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Enlarged Pupils in Dogs

Enlarged Pupils in Dogs

Enlarged pupils in dogs is something that dog owners may not notice at a first glance until they look more closely in the dog's eyes and notice something unusual. But what causes enlarged pupils in dogs? And most of all, should you be worried about it? When should you schedule an appointment to the vet? These are all good questions. Veterinarian Dr. Ivana Crnec discusses enlarged pupils in dogs and some underlying potential causes.

Vet checks a dog's eye

Have your dog with enlarged pupils see the vet

Enlarged Pupils in Dogs 

When it comes to eyes and vision, or better said their anatomy and physiology, dogs are quite similar to us. Just like ours, the dog’s pupils can either constrict or enlarge. The change in the pupils’ size depends on the amount of light they are exposed to.

In a nutshell, when a dog sits in bright sunlight, its pupils are small and almost impossible to spot. On the flip side, when a dog sits in a dark room, its pupils are significantly large and almost covering the entire colored portion of the eye. Basically speaking, the size change is a mechanism that modifies the light intake based on the actual need.

When the pupils are constricted, the condition is known as miosis. When the pupils are dilated, the condition is known as mydriasis. If the pupils are asymmetric, one pupil is larger than the other, the condition is known as anisocoria. The ability of the pupil to either dilate or constrict is medically known as pupillary response.

The size and functionality of the pupils can be indicative of certain problems. Generally speaking, there are three pupil-related signs or abnormalities that indicate a problem:

  • The pupils are not synchronized with the amount of light they are exposed to
  • The pupils lack the ability to change in size regardless of the amount of light
  • The pupils are not equal in size (anisocoria).

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Causes of Enlarged Pupils in Dogs 

A dog's pupils can dilate or become enlarged due to a plethora of triggers and conditions. Some are normal and expected but others are serious and require quick and proper management. When the dog’s vision is at stake, a "wait and see" approach can be fatal and eventually cost your dog its vision. Therefore, if your dog’s pupils do not act normally and you suspect a problem, make an appointment at the vet’s as soon as possible.

The causes of enlarged pupils can be either physiological or pathological. The physiological causes of enraged pupils include being in a dark room and responding to stress.

The pathological causes of enlarged pupils can be classified in several categories: congenital deformities, eye disorders such as glaucoma and progressive retinal atrophy, neurological problems such as seizures and cancer, toxicity, stress and traumatic injuries.

Stress in Dogs 

Although stress is not considered physiological, being able to properly respond to stressful situations is normal. That is why stress is categorized as a physiological cause of dilated pupils.

In dogs, many events can trigger stress. Such events include changes in the environment, new working schedules, adding a new family pet or member.

Stress causes bilateral (affecting both eyes)but temporary pupil enlargement. Other symptoms of stress include hair loss due to excessive licking, behavioral changes (pacing, jumping, lip-licking, whining) and digestive irregularities.

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Congenital Deformities 

Sometimes, in puppies, during the fetal development, the iris does not form completely. The underdeveloped iris results in change in the pupil’s size. The size change can affect one (unilateral) or both (bilateral) eyes.

Enlarged Pupils due to Eye Disorders 


We know a lot about glaucoma in people, but what about dogs? Can dogs get glaucoma? The simple answer is yes. Just like in humans, glaucoma in dogs develops when the eye’s fluid cannot drain properly. As the condition progresses, the buildup of fluid causes painful pressure.

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Different Types of Pain in Dogs

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Unlike in humans, in dogs, glaucoma develops particularly fast and the vision can be lost even with prompt and suitable treatment. An eye with glaucoma will have a permanently dilated or enlarged pupil that does not respond to light changes. Other symptoms of glaucoma include excessive tearing, redness, swelling, cloudiness and obvious pain.

Glaucoma can be treated conservatively (with oral and topical medications) or surgically.

Progressive Retinal Atrophy (PRA)

PRA in dogs is an inherited eye condition, recognized in more than 90 dog breeds. In PRA the retinal cells die and the blood vessels in the choroid layer shrivel. As a result, the vision gradually but progressively deteriorates.

The first sign of PRA is diminished sight in dark conditions or night blindness. The dog is reluctant to go outside when it is dark and bumps into objects while moving in dark rooms. In dogs with PRA, the pupils dilate in an attempt to bring in more light.

Since the condition affects both eyes, both pupils will be enlarged. In addition to being enlarged, the pupils will have an abnormal shine medically known as "tapetal reflection". As a consolation, it should be well-noted that PRA is not painful. Sadly, there is no currently available treatment for PRA.

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Neurological Problems 


Seizures occur when the neurons in the brain are overly active. There are many different types of seizures. The grand mal seizures, affect the entire body and one of the symptoms they cause are dilated pupils.


A tumor affecting the brain or the eye can result in a plethora of ocular changes including pupil enlargement. Depending on the type and location of the tumor, the enlargement can affect one or both eyes.

The most common type of tumor affecting the eyes is the suprasellar germ cell tumor that develops near the pituitary gland. This tumor causes permanently enlarged and unresponsive pupils, diminished vision, lethargy and behavioral changes.


More often than not, toxic ingestions are followed by visible eye changes – the most striking consequence is enlargement of the pupils. Fruit pits, horse chestnuts, foxglove flowers, scorpion venom, marijuana are all examples of toxic agents that influence the central nervous system thus causing temporary pupil enlargement.

Some regularly prescribed drugs can also have a similar effect. Every time your vet prescribes a new drug, ask about the possible side-effects.

Traumatic Injuries 

Traumatic injuries of the head or eyes can have long-term consequences on the dog’s vision. If the optic nerve or the eye tissue are damaged it is likely that the dog will experience vision loss, behavioral changes and pupil enlargement. As the injury heals, the formation of scar tissue can also cause significant damage, potentially leading to neurological issues.

About the Author

Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia.

ivana crnec

She currently practices as a veterinarian in Bitola and is completing her postgraduate studies in the Pathology of Domestic Carnivores at the Faculty of Veterinary Medicine in Zagreb, Croatia.

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