Corneal transplantation

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Corneal transplantation

CORNEAL TRANSPLANTATION

Corneal transplantation, also known as corneal grafting, is a surgical procedure where a damaged or diseased corneal is replaced by donated corneal tissue (the graft). When the entire cornea is replaced it is known as penetrating keratoplasty and when only part of the cornea is replaced it is known as lamellar keratoplasty. Keratoplasty simply means surgery to the cornea. The graft is taken from a recently dead individual with no known diseases or other factors that may affect the chance of survival of the donated tissue or the health of the recipient.

The cornea is the transparent front part of the eye that covers the iris, pupil and anterior chamber. The surgical procedure is performed by ophthalmologists, physicians who specialize in eyes, and is often done on an outpatient basis. Donors can be of any age, as is shown in the case of Janis Babson, who donated her eyes at age 10. The corneal transplantation is performed when medicines, keratoconus conservative surgery and cross-linking cannot heal the corneal anymore.

Cornea transplant.jpg

Medical uses

Indications include the following:

  • Optical: To improve visual acuity by replacing the opaque or distorted host tissue by clear healthy donor tissue. The most common indication in this category is pseudophakic bullous keratopathy, followed by keratoconus, corneal degeneration, keratoglobus and dystrophy, as well as scarring due to keratitis and trauma.
  • Tectonic/reconstructive: To preserve corneal anatomy and integrity in patients with stromal thinning and descemetoceles, or to reconstruct the anatomy of the eye, e.g. after corneal perforation.
  • Therapeutic: To remove inflamed corneal tissue unresponsive to treatment by antibiotics or anti-virals.
  • Cosmetic: To improve the appearance of patients with corneal scars that have given a whitish or opaque hue to the cornea.

Risks

The risks are similar to other intraocular procedures, but additionally include graft rejection (lifelong), detachment or displacement of lamellar transplants and primary graft failure.

There is also a risk of infection. Since the cornea has no blood vessels (it takes its nutrients from the aqueous humor) it heals much more slowly than a cut on the skin. While the wound is healing, it is possible that it might become infected by various microorganisms. This risk is minimized by antibiotic prophylaxis (using antibiotic eyedrops, even when no infection exists).

There is a risk of cornea rejection, which occurs in about 20% of cases. Graft failure can occur at any time after the cornea has been transplanted, even years or decades later. The causes can vary, though it is usually due to new injury or illness. Treatment can be either medical or surgical, depending on the individual case. An early, technical cause of failure may be an excessively tight stitch cheesewiring through the sclera.

Procedure

Cornea transplant one day after surgery.

Cornea transplant after one year of healing, two stitches are visible

On the day of the surgery, the patient arrives to either a hospital or an outpatient surgery center, where the procedure will be performed. The patient is given a brief physical examination by the surgical team and is taken to the operating room. In the operating room, the patient lies down on an operating table and is either given general anesthesia, or local anesthesia and a sedative.

With anesthesia induced, the surgical team prepares the eye to be operated on and drapes the face around the eye. An eyelid speculum is placed to keep the lids open, and some lubrication is placed on the eye to prevent drying. In children, a metal ring is stitched to the sclera which will provide support of the sclera during the procedure.

Pre-operative examination

In most instances, the person will meet with their ophthalmologist for an examination in the weeks or months preceding the surgery. During the exam, the ophthalmologist will examine the eye and diagnose the condition. The doctor will then discuss the condition with the patient, including the different treatment options available. The doctor will also discuss the risks and benefits of the various options. If the patient elects to proceed with the surgery, the doctor will have the patient sign an informed consent form. The doctor might also perform a physical examination and order lab tests, such as blood work, X-rays, or an EKG.

The surgery date and time will also be set, and the patient will be told where the surgery will take place. Within the United States, the supply of corneas is sufficient to meet the demand for surgery and research purposes. Therefore, unlike other tissues for transplantation, delays and shortages are not usually an issue.

Penetrating keratoplasty

Replacement of the entire cornea

A trephine (a circular cutting device), which removes a circular disc of cornea, is used by the surgeon to cut the donor cornea. A second trephine is then used to remove a similar-sized portion of the patient’s cornea. The donor tissue is then sewn in place with sutures.

Antibiotic eyedrops are placed, the eye is patched, and the patient is taken to a recovery area while the effects of the anesthesia wear off. The patient typically goes home following this and sees the doctor the following day for the first postoperative appointment.

Lamellar keratoplasty

Lamellar keratoplasty encompasses several techniques which selectively replace diseased layers of the cornea while leaving healthy layers in place. The chief advantage is improved tectonic integrity of the eye. Disadvantages include the technically challenging nature of these procedures, which replace portions of a structure only 500 µm thick, and reduced optical performance of the donor/recipient interface compared to full-thickness keratoplasty.

Deep anterior lamellar keratoplasty

In this procedure, the anterior layers of the central cornea are removed and replaced with donor tissue. Endothelial cells and the Descemets membrane are left in place. This technique is used in cases of anterior corneal opacifications, scars, and ectatic diseases such as keratoconus.

