Article

SPARE THE CAUTION, SPOIL THE GRAFT

Specialty lens options for visual rehabilitation in post-penetrating keratoplasty patients.

Modern keratoplasty techniques have evolved, becoming targeted procedures to remove and replace only the diseased and affected layers of the cornea. Modern keratoplasty procedures include penetrating keratoplasty (PK), a full-thickness procedure removing and replacing all layers of the cornea; deep anterior lamellar keratoplasty (DALK), removing and replacing corneal tissue while preserving the hosts’ Descemet’s membrane and endothelium; and endothelial procedures such as Descemet stripping endothelial keratoplasty (DSEK) and Descemet membrane endothelial keratoplasty (DMEK), in which Descemet’s membrane and endothelial layers are removed from the host and replaced with donor tissue, the former having a layer of posterior stroma attached to Descemet’s membrane and the endothelial layer while the latter is just Descemet’s membrane and endothelium.

Various techniques can be utilized to dissect the cornea during PK; the primary two techniques are manual dissection using a trephine punch, yielding a perpendicular cut through the cornea, and femtosecond laser-assisted dissection, allowing for more precision, customized size, and incision shapes such as zig zag, mushroom, and top hat configurations (Figure 1).

Figure 1. One-month post-op femtosecond laser-assisted penetrating keratoplasty (PK) utilizing a mushroom incision with a 9.0mm anterior diameter and 7.5mm posterior diameter. Note overlap of tissue at the graft-host junction.

The reasons for keratoplasty are vast. A review of the Eye Bank Association of America’s annual report for the past three years determined that keratoconus is the number one reported indication for PK and anterior lamellar keratoplasty.1-3 The second most common reported indication for PK is a repeat corneal transplant.1-3 For the purposes of this article, we will be discussing contact lens management post-PK.

It has been suggested in the literature that the number of keratoplasties performed for keratoconus is decreasing as a result of corneal cross-linking, which can stop the progression of the disease and effectively prevent advanced levels of keratoconus from occurring,4,5 and the utilization of scleral contact lenses, which allows individuals who have severe keratoconus and individuals who have been deemed contact lens intolerant another option to achieve stable, comfortable visual correction, thus avoiding keratoplasty.6 However, until the day when corneal disease is eliminated, practitioners will need to find options for correcting vision after keratoplasty.

VISUAL REHABILITATION OPTIONS

There are two primary forms of visual rehabilitation for individuals who have undergone successful keratoplasty: surgical and non-surgical. In most cases, non-surgical contact lens therapeusis is preferred.

Surgical options for these individuals can include photorefractive keratectomy (PRK), topography-guided photorefractive keratectomy (TGPRK), implantable collamer lenses (ICLs), and refractive lens exchanges (RLEs). In the correct patients, these procedures can be very successful in improving uncorrected and spectacle-corrected visual acuity (Figure 2).

Figure 2. Difference map of irregular astigmatism two years postoperative femtosecond-assisted PK treated with topography-guided photorefractive keratectomy (TGPRK): Pre-operative axial curvature map showing 4.2D of corneal astigmatism (left), best-corrected spectacle visual acuity of 20/40 with –5.75 –5.25 x 008. Three months postoperative TGPRK axial curvature map showing 0.70D of corneal astigmatism (center), best-corrected spectacle visual acuity equal to 20/25 with –3.00 –0.50 x 115. Difference map showing curvature change as a result of TGPRK (right).

Non-surgical options for these individuals include spectacles, soft contact lenses, corneal GP contact lenses, piggyback lenses, hybrid contact lenses, and scleral contact lenses. Generally, options utilizing GPs are selected for both regular and irregular high astigmatism, which commonly presents after corneal transplantation. The Australian Corneal Graft Registry revealed that more than 20% of individuals who underwent PK had greater than 5.0D of astigmatism, and 13.1% had anisometropia.7

INITIAL CONSULTATION APPROACH

Prior to providing visual rehabilitation for corneal transplant patients, it is important to obtain some baseline information. The history and status of the graft tissue is helpful, including the reason for undergoing transplantation, age of the graft, type of keratoplasty performed, current medication with frequency of use, and any past rejection episodes.

