CONDITION
Glaucoma
Glaucoma describes a group of conditions in which pressure inside the eye becomes elevated, often quite rapidly. The pressure builds when the fluid that normally circulates through the eye cannot drain away properly. Left unchecked, that pressure damages the optic nerve and the light-sensitive structures at the back of the eye, and vision can be lost within hours to days. Owners often notice a red, cloudy, or enlarged eye, sometimes with the dog or cat squinting, holding the eye partly closed, or showing signs of discomfort. The eye may look hazy or bluish, and the pupil may be dilated and unresponsive to light. In many cases the problem starts in one eye, though the risk to the fellow eye can be substantial — without preventive treatment, roughly half of dogs will develop glaucoma in the second eye within about eight months. This page explores the signals that may point towards raised eye pressure, the mechanisms underneath, how the condition is identified and monitored, and the range of approaches used to manage pressure, preserve comfort, and protect remaining vision where possible.
Why this matters now
Glaucoma can appear at any age, but certain patterns are common. Primary glaucoma—where the drainage structures are abnormal from birth—often emerges in middle-aged to older dogs, particularly in breeds such as Cocker Spaniels, Basset Hounds, and certain terriers. Secondary glaucoma, triggered by another eye problem such as inflammation, lens displacement, or tumour, can occur at any stage of life and is the more frequent form in cats. In many cases, the first eye to be affected signals a risk to the fellow eye, with a median time to fellow-eye involvement of about eight months without preventive measures.
The course varies widely depending on the underlying cause and how quickly pressure rises. Acute glaucoma can cause severe pain and vision loss within hours to days, while chronic forms may progress more slowly, with pressure building over weeks or months. Some animals lose vision in the affected eye before owners notice outward changes; others are brought in early because the eye looks overtly uncomfortable. Even with treatment, the condition may continue to evolve, and the risk to the second eye remains a consideration throughout.
Signals & patterns
Early signals
Redness in the white of the eye
The sclera may appear pink or bloodshot, often most visible near the edge of the cornea. This reflects congestion of small blood vessels as pressure inside the eye rises and tissues become stressed.
Cloudiness over the cornea
The normally clear front window of the eye can take on a hazy, bluish, or steamy appearance. This occurs when raised pressure forces fluid into the corneal layers, disturbing their usual transparency.
Squinting or holding the eye partly closed
The animal may blink more often, keep the eyelids narrowed, or avoid bright light. This is a response to discomfort, which can range from mild to severe depending on how quickly pressure has risen.
Tearing or discharge from the eye
Excessive watering, sometimes with a clear or slightly mucoid discharge, can accompany the irritation. Owners may notice wetness on the fur below the eye or a damp appearance around the lashes.
Pupil appears larger or less responsive
The pupil in the affected eye may be wider than the other and slower to constrict in bright light. Raised pressure can interfere with the muscles that control pupil size, leading to this asymmetry.
Later signals
Eye appears enlarged or bulging
Sustained high pressure can stretch the globe, particularly in younger animals whose eye walls are more elastic. The eye may look noticeably bigger than its fellow, a change called buphthalmos or 'ox eye'.
Loss of vision in the affected eye
The animal may bump into objects on one side, hesitate in dim light, or show reduced response to movement in the visual field of that eye. By this stage, damage to the retina and optic nerve may be advanced.
Change in behaviour or appetite
Persistent pain can lead to quietness, reluctance to play, reduced interest in food, or irritability. These signs are often subtle and may be attributed to other causes unless the eye itself is examined.
Hardness or firmness on gentle touch
If pressure is very high, the eye may feel firm compared to the normal eye when very gently palpated through closed lids. This is not a test owners are encouraged to perform, but some notice the difference incidentally.
Click to read about the biological mechanisms
How this is usually investigated
Investigation begins with observation of the eye itself—its size, clarity, redness, and the appearance of the pupil—alongside questions about when changes were first noticed and whether the animal shows signs of discomfort or altered vision. Measurement of the pressure inside the eye follows, often within minutes of arrival, particularly when the eye appears painful or clouded. Further tests then help distinguish whether the raised pressure arose on its own or as a consequence of another eye condition, and whether the structures responsible for drainage and vision remain intact.
Tonometry
Gonioscopy
Ophthalmic examination
Assessment of the unaffected eye
Ocular ultrasound
Options & trade-offs
Management is shaped by whether the eye retains vision, how quickly the pressure rose, and what the owner hopes to achieve—comfort alone, or comfort with the possibility of preserving sight. Different combinations of medical, laser, and surgical approaches suit different animals and circumstances, and what works in one eye or one household may not be practical in another. Many animals are managed with more than one approach over time as the disease evolves.
Medical pressure reduction
Eye drops and sometimes oral medications work to lower intraocular pressure by reducing fluid production, increasing drainage, or both. Drugs may include prostaglandin analogues, carbonic anhydrase inhibitors, beta-blockers, and osmotic agents. Treatment is typically frequent—drops two or three times daily, sometimes more in the early phase—and often involves multiple medications at once.
Trade-offs: Medical control can preserve vision and comfort in some eyes, particularly when started early and when pressure drops meaningfully within hours. Many eyes, however, do not respond durably to medication alone, and pressure may rise again despite treatment. Compliance can be difficult for owners and stressful for animals that resist handling, and the cost of multiple medications over months or years adds up.
