Minimally Invasive Corneal Surgery

4.1 Penetrating Keratoplasty

4.1.1 Introduction

Cornea transplantation is a fascinating surgical procedure for the many reasons to do it, as well as the many ways to do it. Cornea transplant surgeons now have the option of replacing the posterior cornea for primary and secondary endotheliopathies, as well as the anterior cornea for pathology that does not involve the endothelium. In penetrating keratoplasty (PK), fullthickness host corneal tissue is replaced with fullthickness donor corneal tissue. Erasmus Darwin, the grandfather of Charles Darwin, is given credit for his vision of restoring a scarred cornea to its transparent state [1]. His idea, proposed in 1760, led to our present corneal transplantation procedure [1].

The idea to use living corneal tissue for transplantation was suggested by Franz Reisinger in 1818 [1]. He experimented with the procedure in rabbits and chickens but was unable to achieve clear grafts [1]. Bigger reported a successful homograft procedure on a pet gazelle in 1837, and Von Hippel performed the Þrst successful lamellar cornea transplant in 1886 when he transplanted a rabbit cornea into a human recipient bed [1]. Finally, in 1905, Zirm performed the Þrst successful human cornea transplant [1]. The procedure has continued to evolve since then with successes due to advances in surgical instrumentation, donor tissue preservation, and anti-inßammatory agents. For example,

H. M. Skeens ( )

Storm Eye Institute, 167 Ashley Avenue, MSC 676, Charleston, SC 29425, USA

e-mail: skeens@musc.edu

topical Cyclosporin A (Allergan, Irvine, CA) is now available. According to an eye bank statistical report in 2002, there are approximately 33,000 PKs performed per year in the United States [1].

4.1.2 Indications

Indications for PK are both optical and therapeutic. The most common indications for a cornea transplant, including pseudophakic and aphakic bullous keratopathy, are no longer being treated with PK, but as they are secondary endotheliopathies, they are being treated by endothelial replacement only. Cornea ectasias and thinning, disorders with a previous indication for PK, are being treated by deep anterior lamellar keratoplasty (DALK) now. DALK is replacing PK in the surgical treatment of cornea ectasias that do not show fullthickness scar formation. Brießy, DALK is a procedure whereby the anterior layers of the cornea are removed down to DescemetÕs membrane and replaced with a donor cornea from which the surgeon has removed the DescemetÕs membrane. This procedure has the advantage of decreased endothelial graft rejection since the host retains their native endothelium. Other advantages include greater wound stability and less intraoperative complications such as choroidal hemorrhage.

Stromal cornea dystrophies are being treated by this method as well, as are cases of previous viral keratitis with anterior stromal scarring.

While DALK is beginning to become the preferred procedure for some of the mentioned indications in which PK was previously more commonly performed, there are those indications in which PK is still considered the preferred procedure. Full-thickness cornea

scars from infectious ulcerative keratitis, mechanical trauma with full thickness scars, advanced cases of ectasia with full thickness scarring, and regrafts related to allograft rejection or related to high amounts of irregular astigmatism with poor visual acuity, are all indications for PK.

4.1.3Preoperative Evaluation of the Keratoplasty Patient

The main questions to be answered when obtaining a patientÕs history are related to how the current cornea pathology is affecting the vision and lifestyle, as well as the ability and willingness of the patient to undergo the proposed operative procedure. For patients with a congenital opacity, the best corrected visual acuity (BCVA) that the patient has had in his/her lifetime must be determined so that both the surgeon and the patient have reasonable expectations. Amblyopia may limit the BCVA that the patient may obtain and it is important to discuss this prior to surgery so that the patient may make an informed decision regarding whether or not to undergo the procedure. Many patients do not understand that amblyopia often may be the main reason for their limited visual acuity and falsely believe that the surgeon may be able to restore the eye to a 20/20 visual acuity following the replacement of the abnormal cornea. It cannot be overemphasized that this point must be discussed and made clear prior to surgery.

For patients with a cornea scar related to a history of infection, a history of herpes simplex viral (HSV) keratitis may be elicited. An experienced cornea surgeon can often diagnose a history of HSV keratitis upon clinical examination, but the patient may also be asked certain questions that help elicit this history. For example, the presence of periocular skin lesions at the time that the patient Þrst noticed a decrease in the visual acuity would be associated with a history of HSV infection. It is important to elicit this history from the patient because it is recommended that the patient take an oral antiviral medication following PK for prophylaxis of recurrent HSV infection in the donor graft. HSV following PK will be discussed later in the chapter.

Cornea ectasias represent another primary indication for cornea transplant. Examples of ectasias include

keratoconus (KCN), keratoglobus, and pellucid marginal degeneration (PMD). KCN is a noninßammatory, bilateral cornea dystrophy that causes reduced bestcorrected visual acuity and severe astigmatism due to its progressive nature. Management options in the early stages of the disease include spectacles or rigid-gas permeable contact lenses (RGP). If a patient presents with a history of ectasia, it is important to determine if the patient has tried RGP lens wear to improve the visual acuity. The patient must be instructed to try RGP wear prior to further intervention if severe central scarring is not present, and if the patient has not tried this methodology. Reasons for failure with contact lenses include discomfort, a poorly Þtting lens, or problems with lens insertion. The patient presenting with contact lens intolerance or advanced disease with cornea scarring may beneÞt best from cornea transplantation. As mentioned, DALK is now becoming the preferred procedure for cornea ectasia but there are still situations in which full-thickness PK is warranted.