Ask about contact lens history, such as what lens modalities were worn in the past and whether the lenses were a contributing factor to undergoing keratoplasty. Had lens wear been attempted with the graft? If so, what was the reason for discontinuation of the contact lenses?

It is important to understand a patient’s goals and to determine whether those goals are realistic for that individual. Some patients underwent keratoplasty because of contact lens intolerance or in an attempt to live contact lens free; for many of these individuals, however, the goal of clear vision can only be achieved with the use of contact lenses, making their goals incompatible.

It is also important to refract patients; a quality best-corrected spectacle visual acuity will help guide your surgical, glasses, and simple contact lens options. It is critical to pay attention to the amount of cylinder power needed and to perform a Becherer Twist test, rotating the axis until the patient reports blur. This will help practitioners understand a patient’s tolerance to blur with lens rotation.

It is valuable to collect baseline data such as slit lamp findings, anterior segment photographs, topography, optical pachymetry, intraocular pressure, and specular microscopy to monitor the graft over time. Note whether sutures are present, and be aware that removal of sutures may cause significant changes to corneal topography and refractive power, resulting in a need to modify lens design or lens power. General contact lens fitting after keratoplasty can begin as soon as three months post-surgery.

VITAL INSTRUMENTATION

Prior to, during, and after the contact lens fitting process, certain instruments may provide vital information that cannot be obtained by history or physical examination alone. These findings may impact decision-making and guide changes in lens material and design to optimize corneal health.

Specular microscopy is a necessary tool to help understand the endothelial health of the transplanted tissue. Many modern specular microscopes will provide multipoint analysis, increasing the analyzed area of endothelium, presenting a global view, and tracking data points to make monitoring more accurate and repeatable. In corneal donor studies, Lass et al showed loss of endothelial cell density after successful transplantation, reduced from an average starting value of 2,700 cells/mm2 to 800 cells/mm2 at five years and 600 cells/mm2 at 10 years.8 As the cell count decreases, more physiologic burden may fall on the remaining cells to keep the corneal tissue thin and clear. Mishima demonstrated that chronic corneal decompensation may occur when cell densities decrease below 700 cells/mm2.9

Evaluation of not just the density of cells but also the presence of guttae, polymegethism, or pleomorphism may aid in the identification of graft-susceptible complications. Polymegethism or variability in cell size occurs as cells are lost and a smaller number of endothelial cells must cover the same surface area. Pleomorphism is variability in endothelial cell shape away from healthy, hexagonally-shaped cells (Figure 3). Although the function of the remaining endothelial cells is adequate to maintain corneal thickness and transparency, the presence of a stressor such as a contact lens, which acts as a barrier to the oxygen needed for normal corneal metabolism, can be detrimental to the graft health. Thus, prior to contact lens fitting, use of a specular microscope can aid in the evaluation of risks and benefits to guide the selection of contact lens utilized.

Figure 3. Specular microscopy of corneal graft tissue after transplantation. (A) One-year post-PK, normal cell density, note relatively uniform size and shape of cells. (B) 32-year post-PK, severely decreased cell density with obvious pleomorphism and polymegethism.

Optical corneal pachymetry, either by optical coherence tomography (OCT) or Scheimpflug tomography (Figure 4), can be beneficial in monitoring the corneal response to contact lens wear. Corneal edema can occur in grafts that are decompensating as a result of increased metabolic stress on the transplanted tissue. Such an edematous response can be induced by contact lens wear.10 Remember that rejection is a concern in this population and is an immune-mediated response; subclinical edema can be the early signs of rejection. Abou Shousha et al, using an enhancement algorithm on corneal OCT, found that graft rejection could be detected prior to observing clinical signs by analyzing the thickness of Descemet’s membrane and endothelium.11

Figure 4. Optical pachymetry. (A) OCT-based optical pachymetry, showing overall corneal thickness (left map) and epithelial thickness (right map). (B) Scheimpflug camera-based optical pachymetry.

