"He’s blind." Blind? Except for a peculiar gait, the young Ibizan hound seemed as active and inquisitive as the other dogs cavorting in the yard. Still, I had to wonder if it was really fair to raise a blind dog---and a sighthound at that. "You ought to see him run…" his owner continued, "even through the woods." What? Surely this was some kind of dog abuse. Or was the dog really blind? How would you tell? What could have caused him to lose his vision? And just how handicapped is a blind dog?
Most humans who lose their vision face extensive rehabilitation in order to function independently. As a result, people tend to think of blind dogs as similarly, or even more severely, affected. In a survey of 50 owners of blind dogs, half of the owners reported that they had been chastised by friends who considered it cruel to keep a blind dog. 1 The owners of these dogs, however, would tend to disagree.
Most of the blind dogs still led full and active lives. Most still enjoyed going for walks. Surprisingly, about half of the dogs were taken for walks off leash (except when next to a road), with the owners expressing confidence that the dog was safe. Two dogs were even allowed to roam at will (one was killed by a car). Six owners reported their dogs could still play ball by relying on their other senses. The dogs could recognize frequent visitors by scent.
This is not to say that the dogs were unaffected by their blindness. Owners reported having made considerable efforts to contain their dogs in familiar environments. They avoided moving furniture within the home or boarding the dog at a kennel. Despite all efforts, twelve percent of dogs never learned to adequately cope even in familiar surroundings. Roughly half of the owners reported permanent behavioral or temperament changes in their dogs, mostly in the form of greater timidity, caution, and dependence upon the owner. Dogs would tend to walk rather than run, and some dogs would bark for attention. The ability of dogs to cope successfully was not related to their breed or size. Dogs that lost their vision relatively suddenly tended to have greater behavioral changes. Two dogs that had become blind within three months became aggressive. In general, two thirds of the owners felt their relationship with their dog had changed as a result of its blindness, with increased dependence of the dog on the owner, and ultimately a closer relationship.
The complexity of the eye led Charles Darwin, in his Origin of Species, to reflect that its development was one of the most difficult challenges to his theory of evolution. Its function relies upon the interplay of mechanical, optical, chemical, and neural aspects that are themselves extremely complex. That vision is possible at all is nearly incomprehensible; that it is fragile should not be surprising. The list of visual disorders potentially leading to loss of sight is a long one. More hereditary disorders of the eye have been identified than for any other organ of the dog. These disorders can affect virtually every structure of the eye. In addition to trauma and infection, the leading causes of blindness are cataracts, glaucoma, uveitis, corneal erosion, and tumors. 2
The list of ocular and visual diseases is much longer, and includes ---but is not limited to---such well-known or common disorders as entropion and ectropion (in which the eye lid is rolled inward or outward, respectively), distichiasis and trichiasis (in which eyelashes grow from an abnormal lid site or are misdirected, respectively), keratoconjunctivitis sicca (KCS, or dry eye due to decreased tear production), corneal dystrophy (in which opacities develop around the central area of the cornea), pannus (in which vascularized pigment grows over the cornea) , luxated or subluxated lens (in which the lens attachment is compromised, leading to a lens that is totally or partially displaced, respectively), progressive retinal atrophies (PRA, a group of retinal disorders in which the photoreceptors either develop abnormally, incompletely, or once developed, degenerate), retinal detachment (in which the retina tears and becomes partially detached from its underlying layers) , and Collie Eye Anomaly (in which several layers of the eye develop abnormally, leading to blind spots of varying sizes) , most of which can also threaten vision.
Many owners of blind dogs report that their dogs seem to have suddenly lost their vision, but in most such cases the dog more likely gradually lost its vision over a period of time until it finally reached a point at which it could no longer function normally. Often the dog had been exhibiting subtle cues that had gone unnoticed by its owner. These include temperament changes (usually becoming more fearful or aggressive), greater dependence on the owner, lethargy, weight gain, a cautious, often high-stepping gait, and even a tendency to drink large quantities of water due to psychological factors (called psychogenic polydipsia). Sometimes more specific problems will be noted by the owner, such as impaired ability to chase balls, climb steps or curbs, jump up or off of furniture or high places, notice objects on one side but not on the other, or objects nearby but not far away. By performing some simple behavioral tests, an owner can get a better idea of whether a dog may be visually impaired. Several visually guided reflexes can be tested fairly simply:
Other behavioral tests involve more than simple reflex pathways. These tests should be repeated under both bright and dim lighting conditions. For dim lighting, the dog should first be allowed to adjust to the dark for about 15 minutes. A blindfold can be placed over one eye at a time so that each eye can be tested separately.
One such test is to throw an object and see if the dog tracks it visually. The problem with throwing a ball or piece of food is that they either make a noise when they hit the ground or have a strong odor. The solution is to use a cotton ball. Most dogs will attend to this initially, but often become quickly bored with this test so the dog’s response may become subsequently diminished. Another simple behavioral test is to call the dog through an obstacle course of soft furniture, waste paper cans, or other items.
Simple screening tests are not very sensitive, so will probably only uncover dogs that have severe visual deficits. An owner who suspects a visual problem can perform these primitive screening tests at home, but will ultimately want to get the opinion of a veterinarian, and preferably, a veterinary ophthalmologist.
A veterinary ophthalmologist can perform far more sophisticated tests as well as a complete health examination and history. One of the most important pieces of information is the dog’s breed, since so many visual problems have a breed predilection. Color of the dog is also noteworthy; some colors, such as dapple, merle, or white due to albinism can be associated with sensory defects. Age of the dog is another important factor, since many hereditary disorders have a fairly predictable age of onset. Older dogs are prone to a number of visual problems less commonly seen in younger dogs. Because some infectious diseases or toxins, as well as trauma, can result in visual impairment, the dog’s health status, environment, and lifestyle can also provide possible clues.
Simply looking at and in the eye can be suggestive of a visual problem. Many ophthalmic problems involving the lids and cornea can cause tearing and squinting, and such problems, if left unchecked, can eventually result in loss of vision. A pupil that is perpetually dilated, does not respond to light, or does not match its fellow pupil can be a sign of ophthalmic or neurological problems. A displaced lens may be visible through the pupil, and can also cause blindness if not treated.
Most visual problems are not so outwardly apparent, however. Glaucoma may be suspected from outward signs, but should be diagnosed with a tonometer to record the intraocular pressure and a gonioscope to visualize the iridocorneal angle. An ophthalmoscopic examination is necessary to peer into the eye, focussing on the lens or rear surface (called the fundus) of the eye. The fundus has a characteristic pattern of blood vessels that lie over the surface of the retina. Abnormal variations in the pattern of blood vessels are readily apparent to the trained eye, and can also point to specific retinal problems. By injecting fluoroscein into the bloodstream, (a technique called fluoroscein angiography) retinal circulation can be observed. Although an ophthalmoscope or lens is needed to visualize the fundus clearly, the blood vessels are sometimes observable to the naked eye if the dog’s pupils are dilated and light enters its eye along roughly the same path as your own line of sight. Using the same technique, you can also see a reflection from the tapetum lucidum from the upper half of the fundus. Increased reflectivity of the tapetum can be indicative of retinal disorders.
Observation of the dog’s behavior and eyes can provide much information about a dog’s visual health, but more comprehensive electrophysiological tests can often better characterize the nature of visual loss, and detect it at a far earlier stage. The most clinically useful of these is the electroretinogram (ERG).
When retinal cells respond to light, they cause a change in the electrical potential of the eye that can be measured simply by placing an electrode on the surface of the cornea. The drawback is that the dog must be anesthetized in order to stay still and tolerate the contact lens containing the electrode. The normal ERG has a particular shape and consists of three distinctive waves. By performing ERGs under different lighting conditions, the function of the rods and cones can be independently evaluated. In some breeds, ERG screening of young puppies can detect dogs that will later develop progressive retinal atrophy (PRA) sometimes months to years before behavioral or ophthalmic signs are present. The ERG is an essential test when considering a dog for cataract removal when the lens is so opaque the examiner can’t see through it to view the fundus. By shining an extremely bright light into the eye, it’s possible to determine if the retina is functioning, since it would be of little use to remove a cataractous lens if the retina could not respond to light.
A dog can have a normal fundus and ERG and nonetheless be blind or partially blind. In such cases the results of the reflex tests can suggest a neurological problem. In order to pinpoint the location or nature of such a problem, the visual pathways must be understood. Recall from previous articles 4,5 that light passes through the pupil and lens to be focussed on the retina, which contains both rod and cone receptors. These receptors transmit their responses to light by way of several other cell types to the ganglion cells of the retina. The ganglion cells in turn have long fibers, or axons, that all leave the retina at the same place (the optic disc) and travel together as the optic nerve toward the brain.
