Glaucoma is a disease in which the pressure inside the eye (intraocular pressure or IOP) becomes elevated. The elevation in IOP causes irreversible damage to the optic nerve and the retina resulting in blindness.

Normal IOP falls in the range of 10 to 30 mmHg. Pressures above 30 mmHg begin to cause damage inside the eye.

Cause

There are structures in the eye that continuously produce a fluid called aqueous humor. This fluid circulates within the eye and provides nutrients to structures of the eye such as the lens and cornea. It drains out of the eye at a region called the iridocorneal angle or drainage angle.

This region is located around the periphery of the eye, where the iris meets the cornea. The drainage angle is normally a sieve-like structure with many holes through which the fluid can leave the eye.

Types

Primary glaucoma

Primary glaucoma is a breed-related disease. Malformations of the drainage angle such that fluid can not drain adequately from the eye may or may not be present. Although the animal may be born with an abnormal drainage angle, the glaucoma often does not occur until middle age. As well, not every animal born with an abnormal drainage angle will go on to have glaucoma in its lifetime.

It is important to know that in primary glaucoma, both eyes are at risk of developing glaucoma. This type of glaucoma is progressive and eventually results in blindness.

Seondary glaucoma

This can occur in any breed or age of dog. It occurs when another disease inside the eye disrupts the normal circulation and drainage of the ocular fluid. Some causes of secondary glaucoma include:

• lens luxation/displacement — a condition seen most commonly in terrier breeds
• intraocular inflammation (uveitis)
• intraocular tumours
• chronic retinal detachment.

Consequences

The two most important consequences of glaucoma are:

• irreversible damage to the optic nerve and retina causing blindness.
• pain — elevated pressure in the eye is very painful, causing a headache-like feeling and even nausea.

Treatment of glaucoma depends on the cause.

• Treatment of secondary glaucoma requires treatment of the underlying ocular condition. Luxated/displaced lenses can be surgically removed. Intraocular tumours require removal of the eye to control pain and diagnose the tumour. Intraocular inflammation and retinal detachments may be treated in some cases, but they also may require removal of the eye to control pain.

• Treatment of primary glaucoma involves a combination of medical and surgical therapy. Medications are used for reducing the intraocular pressure. Unfortunately, primary glaucoma is a progressive disease and these medications ultimately fail in the long term. Different surgical therapies are used depending on if the animal is blind or still has vision.

• Surgery for blind animals: evisceration and intrascleral prosthesis or enucleation. Glaucoma is a painful disease and blind, painful eyes should have one of these surgeries done to eliminate discomfort.

• Enucleation removes the eye, and the eyelids are permanently sewn shut.

• Evisceration with placement of an intrascleral prosthesis is a more cosmetic alternative to enucleation. In this surgery the internal contents of the eye are removed and a sterile silicone prosthesis is placed inside so the eye can hold its form. Afterwards, the eye looks fairly normal except for scarring of the cornea — giving it a gun-metal grey appearance. The animal can blink and move the eye.

Surgery for visual animals: The anterior chamber to frontal sinus shunt

For visual animals with primary glaucoma, a shunt is placed in the anterior chamber of the eye which drains fluid from the eye to the frontal sinus (a space in the bones of the forehead). The shunt has a valve to control how much fluid is removed from the eye and this valve can be adjusted.

How do I know if my dog is a candidate for a shunt?

If your dog has been diagnosed with glaucoma, your veterinarian can refer you to the WCVM ophthalmology service for evaluation.

We will assess your dogs' eyes and discuss the potential for this surgery with you at that time.

If my dog is having shunt surgery, how long will they be at the Veterinary Medical Centre?

Will my dog require more than one surgery?

A second surgery is very common during the first few weeks following placement of a shunt. Shunts may shift in the eye, become plugged with inflammatory debris, or the valve may require adjusting to allow more or less fluid to leave the eye.

A second surgery is usually a shorter procedure performed to make the adjustments necessary to allow the shunt to work properly. Occasionally some dogs require more than one adjustment following the initial surgery — meaning more short procedures under general anesthesia.

What is the success rate for placement of a shunt?

This is a relatively new procedure and the success is improving as we begin to perform it more frequently.

Some dogs can retain a normal intraocular pressure and normal vision for months to years after placement of a shunt.