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Glaucoma
Glaucoma is a painful condition of the eye which can occur in both cats and dogs. It is caused by increased pressure in the eye. In a normal eye fluid production inside the eye is balanced by fluid draining out of the eye. Most drainage occurs through the angle between the clear cornea and the pigmented iris known as the irido-corneal angle. This drainage angle is made up of a meshwork of fibres known as the pectinate ligament. Damage to this area reduces fluid drainage and the result is increased pressure inside the eye.

In dogs glaucoma is classed as primary or secondary.

Primary glaucoma occurs when there is an inherited disorder in the drainage angle. The normal meshwork is blocked by fibrous sheets of tissue and this is known as goniodysgenesis. It occurs in a number of breeds such as Springer Spaniels, Retrievers and Huskies and these can be examined at an early age to determine if they are likely to develop the disease. Examination is done with a contact lens placed on the cornea to allow visualisation of the drainage angle and this can be a little difficult in some dogs. Since the disease is inherited in both drainage angles both eyes are likely to become affected with glaucoma although frequently one eye will develop disease some time before the fellow eye.

Secondary glaucoma occurs following some other eye injury such as trauma, cataract formation, retinal detachment or tumour and is likely to affect only one eye.

In cats glaucoma is secondary to uveitis (inflammation) or lens luxation.

Symptoms of glaucoma can appear very suddenly. Although the drainage angle is abnormal from birth disease tends to appear suddenly in one eye in middle age dogs. The eye is painful with inflammation of the conjunctiva which becomes brick red instead of the normal almost white colour. The cornea becomes cloudy. The pupil is dilated and does not constrict in response to bright light. Unless treatment is sought early the eye will become permanently blind within days. The majority of glaucoma cases seen in referral practice are blind on presentation. This is because the only way of confirming the diagnosis is by measuring the pressure within the eye using a tonometer. These instruments are expensive and not usually found in most veterinary practices. When one eye is presented with glaucoma it is imperative to examine the drainage angle in the second eye if primary glaucoma is suspected. Medical treatment of the other eye should begin immediately to delay the onset of disease in this eye.

In addition to causing blindness the increase pressure causes pain. Normal pressure is around 15mm/Hg. In glaucoma this can rise to around 60mm/Hg. In humans discomfort occurs with pressures between 25 and 30mm and in dogs pressures over 40mm/Hg are considered to cause pain. Owners are often not aware of this discomfort until the pressure is relieved at which time they find they have a new dog again. If the pressure is not relieved then the eye will actually stretch and can become grossly enlarged.

Treatment of glaucoma is expensive and often disappointing. If the eye is still visual, then medical treatment with drops may be successful. Medication is required two or three times daily and will be required for life. The medication is expensive. Surgery to improve drainage tends to give only short term improvement and is usually used in conjunction with medical treatment. Alternative treatments using laser power or freezing to destroy the fluid producing tissues within the eye have also been tried but so far with limited success. In blind eyes the options are removal of the eye which can be done by your usual veterinary practice. At the Animal Eye Centre we prefer to retain the eye and in these cases the contents of the eye are removed leaving an empty globe. The shape of the globe is maintained with a silicone implant placed into the globe. The result is an eye which, although blind, still looks like a relatively normal eye. It moves with the fellow eye and is closed when sleeping.

In elderly dogs or those with a poor anaesthetic risk the alternative is to destroy the fluid producing tissues by injecting a chemical into the eye. This can be done during a very brief anaesthesia and can be effective but over a period the eye is likely to shrink to a tiny pea like structure.

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