PROGRESSIVE RETINAL DEGENERATION OR PROGRESSIVE RETINAL ATROPHY

Progressive retinal degeneration (PRD) [also known as progressive retinal atrophy (PRA)] refers to several different retinal diseases that cause blindness. There are two distinct patterns to this disease. Some breeds have blindness by abnormal development (dysplasia) of the retina. Other breeds have a slow progressive death of retinal tissue (degeneration). These two types of disease affect many breeds. In general these diseases are inherited. The pattern of inheritance may be different in each breed. In all patients with PRD the outcome is the same--blindness. The age and breed of the patient and what the Veterinary Ophthalmologist finds during the examination are the basis for the classification of exactly what type of condition the patient has. Different breeds of dogs have variations in the age at which the problem starts and speed with which the blindness develops. The condition of PRD has been seen in almost every registered breed and in mixed breed dogs as well. This condition occurs in humans and is known as retinitis pigmentosa. As the name PRD implies, a slow degeneration or death of retinal tissue occurs. It is a slowly progressive disease and the earliest signs may be overlooked. As stated above, PRD or the gene for PRD is known to be passed from parents to offspring even though the parents may have normal eyes and vision. Therefore, identification of breeding animals with or related to patients with PRD is essential to prevent spread of this condition. At this time, researchers are working to find a blood test to identify "carrier" animals.

To better understand PRD, a basic understanding of the function of the retina is needed. The retina is a highly complicated tissue located in the back of the eye. Light strikes the retina and starts a series of chemical reactions that causes an electrical event (nerve impulse). The impulse passes through the layers of the retina to the optic nerve and from there to the brain where vision takes place. In the retina, cells called rods are involved with black and white or night vision and cells called cones are involved with color or day vision. Progressive retinal degeneration may effect either the rods alone, the cones alone or both the rods and cones together. Because PRD is not a painful condition your pet will not have reddened eyes or have increased blinking or squinting. For this reason most clients will not notice the earliest stages of the condition. Some clients will eventually notice an abnormal shine coming from their pet¹s eyes. This abnormal shine is because the pupils are dilated and don¹t respond as quickly to light as pupils of normal dogs. Early signs of PRD include night vision difficulties. Clients often remember their pets seemed disoriented when going out to the yard at night and they had to leave a light on for them. Night blindness may be manifested by a pet that is afraid to go into a dark room. Occasionally these pets will become lost in their own home after the lights have been turned off. The Veterinary Ophthalmologist examines the retina with an instrument called an indirect ophthalmoscope. Specific changes in the retinal blood vessel pattern, the optic nerve head, and the reflective substance within the dog¹s eye (tapetum) are classic for PRD. However in some breeds PRD has little or no early changes. The eyes of these dogs may appear normal on the examination until they are in the later stages of the disease. In different breeds, PRD will progress at different rates. This variation causes difficulty in determining just how long any particular dog will be able to see. There are no cures for PRD although a number of vitamin therapies have been suggested by various people. At this time, none of the vitamin treatments have been effective and some of these 'cures' may actually hurt your pet. Cataracts may occur in some patients with PRD and generally occur later in the disease. Formation of cataracts may interfere with the Ophthalmologist's examination of the retina and make other tests such as an electroretinogram (ERG) essential for accurate diagnosis. In patients with cataracts the diagnosis is made and confirmed by the ERG. This test involves sophisticated instrumentation used to measure the response of the retina when a light is shined upon it. Your pet would be anesthetized for this test. The pet is then placed into a darkened area, a special contact lens with a gold ribbon is placed on the cornea and two tiny needles are placed under the skin. A dim light flash stimulates the retina and this procedure is repeated for 20 minutes. Finally, a bright red, blue and white flash are used for final analysis. A healthy retina will produce a characteristic wave that builds from the time the lights are turned out. The ERG is sensitive enough to diagnose dogs with PRD before they begin to demonstrate signs of the disease. Here is a partial list of breeds affected with different types of PRD:

In summary, PRD refers to a group of inherited retinal diseases which result in the blindness of dogs. Because of the nature of the disease and sometimes the late onset, repeated examinations may be required to detect individuals with the condition. Patients affected should not be used for breeding. Pedigree studies are used to help eliminate carriers of this condition such as the pet¹s brothers, sisters, mother, father and any offspring. How to adjust to having a pet that is blind is important and is discussed in the information on "Dealing with a blind pet.".

Have any questions on this subject? Contact Dennis Hacker via E-mail:

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