What is Glaucoma?
This disease has an estimated prevalence of between 0.07 to 0.5 percent of horses. Appaloosas, horses with recurrent uveitis, or older than 15 years of age are considered at increased risk of developing glaucoma. This is a degenerative, painful condition that can lead to blindness; it is essential that if you are concerned your horse may be suffering from this disease they are seen by your veterinarian.
Glaucoma is a term that covers a group of progressive and debilitating diseases that cause the increase of intraocular eye pressure and ultimately leads to blindness. This often occurs following changes to the formation and draining of the fluid of the eye, or aqueous humor. Causes can vary; however, the most common cause of this condition in horses is recurrent uveitis.
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Symptoms of Glaucoma in Horses
Glaucoma can be a difficult disease for owners to spot, especially in the early stages. Often, the only sign of this condition is slight pupil dilation. Other symptoms may be seen and vary depending on the severity of the disease. These may include:
- Corneal opacity
- Red or painful eye
- Eye closure or squinting
- Reduced vision or blindness
- Excessive production of tears
- Pupil constriction
- Corpora nigra atrophy
- Sequestered pupil
- Retinal detachment in severe cases
- Enlarged or buphthalmic eyes
Causes of Glaucoma in Horses
Aqueous humor is a normal substance found in the eye, which supports the eye health, nourishing the different structures. This is produced by the ciliary body processes of the eye and travels through the eye and is drained through the iridocorneal and uveoscleral outflow pathways.
In many cases of glaucoma the drainage of the aqueous humor is blocked, this can be due to:
- Primary causes – Due to alterations to the internal structure of the eye, leading to abnormal iridocorneal or uveoscleral outflow pathway
- Secondary causes – Due to blocked outflow pathways caused by trauma such as perforation of the cornea, inflammation, neoplasia of the eye or repeated cases of uveitis
* Congenital glaucoma – This has been reported in foals due to developmental anomalies
Diagnosis of Glaucoma in Horses
Your veterinarian will perform a nose to tail examination of your horse, and discuss their clinical history and any concerns you may have. If your horse is showing signs of glaucoma your veterinarian will perform a quick eye examination, using a special light to look into the eye. This will allow them to visualise the fluid in the eye, and may allow them to see blockages, build up or trauma.
If your veterinarian suspects glaucoma they will perform a test to measure the intraocular pressure (IOP). This test is performed using a tonometer, a small handheld instrument.
This procedure is performed by applying a topical anesthetic to your horse’s eye to prevent them from blinking during the examination. Your veterinarian may also apply a nerve block to the auriculopalpebral nerve to further aid in examination. The IOP is then recorded. Normal range is considered 20-28 mmHg, if your horse’s measurement is above 32 mmHg a diagnosis of glaucoma may be made.
Treatment of Glaucoma in Horses
When choosing a treatment plan for your horse your veterinarian will assess their age, any pre-existing conditions they have, and the severity of the disease and symptoms.
The first action taken by your veterinarian will be to provide pain relief for your horse and attempt to reduce IOP. Topical medications such as corticosteroids may be given that can be placed in your horse’s eye to reduce inflammation. Oral NSAIDs may also be given to reduce inflammation and provide pain relief. Beta-blockers and carbonic anhydrase inhibitors may be given separately or in combination to reduce the IOP.
Your veterinarian may feel that the best way to treat your horse is via surgical treatment. The aim of treatment is to reduce the production of fluid in the eye via partial ciliary ablation. This is most commonly performed using diode lasers under general anesthetic or sedation with a local nerve block. If this procedure is done under sedation this may be able to be performed as an outpatient procedure.
If your horse continues to suffer from eye pain your veterinarian may recommend an enucleation (eye removal surgery). This can be done either under general anesthetic or under standing sedation. Following this a prosthesis may be surgically placed.
Recovery of Glaucoma in Horses
Unfortunately, the treatments used in horses with glaucoma are management tools, used to slow the progression of the disease. Although the treatment may maintain eye health for two to three years, the disease will slowly resurface and IOP increase. Your horse will need monitoring and repeated treatments throughout their life.