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VITREOUS DEGENERATION |
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What is vitreous?
The vitreous is a jelly-like material located behind the lens in the eye. It has several responsibilities. First, it helps keep the retina in position. Second, by being a jelly, it reduces torque applied to the retina when an individual shakes their head.
What is vitreous degeneration?
Vitreous degeneration may be one of several conditions: It may refer to liquefaction
of the vitreous which occurs following some types of inflammation. This occurs commonly in horses, dogs and cats following episodes of uveitis.
Alternatively, it can occur in certain breeds of dogs as a primary condition. These breeds include the Shih Tzu, Brussels Griffon, Chihuahua, Havanese, Italian
Greyhound, Löwchen, Papillon, Whippet and is seen occasionally in the Labrador retriever.
Why is vitreous degeneration significant?
Vitreous degeneration may be significant because it has been suggested that it may predispose patients to retinal detachment. Veterinary ophthalmologists are not certain about this, but from work that has been presented, it seems likely that vitreous degeneration is noticed in Shih Tzu dogs prior to retinal detachment. For this reason, patients found to have vitreous degeneration should be monitored for possible retinal detachment by the use of ultrasonography and careful examination of the retina at regular intervals.
If my pet is diagnosed with vitreous degeneration, what should I do?
Your pet may be diagnosed with vitreous degeneration during a routine ophthalmic examination or a breeding examination (C.E.R.F. examination). It is unlikely that a general veterinarian will find vitreous degeneration during an annual examination. If you have a pet which is in the breeds listed above, it is suggested that you have an ophthalmic examination or CERF examination by a veterinary ophthalmologist at some point in the future. Pets who develop retinal detachment in one eye should certainly have an ophthalmic examination.
What can be done if my pet has vitreous degeneration?
There are several viewpoints concerning what should be done. If your pet has not experienced retinal detachment, then regular examinations should be performed. If your pet has had retinal detachment, the Dr. Hacker feels that prophylactic lasering of the periphery of the retina should help prevent retinal detachment in the second eye.
Dennis Hacker
Web Site |
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SUDDEN ACQUIRED RETINAL DEGENERATION (SARD) |
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Sudden acquired retinal degeneration is probably the second most common retinal degeneration in dogs. It has not been reported in cats. Affected dogs tend to be middle-aged or older. Females may be over-represented and many patients seem to be overweight. They are presented for peracute visual loss without signs of inflammation. Clinical examination usually reveals normal PLR's and no observable fundic abnormalities. Non-specific signs of retinal degeneration identical to those seen in PRA are first observed 4-8 weeks after onset of blindness. Occasionally dogs will simultaneously demonstrate signs of canine Cushing's syndrome including PU/PD, polyphagia, weight gain, and panting. Some of these patients will also have biochemistry, urinalysis, and endocrine changes that are supportive of the diagnosis. These clinical signs and biochemical changes usually resolve spontaneously in a short period and do not warrant treatment. Acute vision loss with normal PLR's is also consistent with a diagnosis of cortical blindness and for this reason, some owners will choose to verify the diagnosis. This is done most simply with an electroretinogram that will be extinguished in SARD but normal in cortical blindness. No treatment is possible.
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PROGRESSIVE RETINAL ATROPHY OR DEGENERATION (PRA OR PRD) |
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Progressive retinal atrophy or degeneration (PRA or PRD) is the name for several diseases that are progressive and lead to blindness. First recognized at the beginning of the 20th century in Gordon Setters, this inherited condition has been documented in over 100 breeds, and mixed breed animals as well. PRA is not very common in cats.
Anatomy of the eye
 The eye is a very delicate, yet surprisingly durable organ. It consists of several layers. The cornea is a transparent layer that covers the front of the eye. The iris is the colored part of the eye and it is responsible for letting in more or less light. The lens gathers and 'bends' light in order to focus it on the retina. In between the cornea and lens is an area of fluid which bathes the lens and helps it focus. The retina lines the inside of the eye and converts light into signals which travel down the optic nerve to the brain. A large area between the lens and the retina contains a jelly-like fluid called 'vitreous.' The vitreous gives the eye its form and shape, provides nutrients, and removes waste products.
