Full corneal transplants are not ‘set and forget’ surgical procedures. A variety of corneal problems can occur, some rapidly. Early intervention is important because it can make a positive difference to the outcome.
There are many patients who have undergone a full corneal transplant yet confusion still surrounds their presentation and management. The patients presented here display a variety of clinical findings.
THE FULL CORNEAL TRANSPLANT
Full refers to ‘full thickness’, meaning replacement of a portion of the cornea in its entirety. This includes replacement of Bowman’s membrane, stroma, Descemet’s Membrane and endothelium. The circular shape transplant is familiar to most of us, but many shapes have been tried, even a square shape. While the full corneal transplant has only recently been surpassed as the gold standard of corneal transplantation to restore vision, it remains an important procedure for cases such as trauma, corneal perforations, or full thickness corneal disease.
IMPORTANT CORNEAL TRANSPLANT CONCEPTS
Most full transplants are an optical full transplant in the centre of the cornea to restore vision loss from corneal disease. A tectonic full transplant is used to repair a cornea that is damaged from injury or disease, and this can be anywhere, including the centre or periphery of the cornea. A tectonic transplant may also be used to repair tumour excisions or scleral holes such as occurs with radiation damage. The size of a full transplant varies from six to 8mm, while a tectonic transplant varies from three to 10mm.
The cornea swells unevenly. Sometimes it swells centrally, sometimes peripherally, sometimes just the stroma, sometimes the epithelium, and sometimes full thickness
The extraordinary strength of the cornea never returns after full thickness corneal transplantation. What this means is that the structure and strength of the eye has been permanently and irreversibly weakened after full thickness corneal transplantation, and puts the recipient at risk of serious eye injury from even trivial force.
A full corneal transplant introduces immunogenic responses due to transplanted Langerhans cells, endothelial cells and keratocytes which can incite a rejection response. Modern layered transplant surgery has revealed that the endothelium incites little inflammatory response, while keratocytes incite a more powerful inflammatory response.
ANCHORING THE TRANSPLANT
With advances in modern sutures, 10-0 nylon is the suture of choice to anchor a transplant. Interrupted stitches are multiple individual stitches, usually one per clock hour, and sometimes more. These should be as radial as possible to the transplant, with one knot for each suture. The shape of the cornea cannot be changed once stitches are in place, and small changes can occur each time a stitch is removed. A continuous stitch is a single stitch running the circumference of the transplant, in a zig-zag pattern, with just one knot. The shape of the cornea can be changed early in the postoperative period by adjusting the tension in this continuous stitch, which means postoperative astigmatism can be improved. A variation is the double continuous stitch where two continuous stitches are used. A hybrid stitching pattern is the use of both continuous and interrupted sutures, where additional anchoring points are required ’just in case’.
The nylon stitch doesn’t dissolve, but breaks down over time, to cause irritation and discomfort, and even trigger vessel ingrowth or rejection. Stitches must be removed, and the average time for removal is 18 months.
FULL TRANSPLANT PROBLEMS
1) Asymptomatic surface staining
A surface punctate staining may appear with fluorescein staining in the epithelial over the donor tissue in some cases. This usually occurs early on and settles over time, but the cause is not simply attributable to dry eye. Loss of nerve supply may be a factor and as it disappears as the corneal nerves return to the sub epithelial layer. In most cases, punctate staining is asymptomatic and optically unimportant.
2) Microcystic keratopathy
In some cases, microcysts can develop within the corneal epithelium over the donor tissue. These appear like punctate staining but on retro illumination, the cystic appearance is clearly visible. The microcysts vary in density and distribution, and may degrade the optics of the eye. The cause is unclear.
3) Herpetic keratitis
Any surgical procedure can trigger a reactivation of herpetic disease of the eye. This can occur early on or at any stage after surgery, and lead to ulceration, scarring, suture melting, vascularisation or secondary bacterial infection. Early on the patient complains of irritation, and the typical club shaped dendritic ulcer occurs. If not treated, this can progress to significant corneal ulceration and thinning.
4) Neurotrophic ulceration
This is often referred to as neurotrophic keratitis but it is a primary ulceration. The loss of corneal nerves has an impact on epithelial healing and stability, with slow healing or corneal ulceration. The effect is usually short lived as the nerve supply returns
5) Recurrent keratoconus
Ideally, the donor and host stroma should be perfectly aligned and stay that way, but over time there is slippage and thinning. This usually occurs in the inferior cornea and is exacerbated by rubbing. It leads to irregular astigmatism and usually the only treatment is revision full corneal transplantation.
Early on, discomfort and irritation are the norm, and epithelial and surface irregularity along with corneal filaments are the typical cause. Some patients complain of discomfort long term despite no clinical finding, and patients often attribute the discomfort to the stitches. The cause is unclear.
7) Optical issues – aberrations, distortion, astigmatism
There is always some mismatch between the shape of the donor button and the host recipient bed. This occurs for many reasons, including variability in the strength of tissue, with conditions such as keratoconus and asymmetrical wound healing. Shape mismatch, suture irregularities, wound misalignment and wound healing issues lead to optical aberrations, distortion and astigmatism. Many changes occur during the early postoperative period, which can cause wild swings in the refraction, and further changes occur following suture removal.
8) Corneal folds
Sometimes, despite best efforts, folds develop in the corneal donor tissue. Folds arise from suture anchoring of the cornea, and folds within the visual axis degrade vision.
