The pupil is the black slit/circular shape you see in the middle of the cat’s iris (the coloured part). The of the pupil is to control the amount of light entering the eye. It does this by dilating (becoming large) and constricting (becoming small/slit-like). So, in poor light, the pupil will be dilated, to let in as much available light as possible, when it’s bright, the pupil will be small, to reduce the amount of light entering the eye.
Emotions also play a role in the size of the pupils, when a cat is angry or aggravated the pupils constrict when it is happy or frightened, they dilate.
Pupils should be equal in size and when looked at with a bright light, they should both constrict quickly.
Dilated pupils can also be a symptom of an underlying problem. One or both pupils may be affected, when only one pupil is affected, the condition is known as anisocoria. Some conditions which can cause the pupils to remain dilated include:
Insulinoma – Tumour of the pancreas which causes it to produce too much insulin.
Certain drugs and medications can have an effect on the functioning of the pupil.
Poisoning such as snake bite or tick bite.
Retinal detachment – High blood pressure is the most common cause of retinal detachment in cats, taurine deficiency used to be a cause, but this is extremely rare now. Other causes include glaucoma, certain infections (histoplasmosis), certain poisons which can lead to bleeding behind the pupil, and eventually retinal detachment.
How is the cause diagnosed?
Your veterinarian will perform a complete physical examination and may need to perform some diagnostic tests to determine the cause. He will carefully examine the eyes and obtain a medical history from you, including how long the eye(s) have remained dilated and if there are any other symptoms you may have noticed, is the vision affected? He may need to run some diagnostic tests, including:
Checking the blood pressure
Blood tests to check for diabetes
Ultrasound of the eyes to look for lesions
CT scan to check for tumours or brain lesions
How is the condition treated?
Treatment is aimed at addressing the underlying condition such as glaucoma, high blood pressure etc. If caught early, retinal detachment can be helped, but without immediate treatment, blindness will occur. It is a medical emergency.
If medications or poisons are the cause, the condition may improve once the drugs/toxins are out of the system.
Some types of tumour may be surgically removed, if possible and radiation therapy given.