Progressive Retinal Atrophy (PRA)
Progressive retinal atrophy, or PRA as it is frequently termed, is a hereditary eye disorder. It is inherited as a simple autosomal recessive in most breeds. PRA has been recognized in humans, in most purebred dogs and some breeds of cats.
PRA is a disease of the retina, a tissue that is located inside the back of the eye. The retina contains specialized cells called photoreceptors. These absorb the light focused on them by the eye's lens, and convert the light into electrical nerve signals. The nerve signals are passed by the optic nerve to the brain where they are perceived as vision. In PRA in Abyssinian and Somali cats, the photoreceptors degenerate gradually.
Early in the disease, night vision deteriorates as the cat will face increased problems adjusting its vision to dim light. Later in the process, daytime vision also fails. As the retina degenerates it gets thinner and light will be strongly reflected (hyper reflection). At the same time the pupils become increasingly dilated, in a vain attempt to gather more light, causing a noticeable "shine" to their eyes. The lens may become cloudy, or opaque, resulting in a cataract. Total blindness occurs typically at around 4-5 years but could be delayed for several years.
Affected cats will adapt to their handicap as long as their environment remains constant, and they are not faced with situations requiring excellent vision.
Diagnosis of PRA is normally made by ophthalmoscopic examination. This requires dilatation of the pupil by application of eyedrops. PRA can be diagnosed by ophthalmoscopic changes: increased reflectivity (shininess) of the fundus (the inside of the back of the eye, overlain by the retina); reduction in the diameter and branching pattern of the retina's blood vessels; and shrinking of the optic nerve head (the nerve connecting the retina to the brain).
As there is now a DNA test for the rdAc mutation (causing the most common PRA), one also knows that a cat with double set of this recessive gene (the rdAc) will develop PRA, although there may be no signs at an ophthalmoscopic examination.