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 can not 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 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.
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.
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.
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.
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.
Treatment depends on several factors - the type of glaucoma present, the degree of elevation of IOP, and the extent of visual impairment. Primary open angle glaucoma tends to be slower in onset and may, at least initially, be controlled by medical therapy (drugs) alone. With closed angle glaucoma, which is much more common, there is usually a sudden, rapid elevation in IOP. Ultimately, most forms of glaucoma require surgery.
If vision is present or has just recently been lost, a combination of medical and surgical therapy will be used to try and maintain your dog's sight . Aggressive medical therapy (meaning a combination of anti-glaucoma drugs administered frequently and monitored closely) is used to reduce IOP prior to surgery to prevent further damage to the eye. Some of these drugs will be used as well for additional minor IOP reductions following surgery. The aim of surgery in an eye that is still visual (or potentially visual) is to decrease the production of fluid within the eye, and to improve the drainage from the eye. There are a few different methods that a veterinary ophthalmologist can use to achieve this.
If the eye is irretrievably blind, glaucoma can be treated by removing the globe of the eye (enucleation). This will eliminate the pain for your dog. There are also procedures that can be done that preserve the globe such as placing a prosthesis.
Inherited glaucoma usually occurs in both eyes eventually. Your veterinarian will monitor the pressure in the other eye regularly, and discuss with you recognition of early signs of glaucoma. He or she may also recommend preventive medication for the unaffected eye.
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 edema, 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.
Animals of predisposed breeds should be screened for glaucoma before being used for breeding. Affected dogs and their close relatives should not be bred. Unfortunately, glaucoma does not generally become apparent until after breeding age has been reached, usually 3 years of age or greater.