Among the most common questions I am asked by pet owners is, “My dog’s eyes look cloudy. Do you think my dog is going blind?” It’s a good question, and the answer can be complicated because there are several structures in the eye that can develop cloudiness. To start, it’s helpful to know a few things about how the eye is put together. On the surface of the eye is the cornea. In ophthalmology class in veterinary school, I remember being amazed at the remarkable architecture and function of the cornea. Because the cornea has to be clear so we can see through it, there can be no blood vessels going through it. To get oxygen and nutrients, tears on the outside and fluid (aqueous humor) on the inside bathe the cornea. It is very sensitive to touch, chemicals, and temperature. This means any injury to the cornea is very painful. Dogs and cats have four layers in their cornea. The thickest layer is called the stroma; this is where cells needed for maintenance and repair lie. The cells are parallel and arranged like pages in a book so that light can pass through to the deeper structures of the eye. When the cornea is injured, cells in the stroma can never quite be so perfectly aligned as they were, and the result is a white, no longer transparent, spot in the cornea. Other conditions affecting structures deep to the cornea can cause corneal edema, making the cornea appear white or bluish in color. A condition called keratoconjunctivitis sicca (KCS or “dry eye”), caused by lack of tear production, leads to whitening of the cornea if left untreated. When vets evaluate an animal’s eye for blindness, the cornea is the first place they may see opacity or whiteness.
Beyond the cornea is the aqueous humor, which is also clear in a normal, healthy eye. The aqueous humor is 98% water and also contains electrolytes, amino acids, antioxidants, and immunoglobulins. It provides nutrition to the cornea and the lens, helps to keep the eye inflated, and plays a role in protecting the eye from infection. Glaucoma is a condition of high pressure behind the cornea, resulting from too much aqueous humor. This occurs due to lack of drainage or overproduction. If glaucoma is not controlled, gradual loss of vision leading to blindness will occur. Another condition affecting the aqueous humor is called uveitis, which leads to accumulation of blood, pus, or other cells. The aqueous humor is the second place your vet may see opacity.
The lens, a clear structure in a healthy eye, sits between the aqueous and vitreous humor, suspended in place by a ligament. The iris is very near the lens and controls the amount of light entering the eye. The lens can change shape and allow the eye to focus. Dogs have three times less focusing ability as humans, and cats have about half the focusing power of humans. Their sense of smell (1000x greater than people) more than makes up for it! When the lens becomes white or opaque, does this mean the animal has a cataract? Not always. Normal aging change leads to denser fibers in the lens, which will appear cloudy. This is called nuclear sclerosis and is very common in older dogs. Luckily, the lens is still transparent and animals with nuclear sclerosis are able to see. A true cataract in the lens causes opacity through which the animal will not be able to see. If the entire lens is involved, the animal will be blind in that eye. Cataracts are caused by a number of things, including genetics, trauma, dietary deficiency, toxins, and diabetes (in dogs, not in cats).
Cataract surgery can be done in pets much the same as it is in people. The lens is the third place your vet may see opacity.
The vitreous humor lies between the lens and the retina and is a clear jelly-like substance. If blood or cells enter the vitreous due to aging or a disease process, they will sit there unless removed by surgery. They are often called “floaters.” Finally, the retina is the structure at the back of the eye and acts like film in a camera. Light reaches the retina, where photoreceptors change light into electrical impulses that travel into the optic nerve and then to the brain. A condition called Progressive Retinal Atrophy (PRA) occurs in dogs (rarely in cats), where the photoreceptors in the retina begin to die. This usually occurs slowly, giving the animal time to adapt to loss of vision. Owners may realize their dog can’t see at night or have trouble navigating the stairs. Sometimes owners will comment that their dogs’ pupils always seem dilated. There is no cure for PRA but it isn’t a painful condition. This is a genetic condition and certain breeds of dog are predisposed to develop PRA.
Isn’t the eye an incredibly complicated, wondrous structure? Add to that the fact we vets cannot conduct an eye exam like an optometrist because our patients don’t speak, and you realize that determining the level of your senior dog or cat’s vision is challenging. Sometimes your vet may need to refer you to a veterinary ophthalmologist for an accurate diagnosis if your pet is having eye issues. Keep in mind that if your senior pet has partial or total vision loss due to one of the conditions I’ve written about here, your dog or cat will adapt using his/her other senses. It is helpful for these pets if you keep changes in the household to a minimum (like don’t move the furniture), block access to stairs to prevent injury, and ask family members to keep personal items off the floor. To avoid startling your pet, say his name or snap your fingers when approaching him or asking him to come to you.
Has one of your pets had an eye problem or even gone blind? Post a comment and tell me about it!