Influence of orthok treatment zone diameter and pupil diameter on myopia progression

Paper title: The Role of Back Optic Zone Diameter in Myopia Control with Orthokeratology Lenses

Authors: Jaume Pauné (1), Lina Rodríguez (2,3) and Antonio Queirós (3)

1. Centre Marsden de Terapia Visual, Consulta 156, Centro Medico Teknon, Vilana 12, 08022 Barcelona, Spain
2. Institute Visual Clinic Center, Pereira 660002, Colombia;
3. Clinical and Experimental Optometry Research Lab (CEORLab), Center of Physics, School of Science,University of Minho, Gualtar, 4710-057 Braga, Portugal

Date: Jan 2021

Reference: J Clin Med. 2021;10: 336 [Link to open access paper]

The authors report retrospective observations from children fit with OrthoK in a private medical clinic where the back optic zone diameter (BOZD) in the OrthoK lens design was modified to alter the induced treatment zone diameter. The strategy defining how BOZD was selected for each patient is not discussed, however once selected for inclusion, the study participant data was separated into different groups to investigate the relationship between treatment zone diameter and pupil diameter on change to axial eye length and objective cycloplegic refraction across 1-year. Children with a pupil diameter larger than the OrthoK induced treatment zone diameter were found to have slower axial eye growth and a hyperopic shift in refraction compared to the opposite profile where pupil diameter fell inside the peripheral plus power zone of the treatment zone, who exhibited greater axial eye elongation and increased myopia. These results support previously published clinical research, confirming the need for prospective studies to validate these findings.

Results from this study add further weight to the clinical observation that promoting the plus powered zone induced by OrthoK to fall within the pupil diameter has a beneficial effect on slowing progression of myopia in children.

However, further research is needed, particularly a prospective randomized study design, to add  scientific credibility before this strategy can be adopted as a guiding principle when fitting OrthoK for myopia management.

Study data collection was retrospective with participants not randomized to the different treatment groups

• Gold standard study design is for the study protocol to be established before participant recruitment (prospective design) with participants then randomized between the test and control arms of the study.

Small sample size, particularly in the Full Effect (FE) group

• Larger samples increase likelihood of converging to the mean - the larger sample size in the other groups increases confidence in their mean measurement relative to the FE group.
• However, a power analysis was reported and indicated that sample size was sufficient to detect a significant difference between treatment groups.

Subject inclusion is not defined, just that 71 schoolchildren were included that has attended the optometric clinic; were fit with OrthoK lenses with differing BOZD; and identified by the lead author as exhibiting <0.5mm treatment zone decentration; uniform treatment zone and successful treatment defined as ≤ 0.50D residual refractive error and 6/6 or better uncorrected visual acuity.

• It is standard practice for retrospective analyses that all patients between the defined study data collection dates are included and then excluded if they do not meet the study criteria
• In this case it appears that the lead author chose which patients to include, so it’s not clear if any eligible patients were left out of the analysis, in effect creating selection bias

Detail on how PPR was established is not specified

• What map type was used? Axial, tangential or power?
• How was the maximum plus power identified? Was this by subjective visual inspection or objectively done by a computer
• Was measurement conducted masked from knowing what BOZD was dispensed to avoid selection bias?

PPR width for the evaluation of the relationship between pupil diameter, axial eye growth and PPR was based on on an analysis performed in a different study

• The authors assume that PPR width will follow the same profile as reported in a historic data set
• The data was present, meaning that the authors could have calculated PPR width to establish the range that fit their dataset rather than relying on historical reported data

Axial eye length was measured by ultrasound

• Ultrasound measurement requires placing a probe into contact with the anterior eye surface. This causes the globe to be susceptible to mild compression from differing amounts of pressure applied by the probe, thereby increasing potential for variability in measurement
• Variability in measurement has been reported as 0.06±0.04mm, which needs to be taken into consideration when reported differences between the study groups in this study are within the same magnitude range.

Why did refraction reduce in the small S-BOZD group and increase in the L-BOZD group, while in both cases axial eye growth increased, albeit to a lesser degrees in the S-BOZD group

• It was shown in this study that reducing BOZD led to a reduction in PPRD. Previous studies have shown that altering treatment zone diameter changes spherical aberration,[1] which may offer a possible explanation, given that depth of focus is altered through altering spherical aberration[2]

Future research directions

• The outcomes from this study are in agreement with a previous longitudinal study (Chen et al), which increases confidence that a beneficial myopia control effect is achieved by ensuring treatment zone diameter exceeds pupil size
• However, before changing prescribing behaviour a prospective randomized study is needed to validate that this is a true beneficial effect

To evaluate whether axial eye growth in myopic children progresses differently between children wearing OrthoK lenses with different back optic zone diameters

Retrospective analysis of children attending a private optometric clinic in Spain between March 2012 and October 2016. 71 School children were included, having been selected by the lead author as meeting the study inclusion requirements, they

• ranged in age from 10-15yrs, 30 female, 1 male
• had baseline spherical equivalent refractive error between -0.75 and -6.00D
• had less than -2.00D with the rule astigmatism
• had best corrected visual acuity exceeding 6/6

The lens used was the double reservoir design developed by the lead author, fit according to the lens design fitting specifications with toric design selected where necessary to improve treatment zone outcomes. The lens design offers the ability to reduced BOZD without altering total lens diameter, which was utilized to vary BOZD between participants, though indication of when or why to alter BOZD is not described in the paper.

