Gazette Vet: Through the Looking Glass

Photo: Dr. Alison Clode, DACVO

One of the more common misinterpretations I hear owners make about their older dog’s health is that a cloudy-looking eye means that they are getting cataracts and going blind. I hear it almost daily during many of the check-ups I do on dogs over 10 years of age. “Her eyes are looking a little cloudy these days, I guess that means she’s getting cataracts?”

I don’t expect most dog owners to know the difference, but I am happy to tell them that they are wrong in the majority of these instances, and more likely their dog has “nuclear sclerosis” of the lens, which is a common aging change in dogs over ten years of age, vs. cataracts, which lead to significant vision loss or blindness.

Most people have never heard of nuclear sclerosis in pets, yet it outnumbers cataracts about 50:1 in my estimation in dogs with “cloudy eyes.”

What is nuclear sclerosis?

Nuclear sclerosis is the age-related hardening of the lens of the eye. The lens is an incredible structure in our body. About the size and shape of an M&M, the lens is a perfectly clear focusing device that takes all that we see and focuses the image clearly on our retina, where our brain and nervous system then translate that into a beautiful picture of the mountains, etc. The lens is located immediately behind our pupil. As you look at yourself in the mirror and you see the black hole in the center of the eye, you are actually looking through the lens into the back of the amazing eye. Of course, it’s black, because the back of our eye doesn’t come with built-in lighting—it’s dark back there!

The lens is perfectly translucent, letting light in as if it were passing through smooth glass. The lens is also perfectly shaped at just the correct curvature to focus light on the perfect place on the retina.  As if that weren’t amazing enough, there are tiny little attachments all around the circumference of the lens that can bend its shape to better focus the images we see, depending on if we are looking far away or reading up close. It also has no blood supply—the lens gets all of its nutrients through the passive diffusion of nutrients, versus from the bloodstream like most every other structure in our bodies. The lens (and the eye) is a miraculous structure indeed!

As we age (and dogs and cats age), the lens of the eye starts to become harder and denser, specifically in the center of the lens, the “nucleus.” This is because instead of growing outward with time, the lens slowly grows concentrically inward, causing the center to become denser, or “sclerosed.”  Hence, “nuclear sclerosis.”

Fortunately, nuclear sclerosis does not affect the shape or translucency of the lens, so for the most part, our far vision is unaffected by this change.  However, this does make it more difficult for the lens to change shape when we are focusing our vision close, like when reading. This is why we humans need reading glasses after we hit our 40s!

If we get nuclear sclerosis too, why don’t our eyes start to look cloudy when we turn 50? 

There are a few important differences in our eyes versus dog’s eyes. First, dogs have a reflective retina. The entire back part of their retina is highly reflective so when ambient light enters their eyes, some of it reflects and illuminates the lens. The back of our eye is not reflective, so, typically, when ambient light enters our eyes, it disappears into blackness. Now of course when it’s picture time, we know we can see “red eye” in our human photos. This is when intense light is actually lighting up the back of the eye. When you shine a bright light or flash at a dog’s eyes (or a cat’s, or deer) you will see the bright reflective “eye shine,” which is your light coming back at you.

Second, dogs and cats typically have more dilated pupils then humans. Dogs and cats are more adapted to seeing in lower light conditions, so their retinas are adapted to take in more light and peripheral vision.  So, we are not only seeing more of their lens, it is also somewhat illuminated by the ambient light.  When it is very dense from age-related nuclear sclerosis, we see that as “cloudiness.”


What are cataracts then?

Cataracts are actual opacities in the lens that block light. We most commonly see age-related cataracts where, eventually, water or minerals start infiltrating into the once perfectly clear lens.  Ultimately, cataracts can turn a clear lens into a completely white, opaque structure, though most cataracts cause the lens to have more of a “shower glass” appearance. The biggest difference is that cataracts not only block light from entering the eye, but they also scatter the light that does manage to get through.  So, like when looking through shower glass, cataracts cause a significant distortion of the image behind the glass.

Dogs with cataracts generally show signs of vision loss, especially at night. Becoming clingy and losing confident on nighttime walks, bumping into walls and furniture, or becoming confused about their location are all signs of vision loss. Most dogs adapt quite well to vision loss because their sense of smell and hearing are so good.

Dogs with nuclear sclerosis have no vision deficits that we can tell. However, I suspect they do have significant near-vision loss—but they don’t read or use cell phones, so this is not important. Perhaps they are less confident going down stairs in the dark?

If your older dog’s eyes are getting a bit cloudy, don’t assume they are going blind. In fact, the odds are that they just need some reading glasses. A quick trip to your vet can easily tell the difference between cataracts and nuclear sclerosis.


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