Endothelial keratoplasty

Endothelial keratoplasty replaces the patient’s endothelium with a transplanted disc of posterior stroma/Descemets/endothelium (DSEK) or Descemets/endothelium (DMEK).

This relatively new procedure has revolutionized treatment of disorders of the innermost layer of the cornea (endothelium). Unlike a full-thickness corneal transplant, the surgery can be performed with one or no sutures. Patients may recover functional vision in days to weeks, as opposed to up to a year with full thickness transplants. However, an Australian study has shown that despite its benefits, the loss of endothelial cells that maintain transparency is much higher in DSEK compared to a full-thickness corneal transplant. The reason may be greater tissue manipulation during surgery, the study concluded.

During surgery the patient’s corneal endothelium is removed and replaced with donor tissue. With DSEK, the donor includes a thin layer of stroma, as well as endothelium, and is commonly 100–150 µm thick. With DMEK, only the endothelium is transplanted. In the immediate postoperative period the donor tissue is held in position with an air bubble placed inside the eye (the anterior chamber). The tissue self-adheres in a short period and the air is adsorbed into the surrounding tissues.

Complications include displacement of the donor tissue requiring repositioning (“refloating”). This is more common with DMEK than DSEK. Folds in the donor tissue may reduce the quality of vision, requiring repair. Rejection of the donor tissue may require repeating the procedure. Gradual reduction in endothelial cell density over time can lead to loss of clarity and require repeating the procedure.

Patients with endothelial transplants frequently achieve best corrected vision in the 20/30 to 20/40 range, although some reach 20/20. Optical irregularity at the graft/host interface may limit vision below 20/20.

Synthetic corneas

Boston keratoprosthesis

Boston Kpro type 1 titanium posterior plate

The Boston keratoprosthesis is the most widely used synthetic cornea to date with over 900 procedures performed worldwide in 2008. The Boston KPro was developed at the Massachusetts Eye and Ear Infirmary under the leadership of Claes Dohlman, MD, PhD.

AlphaCor

In cases where there have been several graft failures or the risk for keratoplasty is high, synthetic corneas can substitute successfully for donor corneas. Such a device contains a peripheral skirt and a transparent central region. These two parts are connected on a molecular level by an interpenetrating polymer network, made from poly-2-hydroxyethyl methacrylate (pHEMA). AlphaCor is a U.S. FDA-approved type of synthetic cornea measuring 7.0 mm in diameter and 0.5 mm in thickness. The main advantages of synthetic corneas are that they are biocompatible, and the network between the parts and the device prevents complications that could arise at their interface. The probability of retention in one large study was estimated at 62% at 2 years follow-up.

Osteo-Odonto-Keratoprosthesis

In a very rare and complex multi-step surgical procedure, employed to help the most disabled patients, a lamina of the person’s tooth is grafted into the eye, with an artificial lens installed in the transplanted piece.

Prognosis

The prognosis for visual restoration and maintenance of ocular health with corneal transplants is generally very good. Risks for failure or guarded prognoses are multifactorial. The type of transplant, the disease state requiring the procedure, the health of the other parts of the recipient eye and even the health of the donor tissue may all confer a more or less favorable prognosis.

The majority of corneal transplants result in significant improvement in visual function for many years or a lifetime. In cases of rejection or transplant failure, the surgery can generally be repeated.

Alternatives

Contact lenses

Different types of contact lenses may be used to delay or eliminate the need for corneal transplantation in corneal disorders.

Phototherapeutic keratectomy

Diseases that only affect the surface of the cornea can be treated with an operation called phototherapeutic keratectomy (PTK). With the precision of an excimer laser and a modulating agent coating the eye, irregularities on the surface can be removed. However, in most of the cases where corneal transplantation is recommended, PTK would not be effective.

Intrastromal corneal ring segments

In corneal disorders where vision correction is not possible by using contact lenses, intrastromal corneal ring segments may be used to flatten the cornea, which is intended to relieve the nearsightedness and astigmatism. In this procedure, an ophthalmologist makes an incision in the cornea of the eye, and inserts two crescent or semi-circular shaped ring segments between the layers of the corneal stroma, one on each side of the pupil. Intrastromal corneal rings were approved in 2004 by the Food and Drug Administration for people with keratoconus who cannot adequately correct their vision with glasses or contact lenses. They were approved under the Humanitarian Device Exemption, which means the manufacturer did not have to demonstrate effectiveness.

Corneal collagen cross-linking

Corneal collagen cross-linking may delay or eliminate the need for corneal transplantation in keratoconus and post-LASIK ectasia, however as of 2015 it is lacking sufficient evidence to determine if it is useful in keratoconus.

Epidemiology

Corneal transplant is one of the most common transplant procedures. Although approximately 100,000 procedures are performed worldwide each year, some estimates report that 10,000,000 people are affected by various disorders that would benefit from corneal transplantation.

In Australia, approximately 2,000 grafts are performed each year. According to the NHS Blood and Transplant, over 2,300 corneal transplant procedures are performed each year in the United Kingdom.Between April 1, 2005 and March 31, 2006, 2,503 people received corneal transplants in the UK.

 

Reference:

https://en.wikipedia.org/wiki/Corneal_transplantation

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