• Objective cycloplegic refraction: Grand Seiko Autorefractometer/Keratometer WAM-5500 while wearing the OrthoK lenses
• Axial length:  OcuScan RxP Ophthalmic Ultrasound System using the mean of 10 readings
• Corneal topography: Keratron
  • Points of higher plus power change (PPR) were identified
  • Mean PPR diameters (PPRD) established along horizontal and vertical meridians
  • Pupil diameter under ambient mesopic room illumination, but the authors state that photopic conditions were considered due to the topographers intrinsic light level

BOZD

• BOZD > 5mm (L-BOZD, n=36)
• BOZD ≤ 5mm (S-BOZD, n=35)

PPRD

• PPRD > 4.5mm (L-PPRD, n=36)
• PPRD ≤ 4.5mm (S-PPRD, n=35)

• Pupil diameter < (PPRD - 0.9mm)(NE, n=23)
• Pupil diameter ≥ (PPRD - 0.9mm) & Pupil diameter ≤ (PPRD + 0.9mm)(ME, n=40)
• Pupil diameter > (PPRD + 0.9mm)(FE, n=8)

*0.9mm was chosen by the authors as defining 80% of the mean PPR width of 1.9mm to 2.4mm reported in a different paper (Carracedo G et al. J Ophthalmol 2019;1082472)

Groups were further stratified by using either horizontal, vertical or mean PPRD

BOZD was closely associated with PPRD, indicating that reducing BOZD had the effect of reducing PPRD.

Spherical equivalent refraction was significantly more myopia and axial eye elongation over 1 year was significantly lower in the S-BOZD group compared to the L-BOZD group, indicating that smaller BOZD had a preferential effect on slowing axial eye growth.

Horizontal PPRD was slightly larger than vertical PPRD.

The authors state that the L-PPRD group was characterised by smaller pupil diameters than the S-PPRD group indicating that children in this study with smaller pupils tended to fall into the L-PPRD group and those with larger pupils tended to fall into the S-PPRD group.

Spherical equivalent refraction was significantly more myopia and axial eye elongation over 1 year was significantly lower in the S-PPRD group compared to the L-PPRD group, indicating that smaller treatment zone diameter had a preferential effect on slowing axial eye growth.

Similarity with the BOZD outcomes is not surprising given that the authors revealed a close association between BOZD and PPRD.

Mean PPRD was only significantly different between the NE and FE groups.

Participants where the PPR fell within the pupil zone (FE) demonstrated the greatest myopia control efficacy while participants with the opposite characteristic of the PPR falling outside of the pupil zone (NE) had the greatest myopia progression and axial eye elongation over the 1 year study measurement period.

• Change in spherical equivalent refraction
  • FE: 0.27±0.50D
  • ME: -0.01±0.29D
  • NE: -0.25±0.31D
• Change in axial eye length
  • FE: 0.04±0.10mm
  • ME: 0.11±0.11mm
  • NE: 0.17±0.12mm

Results from this study add further weight to the clinical observation that promoting the plus powered zone induced by OrthoK to fall within the pupil diameter has a beneficial effect on slowing progression of myopia in children. Further research is needed to validate these results, particularly a prospective randomized controlled study.

Paper title: The Role of Back Optic Zone Diameter in Myopia Control with Orthokeratology Lenses

We compared the efficacy of controlling the annual increase in axial length (AL) in myopic Caucasian children based on two parameters: the back optic zone diameter (BOZD) of the orthokeratology (OK) lens and plus power ring diameter (PPRD) or mid-peripheral annular ring of corneal steepening. Data from 71 myopic patients (mean age, 13.34 ± 1.38 years; range, 10-15 years; 64% male) corrected with different BOZD OK lenses (DRL, Precilens) were collected retrospectively from a Spanish optometric clinic. The sample was divided into groups with BOZDs above or below 5.00 mm and the induced PPRD above or below 4.5 mm, and the relation to AL and refractive progression at 12 months was analyzed. Three subgroups were analyzed, i.e., plus power ring (PPR) inside, outside, or matching the pupil. The mean baseline myopia was −3.11 ± 1.46 D and the AL 24.65 ± 0.88 mm. Significant (p < 0.001) differences were found after 12 months of treatment in the refractive error and AL for the BOZD and PPRD. AL changes in subjects with smaller BOZDs decreased significantly regarding larger diameters (0.09 ± 0.12 and 0.15 ± 0.11 mm, respectively); in subjects with a horizontal sector of PPRD falling inside the pupil, the AL increased less (p = 0.035) than matching or outside the pupil groups by 0.04 ± 0.10 mm, 0.10 ± 0.11 mm, and 0.17 ± 0.12 mm, respectively. This means a 76% lesser AL growth or 0.13 mm/year in absolute reduction. OK corneal parameters can be modified by changing the OK lens designs, which affects myopia progression and AL elongation. Smaller BOZD induces a reduced PPRDs that slows AL elongation better than standard OK lenses. Further investigations should elucidate the effect of pupillary diameter, PPRD, and power change on myopia control.

[Link to open access paper]