Control of Postoperative Corneal Astigmatism and Refractive Error
A corneal transplant was once considered successful merely if the graft remained clear. Today, success is also measured based on the refractive outcome. Severe astigmatism may be associated with decreased visual acuity, anisometropia, aniseikonia, image distortion, and monocular diplopia, rendering an otherwise clear graft poorly functional. Many methods have been suggested to reduce astigmatism, including
variation of suture techniques
intraoperative adjustments with qualitative keratometry
improvement of trephines and use of new technology, such as the femtosecond laser (femtosecond laser–assisted keratoplasty [FLAK]), to better match donor and host tissue (however, the efficacy of matching the tissue with the femtosecond laser has not been shown to reduce astigmatism in long-term clinical studies)
selective suture removal or adjustment of the continuous suture using corneal topography and tomography for postoperative management
incisional or ablative refractive surgery
secondary intraocular procedures after the graft has stabilized
The primary method of reducing astigmatism postoperatively is to readjust or remove the sutures. However, before suture removal or adjustment is considered, it is essential to ensure that a smooth and regular epithelium is present. Careful attention to the ocular surface and appropriate management of topical therapy expedite reaching this point. If a single continuous suture technique has been used, the surgeon may redistribute the suture tension at 1 month postoperatively, using corneal topography as a guide. Alternatively, if there is a combination of continuous and interrupted sutures, the interrupted sutures can be removed starting at 1 month. If the patient has only interrupted sutures, suture removal should begin at a later stage to avoid wound slippage or dehiscence. Clinicians must be especially careful with older patients placed on long-term topical steroid therapy, as wound healing is often slower in these patients.
Prior to removal of the sutures, the most critical step is to identify the steep axis using corneal topography, photokeratoscopy, or manual keratometry. For example, in Figure 15-12 the simulated keratometry readings from the topographer show a steep axis of 49.93 diopters (D) at 11 and a flat axis of 44.06 D at 101. The photokeratoscopic image shows clear rings that are oval in contour, with the shorter axis horizontally corresponding to the steep axis. The presence of distinct rings demonstrates the smooth surface indicative of regular astigmatism. Rings that are very irregular or indistinct may indicate irregular astigmatism; in such cases, suture removal is not recommended until clear and stable measurements can be obtained. Surface disruption may distort keratoscopic mires, but a drop of a tear supplement can temporarily make the mires more distinct.
Manifest refraction can aid in confirming the steep axis (plus cylinder). The autorefraction in Figure 15-12 is –9.00 +6.75 at 4°. The manifest refraction is –7.00 +5.00 at 4°, resulting in 20/25 acuity; the good visual acuity confirms the presence of regular astigmatism. Removing the interrupted sutures on one or both sides of the 4° meridian or adjusting the continuous suture will compensate for the induced astigmatism. After manipulation or removal of the sutures, the patient uses a topical antibiotic for 4 days and returns for a follow-up visit in 1 month for corneal topography and manifest refraction.
Figure 15-12 Corneal topography with a Nidek OPD showing astigmatism after corneal transplantation.
(Courtesy of Robert W. Weisenthal, MD.)
If the patient has intolerable anisometropia or significant astigmatism after adjustment or removal of selected sutures, a contact lens can be tried. After removal of all sutures, relaxing incisions—performed with a metal or diamond knife or a femtosecond laser—are effective in treating residual regular astigmatism. The arcuate incisions are placed either in the donor cornea anterior to the graft–host junction or in the graft–host interface at the steep (plus cylinder) meridian. Suture placement at the flat meridian can augment the effect. Laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) have also been used to manage residual anisometropia and astigmatism after transplantation (see BCSC Section 13, Refractive Surgery).
If the patient has a visually significant cataract associated with anisometropia following PK, cataract extraction with appropriate IOL power selection will reduce anisometropia. If the patient has visually significant regular, stable astigmatism with a healthy graft endothelium, a toric IOL is also an option. If the patient has intolerable anisometropia with a clear lens, the surgeon may elect to place a phakic IOL or perform a refractive lens exchange.
Risks of surgical intervention include microperforation and macroperforation, infection, rejection, undercorrection or overcorrection, persistent epithelial defects, and production of irregular astigmatism. Any intraocular procedure following corneal transplantation has the potential to damage the endothelial cells and can lead to graft failure.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.