Dealing with Ocular Pain

Introduction

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 having said that, 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 reflex 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 reflex 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 nonsteroidal 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 ether through nonsteroidals, atropine, classical opiates or tramadol.