In the most general of terms, blindness is the loss of the ability to see. Vision impairments often develop in stages over a prolonged period of time
Causes of Canine Blindness
Any condition that blocks light from getting to the retina can impair a dog's vision. This includes diseases of or damage to the cornea, retina or other structures of the eye. Blindness can be caused by cataracts, glaucoma, uveitis, corneal trauma, corneal ulceration, lens luxation, retinal detachment, retinal hemorrhage, retinal degeneration, retinal atrophy, cerebral (brain) lesions affecting the optic nerve (congenital optic nerve hypoplasia, inflammation [optic neuritis], neoplasia [cancer], trauma, atrophy, abscess, optic chiasm lesions), cerebral swelling (edema), ivermectin toxicity, lead toxicity and inflammatory, infectious or neoplastic diseases of the brain.
Many causes of canine blindness have a suspected genetic basis and may be highly breed and age-specific. Vision disorders seem to be more prevalent in white-colored dogs, including white Boxers and white Great Danes.
Prevention of Canine Blindness
Because of the numerous and unrelated potential causes of canine blindness, there is no realistic way to describe a sensible prevention protocol.
The inactivity of aging dogs is most commonly attributed simply to "old age." However, it may also be due in part to failing eyesight, which can make a dog reluctant to move around, even in a supposedly familiar environment. A blind or visually impaired dog should not be turned loose in unfamiliar surroundings, and the furniture in familiar areas should not be rearranged. Blind dogs have a "mental map" of their environment and typically navigate extremely well within the confines of their familiar home.
The symptoms of blindness in companion dogs can be quite variable, depending upon whether one or both eyes are affected and whether the vision loss comes on suddenly or develops slowly. The symptoms of vision loss are also affected by the underlying reason for the vision impairment.
Symptoms of Canine Blindness
Blindness can affect one or both eyes. If it affects only one eye, it is called unilateral blindness. If it affects both eyes, it is considered to be bilateral. Blindness in domestic dogs can come on suddenly or very gradually. The symptoms of vision loss often include one or more of the following:
Bumping into objects or structures in the dog's own environment
Vision deficits in dim light and darkness (loss of "night vision")
Difficulty finding food bowls, water dishes, toys and other familiar things
Difficulty catching balls or other objects
Exaggerated high-stepping gait
Walking with great caution
Walking with nose to the ground
Reluctance to move
Redness of the eye(s) (+/-)
Dilated (enlarged) pupils (+/-)
Opacity of the eye(s) (+/-)
Dogs whose vision loss happens gradually tend to adjust better and compensate better than those with a sudden onset of vision deficits, especially if the vision loss is very slowly progressive.
Dogs at Increased Risk
Dogs (and cats) of any age or gender may be affected by blindness, depending upon its cause. Some breeds have an increased risk of developing blindness due to primary glaucoma, including the Beagle, Bassett Hound, Bouvier des Flandres, Chow Chow, Cocker Spaniel, Dachshund, Dalmatian, Great Dane, Poodle, Shar-Pei, Malamute, Siberian Husky and certain Spaniel breeds. When the cause of blindness is lens luxation, the breeds at increased risk include Terriers, Spaniels, German Shepherd Dogs, Miniature Poodles, Toy Poodles and Chihuahuas. When blindness is caused by retinal detachment, Shih Tzu's seem to be at increased risk.
Older dogs tend to be predisposed to developing blindness associated with cancer (neoplasia) and retinal detachment. Dogs with frequent access to the out-of-doors may be predisposed to developing infectious diseases and/or trauma associated with vision loss. Dogs with poorly-regulated diabetes mellitus are predisposed to developing cataracts, which can contribute to blindness. Dogs with hypertension (high blood pressure) are at increased risk for developing retinal detachment and corresponding vision loss.
Blindness in dogs is often first noticed by owners when their dog starts bumping into furniture, walls or other objects in its familiar environment. Most causes of blindness are not evident by looking at the eye itself.
