CONDITION
Glaucoma in Dogs
Glaucoma describes a group of conditions in which pressure inside the eye rises above the range the eye can tolerate. That pressure can build quickly or gradually. When it rises suddenly, the retina and optic nerve can be damaged beyond recovery within hours to days, and eyes seen within the first three days of signs are more likely to still have sight than those seen later. When it builds slowly, vision fades over weeks to months. Owners often notice a red, cloudy, or enlarged eye, sometimes with squinting or signs that suggest discomfort. In other cases the eye may look subtly different: slightly wider, or with a pupil that does not respond to light as the other does. Primary glaucoma often starts in one eye, and without preventive drops the second eye followed after a median of about eight months in one study. At home, the red eye of glaucoma can look very like conjunctivitis or uveitis, and eye pressure cannot be judged by looking at or touching the eye; it is measured with a tonometer. This page explores what glaucoma can look like in the early and later stages, what drives the rise in pressure, how it is measured and investigated, and the range of approaches used to manage it. The aim is to help you understand what may be happening and what questions may be worth exploring.
Why this matters now
Glaucoma in dogs can present at any age but is most commonly recognised in middle-aged to older dogs. Primary glaucoma, caused by inherited anatomical abnormalities in the drainage angle of the eye, tends to present in breeds with known predispositions, often affecting one eye initially before the second eye becomes involved weeks to years later. Secondary glaucoma, which develops as a consequence of other ocular disease such as uveitis, lens luxation, or intraocular tumours, can occur in any breed at any age. The condition can develop acutely, with a dramatic rise in intraocular pressure over hours, or may build insidiously over weeks to months in chronic forms.
The trajectory of glaucoma depends on whether it presents acutely or chronically and how rapidly the elevated pressure is addressed. Acute glaucoma produces a sudden, significant rise in intraocular pressure that can cause irreversible damage to the retina and optic nerve within hours to days. In one study of 65 Shiba Inus (104 eyes) with primary angle-closure glaucoma, 87% of eyes seen within 72 hours of the first signs still had sight, compared with 44% of those seen later. Where an eye is already blind, the outlook for sight returning is guarded. Vision is often already lost by the time glaucoma is recognised, because the early signs are easy to miss and a dog copes well using the other eye. Drops alone usually become less effective over time in primary glaucoma, and pressure tends to rise again despite treatment: in the same study, only about 8% of sighted eyes managed with medical treatment alone still had sight at 12 months. The retinal ganglion cells and their axons, which form the optic nerve, are particularly vulnerable to pressure-induced ischaemia. Chronic glaucoma may produce more gradual damage, with progressive vision loss occurring over weeks to months as the sustained pressure gradually compromises retinal function. In many cases, particularly with primary glaucoma, the second eye is considered at high risk of developing the condition, and prophylactic management may be discussed. Advanced glaucoma can lead to buphthalmos, physical enlargement of the globe, as the sustained pressure stretches the scleral wall.
Signals & patterns
Early signals
Subtle redness of the eye
Episcleral congestion, which is dilation of the blood vessels on the white of the eye, can make the white look red, with thick, dark vessels. At home this looks much like the redness of conjunctivitis; separating the two relies on examining which vessels are involved and on measuring eye pressure. The redness may come and go if pressure fluctuates.
Mild squinting or discomfort
Dogs may show intermittent blepharospasm or reluctance to have the area around the eye touched. The discomfort may be subtle, with the dog appearing to squint more in bright light or holding the affected eye slightly narrower than normal.
Slight corneal cloudiness
Early corneal oedema from elevated intraocular pressure may produce a faint bluish haze over the corneal surface. This may be most noticeable in certain lighting conditions and can fluctuate if pressure is intermittently elevated.
Subtle behavioural changes
Dogs in the early stages may show decreased activity, reduced appetite, or seem quieter than usual. These non-specific signs are often attributed to other causes, making early glaucoma easily overlooked without specific ophthalmic evaluation.
Later signals
Obvious corneal oedema
A dense bluish-white cloudiness over the entire corneal surface indicates significant oedema from sustained pressure elevation. The cornea may appear uniformly hazy, obscuring the view of the iris and pupil. In an acute pressure rise this haze can develop within hours rather than weeks.
Fixed, dilated pupil
Elevated intraocular pressure can paralyse the iris sphincter muscle, producing a pupil that is dilated and unresponsive to light. This mydriasis may be the first change noticed by owners, particularly if it produces asymmetry between the two eyes. In an acute pressure rise it can appear within hours, and a wide pupil in a red, cloudy, painful eye is the combination most typical of glaucoma; in uveitis, which can look similar, the pupil is more often small.
Visibly enlarged eye
Chronic elevation of intraocular pressure can stretch the sclera, producing buphthalmos, a visibly enlarged globe. The eye may appear to protrude more than the other and the eyelids may not close completely over the enlarged surface.
