RED eye at night, ophthalmologists delight was the intriguing title of a presentation by Dr David Williams from Cambridge University at a CPD meeting on ophthalmology organised by CEVA Animal Health. The talk turned out to be a fascinating overview of the diagnoses and treatment of conjunctivitis, uveitis and glaucoma in dogs and cats.
Dr Williams explained that with conjunctivitis, there are no changes within the eye itself. This contrasts with uveitis, which may be associated with a constricted pupil, iris-lens adhesions and pus within the eye (hypopyon), and glaucoma, which is often associated with a dilated pupil, corneal oedema and globe enlargement.
In the darkened lecture theatre, Dr Williams showed photographs comparing and contrasting the different conditions and explained the importance of turning off the lights when examining eyes with an ophthalmoscope.
A number of underlying causes of conjunctivitis were discussed, for example distichiasis, which may require removal by electrolysis, and immune mediated lymphocytic-plasmacytic keratitis in German Shepherd Dogs.
Allergies
The significance of dry eye was also emphasised and a Schirmer Tear Test (STT) was recommended in all conjunctivitis cases, including an assessment of the unaffected eye. Dry eye is generally treated with topical cyclosporine although parotid duct transposition was also discussed as an option.
Allergies are an important cause of conjunctivitis, which helps to explain why conjunctivitis is most prevalent in August and least prevalent in January. In some cases, it may be possible to identify the cause and administer topical antihistamines, such as sodium cromoglicate (Opticrom, humanlicensed product containing sodium cromoglicate manufactured by Aventis Pharmaceuticals) before known allergen exposure.
Dr Williams then described a case of a dog that would always get conjunctivitis after chasing the drive-on lawnmower.
Bacterial conjunctivitis is also common and involves both Gram positive bacteria (which form part of the normal flora of the eye) as well as Gram negative bacteria (such as Proteus sp. and E. coli), which Dr Williams suggested are likely to be the faecal flora that dogs may pick up when sniffing each other’s bottoms!
Topical antibiotics were then discussed and it was described that there is now a high level of resistance to fusidic acid in both dogs and man. In a recent study in dogs carried out by Dr Williams, none of the Gram negative bacteria were sensitive to fusidic acid and only 40 percent of the Gram positive bacteria were sensitive (Williams and Harmer, 2006).
Other more broad spectrum antibiotics discussed by Dr Williams included chloramphenicol and gentamicin. Gentamicin is available in two preparations, one with a trometanol base (Tiacil, Virbac) and another with a hypromellose base (Clinagel, Ceva).
A study looking at first-line conjunctivitis cases revealed that, despite containing the same antibiotic, hypromellose-based gentamicin significantly reduced hyperaemia (redness) and irritation when compared to trometanol-based gentamicin, even in cases not associated with infection (Williams and Harmer, 2005) . This showed that choosing an eye antibiotic with a soothing base (such as hypromellose) is an important consideration.
After discussing canine conjunctivitis, feline conjunctivitis was then assessed. It was pointed out that, in contrast to dogs, cats are more likely to have an infectious cause of conjunctivitis, the common ones involving herpes virus, chlamydophila and/or mycoplasma.
In these cases, Dr Williams commonly carries out a therapeutic trial with topical chlortetracycline (Aureomycin, Fort Dodge) three times daily for seven days. If the cat gets significantly better, such that there is no conjunctivitis any more, then the cause is likely to be chlamydophila or mycoplasma, in which case Aureomycin should be continued and, in order to kill the nasopharyngeal bacteria as well, oral doxycycline or amoxicillinclavulunate should be added for three weeks.
If, however, there is a poor response to an Aureomycin trial, then it is likely the conjunctivitis is caused by herpes virus, in which case topical acyclovir (Zovirax Ophthalmic eye ointment) five times a day can be effective as can oral famcyclovir (1/8 of 125mg tablet per day) while oral lysine (500mg tablets crushed up and added to the food or now the new Enisyl oral preparation, newly introduced by Vétoquinol) can be useful in preventing recurrences.
Exotic infections
Uveitis in cats, like conjunctivitis, is often associated with infections, in particular FIV, FeLV, toxoplasmosis and FIP. In the dog, however, Dr Williams explained that most cases are noninfectious/idiopathic although exotic infections, such as ehrlichia, borrelia, leishmaniasis and fungi, should be considered if the owners have been abroad with their pet.
Other potential causes include autoimmune disease (for example Vogt Hoyanagi Harada syndrome in Japanese Akitas), lymphoma and lens induced uveitis (for example secondary to a diabetic cataract).
Dr Williams emphasised that if the uveitis was associated with severe miosis (pupil constriction), it is important to dilate the pupil, which can be carried out by giving one drop of atropine, tropicamide and phenylephrine respectively every 15 minutes until the pupil dilates. Once the pupil has dilated then this can be maintained with atropine three times daily.
Topical and systemic steroids are also needed to reduce the inflammation. If there is a corneal ulcer then topical NSAIDs such as ketorolac (Acular, Allergan –ahuman product not licensed for companion animals) could be considered.
Finally, glaucoma was assessed, which is commonly associated with dilated pupils, engorged scleral blood vessels, pain in the acute phase and blindness.
Although surgical methods are available, Dr Williams prefers a medical protocol involving the topical carbonic anhydrase inhibitor dorzolamide (Trusopt, Merck–again a human product) 2-3 times a day, which reduces aqueous production from the ciliary body, and latanoprost (Xalatan, Pharmacia – yet again a human medication), a prostaglandin analogue which opens up new drainage pathways.
Xalatan can dramatically reduce the pressure although it can constrict the pupil so much that the animal can hardly see.
Dr Williams therefore recommends administering one drop of Xalatan last thing at night. Unfortunately, if glaucoma has been left for 2-3 days then there may be little hope of salvaging vision in the affected eye and the best treatment in a blind painful eye may well be enucleation.
In all cases of glaucoma, however, Dr Williams emphasised the importance of treating the other eye with one drop of Trusopt every day to help reduce the chance of that eye developing glaucoma in the future.
The talk was overall a fascinating overview of the most common ophthalmological conditions in the dog and cat and demonstrated that ophthalmology could indeed be a delight at night rather than a warning in the morning!
