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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’



(Figure 2)

are categorised by

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.

Uveitis is inflammation of the middle

layer of the eye, the uvea. 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 most commonly recognised disease

entity of the equine eye. Uveitis is a

painful eye condition.

A range of clinical presentations may be

seen but in general the clinical signs are

non-specific inflammation of the uvea.

Treatment can be lengthy and complicated.

Prognosis is good with prompt diagnosis

and treatment of simple cases but

complications that interfere with vision

are common with delayed treatment and

severe cases.

Figure 2: Equine cataract

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 might also 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).


Cataracts block the visual image as they

increase in size, but don’t block light.

Congenital (present at birth) cataracts are

seen in foals, often 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 will determine if ERU is

also present; this 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)

Retinal disease and dislocation/


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

(Figure 3)


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 and can

occur in one or both eyes. It can be a

complication of ERU

(Figure 4)


associated with congenitally small

eyes in foals, head trauma, wounds that

cause the cornea to rupture, cataract

surgery or secondary to intraocular


Figure 3: Ultrasound image showing

detachment of the retina

Figure 4: ERU and retinal detachment