The anatomy of a dog's eye is incredibly similar to the anatomy of a human eye in that both have sclera (the white part), irises (the colored part), and pupils (the dark opening in the center of the iris). Just like us, a dog's pupil allows light to pass through the eye and to the retina. In a normal, healthy eye, both pupils will constrict in high light and enlarge in low light. So what does it mean if your dog's pupils are two different sizes?
What Is Anisocoria?
Anisocoria, while a mouthful, is the medical term for when the pupils are two different sizes. Anisocoria, in and of itself, is not a disease but rather a symptom of an underlying condition. There are a variety of reasons your dog may have pupils that have different sizes.
- Corneal ulcer/injury
- A brain or neurological disorder affecting the specific nerves running to your dog's eye (i.e. Horner's Syndrome)
- Glaucoma (the affected eye will have an increased pressure within the eye and will be dilated)
- Degenerative changed to the iris tissue that can happen with aging
- Head trauma
- Exposure to chemicals or toxins
Depending on the underlying cause you may not see any other symptoms or you may see different, accompanying ocular symptoms. Dogs with anisocoria may also have reddened scleras, a clouding or blue-tinted cornea (the outermost layer of the eye), eye discharge, a droopy eyelid, a squinting eye, or rubbing/pawing at the affected eye. Your dog also may be less active than normal.
Regardless of the cause, if your dog's anisocoria has sudden onset it is an emergency that requires immediate veterinary care. Failure to get your dog care right away can permanently damage your dog's vision in the affected eye.
Diagnosing the Reason for Your Dog's Anisocoria
Most eye workups will start with three basic tests: A Schirmer tear test, fluorescein stain, and an intraocular pressure test. A Schirmer tear test will check the ability of your dog's eyes to create tears. There are certain disease processes that will inhibit tear production. Your dog's eyes will actually try to compensate but instead of tears they may produce a thick, sticky discharge. All this to say, you may think your dog just has funky tears when in reality he has a lack of them! Staining the eye with fluorescein dye can illuminate any ulcers on your dog's cornea. The stain will collect in the ulcer itself so that, even when excess stain is flushed out of the eye, the stain will still fluoresce under a black light. Checking your dog's intraocular pressures can check for glaucoma as well as uveitis. If your dog has glaucoma in one or both eyes, they will have higher than normal pressures. If your dog has uveitis in one or both eyes, they will have lower than normal pressures. Rest assured, veterinary medicine doesn't check eye pressures with the dreaded 'air puff' test that is common in human ophthalmology.
If the standard three eye tests don't reveal anything out of the norm, your vet may want to perform more specialized tests. Gentle scrape samples of your dog's conjunctiva can be sent off to an outside lab for histopathology, where a veterinary specialist will look at the samples under a microscope. From this they can determine if there is any abnormal cells present that could indicate a benign (or malignant) growth. Your vet may want to check a blood panel to rule out any systemic illnesses. They may also want your dog to get skull radiographs, an MRI, or even be seen by a veterinary ophthalmologist.
Treating Your Dog's Anisocoria
How best to treat your dog's anisocoria depends entirely on what is causing your dog's anisocoria. A specific treatment will be based on the specific disease. If your dog's anisocoria is stemming from a chemical or toxin exposure, removing the the substance may reverse the anisocoria. Some causes, such as Horner's Syndrome, are self-limiting and the anisocoria may resolve on it's own. For still other causes, such as degenerative conditions, your dog's anisocoria may never get better. Some causes of anisocoria may also require long term medication. If your dog's vision is affected because of their anisocoria their vision may never return back to normal.
Regardless of the reason for your dog's anisocoria, seeking immediate veterinary attention is vital for your dog's vision and eye health. Whether you see an emergency veterinarian or your regular veterinarian, they can help you figure out what is causing your dog's troubling eye symptoms and how best to tackle the problem.