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Artificial lens and optical implants to aid horses’ vision

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01 January 2013, at 12:00am

DR Chelsey Miller, a resident in veterinary ophthalmology at the University of Pennsylvania, recently described some of the recent developments in equine ophthalmology.

Almost all humans develop cataracts as they age but cataracts in horses are not age-related, she said. Cataracts can cause anything from light sensitivity to nearly complete blindness.

Some horses manage perfectly well with cataracts, even when one eye is virtually blind, but a cataract is a greater liability in equestrian disciplines requiring keen depth perception, such as jumping and polo.

Surgical removal of cataracts in humans is a relatively simple procedure. The surgery is more complicated in equines because general anaesthesia is required and, said Dr Miller, cataract removal is in fact the most complex procedure in veterinary ophthalmology.

In the past, the required minimum of two to three months post-operative care and regular medicating of the eye was practically as challenging as the cataract surgery itself.

During this time, horses often got resentful of having the eye area handled and the continual manipulating of the eyelid could damage the surgical site.

Nowadays veterinary surgeons can install a temporary catheter to deliver the eye medications and no direct handling is necessary. If the horse is quiet and not inclined to rub his head, he can even be hand-walked or turned out in a small paddock with a fly mask on during the rehabilitation period, she said.

Retinoscopy

Cataract surgery doesn’t give the horse perfect vision and researchers at North Carolina State University are looking into the use of an artificial lens to improve near- or farsightedness, she said.

Most horses are farsighted but retinoscopy, in which different lens refractions are used to estimate a horse’s vision, can help determine the degree of near- or farsightedness.

Because there are risks, cataract surgery isn’t undertaken lightly and most veterinary surgeons are reluctant to do it if the horse has some vision. “It’s possible you could start with an eye with some vision and end up with no vision,” she said. 

Equine recurrent uveitis (ERU) is a chronic, immune-mediated eye disease that is not yet fully understood, Dr Miller continued. It is essentially an immune response gone haywire. An antigen gets into the horse’s eye and triggers a continuous, inappropriate autoimmune response; the result can be pain, glaucoma, and eventually blindness.

There might be no obvious signs of eye trauma prior to an ERU episode, Dr Miller said, adding that any bout of eye trouble, such as tearing, a scratch or swelling, should be followed up with twice-yearly check-ups because such incidents could set the stage for ERU.

An optical implant impregnated with the immunosuppressant drug cyclosporine had been developed which could reduce ERU flare-ups to fewer than once a year, she said.

“An implant lasts for four to five years, and many horses never need a second one. You can still have flare- ups, you still can wind up with a blind eye, but it’s the best treatment we have so far.”

Spooky behaviour

Some spooky behaviour can have physical causes, Dr Miller said. For instance, horses – as well as alpacas, cows and goats – have a structure called a corpora nigra in each eye.

It’s a pendulous cyst on a stalk that hangs down over the top of the eye but its function is unknown; researchers postulate that it may function as a sunshade of sorts.

A corpora nigra can grow abnormally large and can even move around in the eye. It is reasonable to suspect that a growth that shades part of a horse’s field of vision could well lead to spooking as objects appear and disappear suddenly.

Although diseases of the equine eye are well studied and documented, less is known about the effects of vision problems on horse behaviour, she said, and she referred to the work of Dr Richard McMullen, assistant professor of ophthalmology at North Carolina State University’s College of Veterinary Medicine, who is conducting vision studies on this topic.

“This ground-breaking research could help us learn more about why our equine friends behave as they do,” she said.