Tuesday, April 12, 2016

A closer look at cloudy eyes

If you’re lucky enough to have a dog or cat live into old age, you’ve seen her eyes develop a hazy, milky appearance. It may have alarmed you because you were worried that she had cataracts and might become blind.

What you’re seeing is a normal age-related change referred to as nuclear sclerosis, lenticular sclerosis, or hardening of the lens. Virtually all dogs and cats over seven years of age have some degree of sclerosis, as do most people over 40. Yes, you read that correctly; it will happen to you, too!

In both people and pets, the lens in the eye continues to grow throughout life. There’s nowhere for those extra fibers to go, so the lens becomes thicker and more dense. The difference between seeing through young eyes vs. old eyes may be compared to the difference between looking through a thin piece of glass vs. a thicker chunk of glass. 

The lens focuses light onto a specific area of the retina, which allows us to see clearly. With sclerosis, light is no longer precisely focused on that central part of the retina, and our vision becomes blurry. And that’s what drives us to go to Walgreens for reading glasses in our early 40s. It’s also the reason most people get surgical lens replacement, or cataract surgery.

We need visual acuity to read, drive a car, recognize shapes and colors, and see objects in fine detail. Because dogs and cats don’t need to read the paper, watch TV, or take the car for a spin, the loss of focus isn’t as critical for them. But that’s not to say they don’t experience some loss of vision. Fortunately, it doesn’t seem to be a problem for them in most cases.

If I can see the architecture of a pet’s retina through my ophthalmoscope, it’s likely that her vision is pretty good, even if her lens is quite hazy. Often, owners of such pets will tell me the pet has problems seeing only in low light. But sometimes I can barely see the colorful reflection of the retina and can’t visualize the retinal blood vessels.1 Those pets most likely have blurry vision and may have trouble recognizing people and objects by sight. 

In humans, cataracts are considered to be a form of nuclear sclerosis, but in veterinary medicine, we make a distinction between the two.2 Cataract development can be a significant cause of blindness in dogs and cats, whereas sclerosis causes less severe visual impairment. Although sclerosis occurs in aging dogs, cataracts may occur at any age for a variety of reasons (congenital, traumatic, age-related, metabolic, post-inflammatory). Cataracts are corrected by surgery, whereas surgery isn’t done for sclerosis. In fact, in spite of ads for magical nutritional supplements, there’s no treatment for sclerosis that has been scientifically proven to be effective. 

So rest assured that your senior pet’s milky eyes most likely are not cataracts and probably bother you more than they do her. But don’t forget to turn the light on when she goes down a dark staircase!

1. Dogs have an amazing structure at the back of their eyes called the tapetum. It’s a colorful reflective surface that allows them to see in the dark. Severe sclerosis prevents much light from reaching this reflective surface, so not only does the normally bright tapetum look hazy to me, but also the dog doesn’t get enough light to the back of her eye to allow her to see in low light.

2. We can usually differentiate between cataracts and nuclear sclerosis with a visual inspection of the eyes with a light source. Cataracts can appear as dots, stars, or other shapes and can even have a sparkling appearance. They may start out as a diffuse haze, much like sclerosis, but often progress and become completely opaque.