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Progressive Retinal Degeneration in Cats

By: Dr. Noelle McNabb

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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.

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