What Causes Dilated or Enlarged Pupils in Dogs?

Updated on May 15, 2020
alexadry profile image

Adrienne is a certified dog trainer, former veterinary assistant, and the author of "Brain Training for Dogs."

Have you ever noticed how your dog's pupils will get bigger when frightened or when you are playing with them? Do you ever wonder why this happens?

Enlarged pupils in dogs often reflect their emotions, however, in some cases, pupil enlargement can be a sign of an eye problem or an underlying medical disorder, in which case you want to have your dog see the vet to play it safe.

In this article, we will be taking a closer look at the following topics:

  • How your dog's pupils work and react to light and darkness
  • Why your dog's pupils dilate as a response to fear
  • The reason why your dog's pupils enlarge when he's playing
  • Several medical conditions are known to cause dilated pupils in dogs
  • Discover several fascinating facts about dog pupils

Notice the large pupils in the this French bulldog in a a dark environment
Notice the large pupils in the this French bulldog in a a dark environment

Why Do My Dog's Pupils Dilate in a Dark Room?

The black circle that sits in the middle of your dog's iris (the colored part of the eye) is called the pupil. Just as it happens in humans, your dog's pupil size changes based on lighting conditions. The size of the pupil is controlled by muscles and is based on how much light is present.

In low lighting conditions, your dog's pupils dilate or get bigger so to let more light in. The opposite happens when your dog encounters bright lights. Their pupils will constrict, or get smaller so to let in less light.

If your dog's pupils are therefore dilated in a dark room, that's totally normal. It's a sign that your dog's pupils are working properly.

Indeed, when visiting a good veterinary eye doctor, one of the first things he may evaluate is whether your dogs' pupils constraint with light. If they do, can say therefore assume that your dog's "pupillary response" is in good working order.

Fearful expression in dog, notice the dilated pupils
Fearful expression in dog, notice the dilated pupils

Why Do My Dog's Pupils Dilate When Fearful?

Emotions can play a big part in causing your dog's pupils to dilate. Excitement, surprise, fear, pain and even stress will cause your dog's pupils to dilate and constrict in response to these extreme emotions.

There is a theory about pupil dilation when a dog experiences fear. When a dog gets scared, their survival instincts will kick in, and their fight or flight response is triggered. When this happens, their pupils will dilate, allowing more light to enter their eyes so that their brains can process visual information quicker and more clearly.

When dogs are in life and death situations, every second counts. The automatic dilation reaction happens on a physiological level and is beyond the dog's control.

According to Scientific American, "Stimulation of the autonomic nervous system's sympathetic branch, known for triggering "fight or flight" responses when the body is under stress, induces pupil dilation. Whereas stimulation of the parasympathetic system, known for "rest and digest" functions, causes constriction."

Just as the dog's pupils dilate when they are scared, they also dilate when your dog gets excited. And dogs get super excited when you play with them!

This border collie's eyes are fixated on the ball and are dilated to increase visual acuity
This border collie's eyes are fixated on the ball and are dilated to increase visual acuity

Why Do My Dog's Pupils Dilate When Playing?

Most dogs naturally have an instinct for hunting. Whether it's chasing squirrels, harassing other dogs, or even chasing a toy, their hunting mode will kick in.

When this occurs, their predatory drive responds with a surge of adrenaline. However, in this case, the adrenaline surge is not in response to their "fight or flight" instinct to help save their life. Instead, the adrenaline surge, in this case, is meant to increase the chances of the dog catching its prey.

Similar to humans, dogs can experience good stress and bad stress. Bad stress occurs when the dog is in danger, scared, or feels threatened. Good stress occurs when the dog is challenged, usually physically, in a positive and exciting way.

Good stress can get the adrenaline pumping, ensuring the dog's brain and muscles are flooded with oxygen-rich blood, enabling them to spring into action quickly as needed. While this is happening, the pupils are dilating as well, letting more light in, and increasing their visual clarity.

When dogs are playing, good stress is meant to enhance their senses, as well as their overall performance. This allows their hearing to pick up even the faintest of sounds, while their eyes detect the slightest of movements. You might notice that while this is happening, your dog is quivering in anticipation.

There are several causes of pupil dilation in dogs.
There are several causes of pupil dilation in dogs.

Medical Causes of Enlarged Pupils in Dogs

As the saying goes, "The eyes are the windows to your soul." While we have seen how emotions impact the pupils of dogs, let's also take a look at what medical conditions may cause pupil enlargement in dogs. Of course, this list is not meant to be exhaustive. See your vet for proper diagnosis and treatment.

Eye Problems

  • Glaucoma: Pupil enlargement is often a sign of glaucoma, and therefore the dog's eye pressure should be always measured in dogs showing this sign.
  • Tumors: Dilated pupils in dogs may also be indicative of tumors affecting the dog's retina, optic nerve or even brain.
  • Iris atrophy: In elderly dogs, the appearance of dilated pupils may be due to a condition known as iris atrophy where degeneration of iris sphincter muscle responsible for constructing the pupil fails to work properly.
  • Blindness: Enlarged pupils in a dog that tend to remain that way can also be a sign of blindness of various causes. In particular, a condition known as progressive retinal atrophy is known for causing bilateral blindness in dogs that occurs gradually.

