Goniodysgenesis, pigmentary glaucoma

What is glaucoma?

 Glaucoma is a leading cause of blindness in dogs. It is the result of increased fluid pressure within the eye (elevated intraocular pressure or IOP). If the pressure cannot be reduced, there will be permanent damage to the retina and optic nerve resulting in visual impairment. Complete blindness can occur within 24 hours if the IOP is extremely elevated or can occur slowly over weeks or months if the elevation is mild. Glaucoma is usually very painful.

Glaucoma may be primary (inherited) or secondary to a number of eye disorders including luxation of the lens, tumours of the eye, and uveitis (inflammation of the eye).

Primary/inherited glaucoma causes an elevation of pressure within the eye because of abnormal drainage of fluid through the iridocorneal angle. When the angle at which the iris and cornea join is wide, the glaucoma is classified as open angle. If the base of the iris is pushed forward, the glaucoma is described as narrow angle.

 Goniodysgenesis is characterized by an abnormal sheet of tissue in the angle where drainage normally occurs. This may or may not cause an elevation in IOP and glaucoma.

In pigmentary glaucoma, the obstruction to fluid drainage is caused by an abundance of pigmented cells within the iridocorneal angle and sclera. The increase in IOP is progressive and often results in blindness.

 

How is glaucoma inherited?

Inherited open angle glaucoma is an autosomal recessive trait in beagles. Narrow angle glaucoma is inherited as an autosomal dominant trait in the Welsh springer spaniel. The mode of inheritance for glaucoma in other breeds has not been identified.

What breeds are affected by glaucoma?

Narrow angle glaucoma is much more common than open angle glaucoma. Both types occur in mixed breed dogs as well as purebreds.

Open angle glaucoma: beagle, American cocker spaniel, basset hound, Boston terrier, miniature Schnauzer, Norwegian elkhound

Narrow/closed angle glaucoma: Alaskan malamute, American and English cocker spaniel, basset hound, Chow Chow, Dalmatian, Great Dane, wire and smooth fox terrier, toy, miniature, and standard poodle, Samoyed, Siberian husky, Welsh Springer spaniel

Goniodysgenesis: basset hound, Bouvier des Flandres, American and English cocker spaniel, This is also seen in the Chihuahua, Dandie Dinmont terrier, Norwegian elkhound, toy and miniature poodle, Siberian husky, wire hair fox terrier.

 

Pigmentary glaucoma: Cairn terrier

For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.

What does glaucoma mean to your dog & you?

Primary open angle glaucoma develops slowly over weeks to months. With closed angle glaucoma, which is much more common, there is usually a sudden, rapid elevation in the pressure within the eye. This affects all the structures in the eye. The effects on the optic nerve and retina cause loss of vision.

Glaucoma is moderately to extremely painful. The eye may be red and your dog may paw at it, or rub his or her head along the carpet. The eye may look cloudy due to swelling of the cornea and your dog will be very sensitive to light. The affected eye may seem larger, or appear to bulge out, relative to the other eye. Other more general signs of pain include loss of appetite and depression.

Glaucoma is an emergency. Treatment must be started as soon as possible if your dog's sight is to be saved. Irreversible damage to the retina and optic nerve occur within a few hours of significant elevation of the intraocular pressure.

How is glaucoma diagnosed?

Glaucoma is one of the conditions your veterinarian will suspect if your dog has a painful eye. It is diagnosed by measuring the intraocular pressure with a tonometer. This can usually be done with local anaesthetic drops placed in your dog's eye. To determine the type of glaucoma, gonioscopy is used to measure the iridocorneal angle.

FOR THE VETERINARIAN: Because of the potential for elevated IOP to quickly cause irreversible damage to the visual structures of the eye, the timely diagnosis of glaucoma is very important. IOP should be measured in all red eyes for which the cause is not immediately obvious, and in eyes with unexplained pupillary abnormalities, corneal oedema, or visual impairment, particularly if these signs occur in a dog that is of a breed with a predisposition to glaucoma. (See references below for a good discussion of accurate IOP measurement, as well as therapy).

Although the normal range of IOP varies (based on tonometer and other factors), generally a measurement of >25 mm Hg indicates glaucoma. An IOP of 50 mm Hg or more can lead to permanent optic nerve and retinal damage within hours if not relieved.

How is glaucoma treated?

Preserving vision in an eye with glaucoma is difficult and requires aggressive medical and surgical therapy. Your veterinarian may choose to provide initial emergency medical therapy and refer you immediately to a larger veterinary centre.