Examining the equine eye *H&H Plus*

What does an ophthalmic exam for a horse entail? Ria Chalder MRCVS explains all

You make sure your horse’s feet are trimmed regularly and his teeth are checked routinely, but what about his eyes? Would you know if he was having vision problems?

Several situations call for an equine ophthalmic examination. The most likely is an obvious issue, such as apparent pain in an eye or a change in its appearance. A vet will examine a horse’s eyes during a pre-purchase exam, to look for any abnormalities that may affect his future use or performance. Certain behavioural issues, such as head-shaking, spooking or a recent deterioration in the horse’s confidence, may also warrant an ophthalmic exam.

It is a good idea to arrange an eye check once a year, at the same time as booster vaccinations, so problems can be spotted early and monitored closely.

Assessing vision

An equine ophthalmic examination is very different from what you may have experienced yourself when visiting an optician.

After taking a thorough health history, your vet will observe how the horse interacts with his environment and assess the symmetry of his eyes and face. Any swellings will be noted, as well as abnormalities with eyelid position or ocular discharge.

The angle of the upper eyelashes should be the same on both sides; lashes pointing downwards may indicate pain or a more serious neurological condition. A small amount of clear ocular fluid or dry, black/brown discharge in the corner of the eyes is normal, but excessive fluid or discharge that is white, yellow or green is not, and may signal an infection.

The vet will also check that the third eyelid, termed the nictating membrane, is in its normal position inside the corner of the eye. If it is visible at rest, this may indicate a problem.

Once general observations have been made, the next step is to evaluate the horse’s vision. Determining how much a horse can see is quite challenging. While we may be accustomed to reading aloud the letters we can see on an eye chart, this test of visual acuity, or sharpness, is obviously not an option. Instead, a vet will use a combination of tests to try determine what the horse can see out of each eye in turn.

The first is to assess certain nerve reflexes, or responses to stimulation. If a horse blinks when a threatening hand gesture is made towards his eye, a test known as the menace response, it can be assumed that he could see the hand coming.

Since this protective reaction is developed in the first few weeks of life, a young foal may not have a positive menace response – despite being able to see. The test can also be unreliable if a horse is very anxious, or if nerve damage is preventing him from blinking.

It is important that additional tests are performed and the results are interpreted together. The pupil should constrict, for example, when the vet shines a bright light into the eye. This pupillary light reflex (PLR) reveals more information about the various parts of the eye and brain required for vision.

Inside info

With the hands-off assessment completed, your vet will look more closely at the different parts of the eye using various magnification tools. This stage of the exam should be performed in a darkened stable where possible, to reduce light interference and maximise the amount of observable detail. Sedation may be necessary. Nerve blocks can also be injected under the skin surrounding the eye to paralyse the eyelid muscles, so the horse cannot clamp the eye shut.

A handheld instrument called a direct ophthalmoscope is first used to assess both the retina – the area at the back of the eye which senses light – and the optic disc, where the optic nerve takes information to the brain. At a referral hospital, a head-mounted version of this tool (an indirect ophthalmoscope) will offer a wider field of view. To observe the back of the eye more easily, your vet may administer tropicamide drops to dilate the pupil.

Some vets (especially a veterinary opthalmologist) will also use a slit lamp, which enables much greater magnification and a more in-depth examination of the structures of the eye: the cornea (the clear outer layer at the front), the coloured iris, and the lens, the usually transparent structure located behind the iris. A wide range of conditions can be detected with the slit lamp, including keratitis (inflammation of the cornea), uveitis (inflammation inside the eye) and cataracts (clouding of the lens).

Glaucoma is a condition in which the optic nerve is damaged, usually due to a build-up of fluid that causes high pressure within the eye. While relatively rare in horses, glaucoma can cause blindness. It usually occurs secondary to chronic, repeated episodes of internal inflammation, such as those caused by equine recurrent uveitis.

If glaucoma is suspected, your vet will check the intraocular pressure, or IOP, usually using a rebound tonometer – a handheld device that bounces a lightweight probe off the surface of the eye. The equivalent for humans is a puff of air released onto the eye, known as non-contact tonometry. A high reading in the horse may indicate glaucoma, whereas a low reading may indicate uveitis.

Further tests

If the eye is painful and a corneal ulcer is suspected, your vet may apply an orange dye called fluorescein to the surface of the eye. This harmless liquid turns green and sticks to the parts of the cornea that have lost the top layer, revealing the area of ulceration.

A lack of tears – a condition known as keratoconjunctivitis sicca, or dry eye – may lead to recurrent corneal ulcers or poor ulcer healing. The Schirmer tear test may be used if it is thought a horse is not producing enough tears. For this, an absorbent paper gauge is tucked inside the lower eyelid and removed after a minute to measure the moisture produced.

To investigate suspected infection on the surface of the eye, swabs will be taken from the affected area for laboratory analysis. Any bacteria or fungi present is identified and the appropriate antibiotics selected.

An ultrasound scan may be performed if it is not possible to see into the eye, or if a mass is suspected. For more in-depth investigation, it may be necessary to refer the horse to a specialist veterinary ophthalmologist.

What can a horse see?

A horse’s eyes are positioned on either side of his head. The resulting vision is largely monocular, meaning his brain receives different visual information from each eye to create an almost panoramic view. Although this helps a horse detect an approaching predator, he sees less detail than species such as humans.

A horse has two blind spots: one in front of him and one directly behind. Horses tend to be long-sighted, most likely due to their evolutionary adaptation as prey animals. Although it is generally accepted that they possess some form of colour vision, this appears to more closely resemble that of colour-blind humans.

Another piece of the puzzle

While assessing a horse’s vision remains relatively subjective, one of the more reliable methods is the maze test. This involves blindfolding each eye in turn and encouraging the horse to manoeuvre through an obstacle course of objects such as upturned buckets and hay bales, placed in random positions.

Struggling to negotiate the obstacles without knocking into them may reveal significant visual problems in one or both eyes.

Ref Horse & Hound; 5 November 2020