Other treatments are available for cornea ectatic disorders and these include the use of INTACS (Keravision, Inc) and collagen cross-linking. INTACS are intracorneal ring inserts composed of polymethylmethacrylate. Their placement ßattens the central cornea. Cornea tissue is not removed in the placement of intracorneal ring segments, and the central optical zone of the cornea is not invaded. INTACS are reported to be an effective method for the treatment of low to moderate KCN (mean K reading £ 53) [2]. Shetty et al. performed INTACS implantation in eyes with advanced KCN and good results were noted with respect to visual acuity and corneal topography, without major complications postoperatively [2].

Collagen cross-linking was developed to counteract progressive corneal thinning, and thus the progression of KCN [3]. Collagen cross-linking is carried out via the use of riboßavin (vitamin B2) and UV light on the order of 365Ð370nm wavelengths [3]. Riboßavin absorbs UV irradiation and acts as a photosensitizer for the generation of reactive oxygen species [3]. The combination of riboßavin and UV light thus forms radicals that crosslink collagen [3].

The biochemical characteristics of the normal cornea result from the collagen scaffold that has been formed via bonding between the collagen Þbrils [3]. The exact conÞguration of the collagen lamella in the cornea determines the corneaÕs resistance. The speciÞc cause of KCN is not known, but it is known that there

is a modiÞcation of the corneal stromal collagen lamella in KCN. Collagen cross-linking, through the promotion of covalent binding between collagen molecules, stabilizes the collagen scaffold in KCN [3].

In 1998, the Þrst KCN patients underwent collagen cross-linking [3]. Raiskup-Wolf et al. reported on a series of 480 eyes with progressive KCN that underwent collagen cross-linking with UV irradiation and riboßavin, and were followed for a maximum of 6 years [3]. Their results indicated a signiÞcant improvement in BCVA and long-term stabilization of the cornea with collagen cross-linking [3]. Further discussion of these techniques is beyond the scope of this chapter.

Other indications for PK that can be elicited during the history taking include mechanical trauma with scar formation, cornea opaciÞcation due to limbal stem cell deÞciency (LSCD), regrafts for allograft failure and reasons not related to allograft failure. A history of an infectious keratitis not responsive to medical therapy is an indication for a therapeutic keratoplasty.

Besides eliciting the clinical indications for PK and how the patientÕs lifestyle is affected, the surgeon must determine the ability of the patient to undergo the procedure and to maintain proper follow-up care. For example, in considering the method of anesthesia to be used, the ability of the patient to lie ßat for the duration of the procedure is essential. Pulmonary or cardiac abnormalities could interfere with the ability to lie ßat as the patient may begin coughing or have difÞculty breathing. Severe arthritis could also make it impossible for the patient to remain comfortable in any one position for an extended period of time. These things necessitate general anesthesia.

Preexisting medical problems must be controlled before the surgical procedure. Often the anesthesiologist elicits these conditions and the level of control during the preoperative anesthesia evaluation, but it is helpful if the transplant surgeon, as well, pays attention so as to properly educate the patient prior to the anesthesia exam. Hypertension and diabetes that are not controlled should be stabilized in concert with the primary care physician. Patients should be instructed to take their antihypertensive medications with a small sip of water on the morning before surgery because anxiety is common. It is best to perform surgery on patients with insulin-dependent diabetes early in the day and have the patient take only half of their normal insulin dose before surgery. These patients should eat breakfast promptly postoperatively.

Allergies to medications should be noted in the chart and reviewed prior to surgery to avoid any complications. In particular, an allergic reaction to a preservative in a topical ophthalmic preparation must be identiÞed. Often subconjunctival injections of antibiotic are administered postoperatively and an appropriate list of the patientÕs allergies is helpful. It is also helpful to elicit any history of allergy to iodine as this is typically used in prepping the patient. Chlorhexidine may be used in patients with an iodine allergy.

Aspirin and nonsteroidal anti-inßammatory drugs, and other medications that interfere with blood coagulability are typically not discontinued if they are necessary for a patientÕs well-being. The prothrombin times may be checked and if they are in a reasonably good range, some surgeons advocate stopping anticoagulation 48 h before keratoplasty and resuming treatment on the Þrst postoperative day [1]. Other surgeons do not stop anticoagulation prior to surgery.

Finally, it is critical to establish the willingness of the patient to maintain follow-up care and to be diligent with the administration of all postoperative medications. If a patient does not live in close proximity to the surgeon and is not willing to travel to all follow-up appointments, the surgery should not be performed. The surgeon should tell the patient prior to scheduling surgery what he or she expects postoperatively. Typically the visits for the Þrst year can be detailed to the patient so that the patient will know if they are able to make these appointments. If, for example, an older teenage patient is considering a cornea transplant procedure for a cornea ectasia, it may be better to schedule the surgery when the patient is not in school, but perhaps on summer break. In this way, the critical initial visits may be kept more easily.

The postoperative medication regimen should be detailed to the patient preoperatively in the same way as the follow-up appointments. The patient should be made aware of the number and frequency of administration of all postoperative medications. An elderly patient who lives alone may need to plan to have additional assistance at home in the immediate postoperative period when the dosing of medication is more frequent than will be necessary later in the postoperative period. It is also a good idea to determine how a patient will obtain their postoperative medications. The surgeon may be able to help an uninsured patient with a drug assistance program in the preoperative period so that there is not an issue following surgery, when having the medication is so critical.