Glaucoma in dogs: symptoms and treatment

Overview

Aqueous humour and pressure inside the eye

The front of the eye contains clear fluid called aqueous humour, produced mainly in the ciliary body. It carries nutrients to the cornea and lens. In dogs, about 85% drains through the angle between the iris and cornea. The remaining approximately 15% drains through the tissues of the iris and ciliary body. If drainage slows, pressure inside the eye rises. The drainage angle runs around the front of the eye at the edge of the iris. Persistently raised pressure can permanently damage the optic nerve. Depending on the form of glaucoma, pressure may rise suddenly or gradually. Signs of primary glaucoma often appear in only one eye initially.

Causes

Primary and secondary glaucoma

In primary glaucoma, fluid drainage becomes impaired without another eye disease preceding it. In angle-closure glaucoma, the drainage angle may close suddenly. Abnormal development of the comb-like tissue in the angle can obstruct fluid outflow. These congenital structural changes may become more pronounced as the dog ages. In open-angle glaucoma, pressure may rise slowly even though the drainage angle is open.

Secondary glaucoma can develop following inflammation inside the eye, called uveitis. A lens displaced into the front chamber can block aqueous humour from passing through the pupil. A tumour inside the eye or an eye injury can also cause secondary glaucoma. Glaucoma can also develop after cataract surgery. Lens-induced uveitis was the underlying cause in 14 of 19 dogs treated with combined surgery for secondary glaucoma (73.7%).

Risk factors

Breeds affected by inherited angle-closure glaucoma include American Cocker Spaniels, Basset Hounds, Samoyeds and Siberian Huskies. Severe structural abnormalities of the drainage angle increase the risk of glaucoma. However, an abnormal angle can also be found in a dog that never develops glaucoma.

A change in the ADAMTS10 gene has been linked to inherited open-angle glaucoma in Norwegian Elkhounds. Affected Norwegian Elkhounds in the study were diagnosed at an average age of 6.5 years. There were 151 carriers of the genetic change among the study dogs (25.3%). Blood samples were collected from 596 Norwegian Elkhounds across several countries. The variant identified in Norwegian Elkhounds differs from the variant of the same gene described in Beagles. In one northern European breed screening dataset of Siberian Huskies born in 2005–2020 and examined by gonioscopy, 54 had drainage angle abnormalities (34%). Severe abnormalities were found in 11 of the examined Siberian Huskies (7%).

An OLFML3 gene variant has been linked to severe abnormal development of the drainage angle in Border Collies.

Typical symptoms

Acute glaucoma can cause several visible eye signs within a short time.

• Your dog may squint because of pain.
• Blood vessels on the white of the eye may look red.
• The cornea may swell and look milky or cloudy.
• The pupil may be dilated.
• The eye may water.
• Your dog may lose sight in the affected eye.

In slowly developing glaucoma, the first changes may be too subtle for you to notice. An early sign can be a pupil that reacts to light more slowly than usual. Initially, redness may appear only as more prominent blood vessels. With longstanding glaucoma, the eyeball may enlarge and start to bulge. Pain may show as changes in your dog's behaviour, even without constant squinting.

Complications and consequences

Permanent sight damage and structural changes in the eye

Persistently high pressure can destroy nerve cells in the retina. Damage to the optic nerve can lead to permanent blindness. A sudden, very high rise in pressure can damage sight within just a few hours. Raised pressure can reduce blood flow within the eye. Sight may temporarily disappear at the start of an attack when high pressure interferes with retinal function. In a chronically enlarged eye, the lens may become displaced. Stretching of the eyeball can cause tears in the membrane at the back of the cornea. Raised pressure can cause pain even in an eye that has already lost its sight.

Diagnosis at the vet

Measuring eye pressure and examining the back of the eye

Your vet measures pressure in both eyes with a tonometer. The usual range in a healthy dog is about 12–25 millimetres of mercury (mmHg). The upper limit of normal may vary between approximately 20 and 28 mmHg, depending on the individual and breed.

During an acute glaucoma attack, pressure is typically markedly raised, often to 40–60 mmHg or more. In one dog examined during an acute attack, the tonometer reading was 55 mmHg. Pressure on the neck or eyeball can give a falsely high reading. Pressing on the eyelid during measurement can also make the reading falsely high. A difference of more than 20% between the right and left eyes may be abnormal. An IOPvet tonometer tested in dogs correctly identified only 16 of 125 measurements in which the reference instrument read above 30 mmHg. On examination of the back of the eye, cupping of the optic nerve head may indicate previous glaucoma damage.

Assessing the drainage angle and underlying cause

During gonioscopy, your vet uses a special lens to examine the structure of the drainage angle. This is done before dilating the pupil so that the angle can be assessed correctly. Sedation is usually unnecessary because anaesthetic drops numb the eye's surface. Both the width of the angle and its tissue structure are assessed. High-resolution ultrasound of the front of the eye can examine the drainage area behind the angle.

If a cloudy cornea prevents a view inside the eye, ultrasound can assess the retina and vitreous. Your vet looks for causes of the raised pressure, such as a displaced lens or inflammation. The blink response to a bright light can help assess whether the eye still has a chance of retaining sight.

