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What is Wrong With My Horse’s Eye? Bruce P. Whittle, DVM Eye problems are common in horses. Because a horse only has two eyes, damage to one of them can be very serious. Although there are many conditions that can affect the equine eye, the two most common disorders are corneal ulceration and uveitis. Other disorders such as lacerations of the eyelids and neoplasia can also have negative effects on the eye or vision and require prompt treatment. In order to better understand the seriousness of these conditions, it is important to have a basic knowledge of the anatomy of the eye. The globe of the eye (aka eyeball) is generally spherical in shape. It is bordered by the front transparent portion called the cornea and the rear whitecolored portion called the sclera. The cornea is clear because it is dehydrated, despite the fact that it is surrounded on the outside by the tear film and on the inside by the aqueous fluid in the front chamber of the eye. It becomes cloudy if damage occurs either on its outer or inner surfaces that allows fluid to enter the dehydrated interior of the cornea. Also of interest is the fact that the cornea is less than 1 millimeter thick! Our goal when treating the eye is to keep the cornea clear and intact if at all possible. Behind the cornea is the Normal horse's eye anterior chamber of the eye. It is filled with aqueous fluid that helps maintain the shape of the globe as long as the pressure of the fluid is within a normal range. The back “wall” of the anterior chamber is the iris, the typically brown colored structure with a black hole in it. The black hole is the pupil. It lets light into the back portion of the eye. The amount of light allowed to pass is regulated by the diameter of the pupil that is in turn regulated by contraction or relaxation of the iris. Just behind the pupil is the lens. The lens helps to focus the light entering the eye. This focused light then interacts with the retina. The retina, which lines the back interior portion of the eye, converts the light signals received by the eye into electrical signals that are then transmitted up the optic nerve to be interpreted by the brain. The eye is undoubtedly an incredible structure that we often take for granted. Horses’ eyes can have a multitude of problems. They can become infected, inflamed, neoplastic, or traumatized. It is important that you understand what your horse’s eyes normally look like. This makes it is easier to recognize when something is wrong. The cornea should be clear, the eyelids should conform nicely to the globe, and the area beneath the eye should be dry. You should be able to readily see the entire iris and pupil and the iris should typically be the same color throughout although some horses such as Paints can have a multicolor iris that is normal. The eyelids should also be symmetrically open on both eyes – squinting can be a sign of ocular pain. Owners will often notice a brown blob hanging down from the top of the pupil. This is a normal structure called the corpora nigrans. While we cannot visually examine the back portion of the eye that consists of the vitreous and retina, we can suspect eye problems in an otherwise normal appearing eye if the horse suddenly begins to stumble or appears blind. Any appearance of the eye that is abnormal should prompt an examination by your veterinarian. The longer an eye problem is allowed to go untreated, the less likely it can be treated successfully. While there are numerous things that can happen with the horse’s eyes, some conditions are much more common than others. One of the most common problems of the equine eye is cloudiness of the cornea. This usually indicates that there has been damage to either the inside or outside layer of the cornea, allowing fluid to enter the normally dehydrated middle section of the cornea called the stroma. Your veterinarian will apply a stain called fluorescein to the eye to determine if the damage to the cornea is external or internal. Fluorescein is a special stain that sticks to the corneal stroma but not Flourescein staining ulcer (note green to the corneal epithelium (outer layer of the area just below pupil) cornea). If the stain does stick to the cornea as in the picture to the right, an ulceration of the cornea is present. This can lead to rupture of the eye and possibly blindness if not treated appropriately. Corneal ulcerations are typically caused by trauma, bacteria, fungi, or a combination of these factors. If disease of the surface of the cornea is suspected, your veterinarian may collect samples for either cytology (examination of the cells present under a microscope) or culture (laboratory examination looking for bacteria or fungi). Treatment with a proper medication is very important and is best based upon laboratory testing to determine the most likely cause of the disease. If, on the other hand, the corneal epithelium is still intact, stain will not stick to it. In this case, the cloudiness of the cornea is typically due to disruption of the inner layer of the eye, the corneal endothelium, indicating inflammation within the eye. This is a very serious condition called uveitis that can lead to blindness if not properly treated. This condition is sometimes referred to as “moonblindness”. Your veterinarian will look at the eye carefully with a bright light during the examination to look for any abnormalities and to make sure the iris is constricting (closing) and dilating (opening) in response to varying intensity of light. Sometimes, especially if the eye is cloudy and the interior of the eye cannot be visually examined, your veterinarian will use ultrasound to evaluate the internal structures of the eye. Differentiating uveitis from corneal ulceration is important because the medications used to treat one of these conditions can make the other condition worse. Unfortunately, sometimes a horse can concurrently have both uveitis AND corneal ulceration making treatment decisions more complicated. Uveitis is generally treated with some type of anti-inflammatory agent, an agent such as atropine that dilates the pupil, and topical antibiotics. Lacerations to the eyelid, if not treated appropriately, can lead to long term eye problems because the eyelids play an important role in holding the tear film against the cornea. Lacerations cannot always be sutured but, as with any wound, the best chance of success is within the first 8 hours after injury. Permanent disruption of the eyelids can lead to a problem called “dry eye” due to constant leakage of the tear film. The other fairly common cause of eye problems is neoplasia or cancer. Early treatment of neoplasia of or around the eye is critical because many cancers of the eye of the horse spread readily. There are many possible treatments for neoplasia of the eye or eyelids including surgical removal of the mass, topical chemotherapeutic agents, freezing or cryotherapy and laser excision. Sometimes, due to extensive spread of cancer cells, the only treatment is an enucleation or surgical removal of the affected eyeball and surrounding tissues. Eye problems in horses can be very debilitating. At the first sign of eye problems, you should call your veterinarian because an early diagnosis and an appropriate treatment can be critical. You should not try treating an eye on your own with ocular medications you might have around from another case without a diagnosis by a veterinarian as some medications can make certain conditions worse. Damage to the eye, particularly in the case of uveitis, can be cumulative and lead to blindness. Blindness in the horse can be job-ending as many equestrian disciplines do not allow blind horses to compete. Because of the complexity of ophthalmic problems, your veterinarian may recommend referral to a veterinarian who specializes in ophthalmology and has advanced diagnostic equipment. You can help your horse have a long career by keeping a close watch for eye problems and calling your veterinarian as soon as a problem is identified.