How Canine Blindness is Diagnosed
When presented with a dog suspected of having vision impairment or loss, most veterinarians will perform a thorough physical examination and take a complete history. They also typically will draw blood samples for a complete blood count and a serum biochemistry profile, and will take a urine sample for a urinalysis, to assess the overall health of the dog and the status of the function of its vital organs. The dog's blood pressure may also be assessed, because high blood pressure (hypertension) can contribute to retinal detachment, which is one of the potential causes of blindness.
The veterinarian will also conduct specific tests to assess the dog's visual capacity. Examination of the function of the cranial nerves is essential to a diagnosis of the possible causes of blindness. This includes assessing: 1) the "menace response" (how the dog reacts to a hand or other object being moved rapidly towards its face); 2) the size and symmetry of the pupils; 3) the pupillary light reflexes (PLRs); 4) the dazzle, palpebral and corneal reflexes; 5) the vestibulo-ocular reflex; 6) facial nerve function/facial sensation; and 7) motor function of the muscles of the eyes and face. Your veterinarian can provide you with the details of these various vision tests.
Another common test of vision is to observe the dog in a dark, familiar room in which the furniture has been rearranged. The same test can then be done with the lights on. A blind dog will react the same way in both situations, while a dog with some vision will react more normally when the lights are on. The attending veterinarian is in the best position to describe the details of these examinations and their results to the owners of affected dogs.
Most veterinarians will also do a thorough neurological examination on dogs with vision loss, and of course will conduct a complete ophthalmic (eye) examination, as well. The ophthalmic exam may include: 1) a fluorescein stain test; 2) assessment of intraocular pressure; 3) evaluation of any opacity of eye structures; and 4) evaluation of all aspects and structures of the eyes after pharmacologically dilating the pupils with special eye drops.
Thoracic radiographs (chest X-rays) may be taken to look for evidence of cancer or systemic infectious disease. Specific blood tests are available to identify specific infectious disease that might contribute to or cause vision loss, such as Lyme disease, Toxoplasmosis, Bartonellosis or systemic fungal infections. Ocular ultrasonography (ultrasound of the eyes) can also be done.
When the local veterinarian cannot determine the cause of blindness, she typically will refer the owner and patient to a specialized veterinary ophthalmologist for advanced testing. This might include: 1) electroretinography, to assess retinal function; 2) a tap of the cerebrospinal fluid; 3) vitreous centesis, with culture, titers and cytologic (cellular) assessment of the sample; 4) evaluation of visual-evoked potentials; 5) magnetic resonance imaging (MRI); and/or 6) computed tomography (CT scan). These are very advanced diagnostic procedures.
A diagnosis of vision impairment or loss should not be taken as a catastrophe or a death sentence. Most dogs normally rely much more on their senses of smell and hearing than they do on their sense of sight. As vision fails, a dog's senses of hearing and smell become even more acute, which usually helps the dog navigate familiar rooms and areas.
The outcome of any treatment for blindness is highly variable and depends almost entirely upon the underlying cause of the vision loss. The goals of treating vision loss are to resolve any underlying cause if possible so as to restore sight, and to help the dog maintain a happy, enjoyable, safe, pain-free, high-quality life.
Most authorities recommend against administering systemic corticosteroids as a form of empirical therapy – which is treatment before the precise cause of the condition is determined - because premature administration of drugs that suppress the dog's immune system (like steroids) may make it more difficult to diagnose neoplasia (cancer) and other inflammatory disorders, and may actually prompt the proliferation of infectious diseases. The primary cause of a dog's blindness must be identified for effective treatment or management to begin. In some cases, especially if the vision loss is irreversible and the dog is painful, removal of the affected eye (which is called "enucleation") may be a viable alternative.
The prognosis for dogs with vision loss is highly variable and depends almost entirely on the underlying cause of the condition. For example, dogs that are born blind (have congenital blindness) probably will never be able to see. On the other hand, dogs that develop acute vision loss may have their sight restored, especially if there is rapid diagnosis and aggressive treatment of the cause of their vision loss. Unfortunately, in many cases, blindness is irreversible. However, most causes of blindness are rarely fatal, and affected dogs usually are able to live relatively normal, full and functional lives.