Glaucoma in dogs is caused by a buildup of fluid inside the eye that can’t drain properly, raising internal eye pressure beyond the normal range of 20 to 28 mmHg. This pressure damages the optic nerve and, without treatment, leads to pain and blindness. The underlying reasons for that drainage failure fall into two broad categories: inherited structural problems the dog is born with (primary glaucoma) and damage from another eye disease (secondary glaucoma).
How Fluid Buildup Damages the Eye
A dog’s eye constantly produces a watery fluid called aqueous humor that nourishes internal structures and maintains the eye’s shape. In a healthy eye, this fluid drains out through a tiny mesh of tissue at the angle where the iris meets the cornea, keeping pressure balanced. Glaucoma develops when something blocks or slows that drainage. Fluid keeps being produced but has nowhere to go, so pressure climbs. When it exceeds roughly 40 to 50 mmHg, the situation becomes an emergency requiring immediate treatment to prevent permanent nerve damage.
Primary Glaucoma: Inherited Drainage Defects
Primary glaucoma occurs when a dog’s drainage system is structurally abnormal from birth, even though no other eye disease is present. The most common form in dogs is primary closed-angle glaucoma, which is linked to a developmental defect called pectinate ligament abnormality. This is a malformation of the tiny fibers that span the drainage angle inside the eye. Instead of forming an open, comb-like structure that lets fluid pass through freely, these fibers are thickened, fused, or sheet-like, progressively restricting outflow. A veterinary ophthalmologist can detect this abnormality using a specialized lens exam called gonioscopy, sometimes years before pressure actually rises.
A less common inherited form, primary open-angle glaucoma, involves a drainage angle that looks normal on exam but functions poorly at the microscopic level. This type tends to progress more slowly and has been documented in Beagles, Petit Basset Griffon Vendéens, and Norwegian Elkhounds.
Primary glaucoma almost always affects both eyes eventually. If one eye is diagnosed, the second eye carries a high risk of developing the same problem over time.
Breeds at Higher Risk
At least 42 breeds are considered predisposed to primary glaucoma. In North America, the highest-risk breeds include the American Cocker Spaniel, Basset Hound, Chow Chow, Shar-Pei, and Boston Terrier. Shih Tzus, Beagles, and Shiba Inus show up frequently in studies across multiple countries. Breeds particularly prone to pectinate ligament abnormalities include English Springer Spaniels, Flat-Coated Retrievers, Great Danes, and Samoyeds.
Other breeds documented in clinical studies include Maltese, Pomeranians, Pekingese, Toy Poodles, Miniature Pinschers, Yorkshire Terriers, Labrador Retrievers, Siberian Huskies, Newfoundlands, and Vizslas. Mixed-breed dogs are not immune and appear in glaucoma case records regularly. If your dog belongs to a predisposed breed, screening exams by a veterinary ophthalmologist can catch drainage angle problems before pressure rises.
Secondary Glaucoma: Other Eye Diseases That Block Drainage
Secondary glaucoma develops when a separate eye condition physically obstructs the drainage pathway. It’s the more common form overall, and unlike primary glaucoma, it often affects only one eye (whichever eye has the underlying disease). A study reviewing 156 cases of secondary glaucoma identified the leading triggers and their frequency:
- Uveitis (inflammation inside the eye): By far the most common cause, responsible for about 45% of cases that weren’t related to surgery. Inflammation produces debris, proteins, and inflammatory cells that clog the drainage angle. Uveitis following cataract surgery accounted for an additional 19% of cases.
- Lens dislocation: About 15% of cases. When the lens slips out of position, it can physically block the pupil or push against the drainage angle. Vitreous gel from behind the lens often adheres to the lens capsule, creating a seal that traps fluid behind the iris.
- Intraocular tumors: Roughly 11% of cases. Tumors inside the eye can grow large enough to obstruct drainage directly or trigger inflammation that does so indirectly.
- Bleeding inside the eye (hyphema): About 3% of cases. Blood cells and clots can physically block the drainage meshwork.
- Cataracts: Around 2.5% of cases. A swelling, mature cataract can push the iris forward into the drainage angle, cutting off outflow.
Other recognized triggers include intraocular cysts and a condition called ocular melanosis (or pigmentary glaucoma), where pigment-laden cells gradually infiltrate and clog the drainage tissue. This pigmentary form is especially well known in Cairn Terriers and some other breeds.
Why Uveitis Is the Leading Trigger
Uveitis deserves special attention because it drives nearly half of all secondary glaucoma cases. It refers to inflammation of the middle layer of the eye, which includes the iris and surrounding structures. Uveitis can result from infections (tick-borne diseases, fungal infections), immune-mediated conditions, trauma, or lens protein leaking through a damaged lens capsule. When the eye becomes inflamed, the drainage angle gets flooded with proteins, white blood cells, and fibrin. These substances physically block fluid outflow. In severe or chronic cases, scar tissue forms across the drainage angle, creating a permanent blockage even after the inflammation resolves.
Dogs that undergo cataract surgery are at particular risk. The surgical procedure itself can trigger inflammation, and even with careful postoperative management, some dogs develop enough uveitis to push eye pressure into the danger zone.
How Glaucoma Is Managed
Treatment focuses on lowering eye pressure as quickly as possible and maintaining it within a safe range long-term. Most dogs are started on eye drops that work through different mechanisms. Some reduce the amount of fluid the eye produces, while others help open alternative drainage routes. Your vet may prescribe a combination of drops to attack the problem from multiple angles, and these typically need to be applied two to three times daily.
In an acute crisis, where pressure spikes suddenly and the dog is in obvious pain (squinting, tearing, a visibly red or cloudy eye, dilated pupil), emergency treatment aims to bring pressure down within hours. This often involves intravenous medications that pull fluid out of the eye osmotically, combined with aggressive topical drops.
When medications alone can’t control pressure, surgical options exist. These range from procedures that reduce fluid production inside the eye to implants that create new drainage pathways. For eyes that have already lost vision and remain painful despite treatment, removal of the eye is sometimes the most humane choice, and dogs adapt to life with one eye remarkably well.
What Makes Some Cases Progress Faster
The speed at which glaucoma causes permanent damage varies significantly. Acute closed-angle glaucoma, the type most common in predisposed breeds, can destroy vision within 24 to 48 hours if pressure isn’t brought under control. The optic nerve and retina are extremely sensitive to sustained high pressure. Secondary glaucoma from chronic, low-grade uveitis may progress more gradually, but the damage is cumulative and often irreversible by the time it’s detected.
Dogs don’t communicate vision changes the way people do, so early signs are easy to miss. A slightly enlarged or cloudy eye, increased redness, visible third eyelid, or behavioral changes like bumping into objects on one side may be the first clues. In breeds known to be at risk, regular pressure checks during routine vet visits can catch rising pressure before clinical signs appear, giving you the best chance of preserving your dog’s sight.