Dogs and other mammals are built symmetrically. This means that their eyes and pupils will naturally be of identical size. Even if you shine a light in only one of your dog’s eyes, both pupils will naturally constrict or dilate. If you notice that your dog’s pupils are of different size, regardless of the lighting, there is a problem. Here are some possible causes:

When the pupils in the eyes are different sizes, it is an illness that is called anisocoria. The affected pupil can be either larger or smaller than normal. Finding out whether the abnormal pupil is larger or smaller will help determine the underlying cause. Depending on the actual reason, anisocoria is not usually painful, but it will not resolve unless the underlying issue is fixed. Your pet should be brought to the vet, as some causes of different sized pupils can cause blindness that may not be reversible.

Like all other muscles, the pupils receive impulses from the brain through the nerves. Any issues can occur from the brain or through the nerves. This can cause the message from the brain about when to constrict or dilate to get lost. In some cases, the only noticeable symptom will be different sized pupils. Other signs may include redness in the affected eye, a cloudy or bluish cornea, discharge coming from the eye, a droopy eyelid in the affected eye, the dog may be squinting or rubbing at its eye and your pet may be less active than usual.

Head trauma, including concussions, are common neurological causes of different sized pupils. In the case of head trauma, anisocoria can indicate a lesion that is affecting either one side of the brain or will show that one side is more affected than the other. If the pupils are unresponsive to light changes, it could represent a compression or disruption of the oculomotor nerve tracts.

There are two types of eye cancer. Myelocytic cancer is caused by abnormal melanocytes and does not spread. The second type of cancer is called myelocytoma tumors which do spread. The type that is more commonly found in dogs is myelocytic, and it will usually originate in the pet’s iris. There are also eyelid, limbal, choroid and conjunctival tumors, which are all rare causes of anisocoria. Tumors are seen most often in dogs who are heavily pigmented. Signs can include weeping and redness of the eye and eventually the formation of a lump. Ocular cancers are known to be painful. These tumors can grow and bulge into the fluid within the eye, called the vitreous fluid. If they grow large enough, it can cause inflammation, an increased ocular pressure, optic nerve compression and retinal detachment, all of which can eventually cause blindness. 

This is a disease that occurs as pets age and can begin to appear in middle-aged to older dogs which can, among other things, decrease your pet’s ability to constrict the pupils with the change of light. Your dog can be experiencing a stromal thinning or a sphincter muscle atrophy. Thinning of the iris stroma is more often seen in select breeds such as the Chihuahua, the Toy Poodle and Yorkshire Terrier. This can make the iris look like lace and make the pets appear to have numerous pupils. This disease is not as serious as others, but the inability to constrict the pupil can lead to an excess of light entering the eye and therefore you may notice your dog squinting in the light.

This is a condition that is caused by an increased pressure in your pet’s eye. This can be a result of the fluid within your dog’s eye not draining properly, which will lead to a buildup of pressure. If left untreated, it can cause an enlargement of one eye and can very quickly lead to blindness that will be permanent. Glaucoma is thought to cause a pain similar to headaches and migraines. This can cause your dog to have a smaller appetite, be irritable, be rubbing at their eye or not wanting to play. Some other signs to watch out for include a bulging eye or dilated pupil. Squinting or keeping their eye closed, fluttering eyelids and a loss of vision occurring in one eye may be seen. This disease is inherited primarily and can occur commonly in dogs, more so in certain breeds including Jack Russell, Shih Tzu, Cocker Spaniel, Siberian Husky, Chow and Basset Hound. It can also occur secondarily as a result of inflammation to the eye which is called uveitis, ocular cancer, advanced cataracts, chronic retinal detachment and displacement of the lens.

If your dog has a scratch, dry eyes or something stuck in their eye, they may be experiencing ocular pain. Firstly, take a look in your dog’s eye in an area that has good lighting. A healthy eye is bright and clear, has equal sized pupils, no crusts, discharge or tearing and the area around the pupil should be white. You can check the lining by gently rolling the lower eyelid down with your thumb. It should be pink, not white or red. Anything other than what was mentioned should be noted as unusual. This includes crusts and discharge, tearing around the eye, different sized pupils, cloudiness or a change in eye color, closed eyes, a red or white lining and the third eyelid is visible. You can clean your pet’s eyes by taking a damp cotton ball and gently wiping outwards from the corner. Visit your vet if you notice that your pet often has runny eyes and discharge, as it can be a sign of infection.

