What Does the Literature Tell Us About Scleral Lenses?
BY S. BARRY EIDEN, OD, FAAO
Over the past decade, scleral lenses have made a dramatic resurgence in the contact lens field. Highly oxygen-permeable materials and innovations in scleral lens designs and fitting methods allow practitioners to now provide patients with optimized vision, comfort, and physiological response with scleral lenses.
Scleral lenses are typically considered for irregular corneas and for more advanced ocular surface disease; however, as practitioner comfort with scleral lens fitting has increased, there has been greater interest in using sclerals for less severe anterior segment disease and even for non-diseased eyes. The optimized centration, minimal movement, and associated comfort of scleral lenses make them potentially attractive for simultaneous multifocal correction in normal eyes (Davis and Eiden, 2015), for instance.
So what has appeared about sclerals in the recent literature? We will review some recently published papers that can give us greater insight into the performance, benefits, and limitations of scleral contact lenses.
Where Have We Come From?
A comprehensive review of the literature recently identified 184 references that were directly related to scleral lenses (Schornack, 2015). Most of the articles published before 1983 presented lens design and fabrication techniques or indications for scleral lens therapy. Case reviews published after 1983 identified major scleral lens indications as well as visual and functional outcomes. Statistically significant improvements in visual acuity, vision-related quality of life, and ocular surface integrity were reported.
More recent publications have looked more specifically at the fitting characteristics, optical performance, and potential physiological impact on the anterior ocular structures that scleral lenses may have.
Where Are We Now?
Van der Worp et al (2014) reported on the status of modern scleral lenses. They noted that while sclerals were indicated only for diseased eyes in the past, today the indications are expanding and now include less severe and even non-compromised eyes.
The same article presented a review of scleral lens nomenclature in which the authors indicated that all lenses that partly or entirely rest on the sclera are considered scleral contact lenses; however, the Scleral Lens Education Society recommends further classification. When a lens partly rests on the cornea (centrally or peripherally) and partly on the sclera, it is called a corneo-scleral lens. A lens that rests entirely on the sclera and fully vaults the cornea is classified as a scleral lens. Further distinctions of the scleral lens group include mini-scleral (up to 6mm larger than the horizontal visible iris diameter [HVID]) and large scleral lenses (greater than 6mm larger than the HVID, up to 25mm in diameter).
The authors further stated that adverse events with these lenses are rare, but clinicians need to be aware of them to avoid further damage in eyes that often are already compromised. Additionally, the authors reviewed the current applications of scleral lenses for non-diseased eyes, such as managing corneal astigmatism, reducing high-order aberrations, and providing stable lenses in demanding environments.
Advanced Fitting Technologies
Gemoules (2008) was the first to describe the use of anterior segment optical coherence tomography (AS-OCT) in the design and fitting of scleral lenses. He described nine cases in which high-resolution AS-OCT was used to design successful scleral lenses without the use of diagnostic lenses. Two of the cases had also been previously fit with diagnostic scleral lenses to compare the efficacy and efficiency of the two methods. The author indicated that empirically designing scleral lenses based on AS-OCT seems to offer a more accurate and efficient alternative to trial fitting.
More commonly, however, practitioners today utilize AS-OCT to evaluate the fitting characteristics of diagnostic scleral lenses by measuring the corneal vault and the “landing” characteristics on the bulbar conjunctiva. Luo and Jacobs (2012) reviewed the literature regarding AS-OCT in contact lens fitting, including the history, the current and potential ophthalmic applications, and the most recent advancement in three-dimensional AS-OCT. While there is very limited literature on OCT in contact lens fitting to date, they anticipate increased use of AS-OCT in the near future as more practitioners gain access to this technology.
Does Vault Matter?
Anecdotal reports have suggested that visual acuity suffers with greater degrees of central lens vaulting. On the other hand, similar reports and discussions among practitioners have suggested better comfort responses and therapeutic success for corneal surface disease patients when scleral lenses have greater vault. However, these impressions may not be valid. Sonsino and Mathe (2013) conducted AS-OCT measurements of scleral lens vaulting on 20 eyes of 12 consecutive patients who were successfully fit with these lenses for the treatment of dry eye disease. The mean vault was 380µm centrally, with a high standard deviation (SD) (±110µm). There was no correlation of better acuity or better comfort with differing vaults. They concluded that with a large SD in average vault, precision in central vault does not seem important in scleral lenses for successful fitting for dry eyes.
Does this mean that scleral lens vault does not matter? Not necessarily. Compañ et al (2014) investigated the corneal oxygen availability during wear of scleral lenses of various Dk materials, lens thicknesses, and post-lens tear thicknesses. Additionally, the researchers clinically measured corneal swelling response to scleral lenses fit with variable post-lens tear thicknesses. They simulated the partial pressure of oxygen across the cornea behind scleral lenses made in different lens materials (Dk range of 75 to 200) and different lens thicknesses (100µm to 300µm). Post-lens tear film thicknesses ranged from 150µm to 350µm. Eight healthy subjects were fit randomly with a scleral lens with a thin and a thick post-lens tear layer in two different sessions for a period of three hours under open-eye conditions. The lenses that had a Dk of less than 125 and a center thickness of more than 200µm depleted the oxygen availability at the lens-cornea interface below 55 mmHg for a post-lens tear film of 150µm. For a post-lens tear film thickness of 350μm, no combination of material or lens thickness will meet the criteria of 55 mmHg. Their clinical measures of corneal edema showed that edema was significantly higher (P < 0.001, Wilcoxon signed-rank test) with the thicker compared to the thinner post-tear layer (mean ± SD, 1.66% ± 1.12% versus 4.27% ± 1.19%). The authors concluded that scleral lenses must be comprised of at least 125-Dk material and up to a limit of 200μm thick to avoid hypoxic effects even under open eye conditions. The post-lens tear film layer should be below 150μm to avoid clinically significant edema. Other studies support these findings (Jaynes et al, 2014; Michaud et al, 2012) and caution us to be wary of the potential oxygen deprivation influences of scleral lenses.
Scleral Lenses Have to Settle
Kauffman et al (2014) evaluated the mean total settling and mean rates of settling for three scleral designs. Measurements of central lens vault over the cornea were taken with AS-OCT at the geometric pupil center. Although the rates of settling were not linear, analysis of variance revealed a significant difference in the average settling rates (p < 0.05) among the groups. The authors concluded that the amount of settling varied significantly among the three lens designs. Settling rates were greatest shortly after application. Larger lenses were estimated to be settled by eight hours, whereas smaller lenses may take even more time. This short-term study indicates that we should carefully consider the settling patterns of individual scleral designs. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #234.
Dr. Eiden is president and medical director of North Suburban Vision Consultants, president and founder of the International Keratoconus Academy of Eye Care Professionals, and co-founder of EyeVis Eye and Vision Research Institute. He is an adjunct faculty member at The University of Illinois Medical Center as well as at the Indiana and Illinois Colleges of Optometry and Pennsylvania College of Optometry at Salus University. He is also a consultant or advisor to CooperVision, Alcon, B+L, Visionary Optics, Alden Optical, Oculus, Oasis Medical, Paragon Vision Sciences, and SpecialEyes.