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20
OPHTHALMOLOGY
VETERINARY PRACTICE FEBRUARY 2016
Dealing with ocular pain
IF any of you have been unlucky
innervated area of the body, at least in
enough to experience a corneal
the human. This may not be the case
ulcer, you will know how
in all dog or cat breeds – we know
excruciatingly painful such a
that brachycephalic animals have a
trauma can be. And yet every week
lower number of corneal nerves – and
I see a dog or cat with a strikingly
this may explain why corneal ulcers
similar
ulceration,
Dr DAVID WILLIAMS
yet with a
discusses ways of assessing ocular
wide open
pain in animals and the panoply
eye and no
of pain-relieving treatment
apparent
options, and includes a reminder
pain.
to get to the underlying cause of
To move
the problem…
to another
example of a potentially painful
seem less painful in many companion
ocular condition, one of the problems
animals than in people.
with primary open angle glaucoma
Yet ectopic cilia, where eyelashes
in humans is that the condition is
growing out from the meibomian
pain-free and it is not until significant
glands of the lid at right angles to the
blindness ensues that the problem is
corneal surface, can be exceptionally
made evident. Yet glaucoma in many
painful.
dogs is an acutely painful condition.
One of the things that we see in
How are we to correlate ocular pain
such cases is a miosis, a constriction of
in people, where nociceptive signs can
the pupil. This seems to occur through
be reported verbally, and pain in ocular an antidromic reflex in the trigeminal
conditions in animals where such
nerve which supplies the sensory
overt reporting of the pain is clearly
nerves to the ocular surface.
impossible. How are we to assess
Severe ocular surface trauma leads
ocular pain in animals and
to a breakdown of the
what are the best ways of
blood aqueous barrier
treating it?
and a spasm of the iris
Pain is defined as an
and ciliary body muscles
unpleasant sensory and
which itself can lead to
emotional experience
substantial ocular pain.
associated with actual or
This is without doubt the
potential tissue damage,
case in uveitis, intraocular
or described in terms
inflammation which can
of such damage. We can
be particularly painful.
relatively readily determine
Here and in corneal
Figure 1a.
the sensory part of the
ulceration with reflex
painful experience in
ciliary spasm, topical
terms of the anatomy and
atropine can be really
neurophysiology of the
helpful in reducing ocular
nociceptive response, but
pain.
evaluating the emotional
The mechanism of
surface pain
response is difficult in
ocular
Figure 1b.
different humans let alone
resolves around free nerve
in companion animals.
endings in the epithelium
of the cornea and
Causes and cures
topical analgesics such as
proxymetacaine can be
of ocular pain
The ocular surface is, it
helpful in the short term
is said, the most highly
but may be epitheliotoxic
Figure 1c.
if given over a prolonged
period. Topical non-steroidal antiDavid L. Williams, MA, VetMB,
inflammatories can be really useful in
PhD, CertVOphthal, CertWEL,
such circumstances.
FHEA, FRCVS, graduated from
It is not surprising that ocular
Cambridge in 1988 and is now
surface trauma and intraocular
associate lecturer in veterinary
inflammation should cause ocular pain,
ophthalmology there, a position
but another serious ocular condition
he combines with teaching
with pain as a frequent but not
at St John’s College, where
universally present sign is glaucoma, or
he is fellow and director of
rise in intraocular pressure.
studies in veterinary medicine
The acute rise in IOP in most canine
and pathology. He also
glaucoma cases results in substantial
handles referrals in veterinary
pain which can be difficult to manage
ophthalmology. For details see
without a reduction in the pressure,
www.davidlwilliams.org.uk.
but it can be the case as in people with
progressive open angle
glaucoma, that animals,
dog or cat, with chronic
glaucoma, may not appear
to be in pain.
In fact, often when
the raised pressure is
controlled medically or
Figure 2a.
by enucleation, suddenly
owners realise that
what they had taken for
the animal just getting
gradually older is actually
a lethargy induced by
chronic unremitting pain.