Treatment

Lowering pressure and relieving pain

Your vet chooses treatment according to the cause of the pressure rise and whether the eye still has sight. In acute glaucoma, the aim is to lower pressure rapidly with medicines administered by the vet. Some medicines reduce aqueous humour production. Others increase its drainage from the eye. In some acute cases, your vet may give an intravenous medicine that reduces the volume of fluid inside the eye. The effect of this intravenous treatment is temporary. The position of the lens is checked before giving certain drops that strongly constrict the pupil. If the lens has displaced into the front chamber, such a medicine can worsen the pressure rise. During long-term treatment, pressure is measured again so that medication can be adjusted if necessary.

Removing a blind, persistently painful eye can relieve your dog's pain. Of 45 owners whose dogs had an eye removed because of glaucoma, 26 responded to a survey about recovery. Among respondents, 96% were satisfied with the decision to remove the affected eye.

Laser treatment and drainage implants

Laser treatment can reduce fluid production in the ciliary body. Another operation places a small drainage tube in the eye to divert fluid past the diseased drainage angle. Eye surgery can combine laser treatment with a fluid drainage valve. The valve's function is checked at follow-up visits because it helps keep eye pressure under control.

After combined surgery, 39 eyes in 32 dogs were followed up. Before surgery, 23 of these eyes had sight (59%). At 12 months, sight was present in 18 of the 25 eyes whose vision was assessed (72.0%). Pressure was controlled in 31 of the 32 eyes measured at 12 months (96.9%). In this follow-up, pressure was considered controlled at 25 mmHg or below. The median follow-up period was 26 months (12–65 months). At 24 months, sight was present in 12 of 16 assessed eyes (75.0%). Pressure-lowering medicines were used after surgery as needed. A postoperative pressure spike occurred in 18 of 39 eyes (46.2%). Corneal ulcers occurred in 9 of the 39 operated eyes (23.1%). Having sight before surgery was associated with better preservation of vision during follow-up.

Treating the other eye

When primary glaucoma is diagnosed in one eye, your vet may start preventive medication for the other eye while it is still free of symptoms. After angle-closure glaucoma in the first eye, 70.9% of dogs developed glaucoma in the other eye within 5 years. In this patient group, the median time until glaucoma developed in the other eye was 2.15 years. These dogs were first diagnosed with glaucoma at an average age of 8.3 years. The time until the other eye developed glaucoma did not differ significantly between groups receiving different preventive eye medicines. In an earlier clinical study, the median time until glaucoma developed in the other eye was 8 months without preventive medication. In two groups receiving medication, the corresponding times were 31 and 30.7 months.

Prevention

The drainage angles of breeds predisposed to inherited glaucoma can be examined before symptoms develop. The angle's structure can change with age, so the European College of Veterinary Ophthalmologists recommends examination at 1, 4 and 7 years. In one official screening scheme, a gonioscopy result is recorded as official only when an official eye examination is performed at the same time. The scheme advises against breeding from dogs with severe structural abnormalities of the drainage angle. For a dog with mild or moderate abnormalities, it recommends a mating partner examined by gonioscopy and found to have healthy angles. When recording the angle's structure, any narrowing or closure is also noted.

Important for dog owners

Tell your vet when the eye became red or cloudy. Do not use medicines prescribed for a person or another pet without veterinary advice. Wash your hands before and after giving eye drops. Avoid touching the eyelid or the eye's surface with the bottle tip. Keep appointments for pressure measurements even when your dog's eye looks normal. Give prescribed drops as your vet directs, including on days without symptoms, because interrupting treatment can put sight at risk. If the eye becomes redder after medication, contact the vet treating your dog.

Frequently asked questions

Can a dog have glaucoma without a red eye?

In slowly developing open-angle glaucoma, sight may deteriorate gradually and you may not notice the first changes. Your vet can detect raised pressure with a tonometer. Pressure can also be measured during health checks in dogs of at-risk breeds. Sudden deterioration in sight requires an emergency assessment.

Will the other eye be affected too?

Primary glaucoma is often diagnosed in one eye first. Your vet monitors the other eye and may start preventive medication for it. In secondary glaucoma, the risk to the other eye depends on which disease or injury caused the pressure to rise.

Can sight return after an acute attack?

Preserving sight depends partly on how quickly pressure can be lowered. If the eye has already lost its sight when treatment starts, the outlook for recovery is guarded. Prolonged raised pressure can permanently damage the optic nerve.

Does an abnormal gonioscopy result mean my dog has glaucoma?

Gonioscopy can reveal a structural abnormality that increases the risk of glaucoma later on. Not every dog with an abnormal drainage angle develops glaucoma. The more severe the abnormality, the greater the subsequent risk. When assessing glaucoma, your vet also considers eye pressure and the other findings of the eye examination.

Why is a blind eye sometimes removed?

An eye blinded by advanced glaucoma may still be painful. Removal aims to relieve pain when pressure cannot be controlled with other treatment. Owners of dogs operated on for glaucoma reported improvements in their dogs' behaviour by the 2-month follow-up.