Laser cyclophotocoagulation
A laser is directed at the ciliary body, the structure inside the eye that produces aqueous humour, to reduce fluid production by controlled destruction of some of its tissue. The procedure is performed under general anaesthesia and may be repeated if pressure rises again.
Trade-offs: Laser treatment can lower pressure in eyes that have not responded to medication, and it may reduce or eliminate the need for drops. The effect is not always predictable—some eyes require more than one session, and some do not respond at all. Inflammation after the procedure can be uncomfortable, and there is a risk of further damage to an already compromised eye.
Surgical drainage procedures
Operations that create a new pathway for fluid to leave the eye—such as placement of a shunt or valve—aim to lower pressure when medical and laser approaches have not succeeded. These procedures are typically carried out by veterinary ophthalmologists and require general anaesthesia and careful postoperative monitoring.
Trade-offs: Drainage surgery offers the possibility of sustained pressure control in eyes that still have vision, but success rates vary, and complications such as infection, shunt blockage, or bleeding can occur. Not all eyes are candidates, and the cost and need for specialist referral make this option less accessible for some owners.
Enucleation
Surgical removal of the eye eliminates the source of pain when vision is already lost and pressure cannot be controlled by other means. The procedure is performed under general anaesthesia, and the eyelids are usually closed over the socket; some surgeons offer a cosmetic implant to fill the space.
Trade-offs: Enucleation provides definitive relief from pain and removes the need for ongoing medication or monitoring of a blind, uncomfortable eye. Many owners are reluctant to consider it, and the change in appearance can be difficult to accept, though most animals adapt well and quickly to monocular vision if the other eye remains healthy.
Prophylactic treatment of the fellow eye
When primary glaucoma is diagnosed in one eye, medication may be started in the unaffected eye to delay or prevent pressure elevation. The approach typically involves a topical prostaglandin drop applied once daily, aiming to keep the drainage angle open and lower baseline pressure.
Trade-offs: Prophylactic treatment can extend the interval before the second eye develops glaucoma, and some eyes may never progress. Others will develop raised pressure despite treatment, and the benefit of long-term medication in an eye that may remain normal for years must be weighed against the effort and expense involved.
Common misconceptions
"If the eye looks normal and the dog is not squinting, glaucoma cannot be present."
Some dogs with chronically elevated pressure show surprisingly subtle outward signs, particularly in the early stages or in breeds where the eye anatomy makes changes less obvious. Pain may be expressed as quietness, reduced appetite, or reluctance to play rather than visible squinting. Measurement of intraocular pressure is the only reliable way to detect raised pressure before the eye becomes visibly abnormal.
"Once the pressure is lowered with drops, the eye will recover and treatment can be stopped."
Lowering pressure quickly can halt further damage and relieve pain, but damage to the retina and optic nerve that has already occurred is typically permanent. Most eyes require ongoing treatment to keep pressure controlled, and even with consistent medication, some eyes lose vision or become painful again over time.
"Glaucoma only affects old dogs."
Primary glaucoma often appears in middle-aged dogs—commonly between four and nine years old—and some breeds develop it earlier. Secondary glaucoma can occur at any age, depending on the underlying cause. Age alone does not rule glaucoma in or out.
Related conditions
Corneal Ulcers
Corneal ulcers can develop as a consequence of glaucoma, particularly when raised intraocular pressure stretches the cornea or when reduced tear production and discomfort lead to reduced blinking and self-trauma. Conversely, a deep or infected corneal ulcer may itself trigger secondary glaucoma if inflammation spreads into the interior of the eye.
Uveitis
Uveitis—inflammation inside the eye—can be both a cause and a consequence of glaucoma. Inflammatory debris and changes in fluid drainage may raise intraocular pressure, and conversely, sustained high pressure can trigger or worsen inflammation, creating a cycle that complicates management of both conditions.
Lens Luxation
Lens luxation occurs when the lens shifts out of its normal position, and this displacement can block the drainage pathways inside the eye, leading to a rapid rise in intraocular pressure and secondary glaucoma. In some cases, chronic glaucoma may weaken the fibres holding the lens, making luxation more likely over time.
Retinal Detachment
Glaucoma can lead to retinal detachment when prolonged high pressure disrupts the blood supply or structural integrity of the retina, or when rapid changes in pressure cause fluid to accumulate beneath the retinal layer. Retinal detachment may also complicate the management of glaucoma, particularly after surgical intervention.
Hyphaema
Hyphaema—blood in the front chamber of the eye—can obstruct the normal drainage of fluid, leading to a secondary rise in intraocular pressure and glaucoma. Glaucoma itself, especially when pressure rises suddenly, may cause small blood vessels in the eye to rupture, resulting in hyphaema.
Animals with glaucoma in one eye live with the possibility that the second eye may eventually be affected, and understanding what early changes might look like—dilated pupil, slight cloudiness, or subtle discomfort—can be useful for owners who monitor their pet closely. The broader Pain & Mobility section explores other conditions that may alter behaviour or activity in ways that overlap with eye pain, and some owners find it helpful to read about lens luxation or uveitis if those terms have come up during investigation. The timeline and goals of treatment often evolve as the disease does, and conversations about what is working, what has changed, and what feels manageable tend to be revisited more than once.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS