Situated at the front of the eye, the cornea is the transparent dome overlying the colored iris of the eye. The cornea is composed of thin layers of tissue that allow light into the eye and focus the rays of light entering the eye. The cornea and the lens of the eye, a separate structure located just behind the iris, are responsible for creating a sharply focused image on the back of the eye so that we can see clearly. The cornea is responsible for roughly two-thirds of the focusing power of the eye, with the lens responsible for the remainder.
Corneal transplants are performed for treatment of diseases affecting the clarity or contour of the cornea.
Fuchs corneal endothelial dystrophy and corneal edema
after cataract surgery are conditions for which corneal transplants are commonly performed in the United States. Corneal transplants are often offered as a treatment of corneal scarring resulting from infections, including herpes simplex virus (HSV-1) or bacterial infections of the cornea. Corneal transplants are also used to treat advanced stages of keratoconus, a disease causing abnormal protrusion of the cornea and severe distortion of the corneal contour. Repeat corneal transplants may be necessary following rejection or failure of a prior corneal transplant.
In 2007 (the last year for which data is available, surgeons performed 39,391 corneal transplants, 36% of which were some form of endothelial keratoplasty, according to the Eye Bank Association of America. Modern eye banking practices and a large supply of donor corneas in the United States allow scheduling of corneal transplant cases on an elective basis, without a waiting period for corneal tissue. Corneal tissue availability and eye banking are particularly strong in the Pacific Northwest.
There are two basic approaches to corneal transplant:
Penetrating Keratoplasty (PK):
Penetrating keratoplasty has been the standard technique for corneal transplantation, and has been performed for over 100 years. It involves transplantation of a full-thickness core of tissue obtained from a donor cornea. This technique of corneal transplantation is commonly used for treatment of keratoconus, corneal scarring, or other corneal diseases that are not limited to the corneal endothelium (inner layer).
Descemet’s Stripping Endothelial Keratoplasty (DSEK):
This newer technique of partial-thickness (posterior lamellar) corneal transplantation is used to treat
resulting from diseases of the endothelium, the innermost layer of the cornea. In the United States, a large proportion of corneal transplants are performed each year for corneal endothelial failure with corneal edema. Many corneal diseases previously requiring a full-thickness, traditional corneal transplant can now be treated with this newer surgical technique. Advantages of this technique include a smaller surgical wound, a more rapid recovery of vision after surgery, and ability to achieve optimal vision after surgery with glasses instead of a contact lens.
Both DSEK and PK are outpatient surgical procedures in which all or part of the patient’s cornea is removed and replaced with donor tissue. All donor tissue is tested thoroughly for disease and infection by an eye bank.
The corneal transplant procedures may be combined with removal of cataracts. Surgery is either performed with a patient awake, but sedated by an anesthesiologist, or, alternatively, under general anesthesia. The choice of anesthesia will depend upon the specific surgery to be performed, as well as patient and surgeon preferences.
Patients lie supine (face up), with the operative eye held open by a speculum. A brief description of two common corneal transplantation techniques is provided:
This traditional cornea transplant involves replacement of a full-thickness, central core of the cornea, extending from the outermost to innermost layer of the cornea. The surgeon first creates a circular incision in the cornea with a trephine, a circular blade. A circular incision of similar size is created in the donor cornea. Very fine sutures are sewn to hold the transplanted cornea in place. A patch is generally placed over the eye overnight. Sutures remain for one to two years, and may be left permanently in some patients. Tension on the sutures usually distorts the contour of the corneal surface, and even when sutures are removed, many patients may require a rigid, gas-permeable contact lens to achieve the best possible eyesight.
In this technique, increasingly popular since 2005, a corneal surgeon creates a small wound through the sclera, the white tissue that forms most of the surface of the eye. The innermost layer of the cornea, the endothelium, is then peeled away, along with Descemet’s membrane, a layer of the cornea that lies between the endothelium and the thick, middle stromal layer of the cornea. The diseased endothelium of the cornea is then removed from the eye and replaced with a thin, partial thickness corneal graft consisting of the innermost layer of a donor cornea. The donor tissue is then held in position by injection of air, without placement of sutures within the cornea itself.
Patients must remain supine for the first 24 hours after their surgery to enable to air bubble to properly position the donor corneal tissue. Visual acuity generally improves over the first postoperative month, reaching its optimal potential within four to six months. DSEK is generally the corneal transplant of choice for diseases affecting only the corneal endothelium, as it requires smaller wounds than PK, and less recovery time is required to achieve optimal recovery of vision. In addition, contact lens wear is not required to achieve the optimal vision in an eye undergoing the DSEK type of corneal transplant.
Systemic immunosuppression is not required after corneal transplantation, as with other organ transplants. Corticosteroid eye drops are, however, extremely important in preventing a corneal transplant recipient’s immune system from rejecting a transplanted cornea. Antibiotic eye drops are often prescribed for about a week after corneal transplantation. Postoperative pain is minimal after any corneal transplantation procedure, and mild pain is usually treated with over-the-counter analgesics.
Corneal transplants can often restore vision to a significant degree. It is important to realize that, in contrast to cataract surgery, corneal transplant procedures tend to be performed for more significant impairment of vision, as corneal transplant procedures and their recovery are much more involved.
Even with successful corneal transplant, other eye diseases can emerge, and pre-existing conditions such as glaucoma can sometimes require additional treatment after corneal transplantation. If the vision in an eye is limited by another eye condition, such as a retinal disease, then the vision in an eye will still continue to be limited by the other eye condition, even if clarity of the cornea is restored through a corneal transplant.
Another consideration regarding the PK technique of corneal transplantation is that this technique often produces an irregular corneal contour even after corneal wound healing is complete. A rigid, gas permeable contact lens is often required to achieve the sharpest vision with this particular type of corneal transplant, due to the irregular corneal contour, as glasses may not be able to address the distortion of images associated with the irregular corneal contour. Many patients in their 60’s and 70’s might find these contact lenses uncomfortable, and some may also have difficulty manipulating and inserting the contact lenses, or in maintaining a daily regimen of lens cleaning, if they have not used them in the past. A patient might have excellent vision after PK is performed and a contact lens is dispensed, but if the patient does not wear the contact lens, then he may not fully experience all the benefits of his corneal transplant procedure. It is particularly important to wear protective eyewear after PK to keep the cornea protected from trauma.
DSEK cannot be used to treat diseases of the cornea that compromise vision through corneal scarring or for diseases like keratoconus that distort the corneal contour—it is a surgical treatment for corneal diseases such as corneal edema in the setting of Fuchs dystrophy or corneal edema after cataract surgery. In addition, DSEK has not been performed for nearly as long as PK, so we have less long-term data on these procedures than we do with PK. One surgical consideration regarding DSEK is that if a DSEK graft is not successful, a PK can still be performed in the future.
Patients with mild corneal edema from Fuchs dystrophy who are planning to undergo cataract surgery should discuss with their ophthalmologist whether DSEK might need to be performed concurrently, as corneal edema may worsen after cataract surgery in some patients with Fuchs dystrophy. In a study of patients who underwent DSEK at the same time as cataract surgery, 93% had vision of 20/40 or better at six months post-op1.
1 Terry M, Shamie N, Chen E, Phillips P, Shah A, Hoar K and Friend D. Endothelial Keratoplasty for Fuchs' Dystrophy with Cataract: Complications and Clinical Results with the New Triple Procedure. Ophthalmology Nov 2008; 16: 1741-1754.
The effectiveness of corneal transplants can vary significantly, depending upon the underlying corneal disease for which the corneal transplant is performed, as well as the presence or absence of other eye diseases that may also potentially limit a patient’s vision.
Corneal rejection is a common occurrence after corneal transplantation, occurring in approximately 20% of grafts for Fuchs dystrophy or corneal edema after cataract surgery. The risk of corneal graft rejection is lower for some diseases such as keratoconus, and is quite higher for some diseases associated with eye inflammation or vascularization of the cornea. Rejection can happen soon after surgery or years later. The patient may notice worsening of vision, or have sensitivity to light, or discomfort. If symptoms are noted and reported promptly to the ophthalmologist, tissue rejections can be treated, usually with steroids, and can often be halted before significant injury occurs in the transplanted cornea. Rejection rates for repeated transplants are higher than for initial transplants.
Other complications of corneal transplantation include, retinal detachment, and the rare but possible risks of infections or bleeding around the time of surgery. All of these complications can be treated with variable success.
What are the risks of not having this treatment/procedure?
There are no significant risks associated with observation of most corneal diseases; however, corneal diseases requiring corneal transplantation for scarring or severe corneal edema will generally persist and continue to limit vision, if surgery is not performed.
Corneal transplantation is not usually an urgent procedure. Notable exceptions include emergent corneal transplants required for large perforations of the cornea that cannot be addressed with less invasive techniques.