Jul 01 2012

Dogs’ and cats’ eyes

pet-eyeWe have had some beautiful weather so far this summer, even though some of the recent days have been more like August than June. It’s wonderful to see everything so green and alive again. Since this is such a visual time of year, this week we’re going to discuss issues related to our pets’ eyes.

Although dog and cat eyes may look a lot like ours, they have some significant differences. Dogs and cats do not see color very well. We know this because they have few cones in the retina. Cones are the type of cell that detects color. Their night vision, however, is quite acute, especially in cats. One reason for this rests in the back of their eye. Have you ever been driving at night and seen the flash of two green lights in the road? You know that there’s an animal crossing in front of you. Dogs, cats and many wild animals have a reflective surface behind the retina called the tapedum. It amplifies the light so they see better in the dark. It is usually blue or green although in blue-eyed pets it is often not pigmented at all and looks red. That’s where the “red-eye” comes from in your pictures that you have to photo-shop out. Our tapedums don’t reflect light particularly well.
cat_greeneyes2One of the most obvious differences is in the shape of a cats’ pupil. When in bright light, it constricts down to a vertical slit, unlike dog’s and ours that just stay round. Either way, the pupil’s job is to control the amount of light entering the eye. Large pupils are “mydriatic” and small pupils are “miotic”. Sometimes, the pupils can be unequal in size, a condition called anisocoria. There are many causes of anisocoria and the first thing to determine if one pupil is inappropriately small, or if the other is inappropriately large. Glaucoma, iris atrophy, retinal detachment, retinal hypertension and any condition which causes partial or complete blindness can make the pupil dilated. Causes of small pupils include uveitis (inflammation of the inside of the eye), Horner’s syndrome (a nerve problem), and corneal ulcers or scratches which cause pain. Any of these conditions can occur in one or both eyes. We will go into detail about some of these conditions in the next column.

One important thing to remember about eyes is that if anything looks out of the ordinary, your pet should be examined as soon as possible. Some conditions are easily treated — conjunctivitis (similar to pink-eye in people) can be managed with antibiotics for example. But sometimes even minor scratches on the eye can become infected and can risk vision loss. Glaucoma (elevated pressure inside the eye) can cause blindness in as little as twenty-four hours if not treated aggressively. So any eye that is red, squinty or just looks odd, deserves a trip to the vet.

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