The colored part of the eye surrounding the pupil is called the iris. The pupil is the opening in the eye that allows light to reach the retina. Light signals are sent from the retina to the brain, creating the sensation of vision. The iris controls the opening (dilation) and closing (constriction) of the pupil in response to changing levels of light.
Temporary pupil dilation that responds normally to light is very common and is associated with benign physiologic conditions such as low light conditions, fear, alarm, or pain.
Symptoms vary from excessively dilated pupils that are non-responsive to light, excessively dilated pupils that are partially responsive to light (pupils can shrink some in response to light), to dilated pupils that respond normally to light.
Multiple causes of abnormal pupil dilation (mydriasis) exist.
Immediately life-threatening causes of dilated pupils are rare. Potentially life threatening associated conditions typically have additional symptoms such as excessive drooling, vomiting, or trouble walking as with toxicities or obvious signs of trauma with head or eye injuries.
Any age, breed, or sex of cat can experience mydriasis. The causes of mydriasis are many and varied. Prognosis depends on the cause. Any cat experiencing abnormal mydriasis needs prompt veterinary attention.
Mydriasis can affect one or both eyes depending on the cause.
Diagnostics may include
The degree of mydriasis and the nature of concurrent symptoms helps direct investigation. Additional tests may be necessary.
Dilated pupils are self evident and not likely to be mistaken for other symptoms. Mydriasis may be very hard to observe, especially in cats with very dark irises.
Associated symptoms vary widely and depend on the underlying cause. Common groups of causes can have similar symptoms and may include: