Inherited Retinopathies

Swedish Vallhund is a healthy breed but there can still be individual cases of eye disease within the population. Increasing numbers of dogs have undergone eye examinations within the last few years and some cases of RD, HC, PPM and J175 (a "code name" (used in Finland) of a retinal disease) have been found. It is recommended that a dog that is used for breeding should at some point in his life be eye examined so the health of its eyes would be known. Below are listed some of the retinal disease found in dogs, mainly: RD and PRA.

The retina is the specialized part of the eye that contains the nerve cells that make vision possible. Traditionally, if one compares an eye to a camera then the retina corresponds to the film. This is a useful mental image because one immediately understands that the film cannot work if the camera itself is broken or has the lens cap on, but the camera is totally useless without film in it. A better analogy today would be to compare the eye to a video camera, and the retina to the silicon chips inside that convert light into an electrical signal that can be sent to a video tape recorder or TV monitor. Because the normal eye is transparent, it is possible to look inside with a special instrument called an ophthalmoscope, and see whether the retina is healthy or not.

Retinal diseases destroy its nerve cells, producing either partial or total blindness, in people, dogs, and other animals. Retinal disease, like diseases elsewhere in the body, can be caused by many agents, including infections, parasites, trauma (that is injury from physical causes), nutrition (diet), and hereditary factors (genes). Also as in diseases affecting other parts of the body, some retinal diseases have known causes, but many others are poorly understood at best. The most important thing that sets retinal disease apart from disease elsewhere in the body is the limited ability of the retina to respond to injury without impairing its own function. Many of the body's defence mechanisms, such as inflammation and other immunity system responses, that are useful in fighting disease elsewhere in the body, can cause irreversible vision loss inside the eye, and specifically in the retina. Even a minor "scar" in the retina for instance can cause a serious problem for vision. For this reason, the body has developed some special techniques to help prevent retinal disease, to limit the amount of scarring possible, and even to convince abnormally functioning retinal cells to commit suicide before they cause further problems. The retina therefore has only a limited number of ways to respond to injury. An unfortunate side effect for ophthalmologists is that many different retinal diseases can look very similar, particularly in the late stages.

Hereditary retinal diseases have a special significance to dog breeders, creating both an unjustified sense of blame and an often illusory hope that these diseases were caused by and could be eliminated by specific breeding practices. Because certain retinal degenerations are clearly hereditary, it is also tempting to assume, in the absence of specific evidence, that other such diseases are too. It is also important to realize that even a disease that is hereditary may have nonhereditary aspects that are important to understand.


Retinal dysplasia Retinal folds and retinal dysplasia are closely related eye diseases that are primarily inherited, suspected to be autosomal recessive.  Other causes include toxicity and infections during pregnancy, including herpesvirus and possibly parvovirus. 
 

The retina is a layer of sensory tissue that is attached to the tissue in the back of the eye.  The retina receives light stimuli from the environment and converts it to an electric signal that is sent to the brain via the optic nerve, which lies behind the retina.  The brain interprets this signal and vision occurs.  Any alterations in the retinal tissue can impair vision.  The retinal tissue undergoes changes until the eye is fully developed at the age of one year.

Dysplasia is defined as abnormal growth or development. Retinal dysplasia forms when the 2 developmental layers of the retina do not unite properly.  It can involve both eyes or one, but the inherited form is almost always bilateral.  It can occur anytime during eye development, from pregnancy to one year of age. 

There are 3 forms of retinal dysplasia:

1.  Focal retinal folds:  These are wrinkles in the retinal tissue in one or more areas.  They cause small blind spots, but rarely cause visual impairment for the dog.  They primarily occur in young dogs with developing retinas, and usually disappear by the age of one year. Young dogs (<1 year of age) with folds only are not at risk of developing dysplasia at a later age.

2.  Geographic retinal dysplasia:  This consists of irregularly shaped areas of the retinal tissue that appear like a rosette upon examination.  This may cause some degree of visual impairment and possibly blindness.  This can develop at anytime up to one year of age and is usually permanent.  In puppies, this can co-exist with folds since folds and dysplasia are difficult to differentiate in the immature retina.  As the folds disappear later when the retinas mature, the dysplasia will persist. Retinal folds rarely cause serious vision problems as they are usually just small blind areas which may not be noticed by the dog. However, large areas of dysplasia (geographic dysplasia) may lead to visual impairment and dogs with retinal detachments may become totally blind.

3.  Generalized or complete retinal dysplasia:  This form exhibits severe retinal disorganization and is associated with detachment of the retina.  Detachment occurs when the retinal sensory tissue separates from the tissue in the back of the eye, thereby causing complete blindness.  This is detectable at birth and is permanent.

Retinal dysplasia is not a progressive disease, but it will continue to develop until the retinas are mature at around one year of age. If you suspect one of your puppies has visual impairment, seek an ophthalmology exam immediately.  Signs would include frequently bumping into objects, less activity, and possibly timid or fearful.

