Your dog's enlarged, or dilated, pupils could result from a variety of causes. Though it's possible that you may have no cause for alarm, pupil dilation can be a sign of some serious eye diseases, so get your dog to the vet as soon as you notice the problem. A "wait and see" approach could cost your dog his vision. Though it could turn out to be a minor issue, it's always better to be safe than sorry when your best friend's eyes are at stake.
Normally, the pupil's size is determined by the amount of light entering the eye. In bright light, the pupils are small, or constricted. In dim light, the pupils enlarge. If one or both of your dog's pupils are dilated in bright light, something isn't right. In a condition called anisocoria, the pupil in one eye is larger than the other. Without a veterinary examination, it's hard to tell which pupil is normal and which is abnormal.
Glaucoma results from too much pressure within the eye. This ocular disease progresses rapidly, so even with prompt treatment your dog could lose his sight. Symptoms include a dilated pupil that doesn't respond to light. Other signs of glaucoma include cloudiness, redness and swelling, tearing and obvious pain in the eye. Depending on the diagnosis, your vet can prescribe medication -- both oral and topical -- for your dog, or perform surgery. Neither treatment necessarily saves vision but can ease pain.
Progressive Retinal Atrophy
Progressive retinal atrophy eventually causes total blindness. Because it occurs gradually, you might not notice the early symptoms. Night vision usually goes first, so your dog might be reluctant to go outside in the dark, or he bumps into objects in dark rooms. Since it affects both eyes, you might notice your dog's pupils dilating as they try to bring in more light. According to VeterinaryPartner.com, an abnormal shine known as "tapetal reflection" might be seen in the dilated pupils. If it's any consolation, PRA isn't painful, and blind dogs usually learn to adjust quite well, with their owner's help.
Other conditions that might cause pupil dilation include cancer, trauma, reaction to medication or the consumption of toxic substances. Anxious, fearful dogs often exhibit dilated pupils. Don't make any assumptions regarding the cause of your dog's enlarged pupils or give him any medication without veterinary approval.
By Jane Meggitt
About the Author
Jane Meggitt has been a writer for more than 20 years. In addition to reporting for a major newspaper chain, she has been published in "Horse News," "Suburban Classic," "Hoof Beats," "Equine Journal" and other publications. She has a Bachelor of Arts in English from New York University and an Associate of Arts from the American Academy of Dramatics Arts, New York City.