Horner’s syndrome happens when there is a dysfunction occurring in the sympathetic nerves in the facial muscles and eyes. Symptoms of this disease include anisocoria, a droopy upper eyelid, a recessed or sunken eyeball and an elevated third eyelid that can be red in color. Sometimes, the dog will also experience difficulty eating on the affected side, along with excessive salivation. Horner’s syndrome occurs when enervation to the eye is disrupted, which could happen for a number of reasons, including damage in the sympathetic pathways running through the chest and neck, which can be a result of a bite wound, tumor, middle or inner ear diseases or intervertebral disc disease. In half of the cases of Horner’s syndrome, the cause is idiopathic, meaning that the underlying reason cannot be found. This can occur in any dog, but Golden Retrievers and Cocker Spaniels are of higher risk.

When you bring your dog to the vet with different sized pupils, your vet will first have to determine which of the eyes are affected and also if the problem is neurological or ophthalmic. The vet will perform a physical examination and look at the structure of the eye. Depending on the findings, your vet may perform further testing including measuring the pressure and tear production of each eye. The vet may add a dye to the cornea in order to spot any injuries or ulcers. Biopsies or conjunctival scrapings may be sent to laboratories for testing and blood tests may be used to find out if the condition is systemic. 

Different sized pupils will be cured by treating the underlying cause. Dog’s who have undergone head trauma will also undergo a physical examination to assess the cardiovascular and respiratory systems, followed by a neurological examination. The physical examination should focus on the pet’s level of consciousness, the size of the pupils, posture, and how the pupils respond to light. Most tumors can be spotted using an ophthalmoscope but an accurate diagnosis of the tumor will require a microscopic examination of the tissue. Either a biopsy, full excision or cellular aspirate can be used to obtain the tissue. 

Treatments will be different depending on the type of tumor but surgery will most likely be required, as it is very rare for cancers to disappear without treatment. After treatment, your dog may need a special type of collar that will keep them from rubbing the area. The operation site will need to be kept clean and you should report to your vet if there is any swelling, bleeding or loss of stitches. Anti-inflammatory drugs, eye drops and ointments may also be part of the treatment procedure. 

Treatments for glaucoma can include several types of drugs which depend on the cause. The goal will be to alleviate the pain and pressure within the eye by reducing the amount of fluid being produced and increasing the drainage. Unfortunately, when glaucoma is discovered and treated, blindness will most likely have already occurred. If the dog becomes blind, the best long-term and most painless option is to remove the eye. Treatment will then be focusing on preventing the glaucoma from spreading to the other eye. In most cases, Horner’s syndrome will resolve on its own but any underlying causes should definitely be treated. Tests will take place in order to determine if a reason for the development of Horner’s disease exists. Sympathetic treatment may include eye drops for the affected eye given every 12 to 24 hours.

Iris atrophy cannot be prevented, but an avoidance of bright lights and sunlight may be recommended as a way to improve your pet’s comfort. To reduce your pet’s chances of developing glaucoma, avoid stresses in their environment.

Using harnesses opposed to collars in order to reduce the pressure on your dog’s neck can help avoid glaucoma as well. Giving your pet antioxidants and nutraceuticals are good ways to reduce the damage done to the cells in the eye. Monitoring the pressure of the eye for dogs who are predisposed to glaucoma or are aging is also a good way to catch the illness before it occurs. Dogs who are long haired can more commonly experience ocular pain from their hair brushing against or getting caught in the eyelid. This can be prevented by keeping the fur near the eyes trimmed. You should also undergo precautions to protect your pet’s eyes from soaps and medications when bathing them or applying ointments.

Depending on the cause of your pet’s differently sized pupils, you will have to pay varying amounts for treatment. Expect to be paying $1500 to treat head trauma and on average $600 for eye injuries. Your pet’s iris atrophy can be addressed for around $1200 and glaucoma for $900. Horner’s syndrome will require about $850 to be treated.