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.
