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Asking the experts about eye problems

Learn from two ophthalmology experts as they are posed questions on common equine eye issues

13 February 2019, at 2:53p.m.

Managing eye disease is an important part of equine practice, and an assessment of the eyes is an essential aspect of a pre-purchase examination. Here I ask two experienced ophthalmologists, Claudia Hartley and Jim Carter, to give their opinion on various eye issues commonly encountered by equine clinicians in first opinion practice.

At a pre-purchase examination, a horse is found to have peripapillary chorioretinopathy (butterfly lesions) in one eye. What criteria do you use to assess the significance of this finding and what would your advice be to the purchaser?

CH These lesions are considered a hallmark of previous equine recurrent uveitis (ERU) attacks involving the posterior segment. However, as far as clinical significance is concerned, if there are normal retinal vessels overlying the depigmented zone, I am encouraged that there is likely to be normal retina over this area too (the retinal vessels sit in the nerve fibre layer of the retina). I recommend re-examination in six months to check there has been no progression in these cases. Where vessels are attenuated or absent, I am very suspicious that the retina is similarly atrophic – suggesting an impact on vision.

JC I will always look for indications within the vitreous and lens for indications of a previous bout of uveitis. It is also important to look at the vascularisation within the retina coming off the optic nerve head and see if it is spanning the butterfly lesion. If there is marked vascular attenuation, then there are going to be significant retinal issues. I would always advise clients that we have found the lesion, and it is not necessarily possible to define how much visual impairment can be related to these lesions, but in those with significant vascular attenuation, you would expect there to be a significant visual field deficit.

Once a second episode of equine recurrent uveitis has been successfully treated in a horse, have you any suggestions to prevent further occurrences?

CH ERU is the most common cause of blindness in horses. So, at this stage, I would always discuss the surgical options for trying to prevent future ERU attacks. These include suprachoroidal sustained-release cyclosporine implant placement and pars plana vitrectomy (either transpupillary or endoscopically). Other management options involve long-term medical strategies (including low dose steroids), but this can come with problems in competing animals and the complications of any long-term medication.

JC Once we have treated our second bout of uveitis and confirmed that we have a patient that conforms to ERU criteria, prophylactic treatment with a low dose of phenylbutazone may have some benefits. Many of these patients potentially have subclinical uveitis grumbling along the whole time. We are not necessarily treating a reoccurrence, rather just a re-flare of the continued disease process. Checking the intraocular pressure is one way of monitoring the level of response to medication. A difference of more than 5mm Hg between the two eyes (the affected eye is lower) is significant. Many of these cases with repeat bouts of uveitis may benefit from a cyclosporine implant which can be surgically placed and can maintain a constant slow release of cyclosporine. This in conjunction with systemic medications can make a significant difference in these patients.

What are the indications for using the topical NSAID ketorolac tromethamine (Acular) in horses? What should the treatment interval be?

CH It is useful in situations where you need anti-inflammatory action, but steroids are contra-indicated eg in corneal ulceration. I actually now prefer the NSAID bromfenac as I feel it is a bit more potent, but it is more expensive. Treatment interval depends on the indication – something low grade may manage with twice daily, but more aggressive inflammation might require as much as six daily (or even q2hourly in a hospital situation).

JC We would typically use this in patients with uveitis as an adjunct therapy to topical steroid usage. The analgesic effect of ketorolac appears to be relatively minimal in comparison to systemic flunixin, so I would not use this instead of a systemic non-steroidal anti-inflammatory but as an adjunct to it. I try to avoid using this drug in the face of corneal ulceration as there may be an increased risk of keratomalacia.

FIGURE 1 How would you respond if you were presented with a large iris cyst at a pre-purchase examination?
FIGURE 1 How would you respond if you were presented with a large iris cyst at a pre-purchase examination?

The eye lesion in Figure 1 was seen at a pre-purchase examination. What would your advice be?

CH This is a large iris cyst and can be treated quite effectively under standing sedation with diode laser deflation. This one could theoretically have an impact on vision when the pupil is small (in bright light). However, it is my experience that many of these cases have no discernible behavioural or vision problems and are an incidental finding. If the owner (seller or even purchaser) is concerned, then treatment is relatively inexpensive and curative so can negate any effect on the pre-purchase examination and value of the horse.