The retina
The retina is the structure affected in PRA. This important part of the eye receives the light gathered and focused by the other eye structures. It takes the light and essentially converts it into electrical nerve signals that the brain, via the optic nerve, interprets as vision. The retina contains photoreceptors, called rods and cones, which help the animal see in darkness (rods) and see certain colors (cones).
What is PRA?
There are multiple forms of PRA which differ in the age of onset and rate of progression of the disease. Some breeds experience an earlier onset than others; other breeds do not develop PRA until later in life.
Normally, the photoreceptors in the retinas develop after birth to about 8 weeks of age. The retinas of dogs with PRA either have arrested development (retinal dysplasia) or early degeneration of the photoreceptors. Retinal dysplastic dogs are usually affected within two months of birth and may be completely blind by one year. Dogs with retinal degeneration are affected from one year to eight years of age and the symptoms progress slowly.
PRA worsens over time. The affected animal experiences night blindness initially because the rods are affected first. The condition progresses to failed daytime vision.
What are the signs of PRA?
Signs may vary depending on the type of PRA and its rate of progression. PRA is non painful and outward appearance of the eye is often normal, i.e.; no redness, excess tearing, or squinting. Owners may notice a change in personality of their dog such as a reluctance to go down stairs or down a dark hallway. This is characteristic of night blindness, in which vision may appear to improve during the daytime. As the disease progresses, owners can observe a dilation of the pupils and the reflection of light from the back of the eye. If the blindness is progressing slowly, the owner may not notice any signs until the dog is in unfamiliar surroundings and the lack of vision is more apparent. In some animals, the lens of the eyes may become opaque or cloudy.
How is PRA diagnosed?
Depending on the form of PRA, characteristic changes in the retina and other parts of the eye may be observed through an ophthalmic examination by a veterinary opthalmologist. More sophisticated tests such as electroretinography may also be used. Both tests are painless and the animal does not have to be anesthetized. If no abnormalities are found during the exam by a board certified veterinary ophthalmologist, the dog can be certified free of heritable eye disease through the Canine Eye Registration Foundation (CERF).
How is PRA treated?
Unfortunately, there is no treatment for PRA, nor a way to slow the progression of the disease. Animals with PRA usually become blind. Dogs are remarkably adaptable to progressive blindness, and can often seem to perform normally in their customary environments. Evidence of the blindness is more pronounced if the furniture is rearranged or the animals are in unfamiliar surroundings.
Can PRA be prevented?
PRA has been shown to have a genetic component. Puppies from parents who have no history of the disease and have been certified free of PRA will have less risk of developing the disease. Affected animals should not be bred and should be spayed or neutered. The littermates or parents of animals with PRA should also not be bred. If your dog develops PRA, notify the breeder, if possible.
In the last several years, DNA is being used to identify which genes are responsible for PRA. In one form of PRA called 'rod cone dysplasia 1' (rcd1), which affects Irish Setters, the gene mutation has been identified.
Dog Breeds Most Commonly Affected by PRA, and/or Have DNA Testing Available
Progressive rod-cone degeneration (prcd) is the most widespread form of PRA and affects many breeds including Poodles, American and English Cocker Spaniels, Labrador Retrievers, and Portuguese Water Dogs. Prcd starts with night blindness and progresses to total blindness at 3 to 5 years of age. The late onset of clinical signs in prcd is particularly devastating to breeding programs because many dogs have already been bred prior to the onset of symptoms. Thus, the development of a genetic test for this disease which could identify both affected animals and those that just carry the gene would be particularly useful. The researchers at the James A. Baker Institute have found a set of genetic markers that usually indicate the presence of the gene mutation that causes prcd in English Cockers, Labrador Retrievers, Chesapeake Bay Retrievers, and Portuguese Water Dogs. Although the exact gene mutation that causes prcd has not been found, every dog that is affected with prcd has two copies of the genetic marker and every dog that does not have the marker is clear of prcd. Unfortunately, this test for the genetic marker is not as accurate in diagnosing prcd as it would be if the actual gene mutation was found. This is because dogs that are not affected by prcd may still have the genetic marker present and have false positive test results. Thus dogs that are positive for the gene marker may be either false positives or may be carriers of the disease or may actually have the disease. The marker test may be done at a very early age, so potential breeding animals may be selected when they are still puppies.
