The Canine Ledger

Glaucoma: The Red, Painful Eye That Can't Wait

Sudden glaucoma can cost a dog the sight in an eye. The signs, which dogs are at risk, why the second eye matters, and what treatment can and can't save.

Written by thecanineledger

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If one of your dog’s eyes is suddenly red, cloudy or bluish, has a large pupil, or your dog is squinting, rubbing it or holding it shut: see a vet now. Phone ahead so the clinic can check the eye pressure as soon as you arrive. Don’t wait to see whether it clears by morning.

In a clinical trial of dogs who had already lost one eye to sudden glaucoma, the second eye in untreated dogs developed glaucoma after a median of eight months. In dogs given preventive eye drops, it took a median of about 31 months (Miller et al., Journal of the American Animal Hospital Association 2000, abstract).

That is the part of glaucoma most owners don’t hear about. The first eye is an emergency. The second eye is a plan.

While a red eye can have many causes, sudden glaucoma is one where hours matter, and an owner can’t tell the difference at home. This page covers the signs, which dogs are most at risk, what treatment can realistically save, and what happens next for the other eye.

1. What glaucoma is

The front of the eye is filled with fluid that is constantly made and drained. Glaucoma happens when it can’t drain properly, so the pressure inside the eye rises. The Merck Veterinary Manual’s owner page explains that “The increased pressure can cause the destruction of the retina and optic disk”, the parts of the eye that sense light and carry the signal to the brain (Merck, Glaucoma in Dogs).

There are two forms owners need to know about:

  • Sudden (closed-angle) glaucoma. Merck: “a sudden increase in eye pressure with severe pain, redness, and loss of vision.” This is the emergency.
  • Slow (open-angle) glaucoma. Merck: “a painless and gradual development of blind spots or loss of vision over a long period of time.”

Glaucoma can also be primary, meaning inherited, with no other eye disease behind it, or secondary, caused by another eye problem such as a cataract, inflammation, or a lens that has slipped out of place.

A 2023 review describes primary glaucoma as “a painful, progressive, and blinding disease reported in many canine breeds” (Sebbag & Pe’er, Animals 2023).

2. The signs

Merck lists what a vet sees in sudden, severe glaucoma: “a dilated, unmoving or slow-moving pupil; redness in the white of the eyes; swelling and discoloration of the cornea; and a firm eye globe.” In plain terms:

What you may seeWhat it is
A pupil that looks bigger than the other one, or doesn’t shrink in lightDilated, sluggish pupil
Red, bloodshot white of the eyeCongested blood vessels
A cloudy, hazy or bluish front of the eyeSwelling of the cornea
Squinting, rubbing, pawing, or keeping the eye shutPain
A dog who seems dull, hides, or won’t be touched near the headPain, shown as behaviour
An eye that looks bigger than the otherStretching from long-standing pressure

Pain doesn’t always look like pain. Merck notes that pain “usually shows itself as behavioral changes and occasional pain around the eye rather than by spasmodic winking.” A dog who is quiet and off their food, with one odd-looking eye, may be in a great deal of pain.

The early signs of slow glaucoma are easy to miss. Merck says most dogs with early long-term glaucoma “are not taken to the veterinarian because the early signs—sluggish to slightly dilated pupils, mild congestion of the veins in the conjunctiva, and early enlargement of the eye—are so subtle that owners are not aware of the changes.”

Merck’s instruction is plain: “If your dog displays any signs of glaucoma, it should be seen by a veterinarian immediately.”

You can’t tell glaucoma from other red eyes at home. An ulcer, a scratch or inflammation can look similar, and some of them can lead to glaucoma. The only way to know is for a vet to examine the eye and measure the pressure. That is why we say go now, for any eye that is suddenly red, cloudy and painful.

Our note: We say see a vet now for any suddenly red, cloudy or painful eye. Merck says “immediately” for signs of glaucoma.

→ Emergency Signs in Dogs: When to Go Now, Not Tomorrow

3. Which dogs are at risk

Glaucoma is not rare. Merck’s owner page says it “occurs in about 1.7% of the dogs in North America”, counting both inherited and secondary forms.

Some breeds carry far more risk. The largest breed study we found used North American veterinary school records. For 1994 to 2002, the breeds with the highest rates of primary glaucoma were (Gelatt & MacKay, Veterinary Ophthalmology 2004, abstract):

BreedShare with primary glaucoma
American Cocker Spaniel5.52%
Basset Hound5.44%
Chow Chow4.70%
Shar-Pei4.40%
Boston Terrier2.88%
Wire Fox Terrier2.28%

The same study found that in most breeds, glaucoma was first diagnosed “in dogs between 4 and 10 years of age.”

