Keratoconus Fitting With Specialty Soft Lenses
In many cases, soft contact lenses provide optimal comfort and vision for patients with keratoconus.
By S. Barry Eiden, OD, FAAO, & Gregory W. DeNaeyer, OD, FAAO
Fitting keratoconic eyes with contact lenses is synonymous with fitting GP contact lens designs. The firm nature of GP lenses allows them to mask corneal irregularity by creating a post-lens tear layer.
Although standard soft contact lens designs can correct ametropia and regular astigmatism in a keratoconic eye, they do not perform as well for correcting irregular astigmatism because their thinness and inherent flexibility cause them to conform to front-surface cornea irregularity (de Brabander et al, 2003; Holden and Zantos, 1981). In addition, standard soft contact lenses typically are not available in parameters that will fit the steeper corneas of many keratoconic eyes. There are, however, specialty soft contact lens designs that not only successfully fit keratoconic eyes but also provide improved visual acuity by correcting for mild to moderate amounts of irregular astigmatism.
Need for Soft Lenses
According to the baseline findings in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study, 65 percent of patients with keratoconus were wearing GP lenses and 73 percent reported comfortable lens wear (Zadnik et al, 1998). Unfortunately, some keratoconus patients cannot wear GP lenses because of discomfort, and their only option is a corneal transplant.
Improved hybrid and scleral lenses now provide alternative options for patients who previously could not wear corneal GP designs. That said, however, in our experience some patients would prefer a soft contact lens design if available. But can a soft lens deliver acceptable vision to these patients?
Yamazaki et al (2006) reported on 66 patients (80 eyes) who were fitted with a specialty soft lens for keratoconus after experiencing reduced tolerance or poor fitting with other designs. The eyes were grouped according to severity of keratoconus, as follows: 15 percent incipient, 53.7 percent moderate, 26.3 percent advanced and 5 percent severe. The results showed that 91 percent of the eyes achieved visual acuity better than 20/40 with a specialty soft lens design for keratoconus. The base curve most frequently used was 7.6mm in 61 percent of the eyes.
Specialty Soft Lens Characteristics
Specialty soft contact lenses for keratoconus (Table 1) are lathe-cut to allow for customization. These lenses are available with relatively steep base curves (4.1mm to 9.3mm, depending on the design) to accommodate the increased sagittal height of a keratoconic eye.
|Accu Lens||Soft K|
|Advanced Vision Technologies||Soft K|
|Art Optical (distributed by B+L)||KeraSoft IC|
|Continental Soft Lens||Continental Cone|
|Gelflex USA||Keratoconus Lens|
|Marietta Vision||Soflex Soft|
|Ocu-Ease/Optech||Ocu-Flex 38 Keratoconus|
|Strategic Lens Innovations Corporation||Soft K|
|United Contact Lens||UCL-55|
|X-Cel Contacts||Tricurve Keratoconus|
|UltraVision CLPL||KeraSoft IC|
|World Vision||Perfect Keratoconus|
Soft contact lenses designed for keratoconus are generally thicker centrally than standard soft lenses, ranging from 0.3mm to 0.6mm. This increased center thickness helps prevent the lens from conforming to the irregular shape of the cornea. In other words, the soft lens starts to behave like a GP, which allows it to mask some mild to moderate irregular astigmatism. Increasing lens thickness, however, decreases the lens's oxygen permeability, thus raising a concern that the eye will develop hypoxiarelated complications.
As lathe-cut silicone hydrogel lenses become available in keratoconic designs, they will be preferable for many patients. A few of these lens designs have adjustable midperipheral curves that allow the practitioner to adjust the movement and fit of the lens independent of the base curve. A lenticular midperiphery and periphery thins the lens design outside the optical zone, thus improving overall oxygen permeability and comfort (Figure 1). These lenses are available in the high spherical and toric powers commonly required for keratoconus. Some soft contact lens designs for keratoconus are available in reversegeometry configurations and, as such, can be applicable to post-keratoplasty cases that have residual anterior surface irregularity.
Figure 1. Profile of a soft contact lens fitted for keratoconus.
Fitting Specialty Lenses
Fitting specialty soft lenses for keratoconus typically requires a diagnostic fitting set because empirical prediction of lens performance in terms of required sphere, cylinder, axis and center thickness is highly unreliable in these cases. If you do not have a fitting set, you can order a lens based on the manufacturer's fitting guidelines to function as your initial diagnostic lens.
The lens should align with the cornea centrally to provide the best vision possible. Use high-molecular weight fluorescein with a cobalt light and Wratten filter to check for bearing or excessive vault. Edge fluting or lens movement of greater than 1mm indicates the lens is fitting excessively flat. Perform a sphere-cylinder over-refraction to determine power. If the patient is not achieving his best potential vision with over-refraction, consider performing topography over the lens to measure the amount of residual irregularity. Increasing the center thickness by 0.1mm to 0.3mm may be necessary to improve vision if significant irregularity remains. Discuss specific adjustments for a particular lens design with the manufacturer's consultants to fine-tune the final parameters.