Evidence of vision loss
Dogs may bump into objects on the affected side, show reduced confidence in unfamiliar environments, or fail to track visual stimuli. Vision loss may be particularly noticeable in dim lighting if the retinal damage has progressed significantly.
Click to read about the biological mechanisms
How this is usually investigated
Investigating glaucoma centres on measuring intraocular pressure and assessing the structures within the eye that regulate aqueous humour dynamics, as well as evaluating the degree of damage to pressure-sensitive tissues.
Tonometry
Gonioscopy
Ophthalmoscopy
Ocular ultrasound
Electroretinography (ERG)
Options & trade-offs
Management of glaucoma in dogs aims to reduce intraocular pressure to a level that preserves retinal and optic nerve function while maintaining comfort. The approach depends on whether the eye retains visual potential, whether the glaucoma is primary or secondary, and the individual's response to initial treatment.
Topical pressure-lowering medications
Various eye drops can reduce intraocular pressure by either decreasing aqueous humour production or improving its outflow. Classes include prostaglandin analogues, carbonic anhydrase inhibitors, beta-blockers, and combinations thereof. None of these drops has a UK veterinary licence for dogs; they are human medicines prescribed under the veterinary medicines cascade. Multiple drops may be used concurrently to achieve adequate pressure reduction.
Trade-offs: Long-term topical therapy requires consistent, often frequent administration which demands significant owner commitment. Some medications cause local side effects such as miosis (pupil constriction), conjunctival hyperaemia, or discomfort on application. Prostaglandin analogues, which lower pressure quickly in primary glaucoma, can worsen glaucoma caused by uveitis or by a lens that has moved forward into the front of the eye, so the underlying cause shapes which drops are suitable. The effectiveness of medical management alone often diminishes over time, particularly in primary glaucoma, as the underlying drainage abnormality persists.
Systemic pressure-lowering medications
Intravenous mannitol or oral carbonic anhydrase inhibitors may be used in acute glaucoma crises to rapidly reduce intraocular pressure while other treatments are initiated. Mannitol and the oral carbonic anhydrase inhibitors are human medicines with no veterinary licence for dogs in the UK and are given under the veterinary medicines cascade.
Trade-offs: Systemic medications can have broader side effects including increased urination, electrolyte changes, and gastrointestinal disturbance. Mannitol requires intravenous administration in a clinical setting and provides only temporary pressure reduction. These are typically bridge therapies rather than long-term solutions.
Laser cyclophotocoagulation
A laser is used to selectively destroy a portion of the ciliary body tissue that produces aqueous humour, reducing the overall rate of fluid production within the eye. This can be performed using transscleral or endoscopic approaches.
Trade-offs: The degree of pressure reduction is variable and difficult to predict precisely, as the amount of ciliary body tissue that needs to be treated varies between individuals. Over-treatment can lead to persistently low pressure and a shrunken, non-seeing eye (phthisis bulbi), while under-treatment may require repeat procedures. The procedure typically requires general anaesthesia.
Gonioimplant (drainage device)
A small tube or shunt is surgically placed to provide an alternative drainage pathway for aqueous humour, bypassing the dysfunctional natural drainage angle.
Trade-offs: These devices require specialist surgical placement and can be associated with complications including tube obstruction, implant migration, or excessive drainage. Long-term success rates vary, and the devices may eventually require revision or the addition of medical therapy.
Enucleation or intrascleral prosthesis
When an eye is irreversibly blind and painful, removal of the eye (enucleation) or placement of a cosmetic intrascleral prosthesis may be considered to eliminate ongoing pain. The prosthesis maintains the external appearance of an eye within the scleral shell.
Trade-offs: Enucleation removes the source of pain permanently, but involves adapting to the altered appearance. An intrascleral prosthesis preserves cosmetic appearance but requires that the outer shell of the eye is healthy and that an intraocular tumour or infection is not suspected. Both options eliminate the need for ongoing glaucoma medication in the affected eye. Dogs typically adapt well to monocular vision.
Preventive treatment of the second eye
When primary glaucoma is diagnosed in one eye, daily drops may be started in the other eye to delay a pressure rise. The drops most often used reduce fluid production inside the eye, such as dorzolamide, timolol or a combination of the two. In the UK these are human medicines with no veterinary licence for dogs, so they are prescribed under the veterinary medicines cascade.
Trade-offs: In one multicentre trial using older drops, untreated second eyes developed glaucoma after a median of about eight months, compared with around 31 months in treated eyes. Preventive drops delay rather than reliably prevent the disease, so a treated eye can still develop a sudden pressure rise, and treatment involves daily handling and ongoing cost.
