Lots of us spend plenty of time admiring our pets, so it can be alarming if you suddenly notice something about them has changed. ‘Anisocoria’ is a condition where the pupils appear as different sizes and is a symptom rather than a disease itself. It may be that the abnormal pupil is smaller, or larger than normal and it can be caused by several processes in the body. Here we will outline a few of them.
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In the eye, the pupil is the black, elliptical-shaped part that allows light to pass through to the back of the eye (retina). The pupil changes shape according to how much light is sensed; and is under the control of a complex nerve pathway that passes through the brain and spinal cord. Sometimes anisocoria is a result of something interrupting this pathway; or it may be due to a problem with the eye itself.
Causes of anisocoria
Anisocoria may be caused by many conditions.
Injury to the eye
This could be something like a corneal ulcer, where the pupil constricts (gets smaller) due to pain and inflammation in the eye. This can also happen with damage to the retina or when the pressure inside the eyeball is too high, a condition known as glaucoma. Iris atrophy, a condition where there isn’t enough tissue forming the iris is associated with old age. And it can cause the pupil to appear a different size.
Occasionally anisocoria is a result of interference of the nerve pathway we mentioned before. This may be due to a mass pressing on one of the nerves involved, or with cancer of the eye. Infectious diseases such as leukaemia or viruses, drugs and toxins can also have an affect on pupil size and lead to anisocoria.
Horner’s syndrome
Horner’s syndrome is a set of symptoms that all occur together in cats. And constriction of the affected pupil or anisocoria is one of them. Other symptoms include drooping of the upper eyelid, protrusion of the third eyelid and sinking of the eye into it’s socket (enophthalmos). Horner’s syndrome occurs due to interference with the sympathetic nerve supply to the head. The sympathetic nervous system is the ‘fight or flight’ part of the autonomic nervous system responsible for subconscious control of normal body functions. The sympathetic nervous system may be damaged by things like bite wounds, ear infections, bruising behind the eye, skull fractures or even tumours.
Diagnosing the cause of anisocoria
As anisocoria is a symptom rather than an illness, diagnosing the underlying problem is most important.
Your vet will carry out a thorough examination of your cat and check for other signs of illness, they may apply a stain to their eye to look for any scratches or ulcers on the surface. They will likely carry out a full eye exam and perform tests to check nerve supply. They may recommend measuring the pressure inside the eye (intraocular pressure). Depending on your pet’s history and the findings of the exam, they may advise blood tests or imaging to look for other signs of disease. In some cases your vet may recommend referral to an eye specialist (ophthalmologist) for further investigations.
Treatment
If an underlying cause of anisocoria is found, the treatment of this can cause the symptoms to ease. However, because there are so many possible causes, with some easier to treat that others, this may not be a straightforward process. And it’s difficult to say whether your pets eyes will return to normal or how soon. Painful conditions of the eye may be treated with topical medications like eye drops in addition to pain relief. If a drug or toxin is the cause then the symptoms may subside by themselves after a period of time. Internal illnesses or infections may be treated with oral medications or may require further investigations. Your vet will be able to discuss with you the most appropriate course of action for your pet.
What should I do if I notice my cats pupils are different sizes?
As there are so many potential causes of anisocoria, including some that are serious or rapidly progressing, you should contact your vet straight away if you notice your pet’s pupils are different sizes. Taking a picture can be very helpful as sometimes they can return to normal or continue to change before the vet can look at them. Your vet may be able to put your mind at ease or recommend further testing if needed.
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