Dog - Glaucoma

The size and shape of the normal eye is maintained by intraocular pressure (IOP). This is regulated by the amount of fluid within the eyeball. The fluid, called aqueous humor, is constantly produced by a structure called the ciliary body. It contains nutrients and oxygen which are used by the structures within the eye. The excess fluid is constantly drained from the eye through the area between the cornea and the iris. This is called the drainage angle, or iridocorneal angle.

The pressure remains more or less constant since the production and absorption of aqueous humor is equal in normal circumstances.

What is glaucoma?

Glaucoma is an increase in IOP, i.e. raised pressure in the eye. This is measured using an instrument called a tonometer. The cause of glaucoma is usually related to inadequate drainage of aqueous humor rather than its over production. Glaucoma can be primary or secondary.

Primary glaucoma results in increased intraocular pressure in an otherwise healthy eye. Some breeds are more prone than others. It occurs due to anatomical abnormalities in the drainage angle which have been present from birth. For example in narrow angle glaucoma there is a shallow anterior (front) chamber which causes the iris to block the angle which in turn interferes with the fluid drainage. Abnormalities within the drainage angle itself (goniodysgenesis) can lead to a raised IOP in middle age and this is the most frequent form of primary glaucoma encountered in dogs. Breeds such as the Basset Hound, Flat Coated Retriever and Springer Spaniel are particularly prone to primary glaucoma.

Secondary glaucoma results in increased intraocular pressure due to disease or injury to the eye. This is the most common type in both dogs and cats. Causes include:

• Severe inflammation (uveitis) or infections within the eye resulting in debris and scar tissue blocking the drainage angle.
• Anterior dislocation of the lens. The luxated lens falls forward and physically blocks the drainage angle or pupil such that fluid is trapped behind the dislocated lens.
• Tumours which cause physical blockage due to their size.
• Intraocular haemorrhage (a bleed in the eye). The blood clot can physically prevent aqueous drainage.
• Damage to the lens. Lens proteins leaking into the eye as a result of a ruptured lens can cause a reaction which results in blockage of the drainage angle.

What are the signs of glaucoma?

The commonest signs noted by owners are:

• Ocular pain: your dog may keep the eye partially closed and initially start to rub at it. He may shy away as you go to stroke the affected side of the head.
• As the pressure increases a watery discharge will be noted and the dog becomes depressed.
• Physical swelling of the eyeball becomes obvious. The white of the eye looks red and engorged (full).
• The cornea or clear part of the eye may become cloudy (usually bluish in colour).
• Blindness. Since the increase in pressure will compromise the delicate blood vessels and nerves within the eye, blindness can occur very quickly.

All these signs can occur very suddenly with acute glaucoma. In chronic glaucoma they develop more slowly. They may have been present for some time before your pet shows any signs of discomfort or alteration of appearance or habits.

How is glaucoma diagnosed?

Veterinary diagnosis depends upon accurate pressure measurements, (tonometry) and the internal examination of the eye using special instruments. Careful examination of both eyes is required.

Acute glaucoma is an emergency. Sometimes specialist help and advice are necessary.

Treatment

Each case has to be treated on its merits. Initially it is important to reduce the IOP as quickly as possible. For this special drugs will be prescribed.

It is also very important that we examine your pet fully and treat any underlying disease which may be responsible for the glaucoma (an intraocular tumour or a dislocated lens for example).

Medical treatment might not be effective long term and often has to be combined with surgery. Analgesics (pain killers) are prescribed to control pain and discomfort associated with the condition.

Veterinary ophthalmologists employ various surgical techniques to reduce intraocular pressure. Should such a procedure be necessary, this will be carefully discussed with you. In the meantime it is important that medication is carefully and conscientiously administered. If you do have any difficulties, please call us without delay.

Prognosis

Unfortunately glaucoma is a very serious ocular disease. It can result in blindness. Some patients require removal of the eye if it is permanently blinded by the raised pressure as the pain this causes sometimes cannot be controlled. If the condition is diagnosed early and treated appropriately, many dogs will maintain some useful vision for long periods, but lifelong treatment is frequently necessary. Primary glaucoma frequently involves both eyes although not necessarily at the same time. As such, vigilance is required for the second eye – regular pressure checks might be advised.