Overview of Feline Progressive Retinal Degeneration (PRD)
Progressive retinal degeneration or atrophy (PRD, PRA) is premature degeneration (deterioration) of the photoreceptor cells of the retina. There are two types of photoreceptors in the retina and these are the light-sensitive rods and cones. They are responsible for detecting light and converting it into an electrical signal that travels to the brain. When the photoreceptor cells deteriorate, vision is lost because the animal has no way to generate an image from the light reaching the retina.
PRA/PRD is in cats initially affects the rods. The rods are responsible for dim light vision; therefore, the cat loses its nighttime vision first. The disorder is progressive and eventually the cones are affected. Over time, the cat slowly goes completely blind. The disease affects both eyes at the same time.
Most cats are seen in the late stages of disease and have advanced changes in their retinas because they compensate very well as their vision slowly deteriorates. Sometimes the blindness can appear to be sudden in onset (even though it has been developing for months) because the cat may show almost no signs until the last bit of vision has been lost.
PRA in cats is rare in the United States. It is seen most often in purebred cats, such as the Abyssinian, Persian, and Siamese. It is seen sporadically in domestic shorthair and other mixed breed cats. In the Abyssinian, the disease is inherited as a dominant trait, but the inheritance pattern is unknown for other cats.
Below is an overview of Progressive Retinal Degeneration (PRD) in Cats followed by more detailed information on the diagnosis and treatment of this disease.
What to Watch For
Dilated pupils
Bumping into objects, reluctance to jump up onto objects, reluctance to go outside, or other signs of blindness
Poor vision in dim light or darkness
More readily visible eye shine from the back of the eye due to dilation of the pupils
Diagnosis of Progressive Retinal Degeneration (PRD) in Cats
Diagnostic tests are needed to recognize PRA/PRD and exclude other diseases. Your veterinarian will probably take a complete medical history and perform a thorough physical examination.
A complete ophthalmic examination is indicated and involves all of the following tests. Your veterinarian may refer your cat to a veterinary ophthalmologist for completion of some of these tests:
Tests to evaluate vision, such as observing the cat as he navigates an obstacle course in both bright and dim light, and certain neurologic reflex testing
Pupillary light reflex testing
A Schirmer tear test and fluorescein staining of the cornea
Tonometry to measure the pressure within the eye
Specialized examination of the front chamber of the eye, the iris and lens, the vitreous and the retina.
If your veterinarian is concerned that some disease other than PRA is the source of the cat’s blindness, then medical tests to rule out other causes may include the following:
A complete blood count (CBC) and serum blood tests
A feline leukemia virus (FeLV) test, a feline immunodeficiency virus (FIV) test and a toxoplasmosis titer to look for the presence of these infectious diseases
Plasma or blood taurine levels
A measurement of systemic arterial blood pressure to rule out high blood pressure
Possibly chest and abdominal X-rays
PRA can sometimes be confirmed at the time of retinal examination because it causes characteristic changes in the appearance of the retina. Early stages of the disease can be more difficult to diagnose, and in that instance the disease can be detected with the following test:
An electroretinogram to evaluate the function of the photoreceptor cells when they are stimulated with flashes of light. If the electroretinogram is abnormal, then the retina is diseased. If the electroretinogram is normal, then the origin of blindness is somewhere other than the retina.
Treatment of Progressive Retinal Degeneration (PRD) in Cats
No therapy is available to prevent, slow the progression of, or reverse the degenerative changes of PRA.
Early diagnosis of PRA using electroretinography is most important in catteries to eliminate individuals from the breeding pool that are either clinically affected or represent genetic carriers of the disease.
Home Care
Care consists of providing a consistent and safe environment for pets with vision loss. Considerations include:
Establish a known location for the food and water bowls and guide your pet to them until he can memorize the locations.
Place barriers across staircases, over hot tubs and around pools.
Restrict activity on balconies; your cat could fall through the space between the guardrails.
Avoid changing the location of the furniture and leaving chairs or other objects out of place in the house. Your cat will memorize a familiar (stable) environment in a relatively short time.
Purchase toys that contain bells or other noisemakers to encourage and help cats to play.
Affected cats lose their vision in dim lighting first. Therefore, night-lights or plug-in hall lights are helpful for negotiating in the house at night.
It is important to realize that vision is the least important of the cat’s three major senses. The sense of smell and the sense of hearing are more highly developed in cats than in people, and they rely heavily on these two senses. Cats that are blind are not as handicapped in the their activities as you would expect. At times they can act very normal. Because PRA develops so slowly, most cats adapt very well to their vision loss and remain happy and active pets.
Preventative Care
No preventive care is available for an individual because PRA is genetic. Do not breed affected animals.
Breeders can obtain certification from a veterinary ophthalmologist that certifies an individual is free of inherited eye disease. The certification is valid for a period of one year from the time of examination.
