October 2023
Column
Fooling the Foveola
The theme of the Orthokeratology Society of Oceania (OSO) meeting held last month at the Gold Coast (Australia) was 'New Frontiers'. Feedback from the meeting suggests that with regard to myopia progression, we may be heading in a new direction. Historical wisdom over the last few decades from the revolutionary work of Earl Smith III indicates that if the retinal macula is eliminated, myopia still progresses in primates. Hence, it must the peripheral retina that is leading the myopia development. The newest findings indicate that his may not be entirely true. It could be true for myopia development, but the mechanism for myopia control – hence, the slowing down of the axial length elongation in existing myopes – may be a different pathway. In fact, work from different research groups – independently from each other – indicates that it may be only a very small portion in the central retina that is involved. Could it be that it is not just the macula (5500um in diameter) or the fovea (1800um), but the small area of the foveola (350um), where our interventional optics should be applied? Ray tracing imaging studies at Pacific University seem to indicate that the most popular myopia management lenses indeed project their image over that small portion of the retina (the foveola). The 'type of optics' involved could be anything that causes optical confusion (plus or minus or even another form of optical confusion). Pioneer in the field of myopia Tom Aller may have said it right in the past: 'Any Damn Plus, Any Damn Place' (or other optical confusion) on the retina could be the new road to explore in managing myopia. In his view, 'confusing' the retina when faced with conflicting signals may result in stopping or slowing down of growth . Maybe we have been fooled all this time by focusing on location (central or peripheral) or even type of optical confusion when just any confusion potentially anywhere (including the foveola) could 'fool' the retina into producing our preferred outcomes.

Scleral Lenses
Vault, Vergence & Vision
To stay with the remarkable optical effects of contact lenses: a paper by David Piñero et al looks at the optical impact of vault increase in scleral lenses. They also used ray-tracing simulations in three eye models with different levels of primary spherical aberration. Two different scleral lens geometries were used, and fluid reservoir thicknesses between 50–250µm were tested. Interestingly, the impact was measured not only for a distant object, but also for intermediate and near objects (vergence demands from 0.00D to 3.00D). They found, as in previous studies, the thinnest vault (50µm) to provide the best ocular optical quality in all three eye models for distance viewing. Lens decentration did not cause significant variations in the results in this study. However, the most important finding was the impact of vault variation on the optical quality achieved for different vergence demands. According to the simulations, vergence demands representing near vision improved with a thicker vault. This may be explained by the changes induced in high-order aberrations leading to an increase in the depth of focus and, more specifically, by changes in primary spherical aberration. It should be outlined, the authors explain, that controlled changes in primary and secondary spherical aberrations may be beneficial for presbyopes due to the enlargement of the depth of focus generarated – but further investigation is warranted to explore this more.
Scleral Lens Replacement Period
Surprisingly, little is known about the average replacement time frequency of scleral lenses. This study looked at this from a tertiary care hospital standpoint, and found that in a cohort of two hundred fifty-one251 patients (120 men and 131 women) with an average age of 57.1±17.4 years (range 9–93 years), the average replacement period to bewas 23.9±14.3 months or (1.99±1.19 years). The range was pretty large: from 5 to 2617 days. An interesting finding was that patients with greater scleral lens experience had a statistically significant increase in their average scleral lens replacement period;: for every one year of additional experience wearing scleral lenses, the average replacement period increased (statistically significantly) by 30.7 days. No statistically significant correlation was found between average scleral lens replacement period and sex, diagnosis, prior outside scleral lens treatment, lens brand, or lens diameter.
Topography
Topographic Findings in Thyroid Disease
This study aimed to evaluate corneal topographic findings in patients with thyroid eye disease (TED) using a Scheimpflug imaging system. Compared with control eyes, several corneal topographic indices showed marked differences, including the Inferior-Superior Index, Keratoconus Prediction Index, Surface Asymmetry Index, Surface Regularity Index, simulated keratometry astigmatism and anterior instantaneous astigmatism axis. The anterior instantaneous astigmatism axis was more oblique in the TED group than in the control group. However, the corneal topographic parameters did not differ significantly among the different thyroid dysfunction groups. In conclusion, corneas of patients with TED are suspicious and keratoconus-like, and TED can increase anterior corneal astigmatism in the oblique axis, albeit with large variations in the extent of change. These TED-related corneal changes may aid in the early diagnosis of TED, especially in “silent” TED.
Recovering Original Corneal Curvatures in Orthokeratology
What if you want to know the original corneal curves in current ortho-k lens wearers? Not much research has been done on the prediction of the initial corneal curvature based on the current corneal topography of patients being managed with ortho-k. This could be of great importance when the initial parameters are missing. In this study, machine learning was applied to predict the initial corneal curvatures. A total of 497 right eyes of 497 patients undergoing overnight orthokeratology for myopia for more than one year were enrolled in this retrospective study. Corneal topography was obtained by a Placido disc corneal topographer (Sirius, CSO). The original flat keratometry value (K1) and the original steep K (K2) were set as the targets of calculation. The results of the complex algorithm calculations showed that K2 after one year of orthokeratology (K2after) was most important in the prediction of K1 and K2 – hence, the original curvature values. The authors state that machine learning can be applied to predict the corneal curvature for those who cannot provide the initial corneal parameters in the outpatient clinic, providing a relatively certain degree of reference for the refitting of the ortho-k lenses.
Webinar
Toric Cornea in Ortho-k
A key element in ortho-k lens fitting and effectiveness today is the management of corneal astigmatism. Studies have revealed that relatively small changes on the ocular surface can give rise to lens decentration, with suboptimal outcomes in terms of vision and myopia control effectiveness. Changing to a toric (or more complex) back-surface lens design could be very beneficial. In the recent August 2023 GPLI webinar (accessible with membership), Melanie Frogozo looks into 'Approaching the Toric Cornea with Orthokeratology.’
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.