When it comes to blindness, typically the first thing dog owners notice is the dog's reduced ability to see at night and a reluctance to go up or downstairs.

The affected dog may also have dilated pupils and when the dog's eyes shine in the dark they may show an abnormal tapetal reflection through the dilated pupils, explains Dr. Rhea Morgan, a board-certified veterinary ophthalmologist.

Head Trauma

You may have noticed in movies how after taking a blow, medical personnel check a person's eyes for signs of pupil dilation. Dilated pupils after head trauma may be an indication of internal bleeding in the brain or a neurological problem that requires immediate intervention. Treatment to decrease intracranial pressure should be initiated as soon as possible.

Brain Lesions

The brain is responsible for making the dog's pupils change shape, so pupils that remain dilated even in lighted conditions can signal that the dog's brain is not working normally and there may some tumor affecting the dog's brain or the retina or optic nerve.


Seizures take place when the neurons in the dog's brain are overly active. Dogs can be affected by different types of seizures. Grand mal seizures, in particular, are known for affecting the dog's whole body and one of the signs noticed are dilated pupils.

Ingestion of Toxins

Ingestion of certain types of toxins may cause a dog's pupils to dilate. Could your dog have picked up a psycho-therapeutic or recreational drug or pill off of the floor? These drugs are known to cause pupil dilation.

Other toxins known to cause pupil dilation in dogs include organophosphate poisons used often in gardens as an insecticide, nicotine, certain fruit pits, horse chestnuts, foxglove flowers and scorpion venom, to name a few.

If your dog's pupils are dilated and you cannot find a reason for it or if they appear dilated for an extended period of time, please play it safe and see your vet.

As mentioned, your dog's eyes can alert you of health problems before other symptoms may occur. Your dog could be ill or injured, so please see your vet as soon as possible. Complicated cases may need a referral to a board-certified veterinary ophthalmologist.

5 Fascinating Facts About Your Dog's Pupils

  • Contrary to what you may have believed in the past, your dog's pupils aren't actually what they seem to be.What you are really looking at when you look into your dog's pupils are actually dark holes
  • .Pupils appear black for the simple fact that light rays entering the pupil are directly absorbed into the tissues on the inside of the eye.
  • Your dog's pupils work pretty much like the shutter of a camera, changing their shape depending on how far away the eye focuses.
  • If neither pupil constricts (becomes small when a light is shined in it), the pupils are said to be "fixed," which can be a sign of a brain problem/
  • Doctors use the abbreviation "PERRLA" to depict healthy pupils. These letters stand for "Pupils, Equal, Round, Reactive to Light and Accommodate.

Did you know?

The medical term for dilated pupils affecting both eyes is mydriasis, while the term anisocoria is used to depict pupils of a different size.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2020 Adrienne Farricelli


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    • alexadry profile imageAUTHOR

      Adrienne Farricelli 

      5 days ago

      Hi Peggy,

      When I worked for the vet, we were always instructed to urge dog owners whose dogs had eye problems to make a same-day appointment considering that some eye conditions in dogs may lead to serious complications and even blindness if left untreated.

    • Peggy W profile image

      Peggy Woods 

      2 weeks ago from Houston, Texas

      None of our dogs have ever had eye problems, and you are wise to direct people to seek veterinary evaluation if they ever detect something wrong. So sorry to hear that you lost your own dogs. Those you board and train are lucky to have your expertise.

    • alexadry profile imageAUTHOR

      Adrienne Farricelli 

      2 weeks ago

      Yes, I got your first comment, just been busy lately and haven't been able to approve them as soon as they arrive. My dogs got dilated pupils when I would toss a ball to them. Their faces would be happy and eager to chase. Luckily, they never developed any eye disorders.

    • alexadry profile imageAUTHOR

      Adrienne Farricelli 

      2 weeks ago

      Hi Devika,

      Sorry to hear about the loss of your first dog. It's so tough losing them. I am still struggling with the loss of my dogs and keep busy boarding and training others peoples' dogs.

    • DDE profile image

      Devika Primić 

      2 weeks ago from Dubrovnik, Croatia

      I know I commented on this hub but do not see it here. It is an informative and a well told hub. Dogs are amazing friends but such disorders do make me see their stranger appearances.

    • DDE profile image

      Devika Primić 

      2 weeks ago from Dubrovnik, Croatia

      You impress me with your articles about dogs. I think you deserve more readers. In detail about dogs and so much I have learned from you. I had my first dog in 1982 it was killed by a negligent driver. Since that day I decided to not have a dog again. Then 12 years later I decided to get one and rescued it fro the kennels it felt different and tried to not get close to this dog. A year later I got another dog this one I got close to and when I left my birth country to move to Croatia I could not take him with and that was the end of my closeness to dogs. It was terrible to leave my pets behind. I read your hubs and wonder how much more I could have applied to my dogs from your informative hubs.

    • bodylevive profile image


      2 weeks ago from Alabama, USA

      Wow! Never saw this disorder before. I hope it can be fixed. I love dogs, I have 2.

    • Pamela99 profile image

      Pamela Oglesby 

      2 weeks ago from Sunny Florida

      This is an interesting article with good information about the pupils of dogs.


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