Lynette Johns OD FAAO FSLS
Geunyoung Yoon PhD
Ramkumar Sabesan PhD
Higher order aberrations (HOAs) are often the culprit behind sub-optimally corrected keratoconus patients who are wearing either corneal (R)GP lenses or scleral prosthetic devices (SLPD). Due to these HOAs, a patient with keratoconus may have adequate pinhole acuity, but sphero-cylindrical over-refraction may not provide the same acuity level. Even with optimized Snellen acuity, the patient may still complain of ghosting, starbursts, shadows or doubling due to uncorrected HOAs.
The Boston Foundation for Sight and the University of Rochester are collaborating in a study to investigate the feasibility of correcting HOAs using wavefront-guided optics in a scleral lens prosthetic device. Recruited patients wore back-surface toric devices for increased stability and limited movement over pharmacologically dilated pupils. The front surface consisted of spherical optics with no eccentricity. Baseline HOAs through this scleral prosthetic device with spherical optics were measured with a custom-developed Shack-Hartmann wavefront sensor and incorporated into a custom corrective device. The static rotation and the horizontal and vertical decentration of the device on-eye were measured simultaneously and incorporated into the design of the corrective device so that the HOA correction was appropriately aligned to the pupil. Optical correction and visual performance were compared between the scleral prosthetic devices with spherical and wavefront-guided optics.
A 37-year-old male with severe keratoconus had worn scleral devices since 2005 because of corneal (R)GP intolerance. His simulated keratometry readings were OD 71.3/88.3 @ 82° and OS 57.5/81.4 @ 62°. At each annual visit, we attempted varying levels of front-surface eccentricity to optimize his best-corrected visual acuity (BCVA). His left and right eye measured 20/30-2 with poor subjective quality despite the aspheric optics. His left eye developed hydrops, which was resolved but left a residual scar outside the visual axis. The right eye did not have any media opacity. Both of his native devices had 0.8 front-surface eccentricity to optimize his acuity. A spherical-optic baseline device was made with the same fitting characteristics (diameter 18.0 mm, sagittal height 6.10 mm with 0.3 mm back-surface toricity from the flat edge, base curve 7.9 mm, power +2.50), but with no front-surface eccentricity for the right eye. The HOA root mean square (RMS) through this device was 1.67 µm for a 6 mm pupil. Natural mesopic Snellen visual acuity with an over-refraction and the spherical testing device was 20/44.
We fabricated the wavefront-guided corrective device by combining the HOA and device movement measurements obtained with the spherical-optics device. The RMS with the wavefront-guided corrective device was improved nearly 8-fold to 0.22 µm for a 6 mm pupil. The mesopic Snellen acuity improved to 20/28.5. Although the optical quality with the corrective device was comparable to normal levels, the acuity was significantly worse than normal acuity. For comparison, in 4 normal eyes with similar RMS of 0.25 µm, Snellen acuity of 20/10.7 was measured.
Wavefront-guided correction with scleral prosthetic devices has many benefits for patients with keratoconus. The stability of the device ensures proper placement of the optics, once any decentration and rotation are factored into the design. As a result, this case demonstrated a significant reduction in the HOAs with a custom wavefront-guided correction. However, the BCVA with the wavefront-guided correction was significantly poorer than that of normal subjects, even though the optical quality was excellent. This demonstrates that a patient with keratoconus who has long-term neural adaptation with optical blur due to HOAs may not elicit the maximum visual benefit upon optical correction. However, this neural deficit may be overcome given sufficient time to re-adapt with corrected optics, which may eventually improve overall visual performance.
Dr. Lynette Johns has been the senior optometrist at the Boston Foundation for Sight since 2005.
She is a graduate of the New England College of Optometry where she completed a residency in cornea and contact lenses. She is adjunct clinical faculty at the New England College of Optometry.
Dr. Johns is a fellow of the American Academy of Optometry and a fellow of the Scleral Lens Education Society.
Professor Geunyoung Yoon has been directing the advanced physiological optics laboratory at the University of Rochester since 2001. He graduated from Osaka University in Japan where he obtained his MS and PhD. Dr. Yoon has been the principal investigator on developing wavefront-guided soft contact lens for patients with keratoconus. His research focus also includes supernormal vision, presbyopia correction, high-resolution noninvasive ocular imaging and tear/dry eye research.
Ramkumar Sabesan is a postdoctoral researcher at the Flaum Eye Institute, University of Rochester. He obtained his Bachelor of Technology in Engineering Physics from the Indian Institute of Technology, Delhi. Subsequently, he obtained his PhD from the Institute of Optics at the University of Rochester. His research has focused on the diagnosis and correction of disorders affecting the visual system using advanced optical methodologies such as customized ophthalmic lenses and adaptive optics instrumentation.