Cats have very distinctive eyes; the iris or coloured part of the eye. It can range in colour from blue to greens or hazel or copper appearance. The pupil is the black part of the eye seen centrally which controls how much light is let into the eye. The pupil can appear as a vertical slit to almost fully round when it is dilated. A cat’s pupil will respond to the amount of light available, so in dim or dark settings the pupil will be dilated to allow the maximum amount of light into the eye. In bright light the pupil will be much smaller and slit like. This can mean the appearance of a cat’s eye varies depending on the conditions.

A cat’s pupils may be dilated at times other than dim lighting giving them a wide-eyed starry look. There are a few possible causes for this:

An excited or playful cat may have wide dilated pupils as they get ready to play. 

However, fear or surprise could also lead to the same result. Changes that occur due to excitement or fear should be relatively short lived. As the cat relaxes the obviously large pupil should return to a smaller size. A young playful cat is much more likely to display wide pupils because of excitement or fear as they are naturally more inquisitive and playful. A more relaxed sedate cat may not often dilate its pupils due to excitement. 

If the pupil remains dilated for an extended period of time it is important to rule out any underlying problems. 

Persistently dilated pupils could be a clue that your cat is experiencing pain somewhere in its body. If the eye itself is painful it would usually only be the affected eye that is different. The pupil would likely be constricted rather than dilated. 

If your cat’s eyes are persistently dilated and there doesn’t appear to be an obvious cause, then it would be prudent to arrange a check-up with your veterinary surgeon. Your vet will perform a physical examination of your cat. They will look not only at the eye but looking to rule out any other potential causes of large pupils.

There are medical causes for dilated pupils which should be ruled out if you notice the pupils are persistently large.

Uneven pupils

One dilated pupil and one constricted pupil, known as anisocoria is another potential reason for seeing a dilated pupil. In some health conditions the pupils may appear uneven, with one pupil larger than the other. If you notice this is the case with your cat, then it is best to arrange a check-up with your vet to check them and assess vision. The causes of anisocoria are wide and varied, from conditions affecting the eye itself such as glaucoma to nerve or brain injury. 

High blood pressure

Hypertension or high blood pressure is one potential cause for dilated pupils. High blood pressure in cats is relatively common and can accompany other disease processes (known as secondary hypertension). Kidney failure, and an overactive thyroid gland are diseases which are associated with secondary hypertension. High blood pressure can also occur alone (primary hypertension). If increased blood pressure is to blame for your cat’s dilated pupils it is likely that you will have noticed other signs or symptoms which would alert you to your cat’s ill health such as a change in weight, change in appetite or poor coat quality.

Poor vision

In rare cases there could be a significant deterioration in your cat’s vision leading to dilated pupils. Blindness may also present as wide dilated pupils. In the case of sudden or acute blindness another disease process may be the underlying cause. 

So do I need to worry?

Temporary dilation of the pupils is a normal physiological response to low light or stimulating or exciting situations for your cat. However, if you notice persistently dilated pupils it is important to arrange a check up for your cat with the vet. 

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