Before reaching the brain, the optic nerves from the two eyes join together in the optic chiasm, which can be likened to a neural intersection. Here some of the axons cross over to the opposite ("contralateral") side of the brain, while other axons continue onward on the same ("ipsilateral") side. This partial crossover may seem peculiar, but is a vital aspect of binocular vision and depth perception. In an animal with forward placed eyes, there is some overlap of the visual fields so that some parts of the visual world are seen by both eyes. This crossing allows fibers that receive information about the same point in visual space to project to the same part of the brain. In a species with virtually no overlap of visual fields (such as the rabbit, with laterally placed eyes) 100% of the fibers cross; in a species with extensive overlap (such as humans, with frontally placed eyes) 50% of the fibers (those from the nasal half of each retina) cross. In dogs, with obliquely placed eyes, 70 to 75% of the fibers cross. 6 Whether or not this percentage changes in dogs with different head configurations, and thus different degrees of binocular overlap, has not been investigated (flat-faced "brachycephalic" breeds generally have more frontally placed eyes than do long-nosed "dolichocephalic" breeds). Reported values of binocular fields in dogs range from 116 degrees in a "ratter" to 78 degrees in a setter. 7,8
After leaving the optic chiasm, the fibers continue on to the brain by way of the optic tract, making contact with cells in three different parts of the brain: lateral geniculate nucleus (LGN), superior colliculus, and pretectal nucleus. Each of these centers is responsible for different processing of the visual input. Most of the fibers project to the LGN, which performs some neural processing and then in turn routes the signals by way of the optic radiation to the visual cortex. This route is considered the primary pathway responsible for conscious vision. Visual loss due to neurological problems often results from injury to or tumors on parts of the visual pathways. If a dog is blind in only one eye, the problem must be in that eye or its optic nerve. If a dog is blind in both lateral visual fields, so that it can only see in a small area that is normally seen binocularly, a lesion that essentially splits the optic chiasm, so that only the non-crossing fibers get through, must be suspected. Such a problem can arise from a tumor impinging upon the chiasm, most often a tumor of the pituitary gland, which is situated just beneath the chiasm. If one side of the dog’s visual field is gone, so that it is blind in the monocular segment of one eye and the binocular segment of the other, the problem is most likely in the optic tract or higher visual center of the brain. Using such processes of deduction, the veterinary ophthalmologist or neurologist can then attempt to pinpoint the exact area in the visual pathway using more sophisticated imaging techniques.
The eye is a sensitive organ that can easily be irreparably damaged. Its complexity, combined with the difficulty to pinpoint small defects in vision in an animal that cannot be tested with a standard eye chart, makes the task of the veterinary ophthalmologist particularly challenging. Dogs seldom give any clues that their vision is deteriorating until it is nearly abolished. In many cases salvage of vision relies upon the astute observation of the owner. A dog with any evidence of visual problems should be seen by a veterinarian as soon as possible, in some cases even if it means a middle of the night visit to the emergency clinic. Many problems can be cured and vision saved if caught in time, but in some cases---most notably glaucoma—that window of opportunity can be small. Remember, "blindsight" is 20/20.
If a dog does lose its vision, it can still lead a full and happy life, as long as the owner makes certain concessions. Dogs have a well-developed sense of vision, and rely upon it to perform many of their services to humans. Yet unlike humans, dogs that lose this vital sense seem to be able to adjust with minimal rehabilitation. Perhaps this is because of the greater sensitivity of their other senses, most notably their abilities to hear and smell, senses that will be the topic of upcoming articles as we delve deeper into the dog’s sensory world.
Self demonstrations: How to see your own blind spot and fundus In a strange bit of design, the retinal axons travel over the front surface of the retina, so that at the optic disc no receptors are present, forming a blind spot. Most students have "seen" their own blind spot by means of a simple demonstration: Place two marks on a piece of paper about four inches apart. Close one eye at look at the mark on your nasal side with the other. Move the paper slowly back and forth from about five to 15 inches in front of your eye. At some point, when the image of the lateral mark falls on your optic disk, it will disappear!
Note that unless you go to some lengths, you are never aware of your blind spot because your brain "fills in" the gap with whatever happens to be surrounding the spot. This is why one or two small areas of the retina (or brain) affected by disease may not initially be apparent to you or your dog—the brain is wonderfully adept at compensating for small areas of visual loss.
Not only are the ganglion cells in front of the receptors, but also a network of retinal blood vessels. You may be able to see your dog’s fundus to some extent by taking it into a dim room so its pupils dilate, then holding a penlight or small flashlight next to your own eye but aimed into your dog’s pupil. You should see the iridescent shimmer of the tapetum, and perhaps a few red blood vessels. How can the dog see "through" these blood vessels? The same way we do; that is, a number of complex neural mechanisms act to make us essentially "blind" to items that are constantly there. If the light were to hit the blood vessels from a different direction they could be made visible. In fact, you can see the blood vessels in your own eye by closing your eye and looking toward your nose as much as possible, then lightly pressing a pen light (wit the beam on) to the outermost corner of your lid, just inside the temporal eye socket.
You can estimate your dog’s field of view by holding a tidbit directly Jiggle the light a bit and behold your own fundus! in front of him so that it attracts his attention, and then quietly and slowly moving an object from behind your dog forward around the side of his head, making note of the point at which it is first noticed. Another simple method is to observe how far to the side of your dog you can move until your dog’s pupils are no longer visible to you, which would indicate the farthest possible extent of lateral vision.