt and fluid retention as well as activation of the renin-angiotensin system, all factors ultimately responsible for increasing blood pressure.
Evaluating systemic blood pressure, as well as performing an ophthalmic examination is useful in diagnosing hypertensive retinopathy. The normal systolic blood pressure for an adult cat is measured between 100 and 160 mmHg, therefore systemic hypertension in the feline is defined as a sustained elevation of systolic pressure greater than 170 mmHg. Ophthalmic examination is necessary to determine the integrity of the retina and its associated vessels as well as to evaluate the pupillary light response. Tonometry, the assessment of intraocular pressure, is also a valuable diagnostic aid, and is facilitated through the use of tonometers that are directly applied to the exterior of the eye to determine its responsiveness to a given applied pressure.
Vision loss due to retinal detachment occurs abruptly and requires immediate attention within the first twenty-four hours in order to preserve some retinal function or allow for the possibility of retinal reattachment. Early treatment usually begins with a topical nitroglycerin eye ointment that can rapidly lower intraocular pressure. Topical ointment is also used in conjunction with an oral medication, initiated to lower systemic blood pressure. A common anti-hypertensive drug prescribed is Amlodipine, which is a calcium-channel blocker that decrease free calcium ion concentration in arteriolar and cardiac muscle, resulting primarily in vasodilation and having a minor effect on reducing cardiac output. Beta-blockers such as, propanolol, act directly on the heart and are used to decrease heart rate, cardiac output, and renal rennin release. Long-term antihypertensive therapy may be required depending on the underlying condition that caused the hypertension. For example, in chronic renal failure blood pressure will need to be maintained medically for the life of the animal. However, after successful treatment of hyperthyroidism, with reduction of thyroid hormone stimulation or secretion, metabolic rate returns to normal and subsequent hypertension is corrected.
The overall prognosis for retinal detachment relies primarily on the early administration of the appropriate medication to lower blood pressure. If the retinas remain detached for more than a day, the prognosis for a return of normal vision is poor. Therefore, client education for individuals who have cats that are predisposed to high blood pressure is essential to facilitate the urgency of immediate treatment.
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