Optical corneal pachymetry, not ultrasonic, is important to understand global corneal changes because ultrasonic measurements provide corneal thickness data only where the probe is touched; thus, portions of the graft where edema is present can be missed. Prior to contact lens wear, obtain a baseline scan and compare at subsequent visits. Obvious changes to corneal thickness may necessitate changes in lens parameters, fitting relationship, or materials.

Corneal topography will aid in guiding the way for lens selection. Five post-transplantation corneal shapes have been described in the literature: prolate, oblate, mixed, asymmetric, and steep-to-flat (Figure 5).12

Figure 5. Axial topography of post-PK corneas. (A) Oblate corneal curvature. (B) Prolate astigmatic corneal curvature. (C) Steep-to-flat corneal curvature.

Keep in mind that curvature is not the same as elevation. A flat curvature can be in the elevated part of the cornea (Figure 6). Zheng et al suggested that differences in elevation of less than 350 microns had an 88.2% chance of success with corneal GP contact lenses.13 Also, note the elevation of the graft-host junction, as it may steer you away from certain lens designs.

Figure 6. Axial curvature map (A) versus anterior elevation map of post-PK cornea (B). Note that the flat curvature regions are the most elevated regions.

SLIT LAMP EXAMINATION

An understanding of the general corneal shape can be obtained simply by looking at the corneal profile and noting graft tilt, elevated graft-host junction, prolate or oblate shape, and sullen or protruding profile (Figure 7).

Figure 7. Graft profiles: (A) prolate contour; (B) proud prolate contour with mild superior protrusion; (C) severe inferior protrusion with steep-to-flat profile, geometry is complex.

Behind the slit lamp, it is easy to obtain a vertical view of the profile; however, Scheimpflug or wide-angle OCT imaging, allowing views from any specified angle, will also aid in understanding the corneal profile (Figure 8). This examination plus topography data will guide diagnostic contact lens selection.

Figure 8. Scheimpflug graft profile images. (A) prolate profile and (B) proud oblate profile with thinning at the graft-host junction.

Be aware of loose, protruding, or exposed sutures, as they can stimulate neovascularization or lead to infection. The immune-mediated inflammatory process of graft rejection can occur in any layer of the cornea, but endothelial rejection is the most common. Clinical signs of graft rejection include conjunctival hyperemia, subepithelial infiltrates, corneal edema, and keratic precipitates on the endothelium (termed a Khodadoust line when they have a linear formation and progress inward from the periphery) (Figure 9).12 Risk of rejection increases with corneal neovascularization in the transplanted tissue, which is thought to destabilize the immune privilege that allogeneic corneal transplants possess.14

Figure 9. Graft rejection approximately three months after femtosecond laser-assisted PK. Note ciliary flush, superior-temporal sectoral corneal edema, keratic precipitates, Khodadoust line, and loose superior temporal sutures. This patient complained of reduced vision, ocular irritation, and redness.

LENS FITTING APPROACH

In general, the goals of contact lens wear in this population are the same as any other; our objective is to maintain ocular health while providing comfortable wear and visual improvement. Above all, in this specific population, the goal should be maintaining graft health. The risk-to-benefit ratio should be considered when fitting contact lenses on corneal transplants. Prudent choices will reduce the risk of complications. Utilize the highest-oxygen-transmissible (Dk/t) material possible to limit risk of edema. Holden and Mertz recommended a Dk/t of 24 to prevent clinically significant hypoxia-induced corneal swelling during daily wear, but that study was performed with relatively healthy corneas, not corneas with decreased endothelial function.15

It is important to monitor closely for the corneal response to contact lens wear. Even the most cautious and prudent choices may still lead to unwanted and unexpected changes to the transplanted tissue. It is important to follow up at short intervals initially, such as weekly, then slowly increase the follow-up interval as long as no deleterious effects (such as edema, microcysts, bullae, or neovascularization) are observed. If changes to the corneal health are observed, the lens design, materials, and lens wear time should be modified to alleviate complications.

