To better understand progressive retinal atrophy, one must have a basic understanding of the function of the retina. The retina is a highly specialized sheet of neurological tissue located in the back of the eye. The retina is analogous to film in a camera; it is responsible for integrating light into vision. Without a normally functioning retina, vision is not possible. Simplified, the eye can be thought of as a light collecting organ that focuses light rays on the retina. As light strikes the retina, a sequence of chemical reactions are initiated. The sequence of chemical reactions propagate an electrical impulse. The impulse passes through the layers of the retina to the optic nerve and finally to the brain (visual cortex) for interpretation. This interpretation by the brain and propagation of a signal by the retina is required for normal vision.

The retinal cells which transform light energy to chemical energy in the retina are known as rods and cones. Rods are responsible for black and white vision, night vision and vision for movements, whereas cone cells are used for color discrimination, vision in bright light and acute focal vision. Most domestic animals (dogs, cats, etc.) have a dominance of rods and therefore have good night vision, peripheral vision and vision for movement. Color vision in dogs is poor compared to people. Cats may have slightly better color vision than dogs. Most birds have excellent color vision.

Retinal atrophy frequently affects both rods and cones though rods are usually affected first. Likewise, gradual loss of night vision is usually the first symptom observed in animals with progressive retinal atrophy. Progressive retinal atrophy refers to a group of retinal diseases that cause blindness in dogs due to abnormal development or slow progressive deterioration of the retinal tissue. The disease affects many breeds of dogs, each in a distinct manner. In general the condition is thought to be inherited but each breed may have a special and individual mode of inheritance. Similarities in the outcome and the observed clinical signs are the basis for group classification. Specific breed variations include age of onset, rate of progression, and other associated ocular signs.

As the name (progressive retinal atrophy or PRA) implies, an atrophy or a degeneration of retinal tissue occurs. In many animals a slowly progressive disease occurs and the early signs may be overlooked. The slow loss of sight is similar to a dimming switch to reduce brightness of light in a room. If the light is slowly reduced over a long period of time, our eyes adapt and the change is not noticed until darkness occurs. A similar situation occurs in progressive retinal atrophy in animals; often the condition is not noticed until the condition is significantly progressed. Unfortunately there are no cure available for progressive retinal atrophy. Identification of affected breeding animals is essential to prevent spread of the condition within the breed.

Most retinal atrophies or degenerations are inherited, although rare cases of nutritional disease have been described. Recently, veterinary ophthalmologists have described an atrophy associated with changes in appetite and water consumption in patients. This disease, known as sudden acquired retinal degeneration (SARD), may affect any breed of dog including mongrel and crossbred dogs.

Clinical Signs of PRA
The early signs of retinal atrophy include night blindness in most cases, which will frequently progress to day blindness. Pupils may be dilated and/or have a slow response to light. Some pet owners will notice a characteristic shine from the eye. This is due to increased reflectivity of an iridescent tissue known as the tapetum located underneath the retina. Night blindness may be manifested in a number of ways, including a pet that is afraid to go out in the dark or go into a dark room. Often these pets will get lost in their own home after the lights have been turned off or they may stay near the light in the back yard at night versus wandering the full extent of the yard as they did previously. As previously mentioned, retinal abnormalities may not be noticed at home until later in the course of the disease. Other well developed senses including olfaction (the sense of smell) and hearing help animals adapt to the slow loss of sight. Often sight loss is not noticed until a change of the pets’ normal environment occurs like when furniture is rearranged in the home or the animal is taken into a different area or is boarded while you are away on vacation. Because PRA can be difficult to identify, routine ophthalmic examination of all pets is recommended. This is especially important in animals that are being considered for breeding.

When the ophthalmologist views the retina with an instrument called an indirect ophthalmoscope, changes can be seen in the retinal blood vessel pattern, the optic nerve and the tapetum (the reflective portion of the eye that is responsible for “eye shine”). However, some breeds characteristically have little or no early visible changes and may appear normal until the later stages of the disease. Some affected dogs show various rates of progression making generalization difficult.

