LASIK Surgery After Corneal Transplant
Even after successful corneal transplant ( penetrating keratoplasty ), surgical outcomes are frequently marred by significant postoperative visual defects, including astigmatism, myopia, anisometropia and occasionally hypermetropia. Refractive correction can often be achieved with spectacles or contact lenses. However, as the population ages, a growing percentage of patients are unable to effectively handle and maintain contact lenses. In these cases, refractive surgery can be considered in lessening the degree of refractive error and diminish visual complaints.
A number of surgical interventions are available to address refractive complaints in post-keratoplasty patients. Of these, LASIK has shown to be the most successful and is associated with the least amount of postoperative complications.
Visual Complaints After Penetrating Keratoplasty
After successful penetrating keratoplasty, a significant percentage of patients do not achieve emmetropia. Indeed, refractive instability is very common; with high degrees of regular and irregular astigmatism, myopia and anisometropia being frequently encountered. Most studies evaluating post-keratoplasty outcomes document mean cylinders of 4-5 diopters (D). Postoperative astigmatism has been reported in the range of 2 D to 8 D , and spherical equivalents in the range of 2D to 12 D.
In one examination of post-corneal transplant patients, less than half (48%, 21 of 43 eyes) achieved refractive error within 2 D of emmetropia. In studies of combined penetrating keratoplasty and cataract extraction, only 39% of patients were within 2 D of emmetropia, 75% fell between -4.00 D and +2.00 D. The range of postoperative refractive error in these studies was fr -9.75 D to +12.88 D.
Spectacles are only appropriate in cases of anisometropia of less than 3.0 D or astigmatism less than 4.0 D. A significant majority of patients who cannot be rehabilitated with spectacles are able to achieve adequate vision correction with contact lenses, which should be considered first-line treatment for visual correction in post- keratoplasty patients. Contact lenses are successful in 80-90% of cases.
Soft contact lenses are appropriate for patients with mild regular astigmatism, but for moderate to severe cases, or in irregular astigmatism, rigid gas permeable lenses are a better choice. Scleral contact lenses can be very helpful in cases of irregular astigmatism or refractive error.
There is a small percentage of patients who have such significant astigmatism that it cannot be adequately corrected with contact lenses. Additionally, some patients are either unable to tolerate contact lenses, or are unable to handle and maintain contact lenses appropriately.The latter is a significant issue in the growing geriatric demographic. For these patients, surgical intervention may be considered.
Only patients who have failed non-surgical visual correction should be considered as candidates for surgical refractive intervention.
All candidates should be evaluated to ensure refractive and tectonic stability. There is no consensus regarding the appropriate time interval between keratoplasty and LASIK or other refractive intervention. Most studies recommend waiting at least 12 months after the transplant. There is general agreement that waiting 3-6 months after suture removal is an appropriate time interval. Some advocate for a longer interval of 2-3 years between transplant and attempting LASIK to allow the transplant an extended healing window and to ensure graft stability, but successful LASIK correction has been performed as early as 8 months postoperatively.
Contraindications for refractive intervention include peripheral corneal vascularization, thin host tissue, wound ectasia and significant graft override or malapposition. The patient should have a minimal corneal thickness of at least 500 μm.
Informed consent should be obtained, outlining the procedure, its risk and benefits as well as the goals of treatment. It should be made explicitly clear that the primary goal of any refractive surgery after penetrating keratoplasty is resolution of myopia and astigmatism sufficient enough to allow for spectacle or contact lens correction. There are limits to the amount of astigmatism that can be treated with excimer laser. Improvement of uncorrected visual acuity (UCVA) is a secondary goal, and is not always reasonable or possible.
A complete ocular history is vital, and particular attention should be paid to any reported incidence of graft rejection, previous ocular herpes, or presence of any ocular or systemic inflammatory condition that could be reactivated by surgery. Ocular diseases such as keratoconjunctivitis sicca and blepharitis need to be fully controlled prior to surgery.
