The Equine Ophthalmic Exam

What Is an Ophthalmic Exam? | Why Should Horses Receive an Ophthalmic Exam? | How Is an Ophthalmic Exam Performed? | What Are the Benefits of an Ophthalmic Exam?

  • An ophthalmic exam is a thorough examination of the horse's eyes and the surrounding tissues.
  • The exam may be performed by your veterinarian or by a veterinary ophthalmologist (an eye-care specialist).
  • The exam is generally non-invasive and painless for your horse.
  • The types of tests performed during an ophthalmic exam depend on the nature of the horse's eye problem.
  • If your horse has a sudden change in an eye (for example, a red, swollen eye), he/she should be seen by a veterinarian immediately.

What Is an Ophthalmic Exam?

During an ophthalmic (eye) exam, a veterinarian may perform a number of tests that can help identify (1) problems with the eyes or (2) underlying diseases that may affect the eyes. Your veterinarian may conduct the exam or recommend that a veterinary ophthalmologist (an eye-care specialist) evaluate your horse.

Why Should Horses Receive an Ophthalmic Exam?

Your horse's eyes should be examined as part of a regular physical exam. However, more thorough testing is needed in the following circumstances:

  • There is an abnormal appearance to one or both eyes, such as swelling, redness, cloudiness, or discharge.
  • Your horse shows signs of pain, such as holding an eye closed or rubbing at the eyes.
  • You suspect that your horse is experiencing changes in vision (for example, because your horse ran into a fence).
  • An eye injury has occurred.

How Is an Ophthalmic Exam Performed?

An ophthalmic exam may include many different tests. The most common tests are briefly described here. Your veterinarian may choose to conduct some or all of these tests, depending on the nature of your horse's problem.

The ophthalmic exam often begins with an evaluation of the horse's vision. The veterinarian may observe how the horse moves around the stall or pasture. Performing the ophthalmic examination in a darkened, quiet area is ideal for evaluating the horse's eyes. A menace test may also be conducted to see if the horse blinks when a finger is moved toward, but without touching, the eye. A pupillary light reflex test is used to evaluate the retina (the sensory membrane that lines the eye's interior), the muscles controlling the iris (the colored portion of the eye), the nerves, and the part of the brain that controls visualization. The veterinarian shines a bright light into each eye to evaluate both eyes for pupil constriction. An ophthalmic exam usually includes a thorough evaluation of the outer eye structures, including the tissues around the eyes, the eyelids, the duct where the tears drain from the eyes, and the nerves that affect the eyes. At the same time, the veterinarian will check the eye for inflammation and infection as well as for foreign bodies and unusual growths. The lens of the eye will also be examined for signs of cataracts. An ophthalmic exam also includes a thorough inspection of the interior of each eye. A few drops will be placed into your horse's eyes to temporarily dilate (enlarge) the pupils. It may take a few minutes for the drops to work. The veterinarian will use a special instrument to examine the interior of the eye, including the retina, the blood vessels, and the optic nerve. If the veterinarian is concerned about tear production, he or she may perform a Schirmer tear test. During this test, a small strip of specialized absorbent paper is positioned in each lower eyelid and held in place for 60 seconds. During this time, the paper absorbs and measures the amount of tears produced. This test can help determine if your horse is producing enough tears to lubricate the eye properly.

Because of their large, protruding eyes, horses often inadvertently scratch their cornea (the clear layer on the front of the eye). Because these painful abrasions or ulcers are not always visible with the unaided eye, your veterinarian may conduct a fluorescein stain test. For this test, a small amount of fluorescent lime-green dye is placed in the eye, and any defect in the cornea absorbs the dye, displaying the location and size of the abrasion.

Another painful condition for horses is equine recurrent uveitis, which is also known as moon blindness. This condition is due to inflammation of the eye's interior. Certain breeds are predisposed to developing this disease; if not treated, it can lead to blindness.

Other diseases, such as glaucoma, can occur in horses. Glaucoma occurs when the fluid pressure inside the eye is too high. This condition is painful and can lead to blindness. If necessary, your veterinarian can check the pressure of each eye by gently tapping the eye surface with a special instrument that looks like a pen. Before testing the eye pressure, the veterinarian places a few ophthalmic anesthetic drops in the eye to numb the eye surface. The pen-like instrument measures the eye pressure. High pressure is a sign of glaucoma, while low pressure may be a sign of uveitis (inflammation inside the eye).

What Are the Benefits of an Ophthalmic Exam?

If you notice any abnormality in your horse's eyes or vision, contact your veterinarian immediately. Many eye conditions are extremely painful or could result in the loss of vision, if not attended to promptly. An ophthalmic test will help identify the source of the problem so that your horse receives proper treatment and pain relief as soon as possible.