Monday, October 21, 2013

Case study: Sally - Progressive Retinal Atrophy (PRA) in cats

 Sally is a 3-year-old Abyssinian cat that was rescued from a hoarding situation. She was one of 84 cats rescued. Eighty of the cats in the household were too ill or debilitated to survive and had to be euthanized. After adoption, it was discovered that Sally was Feline Leukemia positive. She is shy, but sweet, and her owners brought her in to us because she suddenly appeared to lose her sight. She began bumping into things and lost her fear of the dog. If you notice in her photo, her pupils are very dilated, and her owners had noticed this at home, too. Her regular veterinarian sent her to us because high blood pressure was suspected.

What does high blood pressure have to do with eyesight?
The retina lines the back of the eye

The retina is a thin membrane lining the inside of the eye that receives light and translates it into electrical impulses for the brain with special cells called photoreceptors. There are two kinds of photoreceptors - rods that process black and white or dim light vision and cones that process colors or bright light. If the retina or its photoreceptor cells become damaged in some way, the eye has trouble converting waves of light into signals that the brain can understand, interfering with a cat's vision.

 One of the ways that cats can lose their vision is by developing hypertension lesions in the retinas - tiny blood vessels are damaged and regions of the retina lose their ability to function as they lose their blood supply. These lesions look like bubbles in the retina. If a cat's blood pressure remains elevated for a long period of time, the retina can completely detach and lose its connection with the nerves that transmit visual impulses to the brain. Once this has happened, a cat becomes irreparably blind. If the hypertension is diagnosed and treated early, as in these photos, the retinal lesions can heal over time.

Retinal lesions appear like bubbles in the back of the eye

The same eye 2 months after starting blood pressure medication

 Notice that not only are the lesions gone, but the blood vessels appear more visible.

The first thing that we did when Sally came in was check a blood pressure reading. Her blood pressure was 150mmHg, which is a normal pressure in a nervous cat. Hypertension was not the culprit in this case. So, what could be the problem?

The next thing that happened was that Sally received a full physical exam from Dr. Demos, including an eye exam. This is what Dr. Demos saw when she looked in Sally's eyes...
Something very important is missing, here!
There are no blood vessels in this retina!
This is a characteristic feature of something called Progressive Retinal Atrophy (PRA). PRA is a recessive trait in certain breeds of cats such as the Abyssinian, the Somali and the Ocicat. Perisans and Siamese are also often at risk. There are a number of other breeds that have also been identified to carry the mutated gene in small numbers.

Other key features of this disease are a granular appearance to the retina, signs of retinal thinning, and a reduction or thinning of blood vessels. Increased reflectivity of the retina and dilation of the pupils are symptoms that owners can easily see at home. Later stages show pigment changes to the retina and a degradation of the optic nerve. These changes usually happen equally to both eyes at the same time.
Sally's pupils stay dilated, even in bright light

The most common form of PRA in cats is involved a progressive degeneration of the rod and cone receptors in the retina.The cat's eyes develop normally as a kitten, but as the cat gets older, the cat will begin to experience night-blindness and may tend to avoid stairs or darkened areas of the home. It will progress to total blindness by the age of 3-5 years. This is problematic not only for the cats themselves, but for the breeders of high-risk breeds, because often the disease is not apparent until after the age at which many cats have already participated in a breeding program. If you are interested in a purebred cat, it may be a good idea to ask your breeder about PRA and whether their cats have been genetically tested for the mutation.

Unfortunately, this is a problem for which there is no treatment. However, the good news is that cats adapt well to being without their sight due to their superior hearing, sense of smell and sense of touch (whiskers). You can read about the amazing cat, Homer, who caught flies out of the air without the aid of sight!

If your cat starts to show reluctance to move around at night, or has trouble jumping onto furniture, or starts to bump into things, it would be a good idea to have your cat's eyes checked by his or her veterinarian, as well as having a blood pressure exam performed.

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