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The pupil is an opening in the middle of the iris. Pupils dilate and contract to adjust the amount of light that enters the eye. They dilate when a dog is in the dark, and they appear small in a well-lit room. Small pupils are also a sign of confidence, while dilated pupils mean fear or stress. If a dog's pupils stay dilated regardless of the light in its environment, the dog may have a medical problem.
Uneven pupil dilation can be caused from medicated eye drops, but it can also be caused by severe medical conditions such as an aneurysm or a brain tumor. Headaches can result from uneven pupil dilation, or the headache may simply be a symptom of what is causing the uneven pupil dilation.
When the pupil (the dark center of the eyeball) widens, this is called dilation, or mydriasis. Constriction, also called miosis, is the opposite effect---the narrowing of the pupil. Several factors, both normal and unhealthy, cause these responses.
Some eye exam patients balk at the mention of dilating drops. They associate this chemical enlargement of their pupils with time delays and an hour or two of blurred vision. Looking beyond these temporary inconveniences, there are significant benefits to dilating the eyes during an exam.
