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International Newsletter and Forum on Rigid Gas Permeable Contact Lenses, Corneal Shape, Health and Vision |
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T-Sag vs. N-Sag: What is Your Line of Sight?
 We are learning more and more about the shape of the limbus and about anterior scleral shape - first from optical coherence topography imaging and now from newer eye surface profiling techniques that can assist in this way. This was one of the main topics at the Contact Lens Specialist Symposium (CLSS) in Legoland (Denmark) recently. Patrick Caroline described the difference in scleral height using the Eye Surface Profiler (see this link for the 2015 GSLS poster on the topic). It appears that the nasal sagittal height (N-Sag) and the temporal sagittal height (T-Sag) of the human eye differ quite substantially - while the superior and inferior portions of the sclera are more 'on PAR.' What does this mean in terms of lens fitting? That large diameter lenses - including soft and scleral lenses by default - may decenter temporally. Inferior decentration, if present, may be caused by a different mechanism (eyelid pressure and/or gravity). Why make such a big deal out of this? With sclerals, not only may the optical quality be affected, but decentration also induces a prism and has effects on the lens fit (e.g., clearance). Nasal limbal clearance may be exceptionally low compared to temporal limbal clearance. This difference in T-Sag vs. N-Sag may be countered by asymmetric back surfaces (in scleral lenses) or decentered optics (in custom soft lenses). Decentered optics may play a crucial role in delivering the optimal image to the retina. Temporal decentration of lenses adds to the negative effect of the more nasal location of the line of sight compared to the geometrical center of the cornea. This negatively affects the outcome, especially in multifocal lenses and myopia control lenses. If you want my (line of) sight on this - this may play a crucial part in the success of such lenses going forward. |
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CNN
How often do scleral lenses get mentioned in a CNN piece? This did happen on March 17. The reason is a spectacular new innovation: scientists are developing smart contact lenses embedded with miniscule mirrors that can magnify vision by almost three times. 'It is hoped that the lens will improve the sight of people with age-related macular degeneration - the third leading cause of blindness globally' the CNN article states. In a press release dated February 13th, optics specialist Eric Tremblay from EPFL (Ecole polytechnique fédérale de Lausanne) in Switzerland unveiled the latest prototype of a telescopic contact lens and debuted accessory wink-controlled glasses that switch between normal and 2.8x magnified vision. The contacts work by incorporating a very thin reflective telescope inside a 1.55mm thick lens. Small mirrors within bounce light around, expanding the perceived size of objects and magnifying the view, so it's like looking through low magnification binoculars. The basis for all of this is - indeed - a scleral lens.
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Back to Baseline
Researchers from Spain looked at the short-term changes in light distortion in orthokeratology subjects. Twenty-nine healthy volunteers with a mean spherical equivalent refractive error of −2.10 ± 0.93D were evaluated at baseline and on days 1, 7, 15 and 30 of orthokeratology treatment. Average monocular values of all light distortion parameters measured increased significantly on day 1, returning to baseline after 1 week. Spherical-like aberration stabilized on day 7 for all pupil diameters, while coma-like aberration stabilized for smaller pupils only. Contrast sensitivity was significantly reduced on day 1 for all spatial frequencies except for 1.5 cycles per degree, returning to baseline afterwards. In conclusion: despite consistent increased levels of corneal aberrations, light distortion tends to return to baseline after one week of orthokeratology treatment, suggesting that neural adaptation is capable of overcoming optical quality degradation.
Sanz et al, Biomed Research International - 01/2015
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Let's Settle This Once and For All - Cont'd
 The last issue of I-site newsletter covered studies by Bauer and Lotoczky that looked at scleral lens settling on-eye, and by Bray et al showing what this means for the over-refraction. A study by Kauffman et al from the University of Missouri, St. Louis (USA) published in Optometry & Vision Science looked at the same topic. They concluded that the amount of settling varied significantly among three lens designs with various diameters. In line with the previously mentioned studies, it was found that settling rates were greatest shortly after lens application. Larger lenses were estimated to have settled by eight hours. Based on the results of this short-term study, careful consideration should be given to the settling patterns of individual scleral lens designs, the authors stated.
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Corneal Cross-Linking & Sclerals
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Post-CXL
Visser et al studied the subjective and objective outcome of scleral lens tolerance and fitting before and after corneal cross-linking (CXL) for progressive keratoconus. In this prospective cohort, they evaluated 18 unilateral eyes of patients who underwent CXL and had been wearing scleral lenses before the procedure. Data were collected before and one year after CXL. They found no significant change in scleral lens-corrected distance visual acuity. Sixty-one percent of eyes needed a scleral lens fit and/or power change. The wearing time (a median of 16 hours per day) and subjective tolerance were found to be stable. Based on this, it appears that scleral lenses are well accepted in patients who undergo CXL and can be a good indication to restore vision if desired. A slit lamp scleral lens fitting grading scale updated from their previous version is provided in the publication.
Visser et al, Optometry & Vision Science - Jan 2015
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Sclerals: a small step for Salzmann's
Salzmann's nodular degeneration can be a challenging irregular corneal condition to fit with contact lenses. Patients may present with decreased best-corrected acuity but otherwise be asymptomatic - although symptoms may include photophobia, pain, irritation and dryness. Some nodules may also be vascularized. Vision can be improved by using either a specialty soft contact lens or a form of gas permeable correction to mask the irregularity from the underlying nodules. Often piggyback soft lenses are used to improve comfort for these patients. In order to prevent corneal contact with the nodules, scleral lenses may be an excellent fitting modality. Care should be taken with the peripheral clearance, but also with the scleral landing zone. See link below for full case report and details. In summary: sclerals have the ability to correct the irregular astigmatism while at the same time attenuating any dryness and irritation - a huge step for the patient's quality of life.
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Upcoming Events
- World Cornea Congress, April 15-17, San Diego (US)
- AOMC, April 16-19, Houston (US)
- Contact Lens Educational Summit, May 2, Denver (USA)
- EAOO, May 14-17, Budapest (HU)
- IACLE 3rd World Conference, May 24-28, Manchester (UK)
- BCLA, May 29-31, Liverpool (UK)
- Keratoconus management, June 26-27, Hertford (UK)
- EurOK, July 10-12, Budapest (HU)
- ISCLS, Aug 29-Sept 3, St. Gallen (CH)
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I-site is an educational newsletter that is distributed on a monthly basis and provides an update on rigid gas permeable related topics (scientific research, case reports and other publications worldwide). I-site is objective and non-political. Its editor Eef van der Worp, optometrist, PhD, FAAO, FBCLA, FIACLE, FSLS is a lecturer for a variety of industry partners, but is not related to any specific company. Please contact us at: i-site@netherlens.com.
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