The pupil is the black part of the eye in the centre of the iris, where light enters the eyeball. When the eye receives light, the pupil contracts, while it dilates when light is dim or absent. Although the name may not ring a bell, anisocoria in cats is a very typical eye condition in which the pupils of the cat’s eyes vary in size. Unequal pupil size is a symptom of an underlying disorder and treatment is based on diagnosis. Learn more about this condition below.

What is anisocoria in cats?

As mentioned above, the pupils are the black part of the eye, located in the centre of the iris. Anisocoria is a condition in which the pupils of the cat’s eyes are different sizes. The affected pupil may be larger or smaller than the normal pupil.

Anisocoria in cats can occur in two ways:

  • Dynamic anisocoria: when the stimulated pupil contracts more than the other unstimulated pupil following stimulation by a direct light source, which is normal in able-bodied cats.
  •  Static anisocoria: when there is a difference in pupil size despite both pupils receiving the same illumination, this type of anisocoria is completely abnormal.

Symptoms of anisocoria in cats

The most common symptom of anisocoria in cats is a difference in the size or diameter of a cat’s pupils under the same lighting conditions. However, depending on the underlying cause, the eye with anisocoria may undergo other structural changes and the cat may also behave differently.

Here are the other common symptoms associated with anisocoria:

  • The white part of the affected eye is red.
  • The cornea (the outer surface of the eye) is cloudy or bluish in colour.
  • There is discharge from the eye.
  • The eyelid of the affected eye is drooping.
  • The cat blinks or rubs the eye.
  • The cat is less active than usual.
  • Changes in the cat’s vision.

Causes of anisocoria in cats

The causes of anisocoria are many, it is a symptom and in itself is reason enough for an urgent visit to the vet, especially if it occurs suddenly. Although anisocoria in cats can occur as a result of a disorder in the cat’s physiological system, it is usually caused by disease. The most common are as follows:

Horner’s syndrome: consists of a set of symptoms due to a disease or problem resulting from a lack of sympathetic supply. It is manifested by sunken eyes, mydriasis and miosis, as well as other symptoms such as a prolapse of the third eyelid or a difference in the size of the eyes. Continue reading this other article if you want to know more about Horner’s syndrome.
Corneal lesions: damage to the surface layer of the eye can cause ulcers and thus a difference in pupil size due to changes in the affected eye.
Glaucoma: increased intraocular pressure, also known as glaucoma, causes the affected eye to dilate a little more, resulting in one of your cat’s pupils becoming larger than the other. Continue reading this other article if you want to know more about glaucoma in cats.
Feline leukaemia: Feline leukaemia retrovirus can cause a variety of symptoms and blood disorders in affected cats. Clinical signs that may occur include differences in pupil diameter, as well as other symptoms such as anaemia, oral disorders or gingivitis, fever, lethargy, weight loss or infection. Continue reading this other article if you want to know more about feline leukaemia.

Other causes of anisocoric pupils in cats include:

  • Trauma.
  • Uveitis.
  • Ocular tumours
  • Degenerations of the iris.

Treatment of anisocoria in cats

Treatment of anisocoria depends entirely on the underlying cause of the condition and specific treatment is tailored specifically to the diagnosis. Therefore, an accurate and prompt diagnosis is essential for the cat’s early recovery. The anisocoria itself is less important than treating the underlying disease that may cause it.

Some medications can cause problems with the pupils. If your vet discovers that a medication your cat is taking is causing the difference in pupil size, you should stop giving it to your cat. The anisocoria will be cured when the medication has cleared from your cat’s body.

It is important to be aware that some conditions may not be treatable. Some of the underlying disorders that cause anisocoria are not life-threatening. Therefore, they do not require treatment, such as iris hypoplasia or iris atrophy.

Your veterinarian will discuss the treatment options that are most appropriate for your cat’s individual circumstances.