SPECIALTY LENS OPTIONS

Corneal GP contact lenses are the classic option of choice for individuals who have undergone keratoplasty. These lenses are available in high- and hyper-oxygen-permeable (Dk) materials and provide sufficient tear exchange. Louie et al discussed a lens application approach to approximate the corneal contour and evenly distribute the lens weight across the surface.16 Many laboratories offer post-surgical toric and quadrant-specific back-surface designs to address the variable corneal profiles. A large-diameter GP contact lens with an optic zone larger than the graft diameter is highly recommended to achieve an optic zone encompassing the graft and graft-host junction to reduce mechanical interaction between the contact lens and graft.12 Prolate-shaped grafts with a steeper center can benefit from keratoconic designs. General theory recommends reverse geometry designs for oblate grafts, with peripheral curves steeper than the central base curve for better alignment, but also consider a post-surgical aspheric design. It is important to aim for as much edge lift as possible without excessive lens awareness and staining to aid in tear exchange.16

In piggyback lenses, using a high- or hyper-Dk material for the GP lens over a high-Dk soft lens can provide improved comfort and lens stability for those wearing corneal GP lenses (Figure 10). By changing the power of the soft contact lens, the curvature of the cornea can be artificially altered, allowing for a more normalized corneal shape, which can aid in lens stability and lens centration. It is important to be cognizant that most soft prosthetic or custom-painted contact lenses are made in a low-Dk hydrogel material and should be carefully monitored.16

Figure 10. Diagnostic fitting of a piggyback contact lens system for a six-month postoperative femtosecond laser-assisted PK. Note the 10.5mm corneal GP contact lens resting on a high-Dk daily disposable soft contact lens. It is clear to see that the GP lens is resting on the graft-host junction. The GP lens needs to be modified in the following manner: an increase in overall diameter and optic zone diameter both by approximately 1mm, an increase in reverse geometry, and an increase in edge lift. The GP contact lens will be ordered in a hyper-Dk material.

Mass-produced and custom soft contact lenses are a viable option when a graft has a generally spherical corneal curvature with low amounts of astigmatism and low amounts of higher-order aberrations (Figure 11). Many laboratory manufacturers offer post-surgical and irregular cornea soft contact lenses, which have the option for reverse geometry for better alignment and increased center thicknesses to help “mask” moderate corneal irregularity and optical aberrations. Because hypoxia and sequelae are a concern, Dk/t of the lenses should be taken into consideration. The use of high-Dk silicone hydrogel materials is indicated.

Figure 11. High-Dk silicone hydrogel soft contact lens fit on a three-month postoperative femtosecond laser-assisted PK.

Hybrids can correct large amounts of astigmatism with the GP lens center. However, it is important to ensure that the lens is not excessively bearing on the graft-host junction, as the transition zone between the GP lens and soft skirt has a limited clearance. Earlier generations of hybrids utilized a HEMA soft skirt with a low Dk value of 9.3 and should be substituted in favor of the newest generation featuring a high-Dk silicone hydrogel soft skirt.

