A graft is defined as a piece of tissue that is surgically transplanted to a different location in the body.
This technique is often used in equine ophthalmology (the branch of medicine relating to the eye) to treat a number of conditions affecting the cornea, which is the thin, clear window at the front of the eye. The cornea is made of specialised tissue and is around 1mm thick. Grafts are used to repair defects and provide structural or nutritional support to the cornea, and, when possible, to maintain or restore its clarity.
Depending on the origin of the tissue used, grafts can be divided into three types:
● An autograft is taken from one part of the body and transplanted onto another site in the same individual.
● An allograft is taken from one individual and transplanted onto another individual of the same species.
● A xenograft refers to the technique of transplanting tissue into an individual of another animal species.
Surgical techniques
In equine ophthalmology, there are three techniques typically used for corneal surgery. The first is a conjunctival graft; the conjunctiva is the pink tissue that surrounds part of the eye and eyelids. This is probably the most common type of graft performed, used mostly for the treatment of deep or non-healing corneal ulcers.
This graft is relatively easy to perform and provides immediate structural support. Plus, the blood vessels in the conjunctiva deliver nutrition and healing factors to the damaged cornea.
The main disadvantage is that the conjunctiva is not transparent, so a transplant from this area has the potential to interfere with the horse’s vision to some degree — especially if the graft is large or the lesion (the diseased or damaged area) is in the centre of the cornea.
Conjunctival grafts can be divided into different types, depending on their size and shape:
● Pedicle graft — a stalk-like graft from the edge of the eye to the damaged area.
● Bridge or bipedicle graft — two pedicle grafts that originate from opposite sides of the eye and meet in the middle of the lesion.
● Free island graft — when a portion of conjunctiva is separated from its blood supply and sutured directly to the lesion.
● Hood or advancement graft — covers a lesion on the edge of the cornea.
● Complete graft — performed for extensive lesions where the whole cornea needs to be covered.
Some ophthalmologists like to cut back the transplanted flap after four to 12 weeks, although in most cases it will be left in place for the rest of the horse’s life.
The second technique is a corneoconjunctival/corneoscleral transposition. This type of graft is used in lesions of the central cornea, where maintaining as much transparency as possible is desirable. The graft is composed of the peripheral cornea, still attached to the conjunctiva or sclera (the “white of the eye”), which is advanced over the eye surface to cover the defect.
The final technique is a corneal transplant, which involves removing the damaged area of the cornea and replacing it with a fragment of cornea from a donor horse.
The donor cornea can be fresh, from a recently deceased horse, but is more typically obtained when available and stored frozen until needed.
Unfortunately, some degree of rejection must be expected when dealing with corneal transplants in horses, which means that opacification (clouding) of the graft is almost guaranteed. However, this graft is probably the best option when dealing with lesions affecting the deep layers or the whole thickness of the cornea, such as perforated ulcers or deep corneal abscesses that fail to respond to medical therapy.
Materials and methods
Another option is an amnion graft. The amnion is the thin yet very strong veil of tissue that forms the inside layer of the membranes surrounding the foetus while in the mare’s uterus. It can be collected after birth and separated, washed and kept frozen until needed.
A commercial product composed of pig’s bladder submucosa (membrane lining) is also available for use in eye surgery. This has anti-scarring properties and provides numerous growth factors that help repair the damaged cornea.
These substances can be used in two ways. The amnion can be sutured to the surface of the eye temporarily to protect damaged areas of the cornea, such as the surgical bed (underlying site) left behind when removing a corneal tumour or an infected area known as a melting ulcer. In these cases, the amnion is typically removed after five or six days.
Amnion can also be stacked in layers and used to fill defects in the cornea; the tissue is eventually incorporated into the structure and left there indefinitely.
Some grafts can be performed under standing sedation and local anaesthesia, while others require general anaesthesia.
The suture material used to stitch grafts to the cornea can be as small as 0.04mm in diameter (thinner than a human hair), so good lighting and some form of magnification is essential. As with most ocular procedures in horses, the expertise and surgical instrumentation required are highly specialised and transportation to a referral hospital is usually necessary.
On occasions, a tarsorraphy may be required, which is a procedure where the eyelids are sutured together for a few days after the graft is performed to protect the surgical site.
Fighting infection
The main complication following surgery is failure of the sutures that anchor the transplant and subsequently the graft itself.
This usually happens due to infection of the surgical site, so intense administration of antibiotics and antifungals is essential.
Microorganisms and white cells in the cornea can produce enzymes called collagenases that can rapidly destroy the suture material and its anchor points, so anti-collagenase medication is also an important part of the postoperative treatment.
Uveitis, an inflammation inside the eye as a consequence of either the primary disease or the surgical procedure, is also common. Likewise, this requires medication with anti-inflammatories and analgesics.
These drugs must be administered frequently, which requires compliance from the patient. Where this proves problematic the vet can use a subpalpebral lavage system, which is where a small piece of tubing is inserted under the horse’s eyelid and extends down his neck. This facilitates the administration of eye drops without having to manipulate the eye.
The cornea is a common site for injury and disease. Successful treatment of corneal conditions depends on prompt and appropriate treatment, so seek veterinary advice immediately if you notice any changes or apparent damage to the normal clear surface of the eye.
Ref Horse & Hound; 8 November 2018
