Dealing with ocular pain - Veterinary Practice
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Dealing with ocular pain

Dr David Williams discusses ways of assessing ocular pain in animals and the panoply of pain-relieving treatment options, and includes a reminder to get to the underlying cause of the problem…

IF any of you have been unlucky enough to experience a corneal ulcer, you will know how excruciatingly painful such a trauma can be. And yet every week I see a dog or cat with a strikingly similar ulceration, yet with a wide open eye and no apparent pain.

To move to another example of a potentially painful ocular condition, one of the problems with primary open angle glaucoma in humans is that the condition is pain-free and it is not until significant blindness ensues that the problem is made evident. Yet glaucoma in many dogs is an acutely painful condition.

How are we to correlate ocular pain in people, where nociceptive signs can be reported verbally, and pain in ocular conditions in animals where such overt reporting of the pain is clearly impossible. How are we to assess ocular pain in animals and what are the best ways of treating it?

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. We can relatively readily determine the sensory part of the painful experience in terms of the anatomy and neurophysiology of the nociceptive response, but evaluating the emotional response is difficult in different humans let alone in companion animals.

Causes and cures of ocular pain The ocular surface is, it is said, the most highly innervated area of the body, at least in the human. This may not be the case in all dog or cat breeds – we know that brachycephalic animals have a lower number of corneal nerves – and this may explain why corneal ulcers seem less painful in many companion animals than in people.

Yet ectopic cilia, where eyelashes growing out from the meibomian glands of the lid at right angles to the corneal surface, can be exceptionally painful.

One of the things that we see in such cases is a miosis, a constriction of the pupil. This seems to occur through an antidromic re ex in the trigeminal nerve which supplies the sensory nerves to the ocular surface.

Severe ocular surface trauma leads to a breakdown of the blood aqueous barrier and a spasm of the iris and ciliary body muscles which itself can lead to substantial ocular pain. This is without doubt the case in uveitis, intraocular inflammation which can be particularly painful. Here and in corneal ulceration with re ex ciliary spasm, topical atropine can be really helpful in reducing ocular pain.

The mechanism of ocular surface pain resolves around free nerve endings in the epithelium of the cornea and topical analgesics such as proxymetacaine can be helpful in the short term but may be epitheliotoxic if given over a prolonged period. Topical non steroidal anti-inflammatories can be really useful in such circumstances.

It is not surprising that ocular surface trauma and intraocular inflammation should cause ocular pain, but another serious ocular condition with pain as a frequent but not universally present sign is glaucoma, or rise in intraocular pressure.

The acute rise in IOP in most canine glaucoma cases results in substantial pain which can be difficult to manage without a reduction in the pressure, 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 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.

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 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).

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 urbiprofen 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 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.

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 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.

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 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 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.

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