Retinal degeneration is the "thinning" or "atrophying" of the retina, the light-absorbing neurological tissue in the eye. Retinal degeneration is a non-inflammatory condition that occurs when the cells in the retina begin to decline in function, and the tissue atrophies.
The retina is the neurologic tissue layer that lines the back of the eye. The retina is composed of multiple layers of highly interconnected cells that converge to form the optic nerve. The rods and cones are a specific type of retinal cell, referred to as the photoreceptors. The function of the rods and cones is to absorb incoming light and turn this light energy into electrical energy. This electrical signal is then transmitted through the retina and to the brain for interpretation. The rods photoreceptors function for dim light vision, and the cone photoreceptors function for color vision, and visual acuity.
There are many forms of retinal degeneration, broadly categorized by their underlying cause, and age of onset. The term “progressive retinal atrophy (PRA)” is the broad classification for the inherited forms of retinal degeneration. These diseases are the result of an inherited defect in certain genes, often those that control retinal cell function, or transmission of the visual signal through the retina. There are two distinct patterns to this disease; some breeds are affected by abnormal development of the retina, termed “retinal dysplasia.” These dogs develop visual deficits at an early age, and the retina slowly degenerates over time. Other breeds have a later onset, slowly progressive degeneration. The non-inherited forms of retinal degeneration are often secondary to an underlying disease, and can be caused by inflammation, infection, or bleeding in the retina.
Inherited PRA has been seen clinically in almost all canine breeds, and in many breeds we know the gene defect responsible for the disease, and the mode of inheritance. The most common form of PRA, known as progressive rod cone degeneration, is a late-onset degeneration in which the rods are affected first, then the cones also become affected later in the disease.
Whenever your pet is showing signs of a health issue, your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from retinal deterioration or another serious condition, a veterinary specialist is available at an ExpertVet certified hospital.
As previously mentioned, the rod photoreceptors loose function first. Thus, the early signs of retinal degeneration include night vision difficulties. Pet owners often note that their pets seemed disoriented or hesitant when going out to the yard at night, bump into things when in dimly lit conditions, or are less willing to climb stairs at night. Occasionally these pets will even become lost in their own home after the lights have been turned off. Some owners do not notice the signs of retinal degeneration until their day vision also becomes affected.
Because retinal degeneration is not a painful condition, your pet will not have reddened eyes, increased blinking or squinting, or excessive discharge from the eyes. For this reason, most pet owners will not notice the earliest stages of the condition. Once the retinal degeneration is in moderate to advanced stages, the pupil will no longer respond to light normally, and some owners will notice an abnormal shine coming from their pet’s eyes. The tapetum, which is the reflective layer of the retina, will become brighter (“hyperreflective”) as the retinal tissue overlying it thins, resulting in this increased eye shine.
Cataracts often form in the late stages of retinal degeneration. These cataracts can be the first sign noted by the owner, as an increased cloudiness in the eyes. Many owners assume these cataracts are the cause of the visual impairments.
Retinal degeneration is diagnosed by using special lenses to examine the retinal tissue. Classically, a decrease in the size and number of retinal blood vessels and an increased shine to the tapetum are the hallmark findings of inherited retinal degeneration. These changes are very subtle in the early phases of the disease. In the later stages of the disease decreased visual responses and pupil movements may also be noted. The changes are almost always noted in both eyes to a similar degree.
Secondary retinal degeneration is often asymmetrical (different in each eye), may be unilateral (in both eyes), and the retinal exam will show evidence of previous injury. Many different diseases can induce secondary retinal atrophy, so a systemic evaluation (physical examination, blood work, blood pressure measurement, radiographs, etc.) may be recommended by your veterinarian or veterinary ophthalmologist.
If the retinal exam is inconclusive, or cataracts are present which interfere with the retinal exam, additional retinal function testing may be required. A veterinary ophthalmologist can perform an electroretinogram (ERG). This is a test that measures the electrical activity of the retina (similar to an EKG for the heart). The test is performed by placing a small electrode or contact lens on the surface of the eye, and small electrodes in the skin next to the eye. After a short period of acclimation in a dark room, a bright light is flashed onto the retina, and the activity produces a tracing on a computer screen. A healthy retina will produce a characteristic waveform, and dogs with PRA will have reduced or absent activity. The ERG is sensitive enough to diagnose dogs with early stages of PRA before they begin to demonstrate signs of the disease.
In dogs with abnormal vision but a normal ERG, a neurologic work-up may be recommended to identify the source of the vision loss in the brain.
There is no specific treatment for retinal degeneration. If the retinal degeneration is secondary to an underlying systemic disease, then the primary condition should be treated to optimize the pets overall health. These treatment options will be discussed with your veterinarian or veterinary ophthalmologist based on examination and/or test results. As previously mentioned, cataracts may form secondary to retinal degeneration. Cataracts may induce inflammation as they form, which can lead to discomfort, adhesions or scars, and even glaucoma. Treatment of this cataract-induced inflammation is recommended to reduce the risk of these secondary complications.
Once vision loss or ophthalmoscopic evidence of degeneration is present there is a poor prognosis for vision. However anti-oxidant and vitamin supplementation (Vitamin A, Lutein) may be recommended to help slow the degenerative process.
Patients affected with the hereditary forms of PRA should not be used for breeding. Pedigree studies or genetic tests are used to help eliminate carriers of this condition such as the pet¹s brothers, sisters, mother, father and any offspring.