When it comes to the equine eye, we often discuss common diseases such as corneal ulcers, conjunctivitis or uveitis. We do treat these conditions quite a bit, however there are some serious uncommon diseases that we do have to think about when considering an inflamed eye. One of these diseases is glaucoma, which can be set off by any of the above listed issues, or by another underlying problem.
Glaucoma is an increase in intra-ocular pressure (IOP) due to an alteration in the outflow of aqueous humor. Aqueous humor is the liquid in the front portion of the eye between the cornea and lens. It provides nutrients to the lens and cornea as well as removes waste from the front part of the eye. (As a side note, the back portion of the eye is filled with “vitreous humor”, which is thicker in character than aqueous humor). As with most things, the inflow and outflow of aqueous humor must be balanced. When it isn’t, and the pressure builds, we have glaucoma.
So how does this happen? Well, glaucoma is considered either a primary, secondary or congenital disease. Primary glaucoma is most likely genetic, but can be difficult to prove. Congenital glaucoma is usually due to a malformation of the eye and is noted quickly in life. The most common type of glaucoma we treat in the field is secondary. In these cases, IOP increases are predominantly due to outflow obstruction. This can be caused by a lens luxation, scar tissue formation from inflammation, or synechiae (pronounced seh-nee-key-ay) formation (strands of iris that attach to the lens). The most common cause for secondary glaucoma is Equine Recurrent Uveitis, due to chronic scarring at the edge of the cornea.
In the early stages of glaucoma, it can be difficult to make a diagnosis. The presenting sign is usually an eye with some mild edema (fluid within the cornea, which shows up as a white or blue haze on the outside of the eye) and potentially mild pain. There are several other diseases that cause these same signs which are much more common than glaucoma. The only way to truly diagnose glaucoma is by measuring the intra-ocular pressure. Given the rarity of the condition, most equine vets do not carry the equipment required to do this. So for most field vets, glaucoma becomes a diagnosis of exclusion. We should say here that glaucoma is extremely difficult to control in horses and that the disease usually progresses even in the face of early aggressive treatment.
As acute glaucoma becomes chronic, the cornea begins to take on a blue haze. This is because the excess pressure in the eye is stretching the cornea and allowing more fluid to enter it. The cornea may become pitted and have a roughened appearance to it at this point. Not only that, but pressure is also being put onto the optic nerve and retina, causing damage to these structures. This can cause chronic pain, as well as vision deficits. The eye is often noticeably larger than normal at this point, so measuring IOP becomes less critical for diagnosis. The end stage of glaucoma is often near blindness or complete blindness in the affected eye.
Treatment of glaucoma is directed at decreasing the production of fluid within the eye as well as reducing inflammation. Topical drugs such as timolol and dorzolamide are commonly used in our practice for this disorder. Systemic or topical steroids or systemic NSAIDs can help to reduce inflammation in the eye. There are surgical options available, such as ciliary body ablation, which is directed at removing the source of aqueous humor – but these surgeries typically have more side effects than benefits. Enucleation (removal of the eye) is often performed on these horses. While this seems extreme, it’s actually a fairly simple surgery, and horses do very well once the painful, generally non-visual eye is removed. We usually have people tell us that once the eye is removed, they have their old horse back.
Glaucoma is not frequently diagnosed in horses, but we still must consider it in cases of a chronically inflamed eye which is not responding to normal treatments. Unfortunately, the disease process is not well understood in horses and most information is extrapolated from other species. As more becomes known, and more research is done, we hope that we may be able to develop better ways to combat this vision threatening disorder.
