Anisocoria in dogs is the term used to describe a difference in the size of two pupils or uneven pupils. When a dog has anisocoria, your veterinarian will determine which pupil is abnormal, the smaller or larger one. Common causes of a smaller pupil (miosis) include Horner's syndrome, uveitis and corneal ulcer. Causes of a dilated pupil (mydriasis) include glaucoma, iris atrophy, medications and oculomotor lesion (CN III).

What is anisocoria in dogs and what are the mechanisms that regulate pupil sizes?

Anisocoria, or unequal pupil size, is a common condition in dogs. While it can be caused by several possibilities, in most cases it is benign and does not require treatment. However, it is important to have anisocoria evaluated by a veterinarian, as it can sometimes be an indication of a more serious underlying condition.

There are several mechanisms that regulate pupil size in dogs. The most important of these is the sympathetic nervous system, which controls the dilation and constriction of the pupils in response to light levels. Another mechanism is the parasympathetic nervous system, which helps to keep the pupils constricted in low light conditions. Finally, certain medications and toxins can also cause the pupils to dilate or constrict.

While anisocoria is usually harmless, it is important to have any changes in pupil size evaluated by a veterinarian. In some cases, anisocoria can be an indication of an underlying medical condition that needs to be treated.

What are the causes of anisocoria in dogs?

There are several potential causes of anisocoria in dogs. We'll look at the possible reasons based on whether the constricted or dilated pupil is the problem.

Constricted pupil (miosis) as the cause of anisocoria in dogs

There are a few conditions that can cause a constricted pupil, including:

  • Horner's syndrome: A condition which is caused by damage to the sympathetic nervous system. This can be due to trauma, tumours, or other conditions that affect the nervous system.
  • Anterior uveitis (inflammation inside the eye): If only one eye is affected, anisocoria will be present as uveitis will result in the constriction of one pupil with the other being a normal size.
  • Corneal ulcer: An ulcer on the surface of the eye can cause enough pain to result in miosis (constriction of the pupil).

Dilated pupil (mydriasis) as the cause of anisocoria in dogs

There are several conditions that can cause a dilated pupil, including:

  • Glaucoma: This is a condition in which the pressure inside the eye is too high. It can damage the optic nerve and lead to blindness if not treated promptly.
  • Iris atrophy: This is a condition in which the iris (the coloured part of the eye) deteriorates. It is usually an age-related issue and occurs commonly in senior dogs.
  • Oculomotor nerve lesion: This is a condition in which the oculomotor nerve (CN III), which controls the movement of the eyeball and the muscles that control pupil size, is damaged.

How is anisocoria diagnosed in dogs?

Anisocoria is usually diagnosed during a detailed physical examination by a veterinarian. The vet will perform a thorough ocular and neurological examination and may also recommend other tests, such as blood work or imaging, to rule out any underlying medical conditions. The evaluation of the degree of anisocoria in dim and bright light settings allows for the diagnosis of an abnormal pupil, narrowing down the possibilities.

Your veterinarian can also use single-dose ocular medications to better determine the cause of anisocoria which is referred to as pharmacological testing. Examples include phenylephrine, hydroxyamphetamine, and pilocarpine. When utilizing these medications to determine the cause of anisocoria, it's essential to keep usage periods apart by 24 hours since together, they might influence each other's activity.

What are the treatment options for anisocoria in dogs?

The treatment for anisocoria will depend on the underlying cause. In some cases, no treatment may be necessary if the condition is not causing any problems for the dog. If the anisocoria is due to an underlying medical condition, treatment will be focused on that condition.

For example, if glaucoma is the cause of anisocoria, the dog will need to be treated with medications to lower the intraocular pressure. If anterior uveitis is the cause, the dog will need to be treated with anti-inflammatory medications.

In conclusion, anisocoria in dogs is a condition in which the pupils of one or both eyes are different sizes. While usually harmless, it is important to have any changes in pupil size evaluated by a veterinarian as they may be indicative of an underlying medical condition that needs to be treated. In some cases, no treatment may be necessary if the condition is not causing any problems for the dog. If the anisocoria is due to an underlying medical condition, treatment will be focused on that condition. Thanks for reading and if you have any questions about anisocoria or are concerned your dog has uneven pupils, please contact your veterinarian.