Cataracts may form secondarily to progressive retinal atrophy in some animals and are generally associated with the later stages of the disease process. Formation of cataracts may interfere with direct visualization of the retina and make other diagnostic modalities essential. Although cataracts are a surgically correctable condition, removal of cataracts in an animal with progressive retinal atrophy is not indicated, as their diseased retina will still prevent them from seeing. Cataracts; however, can cause inflammation inside of the eye, which should be medically treated.

Electroretinography
Definitive diagnosis of PRA is supported by electroretinography. This noninvasive test involves the use of sophisticated instrumentation to measure the electrical potential within the retina. It is similar to an electrocardiogram for the heart in that they both measure normal electrical impulses and produce characteristic recorded wave forms. A special contact lens with a gold ribbon is placed on the cornea and two tiny needles are placed under the skin around the eye. After a period of dark adaptation, a light stimulates the retina. A healthy retina will produce a characteristic recording. This instrument is sensitive enough to diagnose affected dogs before they begin to demonstrate clinical signs.

Any diagnostic procedure can introduce complications, including anesthetic risks (in the few patients that require anesthesia for diagnostic procedures). However, complications from ERG are very rare. Nevertheless, potential complications include, but are not limited to, inflammation of the pink tissue (conjunctivitis); ocular infections that may affect internal and/or external areas of the eye (intraocular/extraocular infections) and corneal ulcerations (superficial to deep). If any abnormalities are noticed in your dogs eyes following an ERG please notify us immediately so that the condition does not worsen

Genetic testing
Since PRA is an inherited genetic disease, it is possible to identify and test for the defective gene. This test has been developed in many but not all breeds affected by PRA. The test requires a blood sample to be submitted to a lab and dogs that carry the bad gene as well as dogs that will be affected by the disease can be identified. Information on genetic testing can be found at www.optigen.com.

A partial list of breeds affected with progressive retinal atrophy follows:

Breed Type of Retinal Disease Age of Onset

Collie rod-cone dysplasia under a year

Irish Setter rod-cone dysplasia under a year

Cairn Terrier rod-cone dysplasia under a year

Min. LH Dach. rod-cone dysplasia under a year

Norwegian Elkhound rod dysplasia, cone 2 to 3 years

Samoyed rod-cone degeneration 3 years

Cocker Spaniel rod-cone degeneration 2 to 7 years

Miniature Poodle rod-cone degeneration 3 to 6 years

Miniature Schnauzer rod-cone degeneration 3 to 6 years

Akita rod-cone degeneration 3 to 6 years

Schnauzer rod-cone degeneration > 3 years

Golden Retriever rod-cone degeneration

Labrador Retriever rod-cone degeneration

Therapy
Unfortunately, no treatment has been formulated to prevent, treat or cure progressive retinal atrophy. A number of vitamin therapies have been suggested, however, there is no evidence to suggest that vitamins have any therapeutic effect. As stated previously, affected animals should be identified as early as possible and eliminated from breeding programs.

Progressive retinal atrophy is a painless condition. Animals that lose sight from PRA usually acclimate well to their environment with time, as they utilize their other senses to make up for their vision loss. Maintaining a consistent environment for the affected animals will help the acclimation process. For example, frequent furniture rearrangement during this period should be avoided. When animals are taken from their home environment, the use of leads and harnesses are helpful in addition to reassurance to comfort your pet.

Conclusion
Progressive retinal atrophy refers to a broad category of inherited retinal diseases that result in the blindness of purebred dogs. Because of the insidious nature of the disease and in some breeds, the late onset, serial examinations may be required to detect affected individuals. Individuals affected should not be used in any breeding and pedigree studies are indicated to eliminate other potential carriers for these diseases.

If you have any questions about Progressive Retinal Atrophy, please do not hesitate to call us at Eye Care for Animals.