October 2021
Column
An Index (of Refraction)
A recent systematic review by Alicia Sánchez-García et al in Contact Lens & Anterior Eye (2021) helps greatly with indexing corneal structures and with understanding the mechanism behind ortho-k. We tend to think of ortho-k as a change in corneal curvature. But really, it is a (small) change in corneal thickness that leads to a substantial change in the refraction of light. Note: the change in thickness is mostly epithelial in nature. The normal corneal epithelium is about 50 microns thick—or rather, 50 microns thin. A recent study by Ju Zhang et al in Contact Lens & Anterior Eye (2020) found that the corneal epithelial thickness was reduced in the central (2mm) zone in ortho-k by 6.13±1.67μm, or microns, (about 11.78% of the total epithelial thickness at baseline). In this study, manual analysis of the reverse curve zone epithelial thickness was utilized (as opposed to using the standard zones defined). The corneal epithelium in the reverse curve zone was found to increase by 7.60±2.05μm (about 14.66% of the total epithelial thickness). Participating subjects had a mean -3.35±1.09D refraction at baseline (ranging from -0.75D to -5.00D). In another study, the epithelial thickness was about 10μm thinner in ortho-k wearers (35.8±2.8μm) than in non-lens wearers (46.7±4.5μm), according to Swarbrick, Kang and Peguda in Optometry & Vision Science (2020). Some structural changes occur in the anterior stroma as well, but generally it is believed that the change is small and more evenly distributed. Wook Kyum Kim et al in PloS One (2018) found an increase of 2.0, 3.3, and 3.9μm, respectively, in the center, para-center, and mid-periphery of the stroma in ortho-k. Kin Wan et al in OPO (2021) recently found significant reductions in corneal thickness and epithelial thickness, but no stromal thickness change. This leaves the epithelial change as the main factor in the refractive change in ortho-k. These changes are very minor in nature. Go figure, what the power of refractive index is when going from air to cornea.
Global Specialty Lens Symposium
Orthokeratology
Meridian Change
The aim of this study by Jiaqi Zhou et al was to investigate the changes in corneal epithelial thickness along the principle meridians of astigmatic corneas after six months of overnight spherical ortho-k lens wear for myopia. The ortho-k lens-induced changes in the epithelial thickness profiles were evaluated and compared between the steep and flat meridians of corneas with less than 1.50D of astigmatism. After 6 months of spherical ortho-k lens wear, it was demonstrated that the central epithelium thinned and the mid-peripheral epithelium thickened in all four quadrants. In the para-central cornea, the epithelial thickness flat meridian (ETF) thinned more than the steep meridian (ETS). In the mid-periphery, ETS thickened more than ETF at 2.5mm and at 3mm. The results suggest that there is a correlation between the corneal toricity and the remodeling of the corneal epithelium in ortho-k. In line with this, Yan Lian et al in 2013 found that overnight ortho-k caused the central corneal epithelium to thin in both the vertical and horizontal meridians, while the mid-peripheral nasal and temporal epithelium became thicker and the superior mid-peripheral epithelium became thinner. All subjects in this study had myopia of no more than 4.00D and astigmatism of no more than 1.50D. The thickness of the central or mid-peripheral Bowman's layer in either meridian did not change.
Photo by Lisa Yount on Unsplash
Pigmented Arc
The aim of a study by Pei-Wei Huang et al was to determine the intensity of corneal pigmented arcs in ortho-k-treated children and their correlation with epithelial thickness maps. The study included 57 eyes of 29 children (mean age 11.4 years, range 9-15). After initiating ortho-k treatment, the incidence of the corneal pigmented arc was 91.2%, with a mean lens wear duration of 26.1 months. The intensity of pigmented arcs was significantly correlated with lens wear duration, target power, baseline degree of myopia, and location. More on the same topic: Chun-Fu Liu et al found the intensity of pigmented arcs in ortho-k to be significantly correlated with key epithelial thickness parameters, including maximum and minimum epithelial thickness, the difference between them (Min-Max) and the standard deviation. Significant correlation between intensity of pigmented arcs and ortho-k target power was found as well. Anterior segment optical coherence tomography (AS-OCT) can be a useful tool for predicting the intensity of pigmented arcs in ortho-k-treated children. Photo by gerti gjuzi on Unsplash
Structural Changes
Circling back to the systematic review by Alicia Sánchez-García et al (column above), the following reversible changes after ortho-k have been found: reduction in central epithelial basal cells, an increase in height and especially in width of superficial epithelial cells, as well as changes to the central stroma and in the number of active keratocytes. Concerning corneal endothelium, only changes in terms of polymegathism were reported that were compatible with those observed for other types of contact lens wear. There was a reduction of corneal sensitivity during the use of ortho-k as well as in the nervous density of the sub-basal plexus at the central level, with some contradictory outcomes concerning the reversibility of these changes. In conclusion, they state that the quality of published studies evaluating these aspects is good, although more studies are needed to evaluate longer follow-up changes.
Keratoconus
A Pubmed search on "epithelial thickness keratoconus" reveals no less than 46 papers, just for 2020 and 2021 (and still counting). Some highlights of epithelial thickness assessment in keratoconus are mentioned here.
Doughnut Pattern
While pretty much all emphasis in keratoconus detection has always been on overall corneal changes, and even on posterior changes showing before anterior changes - the latest research shows that we should maybe have a second, and closer, look at just the epithelial layer. In line with the items above, epithelial thickness profiles reveal that the effect of keratoconus as we see it with a corneal topographer may in some way be 'masked' by localized thinning of the epithelium over the top of the cone and/or by concentric thinning of the epithelium around the top of the cone. A pattern of central epithelial thinning (to approximately 20μm) with an annulus of epithelial thickening (to approximately 30-40μm) was demonstrated by Franco et al. They called it the "doughnut pattern" of epithelial remodeling in keratoconus to partially compensate for central corneal steepening that occurs with this disease. Photo by nathan-dumlao on Unsplash
Cone Decision Tree
A recent study by Yang et al in the Journal of Cataract & Refractive Surgery looks at keratoconus or 'cone' detection using OCT corneal and epithelial thickness map parameters and patterns. The cross-sectional observational study from the Casey Eye Institute, Oregon Health and Science University in Portland (OR, USA) developed a smart decision tree that could correctly classify all normal eyes (100% specificity) and had good sensitivities for detecting manifest keratoconus (97.8%), subclinical keratoconus (100.0%) and forme fruste keratoconus (73.7%). Photo by Mae Mu on Unsplash
Assessment Techniques++
What about other corneal assessment techniques with regard to the ability to characterize both the anterior and posterior corneal surfaces in keratoconus? Lynett Masiwa and Vanessa Moodley cover this topic in a review paper in Journal of Optometry (open access). Reflection-based and elevation-based corneal imaging systems should be used in conjunction with other assessments such as higher-order aberration measuring systems to improve sensitivity and reliability in the diagnosis of pre-clinical keratoconus. Ultra-high-resolution ultrasound can detect pre-clinical keratoconus. They also mention that the ability to asses the epithelium (and the endothelium) makes anterior surface optical coherence tomography a superior technique for pre-clinical keratoconus diagnosis. And there is a positive correlation between central corneal thickness and corneal hysteresis. Corneal biomechanics should be considered in conjunction with other corneal assessments in the diagnosis of pre-clinical keratoconus, they conclude. Photo by Jorge Bermudez on Unsplash
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. Disclosure: I-site's editor Eef van der Worp, optometrist PhD FAAO FBCLA FIACLE FSLS, receives educational grants from a number of industry partners but is not related to any specific company.