The eye is the window to the soul, or so it has been said. And who among us pet owners haven’t sat looking affectionately into our pet’s eyes, wondering what they see, think and feel. Dogs and cats have excellent vision, but eye diseases in pets are real and can occur just as in humans. Understanding how the eye functions and knowing what is normal and abnormal will help pet owners recognize eye issues before they become severe, improving the chance of a good outcome.
Normal Eye Anatomy
The normal canine or feline eye shares a similar placement in the skull as in humans. As we are all considered predators our eyes are placed in front of our heads to allow us to better hone in on prey. The forward facing location of a predator’s eye limits peripheral vision but from an evolutionary standpoint, extreme peripheral vision is more appropriate for prey animals who must always be on alert, scanning their environment for any threat.
The external structure of the animal eye is certainly familiar to all of us. The eye is globoid in shape, protected by a thick skull and eyelids. The clear part at the front of the eye is our cornea. The cornea acts as a window that allows light to enter the eye. Our pupil, the dark circle in the center of the eye is delineated by the delicate iris. Opening or closing the pupil will allow more or less light into the eye. Many of us know full well how important a normally functioning pupil is. After an eye exam, we may leave the office with fully dilated pupils. The glare of the full sun shining into an eye incapable of adjusting the pupil is often quite painful.
After light enters the eye through the pupil it hits the lens, the lens helps the light focus onto the retina. The retina is a fascinating structure that lines the inner surface at the back of the eye. The retina contains specialized cells capable of reacting to light in such a way that nerve impulses are transmitted to the optic nerve and, from there, to the brain. The brain then translates these impulses into an actual visual image. The eye is a complicated, complex, and elegant structure, and one can imagine any issue in one part of the eye is likely to affect the overall functionality of other structures in the eye.
How Can an Owner “See” Something Wrong?
It is a good idea for any pet caretaker to familiarize themselves with signs of ocular pain. The vast majority of eye related diseases produce some level of pain in the pet. Recognizing a pet’s discomfort and accessing veterinary care immediately can be the difference between saving an eye and literally ”losing an eye.”
Think of what you reflexively do if you get dirt or debris into your eye, you squeeze the eye shut, your eye starts to tear up and you may even be inclined to rub your eye (even though you know better). Why did you do that? Because your eye was painful and felt irritated. Pets respond the same way. A pet with eye pain will squint with the affected eye, sometimes subtly and sometimes holding the eye shut completely, refusing to open the eye. Most painful eyes will produce a discharge. Sometimes the discharge may be watery and tear-like, other times it may be thick and mucoid. Affected animals may rub their eyes with a paw, on the carpet, or on furniture. If your pet is showing the above signs, an immediate call to your veterinarian is recommended. With ocular issues, overreacting is ALWAYS better than underreacting.
What Could be the Problem?
Of all pet ocular diseases, the two most commonly seen by general practice veterinarians are corneal ulcers and conjunctivitis. Corneal ulcers are defects in the clear covering of the eye (the cornea) and may be caused by a scrape, scratch, or infection of the eye’s surface. These injuries are usually quite painful for pets, and the animal will squint and rub. Corneal ulcers are treated with topical antibiotics, oral pain medication, an E-collar to prevent eye rubbing. Sometimes atropine is used to dilate the pupil as painful eyes often have severely constricted pupils that add more pain to the situation. Many routine corneal ulcers that are caught and treated early heal in 3-5 days. More complicated or infected ulcers may take weeks or months to heal and might even require surgery. Interestingly enough, the Boxer breed is predisposed to develop indolent ulcers. Indolent ulcers are ulcers that do not easily heal despite proper infection control. Complicated, non-healing ulcers are often referred to a veterinary ophthalmologist for treatment.
Puffy, swollen, red, and itchy eyes are the hallmark of conjunctivitis in dogs and cats. Conjunctivitis is inflammation of the conjunctiva (a thin membrane that lines the inside of the eyelids and covers the white part of the eyeball- the sclera.) Inflammation of the conjunctiva can be a result of exposure to dust, dirt, or allergens or a result of infection. In dogs, allergies are the main cause of conjunctivitis, while in cats the leading cause is infection with viruses or bacteria. In dogs, conjunctivitis is treated with steroids and topical tear replacers to flush the eye. In cats, steroids are rarely, if ever, used and much harm can come from the inappropriate use of steroids on the feline eye. Cats are treated with either oral or topical antibiotics or antivirals, sometimes both oral and topical medications are used.
Traumatic injury to eyes is also common. Tears to the eyelids may occur secondary to dog fights or playing with sticks. Penetrating foreign bodies such as sticks or thorns are common. Less common but exceptionally disturbing for owners are cases where an eye has proptosed (been pushed out of the socket) usually due to trauma. Depending upon the severity of torn eyelids and foreign bodies, these cases may be referred quickly to an ophthalmologist.
Occasionally an owner recognizes their pet has a bulging eye and perhaps has inferred a pet may have decreased vision in that eye. This hallmark finding is likely due to glaucoma. Glaucoma causes a buildup of fluid in front of the lens of the eye. This painful condition can dramatically affect eyesight, blindness can occur, and the pressure in the eyeball may increase to the point where the eye lens actually luxates from its normal position. Mainstays of treatment include medications to decrease the pressure inside the eye and pain control. Internal pressures are sometimes extremely difficult to get under control and the eye becomes progressively more painful and vision is lost. In such cases, enucleation (removal of the affected eye) is recommended to improve the pet’s quality of life by eliminating the source of pain.
Blindness May be Sudden or Slow
One of the most disconcerting issues an owner may face is dealing with a pet that goes blind. Depending upon the cause, the blindness may occur acutely or it may be a more gradual process. The rapidity of the process depends upon the underlying disease. Cataracts normally cause a pet to become blind more slowly. Owners notice a pet bumping into things, taking longer to navigate around the yard or house. Dogs with reduced vision may seemingly become more fearful or less excited about exploring new places. However, in some diabetic dogs, full, mature cataracts may develop in less than 24 hrs rendering these dogs blind overnight.
Two other causes of acute blindness include Sudden Acquired Retinal Degeneration (SARD) and Immune-Mediated Retinal Detachment Syndrome. SARDS causes total, irreversible blindness in dogs and is seen more frequently in Dachshunds and Miniature Schnauzers. This disease may be immune mediated and there is no known effective treatment.
Another immune-mediated disease, Immune-Mediated Retinal Detachment Syndrome, also occurs in dogs but thankfully often responds to steroid therapy.
A top differential for sudden blindness in cats is retinal detachment. In cats, retinal detachment can occur in response to severe hypertension, which may be secondary to renal disease or hyperthyroidism. Promptly recognizing changes in your cat’s vision and reducing blood pressure as soon as possible may help save some of your cat’s vision, but blindness secondary to retinal detachment is, unfortunately, a frequent outcome.
We’ve covered numerous diseases specifically related to eyes that can dramatically affect your pet’s comfort and vision. When it comes to eyes, one piece of advice rings true, “When in doubt, seek us out.” If you have any concerns about your pet’s eyes whatsoever, please give Longwood Veterinary Center a call.
Written by Corrina Snook Parsons VMD
