First Walk is on Us!

✓ GPS tracked walks
✓ Activity reports
✓ On-demand walkers
Book FREE Walk

Jump to Section

What is Anisocoria?

The pupil appears as a round black area in the center of the eye. It is actually an opening that allows light to pass through the eye to the retina. A normal pupil adjusts to allow different kinds of light to enter the eye, becoming smaller in bright light, or constricting, or becoming bigger in response to dim light, or dilated. As a result of many kinds of medical conditions, one or both of the pupils may be subject to persistent constriction or dilation, which causes one or both of the pupils to be an abnormal size. While not a life-threatening condition in itself, anisocoria is a symptom that may be indicative of a serious or irreversible condition.

Anisocoria refers to pupil asymmetry, or when the two pupils of the eyes in a dog are different sizes. This is because one of the pupils is smaller or larger than the other, and in some cases, both pupils are abnormal, though still differing in size. Often resulting from a dysfunction of the eyes or neurological system, this condition can be temporary, but may need medical treatment if it persists.

Book First Walk Free!

Symptoms of Anisocoria in Dogs

Anisocoria is a symptom, and its presence signals a medical issue. If anisocoria suddenly occurs in your dog, seek medical help immediately, as it could be a sign of a serious and progressive condition which could cause your dog’s vision to be permanently affected. While the most characteristic symptom of anisocoria is the presence of two different sized pupils in the same dog, there are other symptoms that may occur simultaneously. Noting any and all symptoms can give valuable clues to your veterinarian as to the cause. Signs can include:

  • Different sized pupils, one being either bigger or smaller than the other
  • White part of eye, or sclera, is reddened, bluish, or cloudy
  • Eye discharge
  • Droopy eyelid
  • Cloudy cornea
  • Growths near the eye
  • Excessive rubbing of eye or face
  • Squinting 
  • Lessened activity


There are two types of anisocoria seen in dogs, which can occur singularly or simultaneously.

  • Miosis refers to when the pupil is smaller than normal, or is constricted; when exposed to bright light, the miotic pupil may constrict, making it the easier type to diagnose
  • Mydriasis refers to a pupil that is larger than normal, or dilated; the mydriatic pupil is harder to diagnose, as it doesn’t respond to direct, bright light 

Causes of Anisocoria in Dogs

Anisocoria occurs when the pupil is subject to a state of persistent constriction or dilation. There are many causes of anisocoria, most of them due to a neurologic or ocular issue, and can be from a malfunction of the visual, sympathetic, or parasympathetic systems. Chemical toxicities or a vitamin deficiency can also result in pupils of varying size. Diagnosing if the anisocoria is miotic or mydriatic, or both, will lead to a specific cause. Possible causes include:

  • Glaucoma
  • Injury to the eye
  • Corneal ulcer
  • Uveitis
  • Eye disease
  • Posterior synechia
  • Iris atrophy
  • Iris hypoplasia
  • Congenital defect within the iris
  • Eye degeneration
  • Cancer within the affected eye
  • Horner’s syndrome 
  • Brain disease
  • Fibrocartilaginous embolism
  • Head trauma
  • Hydrocephalus
  • Infectious and inflammatory disease, such as meningitis and encephalitis
  • Infection in the middle ear
  • Hepatic encephalopathy
  • Dysautonomia
  • Thiamine deficiency
  • Chemical or toxin poisoning, such as from atropine, phenobarbital, or organophosphates

Diagnosis of Anisocoria in Dogs

If you notice the two pupils in your dog’s eyes are different sizes, it may be a cause for alarm, especially if it happens suddenly. Be sure to tell your veterinarian how long the anisocoria has been present, and any other symptoms you have noticed. Your vet will give your dog a physical examination, including an eye exam. Often, bright light directed into each eye can help determine if one or both pupils are miotic or mydriatic. Further testing of the eyes can include staining the cornea with fluorescent dye to reveal ulcers or injuries, measuring the intraocular pressure of each eye, checking the tear production of the eyes, taking scrapings or biopsies to be analyzed, or an electroretinogram.

