The iris is the colored part of the eye and controls the opening (dilation) and closing (constriction) of the pupil. The pupil appears as a black circle in the center of the eye and is responsible for allowing light to reach the retina. Light signals are sent from the retina to the brain where they are translated, creating the sensation of vision.
Pupils dilate normally in response to changing light levels and as a result of certain physiological changes, such as during the natural fear response. Mydriasis can be unilateral (one eye only) or bilateral (both eyes). Abnormally dilated pupils may be unresponsive to light, or show partial response to light.
In addition to head, brain, or eye injuries, there are many possible causes of abnormal pupil dilation.
Prolonged mydriasis in dogs is uncommon. If mydriasis is the only symptom present, it may not be an emergency, but veterinary attention is warranted. If accompanied by other symptoms such as excessive drooling, vomiting, or trouble walking, emergency veterinary care is required.
In cases of suspected mydriasis, the first step in diagnosis is a complete physical exam, including a thorough ophthalmic exam. Blood work, urinalysis, and diagnostic imaging may also be performed.
Other tests may be recommended based on the presence of concurrent symptoms and to rule specific conditions in or out.
Treatment depends on the underlying cause. Sometimes no treatment is necessary, while other times hospitalization, medication, or even surgery may be indicated.
Normal variation in pupil size may be mistaken for mydriasis. Mydriasis may be difficult to distinguish in dogs with dark-colored irises.
Concurrent symptoms vary widely and depend on the underlying cause.