Wavefront-guided Soft Lenses
Although specialty soft lenses for keratoconus can mask mild irregular astigmatism, we believe that wavefront-guided soft lenses for keratoconus will eliminate more significant amounts of higher-order aberrations and maximize visual acuity. The difficulty of successfully correcting for asymmetric higher-order aberrations is that soft lenses translate and rotate on the eye. De Brabander et al (2003) reported on simulated optical performance of custom wavefront soft contact lenses for keratoconus and found that translation errors negatively affected performance and should not exceed more than 0.5mm. It has been suggested that partial correction of higher-order aberrations may be beneficial when a lens exhibits significant translation and rotation (Guirao, 2002).
Marsack et al (2007) reported on a patient with moderate keratoconus whose habitual lenses were soft torics and who was refitted with custom wavefront-guided soft lenses. The patient had a 1.5-line improvement of visual acuity and a 50 percent reduction in higher-order aberrations. It may also be possible to customize the back surface of a soft contact lens to improve the fit on a keratoconic cornea. Customized back-surface soft contact lenses, as compared to conventional soft contact lenses, improved lens stability by a factor of 2 for translations and a factor of 5 in rotational orientation in a study by Chen et al (2007).
Case #1 A 14-year-old boy reported for evaluation of keratoconus. His manifest refraction was OD +0.25 –3.25 x 015 20/40 and OS +0.50 –1.50 x 165 20/20. Topography showed moderate nipple keratoconus of both eyes. His corneas were without striae or scarring. He was interested in contact lenses for improved vision for lacrosse. After discussing contact lens options, the patient wanted to try a soft keratoconus lens design. He was diagnostically fit using HydroKone (Visionary Optics) soft lenses (hioxifilcon 59%) with a base curve of 8.5mm, secondary curve of 8.6mm, overall diameter of 14.8mm and center thickness of 0.4mm. The lenses had adequate centration and coverage with 0.5mm movement. An over-refraction was performed over the diagnostic lenses to determine power.
After several follow-up visits and one power adjustment of the right eye, the final lens powers were OD plano –1.12 x 085 20/40 (Figure 2) and OS plano sphere 20/20 (final center thickness 0.49mm OU). The power of both lenses compared to the manifest refraction illustrates how effectively the increased center thickness of the lens can mask astigmatism. The patient is scheduled for future corneal crosslinking.
Figure 2. This soft toric lens for keratoconus is well-centered on the patient's eye. The arrow points to the temporal toric marking.
Case #2 A 32-year-old patient who was previously unsuccessful with soft toric and GP lenses was interested in other options. His spectacle prescription was OD –1.50 –3.25 x 093 20/30 and OS –0.50 –3.00 x 079 20/30. Topography showed moderate oval cones on both eyes. Slit lamp examination showed the patient's corneas were clear without scarring. The patient was fitted with HydroKone (Visionary Optics) soft contact lenses (hioxifilcon 59%) with a base curve of 8.5mm, secondary curve of 8.6mm, diameter of 14.8mm and center thickness of 0.4mm. This diagnostic lens showed significant inferior decentration in the right eye, which was fitted first. A second HydroKone diagnostic lens with a base curve of 8.1mm and secondary curve of 8.6mm centered well and had 1mm of movement on both eyes; it was custom ordered after over-refraction. The final lens powers were OD –3.00 –3.00 x 070 20/25 and OS –3.25 –2.25 x 86 20/20 (final center thickness 0.49mm OU, Figure 3).
Figure 3. Fluorescein highlights the fit of this soft toric lens for keratoconus on the patient's right eye.
Case #3 A 22-year-old man was evaluated for symptoms of unilateral blurred vision with his eyeglasses, which was worsening over the past 2 years. Frequent prescription changes for the left eye failed to provide clear vision with eyeglasses. Interestingly, both of the patient's parents were keratoconus patients (with the father having advanced disease requiring penetrating keratoplasty). Manifest refraction was OD +0.75 –1.00 x 95 20/20–, OS –5.25 –1.25 x 65 20/50. Biomicroscopy failed to show any abnormalities in the right eye; however, the left eye had +2 Vogt's striae and a notable partial inferior Fleischer's ring. Pentacam corneal analysis revealed bilateral corneal ectasia that was significantly more advanced in the left eye compared with the right eye (Figure 4).
Figure 4a (top). Case 3: Pentacam corneal analysis OD. Figure 4b (bottom). Case 3: Pentacam corneal analysis OS.
The patient was fitted with a NovaKone (Alden Laboratories) lens (hioxifilcon 54%). This design is available in a series of “IT” values (0 to 4) corresponding to variable center thicknesses that are used to mask corneal irregularities.