JC This lesion is a large iris cyst attached to the ventral granular iridica and more than likely is having some influence on vision in view of its size and location within the visual axis. This visual impairment will become more prominent in increased light intensity as the now constricted pupil will be significantly impaired. We can be certain this is an iris cyst in view of the fact we can transilluminate it as noted in the ventral part of this picture. I would typically advise surgical removal of the cysts either by laser surgery or, in the hands of an ophthalmologist, direct aspiration. This should significantly improve the horse’s vision and potentially performance.

What medications are suitable for subconjunctival injection and what should the treatment interval be if this is the sole route of treatment of an eye?

CH There are rather a lot of medications that can be used subconjunctivally (as well as many that can’t!) but some of the ones I use more commonly would be: dexamethasone sodium phosphate (lasts 8 to 12 hours), methylprednisolone (lasts approximately 1 week), triamcinolone (approximately 3 weeks) and voriconazole (48 hours). Generally, I choose the preservative-free formulations without a harsh pH (closest to 7.4).

JC I do not personally use subconjunctival injections for treating ocular disease, but in most of the species that we treat, if we have to use subconjunctival injections, I would tend to only use medications that have an aqueous base or could be given intravenously. The exception to this would be when we use subconjunctival steroids in some of our small animal patients, which are typically in a suspension rather than an aqueous solution.

Do you consider hyaluronan gel a useful adjunctive treatment for corneal ulcers? When should it be applied?

CH I really like the hyaluronan-containing products. They generally last longer (and people certainly report longer duration of comfort with these dry eye preps) so they don’t need to be applied as frequently as, say, the carbomer polymers or hypromellose agents. They are useful for cases with poor tear films (Rose Bengal staining can help to identify these cases) and indolent ulceration, but to be honest they can be useful in most corneal ulcers (but not full thickness/perforations) as a form of analgesia (soothing in poor tear films, coating exposed corneal nerves abraded by eyelid movements).

JC I routinely use hyaluronic acid-based topical lubricants for treating patients with ocular surface disease as I think there is a significant improvement in the level of comfort when a film quality has improved. These lubricants will also help the movement of the third eyelid and the upper and lower lid moving over the corneal surface, reducing irritation and generally improving timeframe for superficial corneal surface disease to improve. There are numerous different hyaluronan-based lubricants available on the market, and they are typically what I would advise clients to use to help train their horses and get used to topical medications.

A single linear opacity is seen in the cornea of a horse at a pre-purchase examination. What would your judgement be?

CH I’d definitely want to look at it with a slit lamp, but I think you might be referring to linear band keratopathy, which is stretched Descemet’s membrane (DM). These have been suggested to be linked to glaucoma, and definitely occur in some glaucoma cases. However, they have also been seen in apparently normal horses with no history of glaucoma. There are various theories as to their pathophysiology – globe stretching and stretching of DM (glaucoma), birth trauma and blunt trauma to the globe. A big red “glaucoma flag” for me is if they are associated with corneal oedema (Haab’s striae) or if they are multiple or branching. I would always discuss this finding with the client so that they are aware of the risks.

JC The presence of a linear lesion in the cornea does not necessarily mean that there is long-standing disease. Before I can give any advice with regards to the solution we would need to determine exactly what level within the cornea this lesion is – ie epithelial, superficial corneal stroma or Descemet’s membrane. Some of the superficial lesions may be indicative of previous ulceration or trauma, where similar deep lesions may be indicative of previous glaucoma or repeat bouts of uveitis. There is no necessarily safe or dangerous linear corneal lesion. Each must be judged on their individual merits and in light of clinical disease.

Kieran O’Brien, MA, MVB, PhD, MRCVS, worked as a clinician and lecturer at the University of Bristol before moving to Penbode Equine Vets in Devon 20 years ago. He is a columnist for Horse and Hound magazine and author of the book Essential Horse Health.

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Claudia Hartley, BVSc, CertVOphthal, DipECVO, FRCVS, is an RCVS and European Specialist in Veterinary Ophthalmology and Head of Ophthalmology at the University of Bristol.

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Jim Carter, BVetMed, DVOphthal, DipECVO, MRCVS, is an RCVS and European Specialist in Veterinary Ophthalmology and is based at the South Devon Veterinary Hospital.

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