- Marty Smith, DVM
- Holly Nash, DVM, MS
Drs. Foster & Smith, Inc.
PetEducation.com |
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PROGRESSIVE RETINAL ATROPHY |
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Progressive Retinal Atrophy (PRA), is an incurable hereditary eye disease which, as the name implies, progressively attacks and destroys the retina of the eye, causing blindness. The retina is essential to eyesight, for it is here that a visual image is formed before being transmitted via the optic nerve to the brain. A defect in an enzyme causes a chemical compound to form that kills the cells in the retina.
PRA begins with night blindness, followed by gradual loss of day vision and, eventually, total sightlessness. In some affected breeds, vision loss is observed in puppies, and the dogs may become blind before or soon after maturity. In other breeds, the disease can go undetected until the dog is several years old and has passed the PRA gene to subsequent generations of puppies. Like retinitis pigmentosa in humans, canine PRA is not one disease but a group of related ones. All are characterized by malformation or degeneration of the retinal visual cells. Most forms of PRA are caused by different, autosomal recessive gene defects. This means that for offspring to be affected, both parents must carry one copy of the same mutant gene. Both parents could have normal eyesight, but have 1 gene for normal enzyme production that is dominate over the 1 recessive gene for the abnormal enzyme defect. In this case the parents would be considered carriers.
When a carrier is bred to another dog with 1 dominant normal gene and 1 recessive abnormal enzyme defect gene (i.e., another carrier), 25% of the offspring will be afflicted with PRA, 50% will be carriers like their parents, and 25% will possess the normal 2 dominant correct enzyme genes.
If a dog afflicted with PRA is bred to another dog who is neither afflicted nor a carrier, then all the offspring from that breeding will be carriers (possessing 1 dominant normal enzyme gene and 1 recessive abnormal enzyme gene). All of these offspring are then capable of producing PRA if they are bred to another carrier or a PRA afflicted dog.
In some cases, breeders have resorted to producing "test litters" as a means of identifying and removing all carriers and affected dogs from their lines. The disease afflicts an estimated 80 breeds of dogs worldwide.
Research has achieved some success in developing a test for the defective gene. Carriers of the mutant gene for one type of PRA in Irish Setters can be detected with a new blood test developed at the Cornell College of Veterinary Medicine. The unequivocal DNA blood test for rod-cone dysplasia-1 (rcd-1) in Irish Setters gives researchers hope that similar tests for other affected breeds will produce equally promising results.
Meanwhile, the best recourse of breeders and buyers alike is regular eye checks by qualified veterinarians, and honest disclosure of the problem when diagnosed. Until the method of detecting the defective gene in Chesapeake's is available, buyers and breeders alike must depend upon vigilance and careful research of pedigrees to reduce the occurrence of PRA in the breed.
Until development of the first DNA test, PRA can only be detected by electro- retinography testing and requires examination by a canine ophthalmologist. Using special equipment the ophthalmologist will examine your dog's eyes for a thinning of the retina, which causes the dog's eyes to be hyper-reflective. They may also observe a paleness of the optic disk and a reduction of blood vessels in the eyes. This test, which is available at regional veterinary
centers, identifies only dogs affected by PRA - not the clinically normal animals that carry one copy of the defective gene. Thus, a carrier can still be unknowingly used in breeding. Responsible breeders will supply information on the eyes of their dogs in the form of one or more of the following: eye examination reports (preferably from veterinary ophthalmologists), electroretinogram
(ERGs), and examination forms for the Canine Eye Registry Foundation (CERF).
Often, the symptoms of the disease can go un-noticed by owners until it has progressed significantly. Since the disease advances slowly, an afflicted dog can adapt by depending more heavily on his sense of smell and hearing. Often an afflicted dog can cope very well until total blindness occurs. It is not unusual for owners of afflicted dogs to be unaware of the problem until an eye examination is obtained.
