What is the 'hyperopic reserve' in pre-myopia?

The rising prevalence of myopia worldwide elicits an increasing need to delay the onset of myopia, as well as reduce its progression. Several interventions have shown efficacy in slowing myopia progression and many of these treatments are beginning to be incorporated into eye care practitioners’ myopia management strategies. But what also must be considered are the ways in which children at high risk of developing myopia – otherwise called ‘pre-myopia’, can be effectively managed.

The International Myopia Institute (IMI) - Defining and Classifying Myopia report¹ clearly defines the pre-myope as follows.

Pre-myopic children are recognised as individuals who have a nonmyopic refraction, but who have a cluster of risk factors and/or an observed pattern of eye growth that indicates a high risk of progression to myopia. The key risk factors are outlined below, as identified by the IMI.²

1. Family history – one myopic parent increases risk by 2-3 fold, while two myopic parents increases risk by 3-5 fold.^3-6
2. Visual environment – less than two hours day spent outdoors increases risk⁷, especially if combined with more than 3 hours a day spent on near work activities (outside of school time).³
3. Binocular vision – Children with higher accommodative convergence (AC/A) ratios, typically seen with esophoria, have an increased risk of myopia development within one year of over 20 times.⁴ Accommodative lag may also be a risk factor but there is conjecture.⁵ Intermittent exotropia has also been associated with a higher incidence of myopia.⁶
4. Current refraction – the most significant risk factor of this lot for future myopia is if a child exhibits +0.75D or less of manifest hyperopia at age 6-7.⁸ This risk is independent of family history and visual environment. New data indicates that this widely known figure may be higher for children of East Asian ethnicity, compared to the multi-ethnic North American cohort (more info below) from which this was originally determined. Understanding this figure provides the concept of 'hyperopic reserve'.

Pre-myopes are an important population as these represent children who do not yet require vision correction, but are at imminent risk of doing so based on their risk factors and growth trajectory. There is a need to investigate interventions for pre-myopia, as children with early-onset myopia have a longer course of the disease, and they are more likely to develop higher levels of myopia.⁹ 

Intervention matters, because the overall impact of myopia prevention appears to outweigh that of short-term myopia control in children who are already myopic. This is because delaying the onset of myopia by 1 year can reduce final myopia by around 0.75D - similarly effective as 2-3 years of myopia control with existing modalities.¹⁰

Human eyes are shortest at birth and hyperopia is the predominant refractive error. Myopia arises through childhood commonly due to early emmetropization of the eye, along with a failure of mechanisms to maintain emmetropic status with continued eye growth in older children.¹¹ Compared to emmetropes, children who develop myopia undergo accelerated axial elongation up to 3 years before, with the most rapid growth in the year prior to onset.¹² Within this brief window there is a rapid shedding of age-normal hyperopia that overshoots emmetropia, into myopia.

Identifying ideal candidates is crucial to the accurate application of any treatment. It is well known that the biggest risk factor for developing myopia is being less hyperopic than age-normal – or having a low 'hyperopic reserve'. A child with reduced baseline hyperopia than expected for their age is at greater risk for developing myopia sooner, while more hyperopia has a protective effect.⁸ Directing attention to pre-myopia interventions for children with insufficient hyperopic reserve is an important strategy to reduce the incidence and severity of myopia.

Since a more hyperopic baseline refraction is helpful for reducing the risk of developing myopia, this raises some clinical considerations. What level of hyperopic reserve is required to reduce myopia risk? And does a child’s age and ethnicity affect the protective hyperopic reserve threshold? Three studies have investigated this crucial value in pre-myopia.

In 2023, Chen et al examined longitudinal data among Chinese children for the dose-response relationship between hyperopic reserve and future risk of myopia.¹³ The data spanned visits for over 870,000 patients from 2005 to 2021 and children aged 4 to 18 years. Concerningly, it was found that the mean age of myopia onset reduced by three years over that time, from 10.6 years in 2005 to 7.6 years in 2021. When it came to understanding hyperopic reserve, a dose-response relationship was found, where the risk of myopia increased rapidly when the spherical equivalent refractive error (SERE) was less than +1.50D. With SERE’s less than +1.50D, the risk (hazard ratio) of myopia increased by 0.325 for each 0.10D reduction in hyperopic reserve. In this large cohort of Chinese children, the 'safety threshold' of hyperopic reserve was +1.50D, regardless of age and gender.

