Did you know one of the most common causes of blindness in dogs is cataracts caused by diabetes mellitus? In fact, it’s pretty much inevitable that a dog with DM will develop cataracts. But why does this happen in dogs and not cats?
Are you picturing Wilford Brimley yet?
Diabetes mellitus can be caused by a number of things, but the bottom line is the body either isn’t producing or isn’t using insulin, resulting in high blood sugar.
The sugar involved, glucose, is a small molecule and can diffuse through most cell membranes, so if there are high levels of glucose in the blood, there are high levels of glucose everywhere, including the fluid inside the eyeball. (There is also a lot of sugar in the urine, providing food for bacteria, so urinary tract infections are common.)
Glucose can be converted into sorbitol, but it’s a very slow reaction unless a catalyst (aldose reductase) speeds it along.
One important difference between glucose and sorbitol is that sorbitol doesn’t diffuse through cell membranes very easily. So if glucose gets converted to sorbitol somewhere, that sorbitol molecule is going to stay put.
What’s a lens and why does it matter?
Back to the eyeball — the lens (this page has a good picture of the anatomy of the eye) is a structure that sits in the middle of the eyeball and allows you to focus on things at different distances. It’s made from a bunch of fibers that are put together in such an orderly fashion that the final structure is crystal clear. If something damages that orderly structure, light gets blocked, which then impairs vision. A tiny cataract might just be annoying. If the whole lens is damaged, it will keep light from getting through at all.
[Related anecdote: One of my patients was an elderly bald eagle that came in from a zoo to have its cataracts removed. When it got to the hospital the bird just sat there, as if it were nighttime, and was easy to handle. Then we did surgery and removed the cataracts and suddenly it could see again. Bald eagles are big strong jerks with massive talons and I had to get medicine in that bird’s eyes twice a day over the weekend until we could send it back to the zoo. I definitely wasn’t getting paid enough for that.]
When lenses go bad
So the animal has diabetes and now there’s too much glucose everywhere, including inside the eyeball and even the cells of the lens. If the glucose molecules remain glucose, they will wander in and out and there won’t be a problem. But if the glucose gets converted to sorbitol, it won’t diffuse back out. More glucose will diffuse in, get converted to sorbitol and get stuck. Pretty soon it’s a sorbitol party, and then water gets pulled in. (If you don’t remember how osmosis works, just trust me on this part.) Once water gets sucked in, all the neatly arranged fibers of the lens end up all over the place, and now you have a full blown cataract.
Cats rule, dogs drool
Both dogs and cats can have diabetes, and both will have high blood sugar. So why do dogs get diabetic cataracts but cats don’t?
Remember the aldose reductase that helps convert glucose to sorbitol? Cats older than four years don’t have much aldose reductase in the lens, and cats younger than four generally aren’t diabetic.
Dogs, though, have a bunch of aldose reductase just sitting there in the lens, waiting to cause problems. The glucose diffuses in, gets converted to sorbitol, the sorbitol builds up, and bam! a few days later the dog is blind from an irreversible cataract.
Treatment
Cats: There’s no treatment for being a cat. You just have to accept them for what they are.
Dogs: As long as just the lens is affected, the whole lens can be removed and the dog’s vision restored. Without a lens they probably won’t be able to focus well enough to read the newspaper, but most dogs are okay with that.
If other parts of the eye besides the lens are affected, or if cataract removal is just not a financial possibility, the dog will remain blind. The good news is that blind dogs do very well. Some owners don’t even realize their dog is blind until they rearrange the furniture. My first dog was completely blind and she managed to get up to such shenanigans that I would have sworn she was sighted if her eyeballs hadn’t been completely removed.
So there you have it. Cats and dogs really are different. Want to read more?
- https://veterinarypartner.vin.com/doc/?id=4951531&pid=19239
- https://www.indyanimaleyeclinic.com/veterinary-information-network-publications/cataract-feline/
During Fun Facts Friday, I talk about something I think is interesting. Do you have questions? Suggestions for a future post? Add a comment below!