Not all retinal degeneration is caused by genetic PRA. non-PRA retinal degeneration does exist and can be distinguished in the early stages of development from genetic PRA. When either type of retinal degeneration is found in an advanced state (e.g., the retina in nearly totally degenerated) it is not possible to distinguish between the two types by eye examination.
This is further reasoning for routine and periodic eye examinations. Also, because the determination of PRA can be subjective and have significant consequences, a second opinion would be a good idea.
PRA is a descriptive term applied to retinal diseases that affect all breeds of dogs. The same clinical signs are present in all PRA affected animals. Affected animals will show night blindness and a progressive loss of day vision.
Many PRA cases can be diagnosed between three and five years of age. It is during this age period subtle retinal changes can be noted by the experienced ophthalmologist. Even though the same clinical signs will be present in all PRA affected animals, the age of onset of disease differs from breed to breed. The onset period is divided into three approximate age groups: early, middle, and late. This late-onset form of the disease is now called Progressive Rod-Cone Degeneration
(PRCD). PRCD is inherited RECESSIVELY.
RECESSIVE INHERITANCE
An affected animal can only be produced if both parents are affected, both carry the defective gene, or one parent is affected and the other carries. All the offspring of a PRA affected dog will carry ONE PRA gene. The status of the second parent will determine whether the offspring will be affected or carriers. Statistically, when two carriers are mated a 1:2:1 ratio (25% will be affected- i.e., 1 in 4, 50% will carry, 25% will be genetically clear-1 in 4) will occur.
All breeding stock should have annual eye exams for life. There are other ocular conditions that affect our breed. some of these conditions are: cataracts, entropion, ectropion, distichiasis, dry eye (geriatric), persistent pupillary membranes, and retinal dysplasia.
Carole Miller & John Mingo Snr.
In Memory of Pets |
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PRA TESTING IN AUSTRALIA
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Dr Ziggy Chester,
BSc BVMS Murd, Cert.V.Ophthal
Animal Veterinary Hospital
Murdoch University
South Street, Murdoch, WA 6150
Phone: (08) 9360 6945; 1300652494
Hours: Tuesday & Thursday mornings by appointment. Cost: $27.50
Email: chester@numbat.murdoch.edu.au |
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RECOGNISED CERF VETERINARY OPHTHALMOLOGISTS |
| New South
Wales |
Victoria |
Dr. Cameran Whittaker
Dr. Jeff Smith
Animal Eye Clinic
104 Spofforth St.
Cremorne, N.S.W. 2090 |
Dr. Rowan Blogg
1A Irymple Av.
East Malvern,
Victoria 3146 Australia
Phone: (03) 9509 7611
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| Queensland |
Western
Australia |
Dr.
Douglas Slatter
Dr. Elizabeth Chambers
109 Haig Road
Toowong,
Queensland 4066
Phone: (07) 3371 5763
Web: www.animaleyeclinic.com
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Dr. Anita
Dutton
Diplomate ACO
Animal Eye Clinic
855 Canning Highway
Applecross
Western Australia 6153
Phone: (08) 9315 5525 |
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OTHER AUSTRALIAN & NEW ZEALAND EYE CLINICS |
| New
South Wales
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New
Zealand |
Dr. Bruce Robertson,
Eyevet Associates,
274 Pennant Hills Road,
Carlingford, N.S.W. 2118
Phone: 9872-9877
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Auckland
Animal Eye Centre
18 Barrack Rd,
Mt Wellington, Auckland
Phone: 0-9-527 7697
eyevet@xtra.co.nz
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| South
Australia |
Victoria
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Adelaide
Veterinary Specialist
and Referral Centre
Dr. R. A .Read,
102 Magill Rd, Norwood, SA 5067
Ph: (08) 8132 0533
Web: www.vetreferrals.com.au/ |
Dr Robin Stanley,
Dr Anu O'Reilly,
Dr Chloe Hardman
181 Darling Road
East Malvern Vic 3145
Phone: (03) 9563 6488
Web: www.animaleyecare.com.au
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| Queensland |
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Animal
Eye Services
Dr Richard I E Smith, Dr Michael E Bernays, Dr Edith C G
M Hampson
Cnr. Kessels Rd & Springfield St.,
Macgregor Qld. 4109
Ph (07) 3422 2010
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