Your pet's eyes are very sensitive to changes in light. The middle part of the eye, called the pupil, is constantly in motion, opening and closing to let in the correct amount of light.
Sometimes, however, the pupils stay open regardless of the amount of light they are receiving. This condition, called dilated pupils, nearly always means that something is seriously wrong. "When one or both eyes dilate in good light, your pet could have something wrong with the nerves connected to the eyes or in the brain or the eye itself," says Erika de Papp, D.V.M., a veterinarian at the University of Pennsylvania School of Veterinary Medicine in Philadelphia.
Nerves are responsible for sensing how much light is reaching the eye and for relaying the message to (and from) the brain, which instructs the eye to make the proper adjustments. When a nerve is damaged or under pressure because of an infection, for instance, or a growing tumor, the eye may not receive the necessary messages and may fail to work properly, explains Dr. de Papp. A swelling in the brain, which can happen following a blow to the head, can also press on the nerves controlling the pupil.
A fairly common condition called glaucoma, in which fluids accumulate in the eye, can also cause the pupil to stay dilated. (Other symptoms of glaucoma may include teariness, redness, or cloudiness of the eye.)
Pets that eat toxic substances such as antifreeze can also develop dilated eyes. If you suspect that your pet has swallowed poison, get her to the vet right away.
In cats, dilated pupils may be caused by a diet of raw fish. Fish contains an enzyme called thiaminase that destroys thiamin, a vitamin necessary for good vision, says Paul M. Gigliotti, D.V.M., a veterinarian in private practice in Mayfield Village, Ohio. In addition, cats given mainly milk or vegetarian diets may become deficient in an amino acid called taurine, which is essential for keeping the retinas healthy. When taurine levels fall, the eyes may dilate.
See Your Vet If...
- You suspect that your pet has gotten into poisonous substances like antifreeze
- Her eyes are red, cloudy, or teary
- Your cat eats raw fish or is on a vegetarian diet
- Your pet won't quit scratching or pawing his eyes
- His eyes have turned blue, gray, or cloudy, or he is having trouble getting around
- His eyes are frequently bloodshot or dry
- There has been a watery or discolored discharge from the eyes for 48 hours or more
- Your pet's eyes are bulging
- The eyelids are swollen or unable to close
- There is a growth on the eye or eyelid
- One or both pupils are dilated, or they don't respond to light
- Your pet seems very sensitive to light
- The eyes are droopy or sunken
- The third eyelids are covering the lower parts of the eyes
- His eyes are continually moving back and forth
- Blood or tiny blood vessels are visible in the center (not the whites) of his eyes
- His eyelid appears to be turned inward or outward
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