Figure 2b.
This just shows us how
important it is to ensure that any ocular
condition which could cause pain is
as well-managed as possible, both by
resolving the ocular disease and by
providing pain relief either through
non-steroidals, atropine, classical
opiates or tramadol.
But before we get into the realms
of pain-relieving drugs and which are
best to use in which potentially painful
ocular conditions, we live today in a
world where an evidence base is held
to be all important.
So we must ask the question: have
we got a decent enough evidence
base for how much pain different
ophthalmic conditions in the dog and
cat cause? To my mind the answer is a
resounding no! Nobody seems to have
put together a pain score for the eye.
So, as a very basic starter for 10,
as Bamber Gasgoine would have
said in the days when he hosted
University Challenge, I’ve put together,
in collaboration with a colleague Agata
Grudzien from Poland who worked
with me a couple of summers ago, a
table of different clinical signs which
might add together to produce a pain
score.
We presented this at the BSAVA
congress as a clinical research abstract
last year but as with so many other
interesting projects, I simply haven’t
had a chance to write it
up for publication in the
peer-reviewed literature.
Here is your chance to
review it now!
We assessed 50
dogs with ophthalmic
conditions likely to
Figure 3.
be painful using a set
of criteria likely to indicate ocular
pain. These included behavioural
changes (decreased appetite and
increased lethargy), those general
signs potentially linked to acute
pain (increased vocalisation and
panting) and specific ocular signs
(blepharospasm, aversion to ocular
examination and tearing). These were scored as absent (0),
mild (1) or severe (2). The mean and
standard deviation of the
signs evaluated are shown
in Table 1.
Neither vocalisation
nor panting seemed to be
evident in animals with
varying degrees of ocular
pain. Increased lethargy
appeared to be more a sign
of ophthalmic discomfort
than was decreased
appetite. Blepharospasm
and aversion to
examination of the eye
were important signs while
photophobia appeared
more severe in uveitis than in corneal
ulceration and glaucoma.
Quantification of ocular pain
may prove to be particularly useful
in defining when dogs with ocular
disease are in need of analgesia and
in measuring responses to pain relief.
More work is needed to evaluate this
scoring system and apply it to a larger
number of dogs. In particular, at
present each sign has been given an
equal weighting but further research
may show that some signs need to be
given greater weighting than others.
But it is one thing being able, in a
semi-quantitative manner, to quantify
ocular pain and quite another to know
how to relieve it. If there is an ocular
surface element to the pain, then a
topical non-steroidal medication such
as ketoroloac in Acular, diclofenac in
Voltarol or flurbiprofen in Ocufen
should be really helpful.
There are just two little caveats here.
One is that a key time when ocular
surface pain relief might be required is
in a corneal ulcer. There are reports of
ulcers turning to melting lesions when
such topical non-steroidals are used. In
truth, such reports involve individual
human cases and a Cochrane database
meta-analysis showed no increased
risk of a melting ulcer when NSAIDs
were used over ulcers with standard
treatments.
The other issue is
that we all know about
spinal wind-up in cases
of long-term pain in,
say, a limb; is there spinal
wind-up in the case of
ocular pain? The answer
is a resounding yes: the
spinal ganglion of the trigeminal nerve
has just the same effects as does spinal
wind-up in the dorsal horn, increasing
pain sensation on the ocular surface.
So using a topical NSAID on its own
may well only have a partial effect on
ocular pain. A systemic medication per
os such as carprofen or metacam may
be a better bet in combating ocular
surface pain. It is difficult to evaluate
continued on page 22
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OPHTHALMOLOGY
VETERINARY PRACTICE FEBRUARY 2016
a very miotic constricted
pupil (Figure 2a). There
is a muddy inflamed iris
here and an irregular
pupil border. One drop
of atropine did nothing
Figure 6a.
and it took a couple
of hours of a drop of
atropine followed by one
of phenylephrine, then
one of mydriacyl and then
back to the atropine before
we prised the pupil open
(Figure 2b).
