What is the function of the retina?
The retina is a light sensitive layer of cells at the back of the eye that contains cells called photoreceptors. When light enters the eyes, it is focused by the lens onto the retina, where it is converted into electrical signals that are sent to the brain for processing and interpretation.
The two main photoreceptor cells of the retina are the rod cells and the cone cells. The cat's eyes contain many more rods than cones. Rod cells are responsible for vision in low light conditions and for detecting and following movement. Cone cells are responsible for detecting color. Cone cells do not work very well in low light.
What is progressive retinal atrophy?
Atrophy means the partial or complete wasting of a body part. Progressive retinal atrophy (PRA), is a group of degenerative diseases that affect these photoreceptor cells. With this disease, the cells deteriorate over time, eventually leading to blindness in the affected cat.
Are there different forms of PRA?
There are two main forms of PRA recognized in cats, an early onset or inherited form, also called retinal dysplasia, which is typically diagnosed in kittens around two to three months of age, and a late onset form that is detected in adult cats, usually between the ages of two to five years. It is common for the late onset form to be called PRA and the early onset form to be called retinal dysplasia.
How does it happen?
With retinal dysplasia, the photoreceptor cells of the retina develop abnormally, leading to an early onset of blindness. Usually, both the rods and the cones are affected.
With late onset PRA, the cells of the retina develop normally, however over time, the cells begin to deteriorate. In the majority of cases, the rod cells degenerate first causing night vision to be affected before daytime vision, but eventually the cone cells also degenerate, leading to complete blindness.
Are any breeds of cats more likely to get PRA?
Currently the only breeds in which this disease has been shown to occur as an inherited condition are the Abyssinian and Persian cats. In Abyssinian cats, retinal dysplasia has been recognized with a dominant inheritance, while late onset PRA appears to be inherited in an 'autosomal recessive' pattern, meaning that the affected cat must have inherited the defective gene from both parents. In Persian cats, a recessive inheritance pattern has been confirmed but the specific gene has not been identified.
"Currently the only breeds in which this disease has been shown to occur as an inherited condition are the Abyssinian and Persian cats."
Since the disease is heritable in these breeds, when a cat develops PRA, it should be removed from breeding programs. Its parents and siblings should also be removed from breeding programs, even if they do not show any signs of the abnormality. The rationale for removing related cats from breeding programs is that the typical inheritance pattern is recessive, meaning that affected cats must have inherited the defective gene from both parents. Unaffected siblings and parents are carriers of the defective gene, but will have also inherited a normal gene, so they will not 'express' or show the abnormality. With selective breeding, PRA has the potential to be eliminated as a heritable condition.
What are the signs of progressive retinal atrophy?
PRA is not a painful condition, so it is rarely noticed in its earliest stages of development. The first sign that is usually noticed in a cat with PRA is night blindness. Affected cats tend to be nervous at night, may be reluctant to go into dark rooms, or may bump into things when the light is dim. Pet owners with cats that are developing PRA often observe that their pet's eyes have become very reflective when light shines on them, and that the pupils are more dilated than normal. Both eyes are affected. If an affected cat never goes outdoors or into unfamiliar environments, the condition may not be observed until the cat is completely blind. In Abyssinian cats, more specific signs when examining the back of the eye (the fundus) can help determine what stage the condition is in.
How does PRA affect my cat?
As PRA progresses, your cat's vision gradually worsens until she becomes completely blind. For the average family pet, blindness is not as significant as it would be in a human. Blind cats will rely more on its other senses and are able to move around well in their home environment, as long as furniture and other objects are not moved around.
How fast does PRA develop?
In most cases, cats experience a complete loss of vision over a period of one to two years.
How is PRA diagnosed?
If your cat appears to have vision loss, based on a general ophthalmic examination that shows sluggish pupillary light responses and has dilated pupils, your veterinarian may suspect PRA. In early stages, it may be difficult to observe any obvious changes to the retina. As the disease progresses, examination of the back of the eye with an ophthalmoscope will show increased reflectivity of a portion of the retina called the 'tapetum lucidum', changes in the optic nerve, and changes in the retinal blood vessels.
"Your veterinarian will recommend additional sophisticated testing such as an electroretinogram (ERG), to confirm the diagnosis and/or rule out other causes of deteriorating vision."
Your veterinarian will recommend additional sophisticated testing such as an electroretinogram (ERG), to confirm the diagnosis and/or rule out other causes of deteriorating vision. In the majority of cases, this means a referral to a veterinary ophthalmologist. The ERG is sensitive enough to diagnose PRA in cats even before they begin to show obvious symptoms. For Abyssinian and Persian cats, DNA testing can be done at the University of California, Davis for the affected genes.
Could the signs my cat is showing be caused by something else?
Blindness due to retinal degeneration can be caused by taurine deficiency in cats. Taurine is an amino acid that cats require as an essential part of their diet. Taurine is normally present in commercial cat foods and deficiency tends to be a problem in cats that are fed imbalanced homemade diets or dog food.
Other causes of slowly developing blindness in cats include slowly progressive cataracts, glaucoma, uveitis, lens luxation, and optic nerve disease. These diseases tend to be painful. A more sudden blindness can be seen with overdoses of the antibiotic enrofloxacin (brand name Baytril®).
What is the treatment?
There is currently no effective treatment available for PRA. To date, the use of antioxidant supplements or vitamins has not shown any measurable effect on this disease, although these supplements are not harmful to your pet and they may reduce stress on the lens cells and delay cataract formation. If a condition such as cataracts or retinal detachment has been determined to be the cause of your cat's blindness, treatment of this underlying cause may prevent further loss of vision.