How to set up baseline data for evaluation of long-term treatment

When prescribing treatments for slowing myopia progression and axial elongation, what to use as the most appropriate reference for future comparison, both regarding the refractive data and axial length (AL), has gained lots of discussion among eye care practitioners. This apparently simple question is surprisingly challenging to answer.

Here, we discuss some of the most clinically relevant questions related to setting up the baseline of refraction and ocular biometric data for the reliable long-term evaluation of treatment efficacy.

BASELINE REFERENCE FOR REFRACTIVE DATA

Most of the clinical trials on the efficacy of myopia interventions use cycloplegic autorefraction as the standard method in collecting refractive data both for baseline and at each annual visit. However, in real world practice, it is not always feasible to perform cycloplegic autorefraction at every follow-up visit. Consequently, many practitioners use dry autorefraction or manifest subjective refraction to compare the changes over time.

Although it is uncommon to see a big difference between cycloplegic and dry autorefraction in pediatric myopes who do not have binocular issues, it is highly recommended to perform a comprehensive exam that includes a cycloplegic autorefraction, dilated fundus exam, and ocular biometry, as all these tests will provide a thorough baseline.

In case when a difference of 0.50D or more is detected between dry and cycloplegic autorefraction, another dry autorefraction should be considered after a full recovery from the cycloplegia, as it reflects the natural ciliary tone after the spasm is relaxed by cycloplegia.

Something to keep in mind: It is not uncommon to observe a small hyperopic shift of refraction soon after the initiation of low dose atropine (LDA), especially for concentrations above 0.025%, which likely contributes to the cycloplegic effect of atropine even at a diluted concentration. It is important to communicate with parents about the transient nature of such a shift, and that it should not be interpreted as a reversal of the child’s myopia by the treatment.

As for the considerations among autorefraction, subjective refraction, and retinoscopy, autorefraction is the preferred choice as it is the most objective and repeatable technique that is least dependent on ambient lighting and pupil size. In contrast, subjective refraction yields the most variable results especially in patients undergoing ortho-k treatment, due to the non-uniform central corneal flattening hence dramatically induced higher order aberration. Consequently, results from subjective refraction are highly dependent on the ambient light and pupil size, as well as patient’s ability for blur interpretation in a highly aberrated optical system.

BASELINE REFERENCE FOR AL

With the technology advancement in interferometry, the precision of optical biometers for AL reading has improved significantly, hence allowing for the detection of subtle changes. However, the repeatability and reliability of optical biometry is highly influenced by the depth of penetration with its infrared laser source.

Optical biometry measures AL up to the retinal layer. As a result, any factors influencing the choroidal thickness, such as diurnal variation, choroidal vasodilation, or chorioretinal inflammations, may impact the reliability of AL measurement.

Although choroidal thickening and associated AL shortening up to 50 µm to 70 µm is frequently reported in the early stages of myopia control treatments,^1,2,3,4 the sustainability of such change is not fully understood. Consequently, the AL measurement prior to the initiation of treatment should be considered as the baseline for further comparison, and the apparent shortening of AL should not be interpreted as the reversal of excessive scleral stretching and thinning, a consequence of the accelerated axial elongation during myopia progression.

FINALIZING A BASELINE

In summary, a comprehensive exam, including cycloplegic autorefraction, a careful ocular health evaluation, and a thorough ocular biometry prior to any myopia interventions is critical for the reliable establishment of baseline for future and long-term evaluation of myopia-controlling efficacy. Practitioners should be cautious in differentiating measurement outliers, as well as common confounders for data collection and interpretation from true, clinically meaningful changes. OM

REFERENCES

1. Prieto-Garrido FL, Villa-Collar C, Hernandez-Verdejo JL, et al. Changes in the Choroidal Thickness of Children Wearing MiSight to Control Myopia. J Clin Med. 2022;11(13):3833. doi: 10.3390/jcm11133833. PMID: 35807117; PMCID: PMC9267297.
2. Loertscher, M. (2013). Multifocal orthokeratology associated with rapid shortening of vitreous chamber depth in eyes of myopic children. Contact Lens and Anterior Eye. 2013;36:e2.
3. Jiang Y, Zhang Z, Wu Z, et al. Change and Recovery of Choroid Thickness after Short-term Application of 1% Atropine Gel and Its Influencing Factors in 6-7-year-old Children. Curr Eye Res. 2021;46(8):1171-1177. doi: 10.1080/02713683.2020.1863431.
4. Breher K, García MG, Ohlendorf A, Wahl S. The effect of the optical design of multifocal contact lenses on choroidal thickness. PLoS ONE. 2018;13(11):e0207637. doi: 10.1371/journal.pone.0207637.

DR. LIU is an associate professor at Pacific University College of Optometry and is the founder of University of California, Berkeley’s Myopia Control Clinic. She received her bachelor’s degree of Clinical Medicine from Peking University, her OD from Pacific University, and her PhD and MPH from University of California, Berkeley. She is a world-recognized clinical researcher in myopia. Dr. Liu is a consultant for Coopervision and Essilor International.