Unequal Pupil Size in Dogs

The pupil is the hole at the center of the eye through which light enters. Anisocoria is a condition in which one of the pupils is abnormal, due either to persistent constriction or to dilation. This is most notable as unequal sizes between the two pupils though additional clinical signs may be present depending on the cause of the anisocoria. Anisocoria itself is not life-threatening, but because it can be a symptom of head trauma or neurological disorders, you should take your dog to the veterinarian if the anisocoria does not resolve.

Anisocoria is a condition in which a dog's two pupils are unequal in size. This is a symptom of a wide range of underlying causes, including head trauma, degeneration of the eye, or exposure to chemicals. Occasionally, the anisocoria will resolve on its own. However, you will need to visit the veterinarian to determine the source and to see whether or not treatment is necessary.

Anisocoria is characterized by pupils that are unequal in size. Because anisocoria itself is a symptom of other conditions, you may notice additional symptoms based on the underlying cause. A dog with anisocoria may be otherwise asymptomatic or may exhibit clinical signs such as:

• Cloudy cornea
• Discharge from the eye
• Redness in the white part of the eye
• Rubbing of the face
• Growths around the eye

Types

The abnormal pupil can be either constricted or dilated. Constricted pupils are referred to as miosis, while dilated pupils are called mydriasis. Anisocoria involving a miotic pupil might be more easily diagnosed in a dark room, as the pupil will constrict when exposed to bright light. A mydriatic pupil does not respond to direct light.

Anisocoria can occur for a number of reasons, including:

• Head trauma or injury
• Brain disorder
• Corneal ulceration
• Glaucoma
• Iris hypoplasia
• Horner's syndrome
• Degenerative disease
• Exposure to chemicals or toxins

Whether the abnormal pupil is miotic or mydriatic may depend on the cause of the anisocoria. Horner's syndrome, for example, is characterized by miosis in either one or both pupils, while early glaucoma initially presents with mydriasis in one eye. There is no breed or age predilection for anisocoria, but certain dogs may be more at risk for the different conditions that lead to it.

The difference between the pupil sizes may be obvious, but it may also be difficult to see. The veterinarian will first conduct a physical examination with a focus on the eyes and their structure. The eyes may be examined in both a brightly lit room and a darkened one, as the pupils may respond differently to varying amounts of light. The initial goal is to determine which eye is abnormal and whether the affected pupil is miotic or mydriatic.

Based on these findings, the veterinarian will perform additional tests to determine the underlying cause of the anisocoria. These tests may include:

• Laboratory testing, such as blood work or a urinalysis
• Measuring intraocular pressure
• Staining the cornea to look for ulcers
• Conjunctival biopsy
• Neurologic testing
• Radiographs of the skull
• MRI for intracranial disease
• Electroretinogram

Treatment for anisocoria is highly variable, as the focus is on its underlying cause instead of on the anisocoria itself. Symptomatic treatment of anisocoria is not typically recommended.

If the anisocoria was caused by exposure to toxins, chemicals, or topical drugs, it may be reversed as the substance is removed from the system. Treatment is not necessary for certain causes, such as Horner's syndrome, which may resolve by itself over time, while degenerative disorders like iris atrophy may not be treatable at all. Talk to the veterinarian about the options that are available for your dog, and discuss which one he or she would recommend.

If your dog received treatment for the condition that led to anisocoria, let him or her recover quietly in a safe, comfortable place. Follow any instructions given to you by the veterinarian, and administer any prescribed medications according to directions. If no treatment was recommended, monitor your dog daily to check whether or not the anisocoria has resolved. Contact the veterinarian if the anisocoria persists, or if you notice any additional symptoms.

The prognosis for recovery varies depending on the underlying cause.

Anisocoria can be a symptom of a serious medical condition. To avoid high vet care expenses, secure pet health insurance today. The sooner you insure your pet, the more protection you’ll have from unexpected vet costs.