The left eye was evaluated with a diagnostic lens with a base curve 1mm flatter than the mean keratometric findings. The first diagnostic lens demonstrated peripheral edge fluting and decentration. A second lens with a base curve 0.4mm steeper resulted in appropriate movement and centration without peripheral edge fluting. The over-refraction resulted in visual acuity of 20/30. The center thickness of the diagnostic lens was IT 2. The final lens for the left eye had a 6.6mm base curve, 8.2mm peripheral curve, 15mm overall diameter, IT 3 center thickness (with peripheral lenticularization) and power of –7.00 –1.00 x 90. Visual acuity with the final lens was 20/20–. The right eye was successfully fit with a conventional soft toric lens from SpecialEyes Laboratories (hioxifilcon 54%) in the following parameters: base curve 8.1mm, overall diameter 14.5mm and power +0.75 –1.00 x 100. The resultant visual acuity was 20/20 OD.
Case #4 A 27-year-old man with a 5-year history of keratoconus had been wearing conventional disposable soft lenses on both eyes with poor vision, after trying unsuccessfully to wear GP lenses. Vision with his habitual lenses was OD 20/20 and OS 20/50. Manifest refraction was OD –2.25 –1.00 x 65 20/20, OS –2.00 –5.00 x 70 20/40+. Biomicroscopy revealed a partial Fleischer's ring inferiorly and +2 Vogt's striae in the left eye. Pentacam corneal analysis revealed bilateral keratoconus, significantly more advanced in the left eye compared with the right eye. Both eyes had inferiorly displaced cones, but the one on the left was larger and much steeper in comparison to the right. Corneal thickness at the apex of each cone was OD 523 and OS 437 microns.
The patient was diagnostically fit with a KeraSoft IC silicone hydrogel lens (distributed by Bausch + Lomb [B+L] and manufactured by Art Optical as part of our participation in a premarket clinical trial). Based on the position and size of the cone, as well as the peripheral corneal curvature, a diagnostic lens with a base curve of 8.2mm and standard periphery was evaluated. The lens edge demonstrated mild fluting, and movement was somewhat greater than desired. A diagnostic lens with steep periphery was selected and demonstrated excellent centration and movement. Over-refraction resulted in 20/20– acuity. We ordered a lens with these fitting parameters and a power of –1.75 –4.00 x 80 (based on lens over-refraction). At dispensing and at the 2-week follow-up visit, vision with the lens was 20/25+, fit was excellent, comfort was described by the patient as “wonderful” and physiological response was excellent.
The KeraSoft IC contact lens is being manufactured initially by Art Optical from the Contamac silicone hydrogel lens material efrofilcon A (74% water, Dk 60). It is an anterior aspheric design lens with a consistent center thickness of 0.4mm, available in a variety of base curve and peripheral curve options. It also can be produced in a reverse-geometry configuration. Unique to soft contact lens designs for keratoconus, the KeraSoft IC contact lens can also be produced with what the manufacturer terms “sector management control,” whereby any two quadrants of the lens periphery can be fabricated steeper or flatter. This is occasionally needed in cases where the inferior corneal periphery is very steep compared with the rest of the cornea.
Contact lens management of keratoconus—and all contact lens cases, for that matter—requires that three criteria are met: adequate vision correction, acceptable comfort and appropriate ocular health response. Contemporary management of keratoconus with contact lenses encompasses numerous design categories, including GP corneal lenses, GP scleral contact lenses, hybrid contact lenses, piggyback lens systems and soft contact lenses. Depending on the degree of corneal distortion and the associated best-corrected visual acuity with spectacles, patients with keratoconus may be able to successfully wear either standard soft lenses or custom specialty soft lenses designed for keratoconus.
Standard soft contact lenses are typically limited to mild cases of keratoconus that can achieve normal visual acuities with spectacle correction (typically better than or equal to 20/25). Although both lathecut and molded disposable contact lenses can be used in these cases, the final power parameters that produce best-corrected visual acuity may or may not be consistent with the manifest refraction. Often, because of unpredictable lens draping over the keratoconic cornea, the spherical, cylindrical and axis parameters of the successful contact lens may be quite different from what was predicted from the manifest refraction.
For cases of more advanced corneal irregularity and reduced best-corrected spectacle acuity in keratoconus, specialty custom lathe-cut soft contact lens designs are required. Centrally thickened designs will mask corneal irregularity and provide acceptable visual acuity in many cases. The primary advantages of these contact lenses include improved comfort and the ability to center well while still correcting vision adequately. The most significant physiological concern with these contact lenses is hypoxia, owing to the significantly greater lens thicknesses used in these designs. The utilization of peripheral lenticularization, greater-than-average contact lens movement and higher oxygen transmission materials addresses this concern.
The fitting of keratoconus patients with contact lenses is not synonymous with fitting GP lenses. In many cases, soft contact lenses can provide optimal comfort, vision and physiological response for patients with keratoconus. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #194.
|Dr. Eiden is president of North Suburban Vision Consultants, Ltd. in Deerfield and Park Ridge, Ill. He is co-founder of EyeVis Eye and Vision Institute, research participant and consultant to numerous contact lens and pharmaceutical companies, and adjunct faculty at Indiana, Illinois, Pennsylvania, and UMSL Colleges of Optometry. You can reach him at firstname.lastname@example.org|
|Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio. His primary interests include specialty contact lenses. He is also a consultant to Visionary Optics. You can reach him at email@example.com.|