Other tests that can help to narrow down a cause can include blood work and urine testing, X-rays or MRIs of the head, and a neurological exam. Once the reason for the anisocoria has been determined, treatment will follow accordingly.

Treatment of Anisocoria in Dogs

Sometimes, anisocoria can resolve on its own, such as when it has been the result of Horner’s syndrome. In other cases, such as with iris atrophy, it may be untreatable. When treatment can be prescribed, it will focus on the cause of the anisocoria, rather than the symptom itself. Treatment can vary considerably, and your vet will discuss the options available for your dog based on the particular reason for his anisocoria.

Treatment for the eye condition uveitis involves the administration of corticosteroids, prednisolone, non-steroidal anti-inflammatories, antimicrobials, or mydriatic-cycloplegic drugs, such as atropine or tropicamide which help to relax the iris muscles. Glaucoma treatment can include topical creams or drops to lower eye pressure and inflammation, along with beta-blocking drugs, diuretics, cholinesterase inhibiting drugs, corticosteroids, and mitotic medications. Surgery may be required in some cases.

Infectious conditions, such as meningitis and encephalitis, are often treated with fluid, electrolyte and oxygen therapies. Medication can be prescribed that is appropriate to the cause of the meningitis and encephalitis, and can include corticosteroids, antibiotics, antifungals, pain medications, and anticonvulsant drugs. 

If exposure to a chemical, toxin, or drug has caused the abnormal pupil size, treatment could include inducing vomiting, gastric lavage, administering activated charcoal, and giving supportive care such as fluids and electrolytes. Often, as the toxins leave the system, the anisocoria resolves.

Recovery of Anisocoria in Dogs

Recovery will depend on the severity of the cause of anisocoria, and can vary considerably. Your veterinarian will discuss with you the chances of your dog’s recovery based on his particular condition. Some eye conditions can lead to blindness which is often irreversible. In some cases, short-term or long-term medication will be prescribed for you to administer at home. You will need to monitor your dog whether or not treatment was administered to watch for signs that the anisocoria is resolving. Allow your dog to recover in a quiet and comfortable place, and contact your veterinarian if the anisocoria persists.

Anisocoria Questions and Advice from Veterinary Professionals

6 Years
Mild condition
0 found helpful
Mild condition

Has Symptoms


About one year ago, my dog had one pupil smaller that the other. I took him to the vet and was told he may have just scratched his eye, but I didn’t do an eye stain, so I still don’t know. I was never told anything concerning, so I didn’t check on it much and just noticed that it got better. About one week ago, my dogs pupils were extremely different in size. I took him to the vet, and was told it could be a brain tumor, neurological disorder, or issues with the eye. The vet told me she couldn’t give me answers. The second vet I saw was more informative and gave me a list of everything that could be going on, and do watch it and see if it gets better. Both eyes now constrict equally when exposed to light, but when in dim light, one pupil is slightly smaller than the other. I think this means that the smaller pupil is the affected one since the other one constricts and dilates just fine, but the other doesn’t dilate as much. It’s just a slight difference thought. I still have no answers and I Ann WORRIED SICK that it’s a brain tumor. He does not have any other symptoms. He does shake his head a lot, but he’s always seemingly done that due to his allergies. I do not think he hit his head or anything like that.

Add a comment to Chico's experience

Was this experience helpful?

Siberian Husky
1 Year
Mild condition
0 found helpful
Mild condition

Has Symptoms

Pupil is small
Pupil is large

My Siberian Husky is 1 year old, and from one day to the other she had one large pupil & a small one. She does have some irritation on her right eye. Even with that, she still plays and behaves normal. What should I do?

Dr. Michele King, DVM
Dr. Michele King, DVM
1611 Recommendations
Thank you for your email. Different pupil sizes can be a sign of infection, neurologic disease, or pain. It would be best to have her seen by your veterinarian, as they can examine her eyes and recommend the best course of treatment.

Add a comment to Blue's experience

Was this experience helpful?