When we discuss eye anatomy, we need to consider both external and internal structures.  So let's work from the outside, in. 

The external parts of the eye are easily visible.  This includes the upper and lower eyelids, the corners of the eye (lateral and medial canthus), and the third eyelid.  We also consider the cornea to be an external structure and this becomes very important in injury and disease.  Keep in mind, there are other structures such as glands and things, however we'll keep that information at bay for the time being.

The internal structures are often visible, however not as easily accessible.  This includes the iris, which can expand and contract the pupil to allow more or less light to reach the lens.  The corpora nigra are attached to the iris around the pupil and are believed to have developed as a way to shade the eye from direct sun.  The portion of the eye between the cornea and the iris is called the anterior chamber.  The entire area behind the iris is referred to as the posterior chamber.  The lens is a pliable structure than is able to distorted to bend light and focus it on the retina, the nerve center of the eye.  The retinal nerves all converge into the optic nerve which goes directly to the brain.  The choroid and  ciliary body contain the blood/nutrient supply for the ocular tissues.  Collectively, the iris, choroid and ciliary body are referred to as the uvea.   

Whew!  That's a lot of stuff going on in such a small structure.  But that's why it's so important!   

Common Corneal Disorders

As we said earlier, the eye is located in a position that makes it very prone to injury.  One of the more common injuries is a corneal ulcer. 

A corneal ulcer is an abrasion of the cornea that removes a portion of the top couple of tissue layers.  A majority of the time, ulcers are induced by some traumatic event.  They can be classified as simple (superficial) or complex.  Either way, they are very painful (have you ever scratched your eye?) and you typically only see a swollen, weepy eye.  It's important to note here that if you encounter this in your own horse, you should contact your veterinarian.  Attempts to look into and/or treat an eye on your own could lead to further damage.  Painful eyes are always an emergency. 

Ulcers are not always readily visible on an initial exam.  Sometimes we will see a white "spot" on the cornea, but other times nothing is really noted.  That is why we need to stain the eye.  A small amount of fluorescein stain (a green dye) is applied topically to the eye.  This stain will adhere to exposed connective tissue in the cornea, and allows us to fully see the size and depth of the ulcer.  If necessary, some debridement may be performed to remove any loose edges of the ulcer prior to initiating treatment.  

Treatment of superficial ulcers is aimed at reducing pain and minimizing infection.  Oral bute or banamine is usually given for pain management.  Topically, ophthalmic antibiotic ointment can be administered to help fight off any bacterial infection.  Atropine, a mydriatic, dilates the pupil to decrease muscle spasm and pain within the eye.  A weeklong course of this therapy is generally enough to combat a simple corneal ulcer. 

 One drug you never want to use on an ulcer is a steroid.  A steroid (such as Prednisolone or Dexamethasone) eliminates the body's defense mechanisms and can make the situation much worse in a short amount of time.  So do NOT use steroids in an eye unless directly told to do so by your veterinarian!  

If severe trauma occurs, or a simple ulcer is not tended to quickly, a more complex ulcer can form.  These ulcers tend to affect more layers of the cornea.  One type  of severe ulcer is called a "descemetocele".  (Deh-seh-met-o-seel) (Descemet's membrane is the second to last layer of the cornea). These tend to appear as white opacities with a black spot in the middle.  That black spot means that there is only one cell layer left between the outside world and the inside of the eye.  If we were to stain this eye, we would see a rim of green dye with a black hole in the middle.  Ulcers such as this put the eye in grave danger of rupturing and need to be treated more aggressive therapies, sometimes including hospitalization.

Remember when we said eyes are always an emergency? And remember when we said do NOT use steroids?  That's because the ulcer can become "melting" - which is pretty much what it sounds like.  Melting corneal ulcers are very complicated and difficult to treat.  They occur due to heavy bacterial and/or fungal populations that are using enzymes to break down the cornea tissue.  This causes the cornea to literally 'melt' and slide off of the eye.  While a very scary situation, the eye can often still be saved with aggressive treatment.  These ulcers require diligence on the part of the owner and veterinarian.

There are other things that can affect the cornea, however these are some of the most common as well as vision threatening disorders. 

The uvea is comprised of the iris, the ciliary body and the choroid.  These structures provide vasculature to the internal structures of the eye and assist with nutrient delivery as well as filtering of the ocular fluids (that's part of the job of the ciliary body.   

Uveitis

When the uvea becomes inflamed, it is called uveitis. (You may have heard this term before).  Uveitis is the leading cause of blindness in horses.  A single event may not always be an issue, however it can predispose the horse to future episodes of inflammation, which is then termed Equine Recurrent Uveitis.  This is often diagnosed on clinical signs, which include pain/discomfort, discharge,

Glaucoma is another disorder of the uvea.  While uncommon in horses, it does occur but is poorly understood.  Glaucoma is an increase in intra-ocular pressure resulting from a problem with aqueous humor outflow (the fluid that fills the anterior chamber.)  It can often be difficult to diagnose initially, as the only sign may be pain and slight corneal edema.  Increased pressures can lead to a characteristic "blue eye" appearance due to accumulating edema.  Treatment is directed at decreasing the production of aqueous humor, however it is often unsuccessful.  The pressure becomes painful and results in death of cells within the retina.  Blindness ensues, but the pain continues.  We often have to remove the eye (enucleate) if we cannot control the pain.  While many people are concerned with this, one eyed horses really do get along really well.  If you would like more information on glaucoma, check out our blog on the subject!

This is merely an overview of things that we see on a (fairly) regular basis.  There are many other issues that can arise from eyes, as this is not an exhaustive list.  Any issue with any eye should be considered an emergency and you should contact your veterinarian for assistance.      

We will be out of town October 10th-11th.  Emergency services will be provided by Dr. Aimee Eggleston.  Her office can be reached by dialing 860-942-3365.

We will also be away from October 29th - November 3rd.  Coverage (for emergency as well as routine appointments) during these dates will be provided by Dr. Liz Fish.  She will be able to be reached on our normal office and emergency numbers.   

For the month of October, receive 10% off Equine Dentistry!  This offer is only valid for the month of October, so get your appointment set up now!