Eyeshine
Eyeshine refers to the reflection of light off the mirror-like membrane, called the tapetum, in the back of the eye. Many animals, including dogs and cats, have a tapetum, while humans do not. The tapetum improves the low light vision of these animals.
There are multiple veterinary conditions where the presence of increased eyeshine may be a symptom:
Retinal Detachment
Retinal reattachment surgery is offered at our Pasadena, California (626-564-0202) and St. Charles, Illinois (630-444-0393) hospitals, where patients are received from around the country for surgical care.
The retina
The retina is a thin membrane that lies against the back wall of the eye. Similar to the way a camera works, images from the outside of the eye are focused by the cornea and lens onto the retina, which can be compared to the film in a camera. If the retina is not properly positioned or becomes damaged, vision is diminished or lost. The retina is only fused to the wall of the eye at its peripheral edges and at the optic nerve. Much of the retina relies on vitreous, a firm molded gel (like Jello), to tamponade it against the back of the eye.
What causes a retinal detachment?
Although there are many types of retinal detachment, there are two main scenarios that require retinal surgery. The first is called vitreal degeneration/dysplasia, where the vitreous humor, instead of forming a semi-firm gel, liquefies. It transforms from a Jello-like consistency to a chicken noodle soup-like consistency and can no longer effectively hold the retina in place. As the eyes move, the swirling liquefied vitreous tugs on the retina, causing tears and fluid flow under the retina, separating it from the wall of the eye. The tears often progress, until the entire retina has peeled off the back of the eye. This type of retinal detachment (rhegmatogenous retinal detachment) is inherited or genetic in many breeds including the Shih Tzu, Italian Greyhound, Boston Terrier, Poodle and Terrier Breeds. Tears may be precipitated or exacerbated by vigorous head shaking during play.
The second main type of retinal detachment occurs secondary to cataracts, cataract surgery or other intraocular surgeries. Inflammation in the eye lead to areas of adhesion between the retina and the vitreous, putting traction on the retina, pulling it off the back of the eye. Retinal detachment can occur due to other causes such as trauma.
Signs of retinal detachment
Because most dogs continue to behave normally as long as one eye has vision, retinal detachment in the first eye is often missed. Patients often present to a veterinary ophthalmologist for vision loss when the retina in the second eye detaches. Signs of retinal detachment include vision loss, a dilated pupil, increased eyeshine, and sometimes blood within the eye. With cataracts, retinal detachments are often detected during pre-operative screening ultrasounds. They may also be identified following cataract surgery during recheck examinations.
How is retinal reattachment surgery performed?
Surgical correction is accomplished by entering the back of the eye via small ports. The diseased vitreous is first removed. The detached retina is then repositioned back against the back of the eye using a heavy oil called PFO. The PFO is replaced by silicon oil, which acts as an artificial vitreous. A laser is also used to strengthen the retina’s attachment to the back of the eye.
What is the success rate?
The two biggest factors that influence the success of surgery are: the length of time the retina has been detached prior to surgery and the cause of retinal detachment. Predisposed breeds, such as the Shih Tzu, tend to have the highest success rates (often >90%) if retinal detachments are caught early. Retinal detachments following blunt trauma or bite wounds often have the lowest success rate (<50%). The veterinary ophthalmologist performing retinal reattachment surgery will be able to further discuss the estimated success rate for your pet, the details and care involved with retinal reattachment surgery, as well as work with your local veterinary ophthalmologist to provide long-term care for your pet’s eyes.
What are some potential post-operative complications?
The success of retinal reattachment surgery has improved over the past decade with state of the art equipment and shorter surgery times. However, complications can arise and include retinal re-detachment and degeneration, glaucoma, chronic inflammation, bleeding inside the eye, corneal ulceration, cataract formation, changes in focusing, or silicon oil migration into the front of the eye.
What if retinal reattachment surgery isn’t a possibility?
While most retinal detachments are not initially associated with discomfort, long term, many dogs with an untreated retinal detachment will develop glaucoma, or an elevated pressure in the eye. As this can become an uncomfortable condition, your pet should continue to receive ocular exams over time. Should glaucoma develop, further treatment will be necessary to maintain comfort.
Progressive Retinal Atrophy
To better understand progressive retinal atrophy, one must have a basic understanding of the function of the retina. The retina is a highly specialized tissue that lines the back of the eye. The retina is analogous to film in a camera; it is responsible for integrating light into vision. Without adequate retinal function, vision is not possible. Simplified, the eye can be thought of as a light-collecting organ that focuses light rays on the retina. As light strikes the retina, a sequence of chemical reactions are initiated, propagating an electrical impulse. The impulse passes through the layers of the retina to the optic nerve and finally to the brain (visual cortex) for interpretation. The brain’s interpretation of the light signal is responsible for what we know as vision.
The retinal cells, which transform light energy to chemical energy, are known as rods and cones. Rods are responsible for black and white vision, night vision and vision for movements, whereas cone cells are used for color discrimination, vision in bright light and acute focal vision. Most domestic animals (dogs, cats, etc.) have a dominance of rods. Color vision in dogs is poor compared to people.
As the name progressive retinal atrophy (or PRA) implies, an atrophy or a degeneration of retinal tissue occurs. Progression of this disease occurs slowly and the early signs may be overlooked in many animals. The slow loss of sight is similar to a dimming switch to reduce brightness of light in a room. If light is slowly reduced over a long period of time, our eyes adapt and the change is not noticed until darkness occurs. A similar situation occurs in progressive retinal atrophy in animals; often the condition is not noticed until the condition is significantly progressed. Unfortunately, there is no cure available for progressive retinal atrophy. Identification of affected breeding animals is essential to prevent the spread of the condition within the breed.