Common misconceptions
"Glaucoma in dogs is the same as glaucoma in humans"
While both involve elevated intraocular pressure, canine glaucoma differs significantly from the most common form of human glaucoma. Human open-angle glaucoma typically progresses very slowly over years, while primary glaucoma in dogs often involves closed-angle or narrow-angle mechanisms that can produce rapid, dramatic pressure elevations. Sudden angle-closure attacks do occur in people but are much less common than the slow open-angle form. Additionally, canine glaucoma tends to be more aggressive and harder to control medically, with a higher proportion of cases eventually requiring surgical intervention or enucleation.
"If the eye looks normal, there cannot be a serious pressure problem"
Intraocular pressure can be significantly elevated before visible external changes develop. Early glaucoma may produce only subtle signs such as slight episcleral injection or mild corneal haze that are difficult to detect without specific examination. Obvious corneal oedema or a wide pupil usually means the pressure is already very high; in a sudden rise these signs can appear within hours, at a stage when sight can still be saved in some eyes. Buphthalmos means the pressure has been high for some time, and the eye is usually already blind. This is why the diagnosis rests on tonometry, the actual measurement of eye pressure.
"Once pressure is controlled, the eye is safe"
Controlling intraocular pressure is the aim of treatment, but it does not remove the underlying cause in primary glaucoma. The inherited drainage angle abnormality persists, meaning that pressure can escape control as the disease progresses or treatment effectiveness wanes. Many dogs with primary glaucoma require escalating treatment over time, and a significant proportion will eventually lose functional vision despite management. The emphasis on pressure control is about slowing the rate of damage and maintaining comfort, not necessarily curing the condition.
Related conditions
Corneal Ulcers
Corneal ulcers can occur as a consequence of glaucoma, particularly when an eye enlarged by sustained pressure can no longer be fully covered by the eyelids and the corneal surface is left exposed, and infected ulcers are among the recognised causes of secondary glaucoma. Both present as a red, painful, squinting eye, and they cannot be reliably told apart without examination.
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Uveitis
Uveitis and glaucoma can be linked in two directions: inflammation inside the eye may trigger a rise in pressure in some cases, whilst in others, chronic glaucoma can lead to secondary inflammatory changes. The signs of pain and redness overlap closely. In uveitis the pupil is more often small and eye pressure is typically low, whereas glaucoma tends to produce a wide pupil and high pressure; an inflamed eye with a normal pressure reading can signal that pressure is starting to rise, and measurement is what separates the two.
Diabetes Mellitus in Dogs
Diabetes mellitus commonly leads to rapidly forming cataracts in dogs. Proteins leaking from a swelling cataract can cause lens-induced uveitis, which can progress to secondary glaucoma, and raised eye pressure is also a recognised complication after cataract surgery.
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Cushings Disease in Dogs
Direct evidence linking Cushing's disease itself to glaucoma in dogs is limited. Dogs with Cushing's disease often have raised blood pressure, and systemic hypertension can cause bleeding and retinal detachment inside the eye, which may lead to secondary glaucoma; corticosteroid eye drops have also been shown to raise eye pressure in dogs predisposed to glaucoma.
Lens Luxation
Lens luxation is a common cause of secondary glaucoma: a lens that moves forward can block the flow of fluid through the pupil and drainage angle, leading to a rapid rise in pressure. Terriers such as Jack Russell, Miniature Bull and Tibetan Terriers carry an inherited form, and chronic glaucoma that stretches the eye can also loosen the lens.
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Conjunctivitis
Conjunctivitis is a far more common cause of a red, watery eye and is often mild. Early glaucoma can look very similar at home; a pupil that stays wide in bright light, a blue haze over the cornea or marked squinting point away from simple conjunctivitis, and only measurement of eye pressure separates the two with confidence.
Hyphaema
Blood in the front chamber of the eye can block the drainage angle and raise pressure, and hyphaema and glaucoma can share causes such as intraocular tumours and uveitis. In long-standing glaucoma, fragile vessels in membranes that grow over the iris can also bleed.
Cataracts in Dogs
A swelling or rapidly forming cataract can leak lens proteins that trigger lens-induced uveitis, which can progress to secondary glaucoma. Raised eye pressure is also a recognised complication after cataract surgery.
Glaucoma represents a condition where the relationship between timing and outcome is particularly significant. The retinal ganglion cells that mediate vision are highly sensitive to pressure-induced damage and cannot regenerate once lost, so once pressure rises sharply the window in which sight can be saved is measured in hours to days. Eye pressure cannot be judged at home: a red, squinting, watery eye can look similar whether the cause is conjunctivitis, a corneal ulcer, uveitis or a pressure spike, and a pupil that stays wide in bright light alongside a blue haze over the cornea is the pattern that points most towards glaucoma. For breeds known to carry predispositions, awareness of the early signs and the potential for bilateral involvement can be valuable. Understanding that glaucoma management is often a long-term endeavour, requiring ongoing monitoring and treatment adjustment, helps set realistic expectations for the trajectory of the condition.
Last reviewed: 10 October 2026 · Dr Alastair Greenway MRCVS