Glaucoma is a pathological increase in the intraocular pressure with associated damage to ocular structures - most notably the optic nerve which carries visual information from the eye to the brain. Optic nerve damage results in blindness if the pressure in the eye is not controlled. Glaucoma causes blindness and can be a very painful disease.
The normal eye
is largely a fluid filled structure which is maintained at an optimum size and pressure by production of fluid by cells in the ciliary body and drainage of this fluid back into the venous blood circulation. In the steady state the fluid production should equal the rate of drainage - this maintains a normal intraocular pressure of about 20mm Hg in the horse.
In horses the commonest cause of glaucoma is intraocular inflammation or uveitis (and especially chronic recurrent equine uveitis). Uveitis can clog up the drainage pathways from the eye and if fluid production continues unimpeded the pressure in the eye rises.
The clinical signs of glaucoma are pain, redness around the white part of the eye (conjunctiva and sclera), haziness in the clear part of the eye (cornea), loss of vision in the affected eye, a dilated and non-responsive (to light) pupil, enlargement of the eye,
dislocation of the lens (due to stretching and breaks in the lens suspensory zonule), degeneration of the optic nerve and retina. Additionally in the horse there are usually signs of uveitis present as well.
The diagnosis of glaucoma is made based on the clinical signs associated with an abnormally increased intraocular pressure - the pressure is measured by the ophthalmologist with a special instrument (tonometer) which is gently touched to the cornea after applying a topical local anesthetic.
Treatment of glaucoma in the horse can be difficult - partly because it results from other ocular disease (uveitis) and partly due to failure of the horse to respond to drugs which may be effective in other animals. Once present glaucoma may need chronic treatment - this also can be difficult to achieve in horses. Surgical treatment (laser therapy to reduce production of fluid in the eye and implantation of a drainage valve to increase fluid drainage) coupled with medical treatment may in some cases preserve vision for long periods of time. Expectations about the chronic and progressive nature of the disease need to be realistic however. Glaucoma is a bad disease and often has a bad outcome for the affected eye.