PRA - Progressive Retina Atrophy



Progressive retinal atrophy, or PRA as it is frequently termed, is a long recognised,
hereditary, blinding disorder. PRA is a disease of the retina. This tissue, located inside the
back of the eye, contains specialised cells called photoreceptors that absorb the light focused
on them by the eye's lens, and converts that light, through a series of chemical reactions
into electrical nerve signals. The nerve signals from the retina are passed by the optic nerve
to the brain where they are perceived as vision. The retinal photoreceptors are specialised
into rods, for vision in dimlight (night vision), and cones for vision in bright light (day and
color vision). PRA usually affects the rods initially, and then cones in later stages of the
disease.




PRA, as an hereditary condition in the dog and showing many similarities to retinitis
pigmentosa in man, has been known since the early part of this century. However, reports 
of hereditary PRA in the cat are rare. Magnusson first described PRA in the dog, in Sweden 
1909 in the Gordon Setter, and called it retinitis pigmentosa. Since that time it has been 
recorded in many breeds of dog and in several countries. PRA was divided into two forms, 
generalized and central, the former being further subdivided into three types, according
to whether the rods and/or the cones degenerate prior to maturation.



Advanced generalized retinopathy in the cat, of unknown aetiology, was recorded by 
Rubin. Retinal degeneration in the Siamese breed was suspected of an hereditary basis and 
this aspect has been further studied more recently. An hereditary retinal degeneration in two 
litters of Persian kittens was described in 1973 by Rubin and Lipton. Feline central retinal 
degeneration was at first suspected of having an hereditary origin, but later was proved to be 
due to deficiency of the sulphur-containing amino-acid, taurine.



It is surprising that there are so few accounts of hereditary retinal degeneration in the cat 
with the amount of pedigree cat breeding that occurs and the multiplicity of reports of proven 
hereditary retinal degeneration in the dog. However, a form of progressive retinal 
atrophy was diagnosed in the Abyssinian cat in Sweden in 1977, and, subsequently, 
elsewhere in Europe. In this autosomal recessive disease, the first clinical signs become apparent
at approx. 1,5 - 2 years of age. A research project was undertaken to present a genetic, 
clinical, electrophysiological and morphological analysis of the disease.  A preliminary report 
on progressive retinal atrophy of hereditary origin in the Abyssinian cat has been presented by 
Narfström in 1982.



In England, a condition of similarly early onset has been described in the Abyssinian cat, 
and breeding studies involving 14 litters have confirmed an autosomal dominant 
mode of inheritance. The first sign of disease is an increase in pupil size in moderate 
daylight between 2-3 weeks of age. The pupillary light reflex, although sluggish, persists even 
late in the disease. Feline retinal dystrophies of much earlier onset have also been reported in 
the United States in the Persian breed and in three generations of domestic cats of mixed breeding.
 


Diagnosis of PRA is normally made by ophthalmoscopic examination. 
This is undertaken using an instrument called an indirect ophthalmoscope, and requires 
dilatation of the cat's pupil by application of eyedrops. Boradly speaking all forms of PRA have 
the same sequence of 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). These changes occur in all forms of PRA, but at different times in the 
different forms. Usually by the time the affected cat has these changes there is already significant 
evidence of loss of vision.



Confirmation of the diagnosis can be undertaken by electroretinography. 
This is an electrical measurement of retinal function somewhat similar to an electrocardiographic 
test of heart function, but with two differences: the electroretinogram (ERG) can only be recorded 
as a response to a flash of light and accurate recording of the ERG requires that the cat be 
anesthesized. In all cats showing clinical evidence of PRA, the ERG is severely diminished or 
extinguished. 



Other causes that might lead to blindness:

Taurine deficiency retinopathy: cats of any breed fed a diet deficient in the 
amino acid taurine (eg. dog food) may develop retinal degeneration. 
Ophthalmoscopically this has a characteristic appearance: it always begins in the area centralis 
and may or may not progress. Ophthalmoscopic evidence of retinal degeneration develops 
only after the cat has been on a deficient diet for several months. The diagnosis here relies on 
history: check for any affected relatives, evidence of abnormal diet, or even test-mating. 
Vitamine A and E deficiency were also mentioned to be a possible cause 
for retinal degeneration as were toxoplasmosis, feline infectious peritonitis, feline 
leukemia virus, tuberculosis and oculomycoses. Idiopathic retinal degeneration may 
occur with some frequency. But histologically these cases seem to be different to PRA.




References:


  • Retinal Degeneration in the Dog and Cat, Veterinary Clinics of North America:
Small Animal Practice, Vol. 20, No. 3, May 1990;


  • Progressive Retinal Atrophy in the Abyssinian Cat by K.C. Barnett, in Journal of Small
Animal Practice, 1982, 23, 763-766;


  • Six cases of progressive retinal atrophy in Abyssinian cats, Journal of Small Animal
Practice, 25, 415-420;


  • Progressive retinal atrophy in the Abyssinian cat: An update by Narfström, in Veterinary
Records, 112, 525-526;


  • Progressive retinal atrophy in the Abyssinian cat, Clinical characteristics by Narfström,
in Investigative Ophthalmology & Visual Science, 26, 193, 1985;


  • An Early-Onset Retinal Dystrophy with dominant Inheritance in the Abyssinian cat by
Curtis, Barnett and Leon in Invest. Opht. & Visl Sc. 28, 131-139;


  • Taurine deficiency retinopathy in the cat by K.C. Barnett in J. of small Animal Practice, 
1980, 21, 521-534;