The corners of the eye are called the medial (inner) canthus and the lateral (outer) canthus. The eyelids are fundamental in spreading the tears over the surface of the eye and maintain the protective tear-film. The third eyelid, a mucous membrane, springs across the eye when it is pulled backwards during blinking.
The cornea (front) is 3mm thick and is only clear because a lipid (fatty) superficial layer, and the salty tears bathing it, maintain a state of dehydration. If the surface is damaged, rehydration of the corneal stroma (middle part) turns the cornea opaque, appearing blue white, termed corneal oedema.
The sclera is a thick white structure, which covers the rest of the eye encircling the cornea apart from the area at the back of the eye (where the optic nerve carries impulses to the brain).
Underneath the cornea the anterior chamber is filled with a clear gel, vitreous and behind it the iris (coloured part) contains a central aperture or hole, called the pupil. The pupil can be opened to allow more light into the eye in dark conditions and closed in bright light to protect the retinal cells. In horses, the pupil is a horizontal ovoid shape and the eyes positioned on either side of the head give almost 360o vision which is helpful if you need to watch out for predators. Another adaptation is the corpora nigra (bumpy structures on the upper edge of the pupil) which are thought to act as a sunshade much like the peak of a hat for animals grazing on planes.
The lens sits behind the pupil and focuses light onto the retina allowing long and short distance vision. This is where opacities called cataracts may occur, which can affect vision. The part between the lens and the pupil forms the posterior chamber. The drainage angle forms the outer edge of the pupil and constantly drains and refills the vitreous in these chambers. Blockages can occur when the eye is inflamed and failure to drain results in a condition called glaucoma (excessive pressure within the eye).
Behind the lens the main body of the eye is filled with vitreous. The retina, which contains the light-sensitive cells (cones and rods) lies on the inner aspect of the back of the eye. The choroid under the retina is full of blood vessels, which provide nutrients to the retina. During trauma or disease the retina can become detached from the choroid, losing its blood supply, resulting in blindness.
How Do We Investigate Equine Eye Problems?
- A detailed history and clinical examination determine if the eye problem is a sign of systemic (whole body) disease.
- A completely dark stable is required for ophthalmic assessments, as reflections and a closed pupil make the examination impossible.
- Fluorescein is a dye used to stain the eye, turning damaged areas of cornea green. Dead tissue does not take up fluorescein, neither does Descemet’s membrane (the final, deepest layer of cells in the cornea), so it should be used alongside careful clinical examination.
- The menace test assesses the horse’s ability to see by testing their response to a potential visual threat. In theory this confirms vision but in reality it is difficult to perform a menace without moving air close to the cornea (which is very sensitive) resulting in blinking even in blind animals.
- Pupillary light reflex looks for pupil constriction in response to a bright light. In a sighted animal, the reflex results in constriction of the pupils in both eyes when light is shone in one eye. This confirms that the whole neural pathway is intact and able to respond to light and confirms vision.
- Ophthalmoscopy allows examination of the back of the eye via the pupil with lenses that can focus on the different layers within the eye, so they can be viewed separately.
- Local analgesia involves blocking the nerves around the eye to relax the eye lids and relieve pain allowing examination and surgery to be carried out in standing, sedated horses without the need for general anaesthesia
- Ultrasound – as a soft tissue structure filled with clear fluid, the eye lends itself well to ultrasound evaluation. Abnormal structures such as cataracts or masses and damage to the back of the eye (detached retina) can be imaged ultrasonographically.
Our veterinary ophthalmologists, based at Pride Veterinary Centre, also provide a range of specialist examinations for eye cases on a referral basis. For more information, please call the practice on 01332 294929.