PRESENTING SIGNS

The owner will present the patient with generalized progressive retinal atrophy (gPRA) because they have noticed that vision is reduced and perhaps the eyes look different. Thus they will have become aware that their pet is reluctant to go out in unfamiliar places, chases the ball less and occasionally bumps into things at home. Normally owners will realize, with hindsight, that things have not been normal for several months. They may report that the eyes look ‘cloudy’ or ‘glassy’. Middle-aged dogs are most commonly affected – certain pedigree breeds such as most spaniel types (especially cocker spaniels) and Labrador breeds (particularly the Labrador retriever) and miniature and toy poodles are most frequently seen. Cats are sometimes affected, most notably middle-aged Siamese.

CASE HISTORY

Some owners feel that the pet suddenly went blind. This is most common if they have been to stay in a different house and noticed that the pet bumps into things, or if it has been in kennels and when brought back home does not seem to be able to see properly. However, when questioned in more detail the owners might be able to recall certain instances where the pet did not react as they would normally do – suddenly barking at a dark rubbish sack on the street, not reacting quickly to another dog or bumping into an open cupboard door in the kitchen for example. Vision is frequently worse at night, but the owners might not have noticed this. Careful questioning – for example, ‘Does he still like going for walks when it is dark in the evening?’ or ‘Will he go out in the garden at night quite happily?’ – might elicit answers such as ‘I have to keep him on the lead at night otherwise he is frightened/gets lost/sits down and won’t move’ or ‘He is happy if I put the outside light on, but won’t go out into the total darkness’. Animals are often presented in the autumn when the clocks go back, and it is darker both in the morning and the evening.

Owners might report that the eyes look different – glazed, glassy, shiny and cloudy are all possible descriptions. Observant owners might have noticed that the pupils are quite dilated in normal light levels. Many notice the clouding, and attribute this to cataract formation which is what they blame for the visual problems. However, it is very important to ascertain whether the eyes looked cloudy before or after they started to notice visual problems.

CLINICAL EXAMINATION

General clinical examination is usually unremarkable, but the ophthalmic examination is not! Dogs might be nervous in the waiting room and can bump into objects on the way into the consulting room. Alternatively, they might walk with their head down, nose to the ground, and have a slightly high stepping gait. Cats will be reluctant to come out of their basket and might put their paws out to feel the edges of the table. Menace responses are likely to be reduced, and in most cases pupillary light reflexes are slow and incomplete. Thus the pupils are likely to be dilated in room light and even with a bright light source full and rapid constriction is unlikely to occur. Cats will retain a better pupillary light reflex than dogs. Both the menace response and pupillary light reflexes should be checked in bright (photopic) and dim (scotopic) light. Reactions are likely to be markedly reduced in the latter. In addition, the dazzle reflex is frequently reduced – the patient probably will still react to the bright light, but less quickly and with less intensity than a normal animal.

The adnexal examination is usually normal, as is the cornea. Dogs might have cataracts – ranging from incipient cortical opacities to total mature cataracts, depending on how advanced the underlying retinal disease is. Providing any cataract is not too advanced, it should be possible to examine the fundus – this will be easier with indirect ophthalmoscopy if some cataract formation has occurred. Initial distant direct ophthalmoscopy will reveal dilated pupils and an increased tapetal reflex (‘eye shine’). This tapetal hyper-reflectivity can be so bright as to almost dazzle the observer when attempting close direct ophthalmoscopy, and reducing the intensity of the ophthalmoscope light will make examination easier. The increased reflectivity is frequently quite uniform over the tapetal area, but in early cases it can be almost ‘granular’ in distribution.

The retinal blood vessels will be narrowed and the optic disc can be small and pale (Figure 48.1). The non-tapetal fundus should not be forgotten, and the normal even pigment of the retinal pigment epithelium might be broken up into a patchy, ‘pavementing’ pattern. The changes in the retina are bilateral and symmetrical.

Figure 48.1 Tapetal hyper-reflectivity and blood vessel attenuation typical of generalized progressive retinal atrophy in a 10-year-old miniature poodle.

In addition to the ophthalmic examination, an obstacle test is very useful. This can be set up in the waiting room or similar and should consist of a selection of solid (e.g. cardboard boxes) and open (e.g. chairs) obstacles. The pet is held at one corner of the room while the owner calls them from the other. The animal is observed first in normal lighting, when they will often negotiate the room well, and then in dim light, when they are probably much more hesitant, and likely to collide with things. The obstacles should be moved around between tests. For cats, a couple of objects can be placed between the cat and the safety of its basket, and its ability to find its way is similarly observed in differing light intensities.

CASE WORK-UP

The presentation, history and ophthalmic examination are frequently sufficient to confirm a diagnosis of generalized progressive retinal atrophy (gPRA). However, if cataract formation is marked, it might not be possible to properly evaluate the fundus, even with mydriasis and indirect ophthalmoscopy. These patients benefit from electroretinography and referral is necessary for this. The procedure is usually performed under sedation, and a bright light source is shone into the eyes, while electrodes pick up the retinal response to the light. Recordings can be made at different light intensities, which will allow rod and cone function to be differentiated. Patients with gPRA will have reduced rod responses initially but the condition does progress to involve the cones as well, and all wave amplitudes will be reduced in such cases.

DNA tests are also available in some canine breeds to confirm the diagnosis and as a screening test for potential breeding stock. Blood or saliva samples can be used and results will indicate affected dogs, carriers and clear, unaffected animals. DNA tests are becoming available in more and more breeds and an up-to-date list of which breeds can be tested is available at www.optigen.com. In addition, both the Animal Health Trust and Cambridge University Veterinary School provide DNA tests for gPRA in some breeds.

Stay updated, free articles. Join our Telegram channel

Join