In-depth Information on Progressive Retinal Degeneration (PRD) in Cats
Poor vision in dim lighting conditions (nyctalopia) is usually the first behavioral sign of PRA. Good vision may be maintained for some time under bright light conditions. The visual impairment induced by PRA eventually progresses to blindness in all lighting conditions, and this clinical course often takes place over 18 to 24 months. As the retinas deteriorate the pupils (hole in the center of the iris) become increasingly dilated and often a greenish-yellow sheen or reflection is noted because the eye shine of the retina is more easily seen through the enlarged pupils.
Other ophthalmic diseases or conditions can mimic the signs of PRD by also inducing blindness. Some of these diseases cause acute blindness, while others cause a slow onset of blindness.
It is important to exclude these conditions before establishing a conclusive diagnosis:
Taurine (an essential amino acid in the diet of cats) deficiency results in retinal degeneration that appears very similar to PRA when it reaches its last stages. This disease is now rare, because since 1988 extra taurine has been added to all commercial cat foods in the United States. The disease may be still be seen in cats that eat dog food or are on poor diets. Taurine deficiency causes a slowly developing blindness.
Retinal detachment is the separation of the retina away from the back of the eye. When the condition occurs in both eyes and involves a major part of the retina, then blindness develops. In the cat, the most common cause of retinal detachments is high blood pressure. This condition is most often seen in older cats, and the onset of blindness is rapid.
Retinal inflammation can produce blindness if the condition affects a major portion of the retinas in both eyes. Retinal inflammation can arise with bacterial, fungal (cryptococcosis, histoplasmosis), protozoal (toxoplasmosis), parasitic (larval migrans), or viral (feline leukemia, feline immunodeficiency virus, feline infectious peritonitis virus) infections. It may also develop in association with other systemic illnesses and tumors. The onset of blindness may be rapid or slow.
Optic neuritis is inflammation of the optic nerve that connects the eye to the brain. Any of the diseases that cause retinal inflammation or brain inflammation may also lead to optic neuritis. When both nerves are affected, the cat is usually completely blind. The onset of blindness tends to be rapid with optic neuritis.
Glaucoma can cause complete blindness if both eyes are involved. It is rare for glaucoma to develop simultaneously in both eyes. Usually one eye is affected first and the cat may act like it has normal vision. If the second eye becomes affected, then the cat may become totally blind.
Toxic damage to the retina may occur with ingestion of certain drugs or toxic plants.
Inherited disorders of the central nervous system (storage diseases), such as mucopolysaccharidosis, result in slow progressive blindness when the retina and visual pathways of the brain are affected. These diseases affect certain breeds of cats, usually develop when the cat is very young, and are quite rare.
Certain diseases and disorders of the brain can cause blindness. The blindness may be either slow or sudden in onset, and the eye examination is often normal.
In-depth Information on the Diagnosis of Progressive Retinal Degeneration (PRD) in Cats
Veterinary care often includes diagnostic tests to confirm the presence of PRD and to exclude other diseases. Tests may include:
A complete medical and ophthalmic history. Information of particular importance includes any known information regarding retinal diseases in related animals; the duration and pattern of onset (sudden or slowly progressive) of vision loss; any physical abnormalities accompanying the vision loss; and any medications currently being given to the pet. A thorough physical examination is performed to determine whether abnormalities are confined only to the eye or involve other organs in the body.
A complete ophthalmic examination is indicated and involves all of the following tests. Your veterinarian may refer your cat to a veterinary ophthalmologist for completion of some of these tests:
– Tests to evaluate vision, such as observing the cat as he navigates an obstacle course in both bright and dim light, and certain neurologic reflex testing
– Pupillary light reflex testing
– A Schirmer tear test and fluorescein staining of the cornea
– Tonometry to measure the pressure within the eye
– Specialized examination of the front chamber of the eye, the iris and lens, the vitreous and the retina.
If your veterinarian is concerned that some disease other than PRA is the source of the cat’s blindness, then medical tests to rule out other causes may include the following:
– A complete blood count (CBC) and serum blood tests
– A feline leukemia virus (FeLV) test, a feline immunodeficiency virus (FIV) test, a toxoplasmosis titer and fungal titers to look for the presence of these infectious diseases
– Plasma or blood taurine levels
– A measurement of systemic arterial blood pressure to rule out high blood pressure
– Possibly chest and abdominal X-rays
– Ultrasonography of the structures behind the eye
– CT scan or MRI of the brain and vision pathways leading to the brain
– Cerebral spinal fluid tap to evaluate the fluid around the brain
An electroretinogram may be indicated if the retina appears to be the source of the blindness based upon the physical and eye examination findings. This test is essential for confirming the diagnosis of PRD.
Treatment In-depth for Feline Progressive Retinal Degeneration
There is no treatment for PRA/PRD. It is very important not to breed affected animals.