There is no treatment for this disease.  The best we can do is to strive to prevent it.  The literature recommends that dogs affected with either of the 2 more serious forms of dysplasia (geographic or complete) should not be bred, as well as their parents and their littermates. The genetic relationship between retinal folds and the other 2 forms of dysplasia is unknown at this time.  Some ophthalmologists suggest that a mature dog with persistent retinal folds may produce more severe forms of the disease in their offspring, secondary to the dog being a carrier of the gene for the more severe forms of dysplasia. 

With their acute senses of smell and hearing, dogs can compensate very well for visual difficulties, particularly in familiar surroundings. In fact owners may be unaware of the extent of vision loss. You can help your visually impaired dog by developing regular routes for exercise, maintaining your dog's surroundings as constant as possible, introducing any necessary changes gradually, and being patient with your dog.


Multifocal Retinal Dysplasia  Linear folding of the sensory retina and the formation of rosettes composed of variable numbers of neuronal cells are the hostological characteristics of MFR. Typically the lesions range from vermiform grey streaks to multiple focal sites of tapetal hyperrreflectivity hitch may or may not be associated with hypertrophy.

Progressive retinal atrophy (PRA) Perhaps the best known hereditary eye disease is PRA (generalised progressive retinal atrophy) in which the retina, the light sensitive membrane at the back of the eye, degenerates from puppyhood during life. The condition is always bilateral and the first sign noticed by the owner is often one of night blindness or poor vision in subdued light, but thus progresses over months or years to total blindness and there is no treatment that will either halt or reverse the degeneration.

PRA is a group of degenerative retinopathies consisting of inherited photoreceptor dysplasia and degenerations that have a similar clinical appearance. The photoreceptor dysplasias inherited as autosomal recessive traits in which clinical signs develop in the first year occur in the Irish Setter, Collie, Norwegian Elkhound, Miniature Schnauzer, and Belgian Sheepdog. The photoreceptor degenerations inherited as autosomal recessive traits in which clinical signs develop at 3-5 yr occur in the Miniature and Toy Poodle, English and American Cocker Spaniel, Labrador Retriever, Tibetan Terrier, miniature Longhaired Dachshund, Akita, and Samoyed. Many other   breeds of dogs are also suspected of having inherited PRA.

Clinical signs vary from the dog first becoming night blind in the early stage of PRA (not able to see in low light surroundings) to the entire visual field in all light levels becoming affected, which is advanced PRA. The pupils are usually dilated, and owners often notice a "glow" and increased "eye shine" from the eyes. All dogs with PRA will eventually develop blindness from advanced PRA, and this time frame until the dog is blind varies considerably from dog to dog, but usually takes at least 6 months from the time of diagnosis, and can rarely take years until the dog is completely blind. Dogs with PRA can develop cataracts late in the disease process. Cataract surgery would never be done, as it would not help the dog to see. However, cataracts can cause pain and damage to the eye.

Electroretinography is often used to investigate and diagnose the condition. Cortical cataracts are common late in the course of PRA in many breeds and may mask the underlying retinopathy. No effective therapy is available. A blood-based DNA test has been developed to detect carrier and affected dogs before clinical signs develop; this test is useful only for PRA in the Irish Setter.

Different breeds of dogs actually suffer from different forms of PRA, but the end result is the same; the rod and cone cells eventually die and the affected dogs become totally blind. Owners of affected dogs usually first notice a loss of night vision, especially when the affected dog is in unusual surroundings; the condition eventually progresses to produce a loss of vision under all light conditions. There is as yet no known cure for PRA.

There are broadly two different types of PRA in the dog: rod/cone dysplasia and rod/cone degeneration. Breeds like the Irish Setter and the Miniature Long-haired Dachshund suffer from rod/cone dysplasia. In this case the rod and cone cells develop abnormally and begin to degenerate even before they are fully mature, leading to a very early age of onset in affected dogs, usually within the first few months of life. In breeds that suffer from rod/cone degeneration, like the Labrador, the Golden Retriever and the Cocker Spaniel, the rod and cone cells develop normally and only begin to degenerate later in life leading to a much later age of onset of the disease, usually anywhere from 3 to 4 years of age onwards.

PRA: CLINICAL SIGNS AND AGE OF ONSET

  Early Onset Forms Late Onset Forms
Vision Problems
  Night Blindness from birth 1 to 5 years
  Total Blindness 1 to 5 years 3 to > 5 years
Electroretinogram
  Rod Dysfunction from birth < 6 months to > 3 years
  Cone Dysfunction from birth to 2 years from < 6 months to > 5 years
Fundus (Eye Exam)
  Early Disease from 8 weeks to 12 months from 1 to > 3 years
  Mid-Stage PRA from 6 months to 2 years from 2 to > 5 years
  Late-Stage PRA from 1 to > 2 years from 3 to > 5 years

With one exception, PRA in all breeds so far studied is an autosomal recessive disorder. That means that to be affected a pup has to receive one copy of the defective gene from both parents. Thus both parents of an affected pup must be either carriers or affected themselves. Similarly, because affected dogs have two copies of the defective gene, all their progeny will be at least carriers. The Siberian Husky is the only known breed, so far, in which PRA is not autosomally inherited.

The minimum evidence required to establish that PRA exists in a breed is documentation that multiple affected dogs have been diagnosed with a consistent disease phenotype, and that there exists between these affected dogs a pedigree relationship that supports a recognizable pattern of inheritance.