Clinical Signs of PRA
The early signs of retinal atrophy include night blindness in most cases, which will frequently progress to day blindness. Night blindness may be manifested in a number of ways, including a pet that is hesitant or afraid to go out in the dark or go into a dark room. Often these pets will get lost in their own home after the lights have been turned off or they may stay near the light in the backyard at night versus wandering the full extent of the yard as they did previously. Pupils may be dilated and/or have a slow response to light.
Some pet owners will notice a characteristic eyeshine. This is due to increased reflectivity of an iridescent tissue known as the tapetum located underneath the retina. As previously mentioned, retinal abnormalities may not be noticed at home until later in the course of the disease. Other well-developed senses including olfaction (the sense of smell) and hearing help animals adapt to the slow loss of sight. Often sight loss is not noticed until a change of the pets’ normal environment occurs. Examples of environmental changes include furniture rearrangement in the home; an animal that is restricted to a different area of your house or is boarded while you are away on vacation, etc. Because PRA can be difficult to identify, routine ophthalmic examination of all pets is recommended. This is especially important in animals that are being considered for breeding.
When the ophthalmologist views the retina with an instrument called an indirect ophthalmoscope, changes can be seen in the retinal blood vessel pattern, the optic nerve and the tapetum (the reflective portion of the eye that is responsible for “eyeshine”). However, some breeds characteristically have little or no early visible changes and may appear normal until the later stages of the disease. Some affected dogs show various rates of progression making generalization difficult.
Cataracts may form secondarily to progressive retinal atrophy in some animals and are generally associated with the later stages of the disease process. Formation of cataracts may interfere with direct visualization of the retina and make other diagnostic modalities essential. Although cataracts are surgically treatable, removal of cataracts in an animal with progressive retinal atrophy is not indicated, as their diseased retina will still result in visual deficits. Cataracts can leak protein within the eye causing inflammation within the eye. Uncontrolled or chronic inflammation can lead to glaucoma (increased intraocular pressure), a painful and blinding disease. Therefore, the retina (and cataracts) should be monitored as they may require topical medications to prevent inflammation or glaucoma.
Electroretinography
Definitive diagnosis of PRA is supported by electroretinography. An electroretinogram “(ERG)” is similar to an electrocardiogram (ECG) for the heart in that they both measure normal electrical impulses produced by the organ of interest. A special contact lens is placed on the cornea and two tiny needles (electrodes) are placed under the skin around the eye. After a period of dark adaptation, flashing lights are used to stimulate the retina. The electrical response of the retina is recorded by the electrodes, which send a signal to a computer. A healthy retina will produce a characteristic wave pattern on the electroretinograph recording. This instrument is sensitive enough to diagnose affected dogs before they begin to demonstrate clinical signs.
Any diagnostic procedure can introduce complications, including anesthetic risks (in the few patients that require anesthesia for diagnostic procedures). In order to obtain accurate ERG recordings, the veterinary ophthalmologist may recommend sedation or anesthesia. Complications from ERG are very rare, and include, but are not limited to, inflammation of the pink tissue (conjunctivitis); ocular infections that may affect internal and/or external areas of the eye (intraocular/ extraocular infections) and corneal ulcerations (superficial to deep). If any abnormalities are noticed in your dog’s eyes following an ERG please notify us immediately so that the condition does not worsen.
Genetic Testing
Since PRA is an inherited genetic disease, it is possible to identify and test for the defective gene. This test has been developed in some breeds affected by PRA. The test requires a blood sample, which is sent to a diagnostic lab for analysis. The blood test can identify dogs that are affected, as well as normal dogs that may pass the defective gene to offspring. Information on genetic testing can be found at www.optigen.com. A partial list of breeds affected with progressive retinal atrophy follows:
Breed Type of Retinal Disease Age of Onset
Collie: rod-cone dysplasia under a year
•Irish Setter: rod-cone dysplasia under a year
•Cairn Terrier: rod-cone dysplasia under a year
•Miniature Long-Haired Dachshund: rod-cone dysplasia under a year
Norwegian Elkhound: rod dysplasia, cone 2 to 3 years
•Samoyed: rod-cone degeneration 3 years
•Cocker Spaniel: rod-cone degeneration 2 to 7 years
•Miniature Poodle: rod-cone degeneration 3 to 6 years
•Miniature Schnauzer: rod-cone degeneration 3 to 6 years
•Akita: rod-cone degeneration 3 to 6 years
•Schnauzer: rod-cone degeneration > 3 years
•Golden Retriever: rod-cone degeneration
•Labrador Retriever: rod-cone degeneration
Unfortunately, no treatment has been formulated to prevent, treat or cure progressive retinal atrophy. A number of vitamin therapies have been suggested, however, there is no evidence to suggest that vitamins have any therapeutic effect. As stated previously, affected animals should be identified as early as possible and eliminated from breeding programs.
Progressive retinal atrophy is a painless condition. Animals that lose sight from PRA usually acclimate well to their environment with time, as they utilize their other senses to make up for their vision loss. Maintaining a consistent environment for the affected animals will help the acclimation process. For example, frequent furniture rearrangement during this period should be avoided. When animals are taken from their home environment, the use of leads and harnesses are helpful in addition to reassurance to comfort your pet.
Progressive retinal atrophy refers to a broad category of inherited retinal diseases that result in gradual blindness. Because of the insidious nature of the disease, serial examinations may be required to detect affected individuals. Affected individuals should not be used for breeding purposes.
