If 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, with a wide-open eye and no apparent pain (Figures 1, 2 and 3). As we’ll see later, this is not universally the case and some animals can have considerable ocular discomfort with an epithelial erosion, but many do not.
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. That’s why people with a family history of glaucoma get free eye checks each year to catch a rise in pressure before blindness occurs. 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 for 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.
Having said that, ectopic cilia, where eyelashes grow 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 (Figures 4 and 5), 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 revolves 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 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 intraocular pressure 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 (dogs or cats) with chronic glaucoma do 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 shows us just 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.
Ocular pain in prey species
Finally, we should talk about ocular pain in rabbits and guinea pigs: prey species which, when I was a student, were a child’s pet nobody spent much money on or really worried about particularly. Thankfully now things have changed, but even so, our recognition of pain in such animals can be woefully inadequate. The whole raison d’etre of one of these prey animals is to cope with any painful focus in the eye and not let it influence behaviour, for fear of being picked out by a marauding predator and eaten!
So maybe a rabbit with glaucoma or a guinea pig with keratitis is actually in substantial discomfort but just not showing it (Figure 6). And who knows, that may be the case with other pets such as dogs and cats. They may not be blinking and squinting, they may not be rubbing their eyes continually, but if a lesion is present that would stimulate pain receptors in us or a dog or cat, perhaps we should be putting in place the precautionary principle and giving them pain relief even though we have not got hard and fast evidence that they need it. Their welfare must be our top concern.