Comparative Clinical Performance of Scleral Lenses with Varying Limbal Clearance in a Keratoconic Population

Abstract

PURPOSE The purpose of this study is to investigate the impact varying the limbal clearance (LC) has on the clinical performance of scleral lenses and the levels of inflammatory mediators in the tear film of keratoconic patients. The specific aims of each chapter are outlined. In Chapter, 3 a group of keratoconic patients were fitted with a scleral lens of both high and low LC, with a constant CCC. In Chapter 4, the subjective response, ocular response and clinical performance of scleral lenses having varying LC were investigated. In Chapter 5, the changes in the level of inflammatory mediators in the tear film of keratoconic patients with scleral lenses wear with varying LC were determined. In Chapter 6, the association between clinical performance and tear cytokine changes that varied with scleral lens wear with low and high LC was discussed. METHODS For Chapter 3: based on the corneal sagittal depth at a chord of 15.0mm, participants were fitted with two sets of scleral lenses with the same central sagittal depth and varying limbal clearances (LC) which differed by 50µm. Lenses were worn in a randomly assigned order for a two-week period. Lens fitting parameters including central and limbal clearances were measured with the Visante™ OCT and compared. In Chapter 4: visual analog scales were completed concerning vision, comfort, burning and dryness. Corneal and conjunctival responses to lens wear, such as limbal and bulbar hyperemia, corneal swelling based on pachymetric values, and corneal staining as seen on biomicroscopy were observed and compared. In Chapter 5: tear samples were collected from both the inferior tear meniscus using the flush tear method, and from the pool of tears in the bowl of the inverted scleral lens using a microcapillary tube. Subsequently, tear cytokine analysis was performed using a multiplex electro-chemi-luminescent array (Meso Scale Discovery, Rockville, MD) instrument. Levels of IL-1, -6, -8, TNF-α, MMP-1 and -9 were compared using a Student t-test statistical analysis. In Chapter 6: correlations between cytokine levels and clinical parameters using the Pearson correlation coefficient (r) were performed. P values of less than 0.05 and p less than 0.10 for tear data were considered to be statistically different. RESULTS In Chapter 3, 11 subjects (22 eyes) were fitted with scleral lenses of a sagittal depth of 4.539±0.240mm for low LC and 4.550±0.243mm for high LC (p=0.877). There was no difference in CCC between low and high LC (p=0.671 for initial CCC, and p=0.475 for final CCC). The initial limbal clearances, before lens settling were 159.9 ±45.02µm for low the low LC lenses, and 194.07±66.10µm for high LC lenses (p<0.05). The final limbal clearances, after lens settling, were123.74±56.68µm and 167.31±69.75µm for the low LC and high LC lenses, respectively (p=0.006). There were significant differences between the nasal and temporal limbal clearances, with more clearance found temporally (p<0.001). In Chapter 4, compared to baseline, both low and high LC lenses resulted in improved subjective responses (p=0.07 low LC, p<0.01 high LC for overall comfort). Greater comfort was reported with scleral lenses with high LC (p=0.013 for comfort, p<0.01 for dryness, p=0.08 for burning) compared to low LC. There was no difference in limbal and bulbar hyperemia between high and low LC lenses. Corneal swelling was noted in all corneal locations and especially at the 6mm zone where there were significant differences for both low and high LC lenses compared to baseline (p=0.004, and p=0.039, respectively). Corneal response to scleral lens wear with either low or high LC appears to result in either peri-limbal staining or negative corneal staining. In Chapter 5 the median volume of tears collected from the flush tear collection was 1.0 µL (Range 0.2 to 6.0 µL). The median volume of tears collected from the post-lens tear film was 5.0 µL (Range 0.2 to 10.0 µL). A statistically significant difference was noted between sample volumes from either collection method (both, p<0.05). Significant differences at the p<0.10 levels were found comparing low and high LC with - TNF-α, MMP-1 and MMP-9 (all p<0.10) from the samples taken from the lens bowl. Scleral lenses with high LC were associated with increased levels of IL-1β, TNF-α, and MMP-1 and decreased levels of MMP-9. In Chapter 6, there was a statistically significant correlation between changes in LC and peripheral corneal thickness in the inferior quadrants. With high LC, a correlation was noted between IL-6 and IL-8 levels. CONCLUSIONS This study illustrated how low and high LC can vary the clinical performance of scleral lenses and physiological responses of the ocular surface in a keratoconic population. The clinical changes, such as hyperemia, corneal thickness, and corneal staining, and subclinical responses in tear cytokine levels are associated with hypoxic and mechanical etiologies. Eye care practitioners must take into consideration individual patients’ ocular condition when determining the ideal limbal zone fitting parameter.