LASIK After Penetrating Keratoplasty

Introduction

Dramatic improvements in microsurgical techniques have resulted in penetrating keratoplasty becoming a more common and successful procedure. It is unfortunate that postoperative visual rehabilitation remains challenging. Most patients will not tolerate more than 3 D of anisometropia, which may cause image size disparity, or astigmatism of greater than 2 D. Refractive anisometropia and high postoperative astigmatism can compromise the patient's return to normal binocular function. This is probably the most common reason for patients' unhappiness following a successful corneal transplant. LASIK offers a new treatment option for the visual rehabilitation of these patients.

Surgical Technique

There are several alterations of a normal LASIK technique when treating myopia or astigmatism following penetrating keratoplasty. An accurate assessment of the patient's true refraction depends upon the donor cornea returning to its normal shape following suture removal. Therefore, we recommend waiting a minimum of 3 months following suture removal and until serial topographies are stable to perform LASIK following PKP. We always perform monocular surgery. After repositioning of the corneal flap, we generally wait 2 minutes for the flap to adhere to the underlying stromal bed in conventional LASIK. In LASIK after penetrating keratoplasty, we wait 5 minutes for flap adherence before removing the speculum but protect the ocular surface with carboxymethylcellulose (Slide 1). Additional use of corticosteroids is indicated due to the potential risk of graft rejection following any surgical procedure on a penetrating keratoplasty.

We also alter our perioperative management for specific indications. Patients with corneal neovascularization are given 1 drop of 1% epinephrine 30 seconds prior to the microkeratome pass to reduce intraoperative bleeding. Patients with a history of herpes simplex or herpes zoster should be evaluated carefully for corneal sensation. Patients with a history of herpes simplex are treated preoperatively with oral antiviral therapy, which is then continued for a minimum of 10 days postoperatively to reduce the risk of recurrent viral disease induced by surgical trauma.

Even with these precautions, problems can occur, and we are rethinking our inclusion of grafted patients with a history of herpes simplex. When performing LASIK following penetrating keratoplasty for keratoconus, we have become more conservative and have avoided creating a flap in patients with ectatic peripheral beds.

As part of the LASIK evaluation in post-keratoplasty eyes, we recommend a careful examination of the corneal graft for evidence of corneal edema. Pachymetry should be reviewed not only for adequate post-LASIK bed thickness, but also for evidence of edema. Specular microscopy may be performed if there is clinical suspicion of low endothelial cell density. When LASIK is performed on a post-keratoplasty eye with Fuchs' dystrophy or pseudophakic bullous keratopathy, or any post-keratoplasty eye in which the endothelial function may be marginal, we recommend an 8.5-mm diameter superior hinge flap, particularly when the peripheral host bed has frank edema. If the pump function of the endothelium (which holds the flap in place) is decreased, it is more likely that the flap will elevate and dislocate following a blink with a nasal-hinged flap compared to a superior-hinged flap. In addition, the smaller diameter flap has less tissue overlying the edematous peripheral bed and is less likely to elevate. We also recommend that patients with endothelial compromise who undergo LASIK wear protective shields for a longer than normal period and be followed closely to reduce the risk of flap slippage.

At the time of surgery, we use our standard LASIK procedure altered only to avoid initiating the incision at the corneal transplant graft-host interface. We believe this creates better wound apposition of the flap to the recipient bed. The corneal scar tissue at the penetrating keratoplasty wound tends to be less pliable than normal corneal tissue. In addition, initiating the LASIK incision away from the wound prevents the need to apply additional pressure directly to the corneal transplant incision and may decrease the risk of wound dehiscence. We also avoid placing the hinge over previous corneal relaxing incisions that can result in free caps. We also are concerned about the long-term implications of creating a flap in patients who have keratoconus as their underlying diagnosis.

Clinical Results

The first large series of LASIK following penetrating keratoplasty was presented by Donnenfeld at the American Academy of Ophthalmology in 1998 and published in 1999.¹ Spherical equivalent anisometropia was reduced from a mean of 6.88±4.4 D to 1.42±1.05 D at the final exam. Best-corrected visual acuity remained the same or improved in 21 of 23 eyes, decreased by 1 line in a patient who developed a nuclear sclerotic cataract, and by 3 lines in a patient who developed irregular astigmatism. Endothelial cell counts measured in 9 patients preoperatively and at 3 months postoperatively demonstrated no statistically significant loss. There were no surgical flap or corneal transplant complications in our initial study. Since that time, we have seen two flap slippage problems (Slide 2), one of which required suturing of the flap. There have been several other studies of comparably large series examining LASIK after penetrating keratoplasty whose results are summarized in Table 1.^1-4

Conclusion

The goal of therapeutic LASIK for visual rehabilitation following penetrating keratoplasty is not necessarily the same as LASIK for the correction of myopia and astigmatism. The primary goal of LASIK following penetrating keratoplasty is resolution of sufficient myopia and astigmatism to allow spectacle correction of the residual refractive error. Many patients following penetrating keratoplasty will have irregular astigmatism, which although not amenable to spectacle correction, can be rehabilitated with a gas-permeable contact lens. We discourage these patients from having LASIK performed if they are at all contact lens intolerant. We anticipate that the development of customized corneal ablations guided by corneal topography or wavefront analysis will successfully treat some forms of irregular astigmatism in the next several years. This will allow even more accurate treatment of post-penetrating keratoplasty refractive errors. This may especially favor PRK in treating post-keratoplasty keratoconus patients.

LASIK is successful in treating post-keratoplasty myopia and astigmatism in most patients. Patients are overwhelmingly able to resolve their anisometropia and achieve binocular function. We advocate a conservative approach to treating refractive errors following penetrating keratoplasty with LASIK. Perhaps patients with Herpes simplex may be considered as relative contraindications, and close observation is required to see the effect of this technique on patients who are post-keratoplasty keratoconus. LASIK offers the corneal surgeon an exciting new tool in the visual rehabilitation of the corneal transplant patient.

References

1. Donnenfeld ED, Kornsein HS, Amin A, Speaker M, Seedor J, Perry HD. Laser in situ keratomileusis for correction of myopia and astigmatism after penetrating keratoplasty. Ophthalmology. 1999;106(10):1966-1974.

2. Forseto AS, Francesconi CM, Nose RA, Nose W. Laser in situ keratomileusis to correct refractive errors after keratoplasty. J Cataract Refract Surg. 1999;25(4):479-485.

3. Webber SK, Lawless MA, Sutton GL, Rogers CM. LASIK for post penetrating keratoplasty astigmatism and myopia. Br J Ophthalmol. 1999;83(9):1013-1018.

4. Preschel N, Hardten DR, Lindstrom RL. LASIK after penetrating keratoplasty. Int Ophthalmol Clin. 2000;40:111-123.

5. Perry HD, Doshi SJ, Donnenfeld ED, Levinson DH, Cameron CD. Herpes simplex reactivation following laser in situ keratomileusis and subsequent corneal perforation. CLAO J. 2002;28(2):69-71.