EQUINE EYE DISORDERS

THE safety of a horse and its ability to do its work depends heavily on its vision. Whilst many horses can cope fairly well with compromised vision, especially where this develops slowly, visual compromise will necessarily preclude the animal from being ridden. In spite of this there are in fact many horses that work well even though they have obvious, compromising eye disease.

The outward evidence of ophthalmic disease is obvious when blepharospasm (excessive blinking), epiphora (overflow of tears), eye-rubbing, head tilt, obvious asymmetry of shape or size when compared to the normal eye, changes in the clarity of the cornea and obviously abnormal discharges are seen.

More subtle changes associated with ophthalmic pain include downturned eyelashes, drooping of the upper eyelid, enophthalmos (eye drawn back into orbit) and photophobia (sensitivity to bright light).

In this article four groups of equine eye disorders will be described. These are:

Keratitis (corneal inflammation) (ulcerative or non-ulcerative).

Equine immune-mediated (recurrent/persistent), traumatic & reflex uveitis.

Cataracts and other lens conditions.

Retinal disease and dislocation/detachment.

KERATITIS

Corneal ulceration is a potentially sight-threatening disorder requiring early diagnosis, laboratory confirmation of micro-organisms and appropriate therapy. Viral, bacterial and fungal species may be involved either as a primary cause or as secondary infection and each requires prompt therapy if serious ocular complications are to be avoided.

Ulceration should be considered in every acute or chronically painful eye and infection should be considered in every corneal ulcer. Fungal involvement should be suspected with a history of corneal injury with plant material or if the ulcer has received prolonged antibiotics and/or corticosteroids with poor or no improvement. Many early cases of ulcerative keratitis present as minor corneal epithelial ulcers or infiltrates with pain, blepharospasm, epiphora and photophobia.

UVEITIS

Uveitis can be grouped into traumatic, reflex or recurrent/persistent types. It can occur as an intraocular primary event or as a result of any other ocular disorder (secondary/reflex uveitis). Immune mediated equine recurrent uveitis (ERU) is the commonest recognised disease entity of the equine eye.

A range of clinical presentations may be seen but in general the clinical signs are non-specific inflammation of the uvea (vascular middle layer).

Clinical signs are severe pain, myosis (reduced pupil size) with varying corneal oedema (blue-white discolouration), blepharospasm and epiphora.

Effective early treatment may restore structural and functional normality. If treatment is ineffective or is delayed then the inevitable damage will be posterior synechiae (adhesion of iris to the anterior lens) and cataracts.

The outcome is dependent on the cause and severity and the speed at which the diagnosis is made and effective treatment instituted. An accurate history and careful clinical examination may establish the cause and this will have a significant implication on the outcome.

CATARACTS & OTHER LENS CONDITIONS

The lens forms part of the focusing system that delivers sharp images onto the retina and has three zones which, from the centre, are the nucleus, the cortex and the lens capsule. A cataract is defined as any opacity (cloudiness) within any of these three layers. The position of opacities and their size/extent will determine the amount of visual impairment. Most horses appear to cope well with ‘minor’ lens changes however behaviour and athletic ability are known to be affected by ‘significant’ cataracts.

Cataracts are categorised as to their level of maturity. Incipient/early cataracts involve small areas of the lens and do not affect vision. Immature cataracts involve more of the lens with increasing effects on vision. Mature cataracts involve the entire lens and cause blindness.

Cataracts block the visual image as they increase in size, but don’t block light. Congenital (present at birth) cataracts are seen in foals, in both eyes. In adult horses, cataracts might be caused by trauma, nutritional deficiencies or toxicities, or be secondary to other conditions such as ERU.

An examination determines if ERU is also present which is especially important when cataract surgery is being considered, since there is an increased risk of complications and a poorer prognosis for vision when uveitis is the cause of the cataract.

LENS LUXATION (Dislocation)

Due to a congenital defect in foals or severe trauma in adults, the lens can luxate forward or backwards from its normal position. Movement of the iris from lens contact, shallow or deep anterior chambers, and aphakic (no lens) crescents (edge of lens seen) might be present.

Cataract formation also might be noticed. Dislocation of the lens into the vitreous humour (gel between lens and retina) might not necessitate surgery; however, movement into the anterior chamber usually requires removal to prevent secondary glaucoma (increased intraocular pressure).

RETINAL DISEASE

Chorioretinitis is inflammation of the choroid and retina. It can be caused by infectious agents, a poorly controlled immune system, trauma or vascular disease. It can be found with or without ERU. It can be seen as focal “bullet-hole” lesions, diffuse (spread out) lesions, horizontal bands in the non-tapetum (non-reflective back of eye) and chorioretinal degeneration near the optic nerve. Active chorioretinitis appears as focal white spots with indistinct edges, and as large, diffuse gelatinous grey regions of retinal oedema (fluid swelling). Inactive chorioretinitis can appear as circular depigmented white regions with hyper pigmented (darkened) centres, or large areas of depigmentation that appear similar to the wings of a butterfly.

Congenital stationary night blindness (CSNB) is found mainly in the Appaloosa, and is inherited as a recessive trait. Cases are also noted in thoroughbreds, paso finos, and standardbreds. CSNB appears to be caused by a failure of neurotransmission in the middle retina. Clinical signs include visual impairment in the dark with generally normal vision in daylight. There is behavioural uneasiness and unpredictability at night.

Retinal detachment is separation of the layers of the retina which can be partial or complete. It is associated with slowly progressive or acute blindness.

It can be congenital in foals or acquired in adults & can occur in one or both eyes. It can be a complication of ERU and associated with congenitally small eyes in foals, head trauma, wounds that cause the cornea to rupture, cataract surgery or secondary to intraocular tumours.

A keen side saddle rider, Carlow native Aoife Byrne Dr.Med.Vet. MRCVS Cert AVP is a graduate of Szent Istvan Veterinary University in Budapest, Hungary, and currently works at Chapelfield Equine Clinic in Norfolk. She acquired specialist equine ophthalmology experience during a four-year stint in Gloucestershire, working alongside Tim Knott of the Equine Eye Clinic.