Read these figures with care. They come from dogs seen at teaching hospitals more than 20 years ago, and referral hospitals see more serious cases. They show which breeds are most affected, not the risk for a dog in your home today.

Cataracts are a risk too. A companion study found that glaucoma linked to cataracts made up 81% of secondary glaucomas, and that in predisposed breeds “nearly 20% of all the cataractous dogs developed secondary glaucoma in at least one eye” (Gelatt & MacKay, Veterinary Ophthalmology 2004, abstract). If your dog has cataracts, ask your vet whether the eye pressure should be checked.

Ask for pressure checks in high-risk breeds. Merck’s professional chapter advises that “repeated tonometry should be routinely performed on high-risk breeds of dogs as part of the annual, general physical examination” (Merck, Glaucoma in Animals). Tonometry is the pressure measurement.

Our note: The breed table comes from 1994–2002 teaching-hospital records. Later work may add other breeds.

→ Choosing a Breed or a Mix: The Health Data Breeders Don’t Lead With

4. What treatment can, and can’t, save

Speed is the point of treatment. Merck’s owner page: “It is important to decrease the pressure within the eye as quickly as possible in order to minimize damage.” Vets use drugs that draw fluid out of the eye and drugs that slow its production, and often surgery as well. “Most glaucomas require longterm management.”

Treatment doesn’t always save sight. Merck’s professional chapter notes that in dogs, surgery to create a new drainage route has “traditionally provided only short-term resolution”, because the channels scar over, though newer drainage devices have “offered better results.” We did not find a reliable figure for how often sight is saved after sudden glaucoma.

The blind, painful eye. When an eye has lost its sight and is still painful, Merck says that “Dogs with end-stage glaucoma with an enlarged, blind eye may need for it to be removed by surgery to relieve pain.” That can sound drastic. It is often the kindest option, because a blind eye with high pressure keeps hurting, and most dogs manage well with one eye.

Our note: “Most dogs manage well with one eye” is widely said by vets. We didn’t find a source that says it.

5. The other eye

Inherited glaucoma usually affects both eyes. Merck’s professional chapter notes that “Bilateral primary breed-predisposed glaucoma occurs in nearly 1% of purebred dogs.” Often one eye goes first.

Preventive drops can buy time. In the trial described at the top, preventive treatment delayed glaucoma in the second eye from a median of eight months to about 31. A 2023 review gathered the studies on this. In the two it lists with an untreated group, the second eye developed glaucoma after a median of 5 and 8 months. With treatment, the medians ranged from 195 days to about 31 months across studies.

Preventive treatment delays it. It may not stop it. The trial’s own summary says the drops “delayed or prevented the onset of glaucoma in the second eye.” Ask your vet what the plan is for the other eye, and how often it should be checked.

The rise can be sudden, even when you’re watching. In a 2026 pilot study, owners of 14 at-risk dogs measured eye pressure at home (Westermeyer & Salmon, Veterinary Ophthalmology 2026). In seven of the nine dogs that went on to develop glaucoma, it came with “a sudden rise in IOP > 50 mmHg that was not preceded by an obvious gradual rise in average IOP readings.” IOP is intraocular pressure. The second eye can go suddenly, so know the signs and act on them at once.

Our note: Home pressure monitoring was a 14-dog pilot. We use it only to show that the rise can come without warning.

6. What we could not verify

Commonly saidWhat we found
“A dog with glaucoma will go blind within 24 to 48 hours”We found no veterinary source for that timeline. The sources say to lower the pressure as fast as possible, without giving a deadline.
“A red eye is just conjunctivitis”You can’t tell at home. Glaucoma, ulcers and inflammation can look alike.
“Glaucoma is a disease of old dogs”Not only. In the breed study, most dogs were first diagnosed between 4 and 10 years of age.
“Treating the other eye will prevent glaucoma”It delays it, and may prevent it. Studies report a median delay, not a guarantee.
“Removing the eye is a last resort for owners who give up”Not how the sources describe it. Merck describes removal of a blind, enlarged eye as a way to relieve pain.
How often treatment saves sightWe found no reliable figure in the sources we read.

Sources

Always confirm with your vet before acting on anything on this page.