Corneal pachymetry should be assessed centrally and at both sides of the graft to ensure adequate thickness. If possible, endothelial cell count should be attempted. Applanation tonometry is mandatory to rule out elevated intraocular pressure.
Video keratography and corneal topographical analysis are optional, but can provide valuable information on corneal thickness and abnormalities. Recently, the use of very high frequency ultrasound has been advocated to obtain high-resolution images within the cornea.
Non-LASIK Treatment for Post-Keratoplasty
Non-LASIK surgical treatment for post-keratoplasty astigmatism includes corneal relaxing incisions, wedge resections, and photorefractive keratectomy.
Wedge Resections and Relaxing Incisions
Both wedge resections and relaxing incisions are highly effective at addressing post-keratoplasty astigmatism, but do not address spherical abnormalities. They are associated with a higher incidence of wound dehiscence and variable wound healing, as well as alterations in corneal topography, refraction and keratometry.
When performed on corneal grafts, radial keratotomy and similar procedures have a high rate of complications and thus are not recommended interventions.
Photorefractive keratectomy (PRK) is effective in reducing spherical errors to improve both uncorrected and best-corrected visual acuity, and can also improve refractive and keratometric astigmatism. However, there is usually a loss of best spectacle corrected visual acuity (BSCVA) of at least one line due to increased corneal haze. Additionally, patients with previous keratoplasty tend to experience significantly more corneal haze than does the general population. The use of mitomycin-C is associated with improved outcomes and less haze, but even with this treatment, results are still less optimal than LASIK. Post-keratoplasty patients who received PRK also have an increased incidence of regression and corneal scarring when compared to those who received LASIK after keratoplasty.
Many studies have consistently demonstrated superior outcomes when LASIK is used to treat residual refractive errors and astigmatism in patients with prior penetrating keratoplasty. Therefore, LASIK is the refractive surgery of choice for addressing visual complaints in post-keratoplasty patients.
Two Step LASIK
Some have advocated for a two-step approach in these patients, especially if the refractive error or astigmatism is significant. The argument for a two-step approach is that corneal remodeling may occur after creation of the cut which could change the biomechanics of the cornea and alter the host-graft interface. In the two-step procedure, the flap is cut and replaced and then the patient undergoes months of topography and keratometry measurements to ensure stability before a second procedure to perform the ablation.
Despite the finding that patients who had the two-step process had better vector analysis of refractive cylinder, there was no significant difference in postoperative uncorrected or best corrected visual acuity when compared to the one-step approach. The two-step approach has the added drawback of additional time spent by the patient at clinic, and a longer duration waiting until vision is corrected, which has significant implications on the patient’s quality of life. Therefore, most studies recommend the one-step approach for most cases.
Complications during LASIK occur at a higher rate in patients with previous penetrating keratoplasty. Typical LASIK complications have all been reported in patients receiving LASIK after penetrating keratoplasty: paracentral flap perforation, flap dislocation, buttonhole flap, hemorrhage into the stroma, irregular astigmatism, corneal perforation , and peripheral epithelial ingrowth.
These patients also experience complications associated with their previous keratoplasty. There is a small but serious risk of wound dehiscence. To minimize this risk, suction time should be kept to a minimum. There are documented cases of post-LASIK graft rejection and retraction of the lamellar flap has also been reported in this patient population.
In five year follow-up studies, visual acuity was 20/40 or better in 86-98% of eyes and 50-85% were within 1 D of intended correction. Despite this, retreatment is sometimes indicated, with reported incidence varying between 9.1% and 51.6%.
Visual complaints after successful corneal transplant are frequent. While many of the mild to moderate cases of ametropia and astigmatism can be addressed with spectacles or contact lenses, severe cases or patients with inability to handle or tolerate contact lenses usually require surgical intervention. Relaxing incisions and wedge resections can fix astigmatism but are not effective at improving cylindrical defects. Photorefractive keratectomy in these patients is associated with a significant increase in corneal haze, even when pretreated with mitomycin-C. Of all the surgical interventions, LASIK is associated with the greatest efficacy and lowest incidence of complications or adverse outcomes.
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