9) Endothelial cell failure
Some transplants don’t work from day one, and the cornea stays swollen; this is called primary corneal failure. In others, there is slow failure over time due to endothelial cell loss, and this occurs without rejection. When the endothelium fails, the cornea swells.
10) Corneal swelling
The cornea swells unevenly. Sometimes it swells centrally, sometimes peripherally, sometimes just the stroma, sometimes the epithelium, and sometimes full thickness. It is impossible to predict the pattern of corneal oedema with endothelial failure. It is not known why some patients show more epithelial oedema, while others show more stromal oedema.
11) Acute endothelial rejection
The endothelial cells are foreign, and subjected to immunologic rejection. This usually presents with sudden redness and pain, but often is preceded by stromal rejection. The cardinal sign is the presence of uveitis and keratic precipitates on the back of the transplant.
12) Suture issues – loose and protruding suture
The nylon stitch is usually covered by epithelium, but exposure is common and occurs in every full transplant recipient. Soon after surgery, the swollen transplant changes its shape, which can lead to a suture knot rotating out of the corneal wound where it was buried at the time of surgery. Other times, the suture slowly degrades and becomes loose. When this happens, the nylon then sits above the epithelium and causes irritation, white cell deposits, redness, and sometimes, blood vessel ingrowth. The suture should be removed.
13) Suture degradation
Sutures degrade over time, appearing as patchy clear zones in place of what was once a uniform black suture.
14) Retained suture material
Sometimes, even after sutures are removed, a suture may be retained deep in the cornea. This usually occurs when the knot just won’t exit the corneal wound. In most cases, this does not cause any problem to the eye. Rarely, slow extrusion occurs, and the suture will need removal.
There is a risk of bacterial infection of a full corneal transplant, which can occur around the sutures, or following epithelial ulcerations. Infections present suddenly, are an emergency, and can lead to transplant failure.
Blood vessels can grow into a corneal transplant for several reasons, but in all cases, there is an inciting inflammatory stimulus. This may be stromal rejection, chronic ulceration or stitch problems. Active blood vessels leak lipid, causing a crystalline deposit in the corneal stroma. Once the stimulus is treated, the vessels settle down and often disappear.
17) Lipid keratopathy
This is the deposition of fat deposits in the cornea from leaky blood vessels, and it may affect vision. The fat slowly resorbs over time once the vessels are treated.
18) Corneal scarring
Any break in the surface of the cornea can result in scar formation. After corneal transplantation, this can develop anywhere on the surface and spread across the cornea. It looks like grey flat patches on the surface, and is covered by epithelium.
19) Damage to the eye
Intraocular structures may be damaged as part of surgery or as part of the original problem. The lens or iris may be damaged, and loss of integrity to the pupil causes significant degradation of vision.
Any full transplant increases the risk of glaucoma. This risk increases over time, and may be accelerated with a steroid pressure response to topical steroid use. Life-long careful monitoring of intraocular pressure is required.
WHAT TO DO
Timing: corneal problems occur rapidly with corneal transplants, and escalate over 24 hours. Intervention must be quick and decisive. Most acute cases will need referral for corneal assessment and management. The care received makes a difference in the outcome.
Urgent Problem; Urgent Review
Any suture protruding above the epithelium needs to be removed and antibiotics commenced. If a single suture is protruding, only that suture needs removal. If a continuous suture protrudes above the epithelium at any point, consideration must be given to complete removal of the suture and covering with antibiotics.
Where there is any inflammation, steroids must be considered. Aside from surface inflammation, look for signs of uveitis. No matter how subtle, any inflammation may be the first sign of rejection, and if this is occurring, prompt intense steroids improves transplant survival.
Antibiotics are a first line choice for any suspicious corneal infection. A thirdgeneration fluoroquinolone will cover most dangerous pathogens. Commence promptly and if infection is suspected, the drop should be used every two hours.
Non-urgent Problem; Non-urgent Review
Lubricants are important for comfort and preservative free lubricants are ideal for managing ocular surface symptoms.
Scarring on the ocular surface that degrades vision can be managed with laser phototherapeutic keratectomy and is an elective procedure.
Corneal irregularities are difficult to manage. It is only once all sutures have been removed that something can be done. In some cases, contact lens correction provides the best vision. Laser vision correction may be suitable where there is low prescription error, provided there is stability and no signs of keratoconus recurrence.
Pressure must be monitored, preferably twice yearly.
Vascularisation, if severe, may be treated with laser, or in some cases, a revision transplant will be required. Lipid slowly disappears once vessels are treated.
Recurrent keratoconus is difficult and a revision of the full transplant is usually required, although in some cases, resuturing may fix the problem.
In summary, the full corneal transplant is a very successful technique for vison restoration following corneal disease. However, it is not a ’set and forget‘ surgical procedure, and requires long term monitoring. Many of the serious issues associated with full transplantation have been removed using modern lamellar transplant surgery, and that will be the subject of the next article.
Dr. Anthony Maloof is an internationally recognised and trained surgical subspecialist in EyePlastic and Transplantation surgery, with significant contributions at national and international levels. A Fellow of the College of Surgeons and College of Ophthalmologists, Dr. Maloof offers surgery for functional and medical disease, trauma, aesthetic and rejeuvenative surgery. he is appointed to Westmead and Sydney Hospitals, and is responsible for trainees in his fields. Dr. Maloof regularly lectures to local and international meetings, and publishes in international scientific journals.