Reducing RLRL intensity maintains choroidal thickening

Paper title: Efficacy of 50% vs. 100% intensity repeated low-level red-light on choroidal thickness and vasculature: A randomized crossover trial

Authors: Zhang S (1), Chen Y (2), Zhang J (1), Zhu Z (1,3), Han X (1), Xuan M (1), Ding X (1), Xiong R (1), Yuan Y (1), Wang Y (1,3,4), Li C (1), Liao L (1), Yin Q (1), Zheng Y (1), Kong X (5), He M (1,2,3,4)

1. State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-sen University; Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Guangdong Provincial Clinical Research Center for Ocular Diseases, Guangzhou, China
2. School of Optometry, The Hong Kong Polytechnic University, Hong Kong, PR China; 
3. Centre for Eye and Vision Research (CEVR), 17W Hong Kong Science Park, Hong Kong
4. Center for Eye Research Australia, Royal Victorian Eye and Ear Hospital, Melbourne, VIC, Australia
5. Centre for Eye and Vision Research (CEVR), 17W Hong Kong Science Park, Hong Kong
6. Second People's hospital of Foshan, Southern Medical University, Foshan, 528000, China

Date: Dec 2025

Reference: Zhang S, Chen Y, Zhang J, Zhu Z, Han X, Xuan M, Ding X, Xiong R, Yuan Y, Wang Y, Li C, Liao L, Yin Q, Zheng Y, Kong X, He M. Efficacy of 50 % vs. 100 % intensity repeated low-level red-light on choroidal thickness and vasculature: A randomized crossover trial. Photodiagnosis Photodyn Ther. 2025 Dec; 56:105240

[Link to open access paper]

Repeated low-level red-light (RLRL) therapy is an emerging intervention for myopia control. Choroidal thickening has been observed with several treatment modalities and is increasingly regarded as an early indicator of treatment response to myopia control. However, it remains unclear whether the choroidal response to RLRL varies with treatment intensity. 

This study aimed to compare the effects of 50% versus 100% RLRL intensity on daily choroidal thickness and vascular parameters in myopic adults (18-40 years), using a randomised crossover design. By capturing high-frequency OCT data across both treatment arms, the study sought to evaluate whether a lower radiant energy dose could elicit a comparable biological effect.

Key points were as follows:

• One month of 50% or 100% RLRL produced comparable increases in choroidal thickness: sub-foveal (+24.8 µm vs +24.9 µm) and macular (+22.2 µm vs +20.1 µm).
• Peripapillary choroidal thickening and increases in vascularity index, luminal and stromal areas were observed in both groups, with no differences between intensities.
• Choroidal thickening developed progressively from week 2 and had plateaued by week 4
• Both groups showed a small but statistically significant axial shortening of approximately 0.056 mm over one month.

This study found that reducing the intensity of repeated low-level red-light (RLRL) therapy without losing short-term structural response may be feasible. Half-intensity RLRL produced comparable increases in choroidal thickness and axial shortening compared to full intensity, indicating that dose reduction may not compromise treatment effect in the short term.

Choroidal thickening has been linked to long-term myopia control in prior studies, suggesting that retinal changes could serve as early indicators of treatment response. The similar response observed with 50% intensity suggests that clinical efficacy may be preserved with lower intensity, although this requires confirmation from longer-term trials.

Using lower-intensity RLRL may offer practical advantages, such as reduced cumulative energy exposure and a potentially improved safety margin. It may also be better suited for long-term therapy or for patients with greater sensitivity to light, although these assumptions remain untested.

The study also observed choroidal thickening in the peripapillary region as well as in the macula, suggesting there could be a global choroidal response to RLRL therapy, rather than a strictly localised retinal effect. This may provide further insight into how RLRL influences ocular growth.

Although this study showed promising short-term results, there are some limitations. The trial involved young adults and lasted only one month, so it is not yet clear if the same effects would be seen in children or over longer periods of treatment. Since most myopia control interventions are aimed at children, future studies will need to test whether lower-intensity RLRL is equally effective in that group.

The suggestion that lower intensity may offer a safety advantage is reasonable, but hasn’t been investigated so far. No adverse effects were reported in either group, but longer-term safety data will be needed, especially if RLRL is to be used regularly over months or years.

It is also unclear whether the short-term changes in choroidal thickness and axial length seen with 50% intensity will lead to the same long-term outcomes as full-intensity treatment. Early structural changes may be useful early markers indicating treatment effect, but they need to be confirmed against refractive and axial length changes over time.

Finally, the finding that choroidal thickening occurred in both macular and peripapillary regions raises interesting questions about how RLRL works. A global choroidal response could help explain how this treatment influences eye growth

Purpose: To determine if 50 % repeated low-level red-light therapy (RLRL) can achieve the same choroidal thickening as the standard 100 % intensity, potentially reducing energy use without sacrificing efficacy in myopia control. This study aimed to investigate the daily choroidal thickness (ChT) and vasculature, as surrogate outcomes for myopia control efficacy, with the RLRL at 100 % and 50 % intensity.

Methods: This prospective crossover randomized controlled trial recruited adults aged 18-40 years with myopia. The study consisted of two 4-week treatment phases separated by a 4-week washout period, with participants randomly assigned to either a 100 %-50 % or 50 %-100 % RLRL treatment sequence. The primary outcomes were changes in subfoveal ChT (sfChT) and mean macular ChT (mChT) measured daily using swept-source optical coherence tomography. Secondary outcomes included macular choroidal vasculature.

Results: The study included 27 participants (mean age, 21.2 years). After 1 month of treatment, both 50 % and 100 % RLRL resulted in significant increases in sfChT, mChT, and peripapillary ChT, with no significant differences between the two groups. Choroidal thickening developed gradually over the treatment period, and daily change profiles in ChT and vasculature parameters were comparable.

Conclusions: This study demonstrates that both 50 % and 100 % RLRL produced similar choroidal thickening, suggesting that 50 % RLRL may offer comparable myopia control efficacy with reduced energy usage.

[Link to open access paper]