Eye injury and disease is fairly common in equine practice, in part because of a horse’s flight response causing it to react quite violently to a sudden noise or unexpected object. Horses will pull their heads back or swing round quite suddenly and bang the facial area into fixed objects in the stable or poke the eye on branches or blackthorns in the hedgerow.
Examining an eye
When a veterinary surgeon undertakes an eye exam, it is usually necessary to have access to a stable or exam room with subdued lighting so that the pupil will dilate, allowing close inspection from front to back. Light sedation is often beneficial as patients with a sore eye will react to bright light, which can be quite painful. Horses also have very strong eyelid muscles, making examination very difficult. Local anaesthetic nerve blocks also help reduce the pain and immobilise the muscles, making it easier to examine the eye thoroughly.
Visual examination of the eye is undertaken using an ophthalmoscope, which allows the vet to look at each layer and chamber in turn, from the cornea at the front, through to the retina at the back.
Staining the eye surface
We routinely apply a stain to the surface of the eye, to look for evidence of damage to the cornea. Normally, the stain does not stick to the cornea. Where there is damage to the surface, the stain will stick, outlining the area damage.
We use the same technology as is used for pregnancy diagnosis and tendon scanning to look at the internal structures of the eye. This is particularly useful when the patient has a painful eye which it is reluctant to open, or when the injury causes the eye to become very cloudy or filled with blood.
COMMON EYE PROBLEMS
Corneal ulcers – One of the commonest eye problems involves trauma to the surface of the eye, the cornea. Superficial and deep abrasions and puncture wounds often result from blunt trauma or when sharp objects such as branches or bits of straw scrape the surface layers off the cornea, resulting in pain, inflammation and secondary infection. An orange stain is dropped onto the eye, which turns green and sticks to areas of damage. Treatment involves topical anti-inflammatory and antibiotic drops. Left untreated, ulcers can progress to cause more severe inflammation deeper in the eye.
Uveitis / Moon blindness – Inflammation of the inner structures of the eye around the pupil is called uveitis (YOU-VE-EYE-TUS) and is a relatively common serious cause of eye injury, which can lead to blindness. One form of uveitis, known as moon blindness can result from the immune system attacking the body’s own tissues, causing recurrent episodes of internal inflammation. Treatment should be prompt and aggressive to avoid cumulative damage to the eye and progressive blindness. We would normally hospitalise a patient with uveitis and insert special tubing through the eyelid to allow the frequent application of several drugs every few hours without causing discomfort and leading to a head shy patient.
Eye tumours – Cancer of the eye and surrounding structures tends to occur in older animals and is more common in those lacking pigment around the eye. Any of the structures can be involved but the third eyelid and conjunctiva are the usual sites of tumour growth. If treated early, many of these tumours can be cured without the need to remove the eye. In our practice we use a combination of surgical removal of the tumour, along with the injection or topical application of a variety of chemotherapy drugs.
Detached retina – Separation of the retina from the back of the eye will cause blindness and usually results from head or facial trauma. Diagnosis of this condition may only be possible using ultrasound examination of the eye as shown in the image.
Eyelid lacerations– Being relatively prominent tissues, the eyelids are very susceptible to trauma when horses turn their heads suddenly, pull back through a fence or have a disagreement with a field mate. Eyelid wounds should always be reconstructed promptly and carefully, ensuring that the edge of the eyelid where the lashes exit, is re-aligned to create a smooth edge. Failure to do this will result in long term problems with the tear film spreading across the eye surface.
Fractures of the orbit – The bones forming the eye socket form a frame around the eye itself, absorbing any direct impact in an effort to protect the eye from damage. Ultrasound examination of the area can help diagnose the fractures, which are often difficult to see on X-ray. Most fractures will heal well, although some may need to be surgically reconstructed with steel wire if the bones have been displaced too much.
Blindness – Partial blindness in horses is relatively common and affected patients cope extremely well with one functional eye. Loss of sight in one eye can be due to eye trauma, cancer or severe infection or inflammation, resulting in a shrunken non-functioning eye, or necessitating surgical removal of the eye, a procedure that we would normally undertake with standing sedation and local anaesthesia, avoiding the risks of a general anaesthetic.