Figure 6b.
Look at the
haemorrhage where the
pupil had been tight
shut: in the time the
inflammation had been
developing, new blood
vessels had grown with
Figure 7.
the synechiae that adhered
the uphill to the anterior lens capsule.
If we had sent the owner home with a
bottle of atropine drops to put in three
or four times a day (or not given what
we said above!) the problem would just
have got worse.
Interestingly, the dog didn’t seem in
much discomfort but maybe that just
shows how difficult it can be to assess
pain in some stoic dogs. Or indeed in
cats: just look at the kitten in Figure 3.
He was found in a shoe-box on the
doorstep of a local clinic one Saturday
morning. The eye, enlarged with a
high pressure, has an horrendous
exposure keratitis where the lids have
been unable to close and protect the
globe. But maybe the trigeminal nerve
endings that should be screaming
“pain! pain!” have died just as has the
optic nerve with a high intraocular
pressure. Even without obvious
pain manifesting itself,
enucleation is the obvious
answer here.
The Basset Hound in
Figure 4a has a blind eye
with engorged episcleral
vessels and a dilated pupil Figure 8a.
(Figure 4b). Even without
a tonometer one can be
reasonably sure that this is
glaucoma. The emergency
treatment used to be
intravenous mannitol but
Figure 8b.
now in most cases the
prostaglandin latanoprost opens up
new drainage pathways and reduces the
pressure: in this case from 48mmHg to
18mmHg in 20 minutes. The reduction
in pupil size (Figure 4c) signals a
successful treatment.
The next dog really is in ocular
discomfort and only
topical anaesthetic
Condition
lethargy vocalisation blepharospasm aversion
photophobia
tearing
and sedation allowed
Corneal ulcer 0.0±0.0 1.0±0.75 1.25±0.89 1.13±0.64 0.63±0.51 0.88±0.83
a good look at the
Uveitis 1.29±0.95 0.14±0.38 0.86±0.69 0.71±0.49 1.0±0.58 0.43±0.53 eye (Figure 5a). Can
you see anything?
Glaucoma 1.25±0.96 0.0±0.0 1.0±0.82 0.75±0.96 0.50±0.58 0.25±0.50
Figure 5b shows
the value of some
Table 1. Mean ± standard deviation of factors evaluated as signs of pain in ocular disease.
magnification as a
the effects of different
drugs in spontaneous
conditions such as corneal
ulcers, uvetis or glaucoma
– in each animal the
degree of the noxious
stimulus may be different
and the penetration of the
drug may be affected by
the damage itself.
Figure 4a.
A set of experimental
studies on pain relief in
laser-induced epithelial
erosions in rabbits has
been published, since
LASIK is such a widely
used technique of
refractive surgery and
Figure 4b.
pain relief after such
surgery is important to
optimise. They show,
perhaps not surprisingly,
that topical non-steroidals
do have anti-inflammatory
and analgesic effect after
laser epithelial ablation.
Figure 4c.
In fact, it is the studies
on human patients who can verbalise
their pain which show more precisely
that, in humans at least, diclofenac
appears the best of the non-steroidal
eyedrops at preventing pain. I must
admit to a preference for Acular,
as it comes in a multidose bottle
easy for owners to use at home, a
feature probably quite as important
if not more so than any complicated
pharmacodynamics.
It’s pretty pointless having a drug
with excellent transcorneal absorption
characteristics if the owners find it
difficult to administer. I don’t think
we pay enough attention to owner
compliance. If you look at people
affected with glaucoma
and their compliance with
eyedrops, it is frighteningly
poor.