Another paper investigating hyperopic reserve was an observational study by Wang et al published in 2024.¹⁴ This study aimed to generate age-specific normative values of hyperopic reserve and annual myopic shift in Chinese children aged 3 to 16 years, based on data collected from 3118 participants over a 1-year period (2015-2016). Values for hyperopic reserve were extrapolated using a backward calculation method. Individuals who were non-myopes at age 17 were taken as a starting point, then looking at their range of refractions at age 16, age 15 and so on, with this stepwise analysis continued until reaching the youngest age group available in the database. Key cut points for the mean hyperopic reserve within this population of Chinese children were +2.08D at 6 years, +1.49D at 8 years and +1.04D at 10 years.¹⁴

By contrast, the well-known CLEERE study, used by the IMI in their 2019 definition of pre-myopia¹, was an observational cohort study conducted in North America with data collected from 4512 children aged 6 to 11 years (36% White, 22% Hispanic, 16% African American, 12% Native American, 14% Asian American). SERE was the single best predictive factor for myopia onset. A hyperopic reserve of at least +0.75D was cited as the cut-off for myopia risk in children aged 6 years, with lower hyperopic reserve for older children.¹²

Notably, the threshold for hyperopic reserve for the CLEERE study’s ethnically diverse population, is much lower compared to the two studies of Chen et al¹³ and Wang et al¹⁴ undertaken in Chinese children. The lower threshold likely stems from ethnic differences between the study populations.

Unlike the CLEERE study’s ethnically diverse cohort (where only 14% were of Asian ethnicity), the other two studies were ethnically Chinese populations, who tend to have higher baseline risks for myopia and faster myopia progression after onset.6 Chen et al¹³ did not find a progressive reduction hyperopic reserve threshold with age, compared with the other two studies - it is unclear why. Altogether, these studies contribute significant data on the identification of pre-myopes and thus the candidates for early intervention.

The main evidence-based strategy for children at risk of developing myopia is increasing outdoor time as a first-line intervention (with sun protection).² Limiting continuous periods of near work (< 30-45 minutes) and close working distances (< 20-30 cm) can be considered, as these are modifiable risk factors.¹⁵

More proactive treatment options with an evidence base are as follows, each showing similar propensity to reduce the rates of myopia onset by about half - meaning half the rate of axial length progression and/or half the rate of myopia incidence.

1. Essilor® Stellest® plano spectacle lenses. For those willing to comply with spectacle wear, optical treatment with plano highly aspherical lenslet target (HALT) lenses have minimal side effects, and efficacy data has recently been shown, provided wearing time of at least 30 hours per week is met.¹⁶ Read more in our Science Summary Spectacles with highly aspherical lenslets for pre-myopia and our newly published Q&A Interview on The ‘hyperopic reserve’ and treatment in pre-myopia - Q&A with Dr Peter Chen.
2. Low-dose atropine. Low-dose atropine has shown efficacy in a handful of studies, although the ideal concentration between 0.01% and 0.05% appears to show age and ethnicity variation.^17-20 Read more in Atropine for pre-myopia.
3. Repeated low-level red light (RLRL) therapy. RLRL therapy also has recent efficacy data for delaying myopia onset in pre-myopic children.²¹ Read more in our Science Summary Can red light therapy delay myopia onset?

While the options are fewer compared to the available optical and pharmacological treatments for slowing myopia progression after onset, the impact of pre-myopia treatment on a child’s final myopia is comparable if not greater than two to three years of myopia control.¹⁰ While we can't be sure if we can 'prevent' myopia onset in children at risk, there is a volume of evidence indicating the crucial value of intervention to delay onset, and the available interventions to do so.