Examination of the eyes is an integral part of the pre-purchase examination (PPE) and encountering “abnormal” ocular findings is common.
A large number of these so-called abnormalities are, in fact, just accepted variations of what is considered normal for certain ocular structures, such as changes in the colour of the fundus, which is the interior surface of the eye, opposite the lens. The significance of these findings may depend on whether they are isolated or accompanied by other signs.
To complicate matters, it is virtually impossible to determine how much a horse can see. Vets are often surprised to come across horses with severe and extensive pathology in their eyes that seem to be able to “see” and perform their duties without any obvious deficits.
To examine a horse’s eyes, the vet will need a dark stable or examination area, a light source and some form of magnification, typically provided by a hand-held ophthalmoscope. On some occasions, to reach a more definitive decision, it may be necessary to apply eye drops to dilate the pupil to assess the back of the eye further. More advanced equipment may also be used, such as an ultrasound scanner or a tonometer, which is an instrument that measures pressure within the eyeball.
Once a vet has detected an abnormality and determined its current significance, the next question is to determine whether it is a problem that is likely to progress or recur in the future. For some conditions, the answer to this question may be straightforward, but, in a large number of cases, the vet will have to make a subjective assessment on the likelihood of the abnormality causing further trouble.
When confronted with an ocular lesion — a diseased or damaged area — during a PPE, the vet will therefore try to classify the lesion into one of these categories: normal eye or normal variation; abnormal eye not significantly affecting the horse — vision or comfort — and not progressive; abnormality affecting the horse now or that will progress to affect his vision or cause ocular pain in the future.
Only those horses whose eyes belong to the first two categories should be considered suitable for purchase.
Further discoveries during PPE can include:
Corneal opacities: the cornea is the clear window at the front of the eye. Opacities, or clouding of the cornea, are a relatively common finding. Some of these, such as linear keratopathy (see box, right), may be benign, incidental findings, or may represent previous episodes of corneal damage, such as a corneal scar, for example, and tend to be non-progressive.
As long as the size or position of these opacities does not affect vision significantly and the rest of the eye examination is normal, horses with these opacities may be considered suitable for purchase.
Corneal oedema (fluid) or any evidence of inflammation within the cornea — as demonstrated by the presence of blood vessels or inflammatory cells — may be indicative of more serious eye disease, however. Purchase probably should be discouraged.
Cataracts: these are frequently found and are a common cause of concern during the PPE. They seldom cause any obvious visual deficits to horses. If the cataract is particularly large or dense, however, the horse should be considered unsuitable for purchase.
Most cataracts do not progress, unless they are a consequence of some other ongoing disease of the eye, most likely uveitis. Some will progress with time, though, and, except for certain clear-cut cases, this information on progression can only be determined with serial examinations every six to 12 months — something the potential purchaser should be aware of. While cataract surgery can now be performed at a number of centres, under no circumstances should a horse be purchased subject to surgical treatment.
Uveitis: this refers to inflammation of the uvea, the middle layer of the eye. Uveitis is a common condition of horses and, in a large number of cases, it will recur at regular intervals and cause ongoing damage to the inner structures of the eye.
The diagnosis of uveitis is relatively straightforward during an episode of active inflammation, but it is important the vet identifies signs of previous episodes when the examination is carried out during a quiet stage of the disease. Horses with signs of recurrent uveitis should be considered unsuitable for purchase.
Fundic changes: findings on the retina and other structures of the fundus are relatively common and, in a large number of cases, represent variations of normal. Congenital defects of the retina, called colobomas, are encountered occasionally, but rarely will these cause any significant visual deficit.
Other common lesions encountered are “bullet hole” or “butterfly” lesions, which represent areas of previous damage and scarring of the retina. Their significance will vary depending on their size, number and position within the retina, along with other findings of the ocular examination, such as the shape and colour of the optic nerve or the number of retinal blood vessels.
Cancerous changes: a number of tumours can grow on and around the eye or the eyelid. Sarcoids, melanomas or squamous cell carcinomas commonly affect the eye and periocular structures.
There are various treatment options available for these tumours, but, in most cases, they tend to be problematic in this location. The potential buyer should be discouraged from purchasing any affected horse.
Further problems that spell trouble and should probably be avoided are conditions that cause pain, redness, inflammation or discharge from the eye, or situations in which the eyes appear different in size.
For certain ocular conditions detected during a PPE, the vet will be able to make an immediate decision with regards to their significance, likelihood of progression and the horse’s overall suitability for purchase. In some cases, however, this may not be so easy and a referral to a specialist centre may then be necessary to obtain a second opinion from an equine ophthalmologist.
Ref Horse & Hound; 28 September 2017