Devoted to Basenjis as Pets
By Gregory M. Acland, B.V.Sc.; Dip.ACVO, Center for Canine Genetics & Reproduction, James A. Baker Institute for Animal Health, Cornell University, College of Veterinary Medicine, Ithaca, New York 14853-6401 (originally written 2001)
(Reproduced with kind permission of the author.)
Progressive retinal atrophy, or PRA as it is frequently termed, is a long recognized, hereditary, blinding disorder. The first modern description of this problem was in Gordon Setters in Europe, in the early years of the twentieth century, but since then PRA has been recognized in most purebred dogs. PRA is a disease of the retina. This tissue, located inside the back of the eye, contains specialized 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 specialized into rods, for vision in dim light (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. In human families the diseases equivalent to PRA (in dogs) are termed retinitis pigmentosa.
In all canine breeds PRA has certain common features. Early in the disease, affected dogs are night-blind, lacking the ability to adjust their vision to dim light; later their daytime vision also fails. As their vision deteriorates, affected dogs will adapt to their handicap as long as their environment remains constant, and they are not faced with situations requiring excellent vision. At the same time the pupils of their eyes become increasingly dilated, causing a noticeable “shine” to their eyes; and the lens of their eyes may become cloudy, or opaque, resulting in a cataract.
Diagnosis of PRA is normally made by ophthalmoscopic examination. This is undertaken using an instrument called an indirect ophthalmoscope, and requires dilatation of the dog’s pupil by application of eyedrops. Broadly 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 breed-specific forms. Usually by the time the affected dog has these changes there is already significant evidence of loss of vision.
Dr. Acland had the following exchange with an owner:
Owner: “I’d like to know the time involved for the progression from night blindness to total blindness. … I noticed something wrong a couple of weeks ago … in dim light. Do you guys think there is any point in having all the tests done for PRA if the vet thinks she has it?”
Dr. Acland: From what you describe, ___ certainly has the characteristic signs of PRA in Basenjis, and is developing them at the age that fits what little is really known about Basenji PRA.
Most dogs, including most Basenjis, that are affected with adult onset PRA show fairly consistent patterns of development of the visual problems. There is not a great deal of difference at the clinical level between PRA in say Poodles, Portuguese Water Dogs, etc. and Basenjis, even though it is not yet known whether the disease in Basenjis is caused by the same gene.
The initial visual loss is, as you have noticed in _____, night blindness. Once difficulty in seeing in dim light is noticeable, progression to total night blindness is fairly quick (less than 1 year or so). So I expect that _____ will become totally night blind in the near future, if she has not already.
Vision in daylight is more gradually lost. The initial loss is in the peripheral field of vision, and the dog develops tunnel vision. Unless the dog is used for high visual performance tasks such as agility or guidance work, the tunnel vision may not be detectable to an observer.
Despite the tunnel vision and nightblindness, many PRA-affected dogs retain an adequate forward daylight vision for an extremely long time, sometimes for their entire natural life.
In many dogs however PRA leads to cataracts, an opacity of the lens. This certainly happens at least in some affected Basenjis. This complication is what usually causes the final loss of vision, because the first part of the lens to become opaque is usually in the central visual axis. It is difficult to tell ahead of time when this will happen, as the age varies greatly from dog to dog.
Traditionally, it was not considered wise to remove cataracts from PRA-affected dogs, because the underlying retinal disease was going to cause blindness anyway. This is no longer generally accepted. It is now worth considering to examine PRA-affected dogs on a regular basis, and to remove the lens at the first sign of cataracts developing, as long as the dog appears to have remaining useful vision. This achieves two things: first it extends the period during which the dog will be able see sufficiently to get about; and secondly it removes the possibility of the cataract developing to the point where it causes secondary problems such as uveitis and glaucoma.
If your veterinarian is confident in the clinical diagnosis, I would not recommend having an ERG done. This is unlikely to yield any information worth the stress on the dog and your wallet.
May I ask who made the diagnosis? Has _____been seen by a veterinary ophthalmologist or by a general practitioner? If she has not been seen by an ophthalmologist you should arrange this, as _____’s progression ought to be checked by a specialist to assess the development of cataracts and other possible complications.
Article copyright © 2001 by Gregory M. Acland. All rights reserved