Scleral contact lenses rest on the conjunctiva and vault the entire cornea as well as any corneal irregularities (Figure 12). With scleral lenses, it has been suggested that tear exchange is at a minimum;17,18 thus, oxygen must permeate through a relatively thick contact lens and a post-lens fluid reservoir to reach the cornea.19 There is concern for insufficient oxygen transmission leading to clinically significant corneal edema.19 It has been shown in the literature that stromal edema can occur during scleral wear.10,20 These studies were performed on normal corneas and not on physiologically compromised eyes, which, by reason, should be approached with extra caution. In the literature, a minimalist fitting philosophy has been suggested utilizing the highest-available-Dk material, central contact lens thickness not to exceed 250 microns, and lens clearance under 200 microns.21 This has been proposed to theoretically deliver sufficient oxygen to the cornea to avoid hypoxia complications. Although not clinically studied and proven, a rule of thumb has been adopted to exercise caution with scleral contact lens wear when endothelial cell density is below 800 cells/mm2.22 Although case reports and small-scale studies of scleral lens wear over corneal grafts have been performed showing some complications, larger-scale studies are necessary.23,24

Figure 12. Profile view of a scleral lens vaulting a 28-year-old corneal transplant. Due to extremely complex geometry, all other contact lens options had failed. Transplanted tissue showed reduced endothelial cell count with significant polymegethism and pleomorphism. Prudent choices were made to reduce risk of complication by utilizing hyper-Dk material, reduced apical clearance, and reduced lens thickness while promoting tear exchange. The patient is followed every three months.

PATIENT EDUCATION

Patient education is extremely important with graft patients. It is important to emphasize the need for regular follow up and for urgent follow up with visual blur, redness, light sensitivity, irritation, pain, or foreign body sensation. Patients may think that their symptoms are related only to new contact lens wear, so educate them that many of the symptoms of an ill-fitting contact lens are similar to those associated with graft rejection.25

FOLLOW-UP APPROACH

It is critical to perform a careful history and to pay specific attention to patients’ quality of vision. Patients experiencing lens-induced complications may describe a spread of colors around bright lights, called Sattler’s veil, which results from a diffraction phenomenon caused by mild, central, microcystic edema secondary to hypoxia occasionally observed with hard contact lens wear. Understand each patient’s wearing patterns and correlation with visual challenges.26,27

Monitor for changes with comparison to the baseline. Remove the contact lens at every follow-up visit and closely evaluate the graft for changes (Figure 13). If edema is present, it is important to differentiate between immunological-mediated edema and a contact lens hypoxic-stress-induced change. Edema from an immunological-mediated process signifies a rejection event, typically including presence of circumlimbal injection, infiltrates, and keratic precipitates, which necessitates aggressive immunosuppressive therapy.

Figure 13. Scheimpflug camera-based optical pachymetry, difference maps. This case shows the effect of scleral lens wear on a three-year-old full-thickness corneal transplant. The patient had failed other contact lens modalities due to an elevated graft-host junction. (A) Baseline pre-lens-wear thickness map (left); one year of scleral lens wear post-lens-wear thickness map wearing a 0.42mm center thickness lens, 100 Dk material, 200 microns of apical clearance, minimal tear exchange, average daily lens wear of 14 hours (center); and a difference map showing global increase in corneal thickness (right). The cornea showed no sign of rejection or decompensation but did have mild microcystic edema. The decision was made to reduce physiologic stress from the lens by changing its parameters, material, and fitting relationship. (B) Eight weeks of scleral lens wear post-lens-wear thickness map wearing a 0.2mm center thickness lens, 160 Dk material, 50 microns of apical clearance, moderate tear exchange, average daily lens wear of 14 hours (left). One year of scleral lens wear post-lens-wear thickness map wearing a 0.42mm center thickness lens, 100 Dk material, 200 microns of apical clearance, minimal tear exchange, average daily lens wear of 14 hours (center). Difference map showing global decrease in corneal thickness (right). After the parameter changes, the cornea was clear and thin without obvious signs of edema.

As grafts can change with time and in response to contact lens wear, it is of utmost importance to follow these individuals closely. During the initial fitting process, an appropriate follow-up visit should occur every one to two weeks. Once a contact lens is finalized and all is well after a six-week follow-up visit, a three-month follow-up visit can be made. Once the patient is a habitual contact lens wearer, follow up every three to six months with much shorter time in between visits, six to eight weeks, in high-risk patients. CLS

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