In a recent survey on
pharma-adherence (that’s
to say patients adhering to
their drug regime rather
than drugs adhering to
Figure 5a.
the cornea!) 27% of
patients self-reported
poor compliance and who
knows how many lied
about their appropriate
use of medication; 95%
of the problems involved
difficulties administering
the drops.
I bet you would find a
similar issue in our patients.
It might be easy for us to
Figure 5b.
put drops in a dog’s eye
when it is stock still with
fear in our consulting room,
but how many owners find
it as easy to put the drops in
themselves when at home
where the animal feels more
ready to play up? One of
my new year’s resolutions is
to put together a survey of
exactly that question!
Interestingly, a veterinary
practice I work with allows
owners with dogs where
treatment of eye conditions
such as dry eye is not
succeeding to bring their
dogs in each time they need
a drop and get nurses to
ensure that medication is
given. It is remarkable how
many poorly controlled
eye conditions do improve
dramatically once the
medication really is getting
into the eye! That could
work with any disease of
course but is particularly useful where
there is ocular pain.
We could continue for some time
with these general comments, but
perhaps the best way to finish is to
look at a few specific examples and
how we deal with ocular pain in each
case.
First in comes a young cocker
spaniel with an exceptionally painful
eye (Figure 1): so painful he snarls and
snaps as soon as you try to look at it –
note the muzzle we had to put on him
to get the photos! Completely unlike
him say his owners: something really
must be wrong. But all that is evident is
one pupil significantly smaller than the
other.
The affected eye with
the miotic pupil (Figure
1b) can still see but the
intraocular pressure is
12mmHg compared
with 18mmHg in the
normal. This is a reflex
uveitis probably after a
transient injury to the
ocular surface. Atropine
dilates the pupil here but
also paralyses the ciliary
muscles, spasm of which is
causing most of the pain.
The pain is sufficient
that a dose of tramadol
would be worth adding
into the mix. A quick (and
effective) fix!
Next is a labrador with
short stubby hair is poking
out of the conjunctiva.
This is an ectopic cilium
and needs sharp knife
dissection to remove it and
the intense pain associated
with it.
Our next patient, an
elderly cat, certainly has
discomfort as can be
seen from the narrowed
palpebral aperture (Figure
6a). The reason for this
is the raised black lesion
in the cornea – a corneal
sequestrum (Figure 6b).
I use the degree of ocular
discomfort as my signal
as to whether surgery is
required, and a superficial
keratectomy is needed
here, probably with a
corneoconjunctival transposition graft
– but that needs referral to a specialist
centre.
A good lubricant such as Bayer’s
Remend is called for to soothe and
lubricate the roughened corneal surface
until surgery can be undertaken.
Amazingly, the boxer with the
corneal ulcer shown in Figure 7
seems unconcerned, although it will
need topical anaesthetic before we do
a grid keratotomy or diamond burr
debridement to promote its healing.
I make sure that these animals have
good systemic non-steroidal pain relief
after such a procedure: maybe in the
consulting room they are alert and
wide-eyed but once at home they may
well be significantly more obviously
showing pain.
This is the point to be sure to
mention the importance of a buster
collar to avoid self-trauma:
I do like the floppy blue
ones these days rather than
the firm white plastic ones.
Maybe there is another
little student project to
assess owner and animal
acceptance of each sort!
Chronic pain is a big
problem in human patients
and in our animals as well.
The aged cocker spaniel in
Figure 8 just seems to put
up with the ocular surface
irritation caused by the trichiasis and
also the mild dry eye – his Schirmer
tear tests are 8 and 10mm/min. His
owner is delighted by the improvement
in his demeanour the moment she
picks him up after his Stades procedure
to remove the eyelashes, leaving a bare
strip of subcutis which will granulate
leaving a hair-free eyelid edge.
Which only leaves the poor rabbit in
Figure 9. This extreme exophthalmos
caused by a retrobulbar abscess could
potentially be treated with orbital
amoxycillin beads but I felt that,
though as a prey species it was showing
no pain, euthanasia was the best