What Are the Causes of Enlarged Pupils in a Dog?

By Quentin Coleman

Normally, your dog's pupils shrink in response to bright light and dilate in darker environments. This mechanism allows healthy eyes to limit light intake depending on the need. However, there are many health conditions that cause unnatural dilation of the pupils, including deformity, disease and injury.

Examine your dog's eyes regularly as a precaution. Familiarize yourself with the normal appearance of his eyes so you can recognize abnormal dilation if it occurs. Consult a veterinarian if you notice any change in the size of your dog's pupils, especially if other unusual symptoms are present.

Eye Disorders


As a common cause of blindness in older dogs, glaucoma is a prevalent condition that can affect any breed. It's caused by increased pressure inside the eye, which forces the aqueous humor out of place. This change results in redness, itchiness and inflammation of the eye, and is rather uncomfortable for your pet. Abnormal dilation of the pupil, even when exposed to bright light, is another classic sign of glaucoma.

Retinal Degeneration

Progressive retinal atrophy is less painful and irritating than glaucoma, but it is also a serious condition that threatens your dog's vision. While the gradual degeneration inevitably leads to severely limited vision as the dog ages, it does not cause any life-threatening issues. The damage from degenerative disorders usually is not repairable.

Neurological Problems


Also known as epilepsy or convulsions, seizures are a complex neurological issue caused by excessive activity of neurons in the brain. These violent episodes provoke muscle spasms, involuntary movement and alarming changes in behavior. Grand mal seizures, which encompass the entire body, can cause dilation of the pupils as well as incontinence and drooling, according to Long Beach Animal Hospital in Long Beach, California.

Seizures may be primary or idiopathic, with no discernible cause, or a single symptom of an underlying condition.


Tumor growth in the brain or inside the eye itself can cause a variety of ocular symptoms, including unnatural dilation of one or both pupils. Suprasellar germ cell tumors, which usually form near the pituitary pituitary gland, are particularly detrimental to your dog's eyes. Unresponsive, enlarged pupils are a common symptom of this disorder. Cancerous growth can cause behavioral changes, lethargy and diminished eyesight, as well as other symptoms.

General Causes


Poisoning from toxic substances can cause visible changes to your dog's eye function, including enlargement of the pupils. Cyanide-containing stone fruit pits and horse chestnuts are both known to dilate the pupils of dogs when ingested, according to Granville Island Veterinary Hospital in Vancouver, British Columbia. A variety of toxins that impact the central nervous system can cause pupil enlargement, including the foxglove flower, scorpion venom and marijuana.

Ask your vet about possible side effects of any medication he prescribed to your pet. Some of these substances, such as the stimulant phenylpropanolamine, cause pupil dilation as a side effect.


Changes in living circumstances, like the addition of a new family member or pet, are among the many events that can trigger stress in your dog. Pets are prone to this condition just like people, and it can have consequences on their overall health. Stress can cause dilation of the pupils, as well as other symptoms, including:

  • Skin damage and hair loss from excessive grooming.
  • Repetitious activity, such as jumping or pacing.
  • Nervous behavior, including frequent lip-licking and whining.
  • Digestive irregularity.

The signs of stress may be subtle and difficult to diagnose on your own. Pay attention to any changes in your dog's behavior, especially when there are big changes going on in his life.

Congenital Deformity

Your dog's enlarged pupil may not be the result of any current health condition, but rather a product of a congenital defect. In some puppies, the iris of the eye does not completely form, which can result in a unilateral or bilateral change in pupil size, according to VCA Animal Hospitals.

Traumatic Injury

Damage to the head or eye can have immediate or long-term impact on organ function. Damage to the ocular nerves or eye tissue can cause serious vision loss, pupil dilation and changes in behavior. Scar tissue from these injuries can have a lasting impact as well, which may lead to seizures and other neurological issues down the road.