Axial Length
Axial Length Growth And The Risk Of Developing Myopia In European Children
Summary
As a child’s eyes grow, their final eye shape and their final refractive correction is influenced by a mix of genetics, the environment and what the child does with their time, and continued eye growth increases risk of becoming highly myopic in later life. Being able to identify what stage of growth a child is at by assessing their growth against a reliable percentile chart would be valuable in trying to predict future level myopia of myopia in a child and guide interventions towards reducing rate of eye growth.
This population-based study set out to produce a percentile growth chart for axial length based on the data collected from European children and adults, and in doing so they found a stronger correlation between the refractive error and axial length in myopes compared to the same measurements in emmetropes. A significant relationship was also found in myopes between spherical equivalent refraction and both corneal curvature and axial length. The authors conclude that percentile growth charts of AL can be used as a key predictor for monitoring paediatric eye growth.
Clinical relevance
- The spherical equivalent (SE) was significantly related to the corneal radius (CR) and, on average, myopic children had steeper CR compared to emmetropes and hyperopes. This was confirmed by the same finding from the adult subjects.
- Compared to emmetropes and hyperopes, myopic children had a faster eye growth rate and a myopic shift seemed to occur at approx. 9yrs old.
- The axial length findings suggest that if a child seems to be moving up through the percentile groups, there is a good chance they will become myopic.
- What percentile a child is at when aged 6yrs could be a good predictor of their myopia by age 9yrs old.
- All of these finding help eye care practitioners identify those with a higher risk of fast progression when we look at the SE, CR and age of the child in practice.
- If we had growth charts to refer to in practice, it makes it easier to demonstrate to parents and patients what track a child’s likely progress could be.
Limitations and future research
Tideman et al were aware of some aspects of their study which may affect their results:
-
- The Generation R study (children aged 6-9yrs old) and the Rotterdam study (adults of 57yrs old) were both carried out on the Netherlands, while the ALSPAC study involved 15yr olds in UK. The authors admitted this may have geographical differences.
- There was no data for 20-25yr olds to bridge the gap in between the ages of the cohorts. This could mean that the growth curve trends could be under-estimated for 15yr olds.
- There was a difference in the instruments used to measure AL. Although this could have introduced a slight difference, the authors felt it shouldn’t be significant enough to change the results
- The authors also suggested that a follow-up of children after 2010 could reveal if they have a steeper growth curve compared to those from their study.
- An interesting finding was that height may be linked to axial length growth. The authors found a strong correlation with height and AL growth for 6yrs olds. There was slightly less for 9yr olds but it was still a significant correlation. However, they found there was no difference with the refractive error for boys or girls, suggesting no gender difference. Future research may explore this further and confirm if axial length growth may have a proportional link to height growth rates.
Association Of Axial Length With Risk Of Uncorrectable Visual Impairment For Europeans With Myopia
Summary
Myopia is increasingly frequent in many developed countries and we now know it isn’t simply a matter of poor vision which we ‘fix’ with spectacles or contact lenses. It can lead to myopic maculopathy, glaucoma, cataract and retinal detachment and having 'high' myopia increases these risks further. These eye conditions can all potentially lead to reduced vision at best and blindness at worst. As myopia increases, the eye’s total axial length has been seen to increase too, and it is this elongation which leaves the eye at risk from ocular pathologies.
The classification of what constitutes ‘high’ myopia can vary. The World Health Organisation (WHO) decided on a threshold of -5.00D and stronger in 2015 (1). For the purposes of their study, Tideman et al classed participants as being highly myopic when they had a spherical equivalent (SE) of -6.00D and an axial length (AXL) of 26mm or more.
This study used cross-sectional data collected from population-based studies undertaken by the Rotterdam study (I, II & III) and the Erasmus Rucphen Study. These results were combined with case-control data from the Myopia Study, with participants across all studies being 25 years or older. The authors wished to explore a link between visual impairment as a result of increased AXL, refractive error and to be able to apply this to the wider population.
Clinical relevance
It’s all about the risk
- We can see how myopia is likely to be a leading cause of blindness in the near future after the study findings suggest that “visual impairment will increase 7- to 13-fold by 2055 in high-risk areas”.
- We can also be more aware of an individual’s risk when we can see that those with an AXL of 26mm or greater. Those with longer axial length had a higher chance of having myopic-linked vision impairment - this increased from a 3.8% chance if the AXL was less than 26mm to a 25% chance if the AXL was 26mm or longer. An AXL of 30mm or longer was associated with a 90% chance of vision impairment in their lifetime due to complications from myopia.
- Eye Care Professionals are in a prime position to communicate this robust information we have on the risks associated with increased myopia and what their options are to reduce that risk.
- Start considering myopia management early.
- The participants who were most at risk of visual impairment were in the older age group and they had therefore had longer eyes and higher myopia for a longer time.
- Don’t forget the lower myopes!
- We need to consider all myopes as potentially being at risk of axial elongation and an increasing risk of impairment later in life, as we know from this and other studies that those who ‘only’ have -1.00D refractive error will already have double the risk of myopic maculopathy and posterior sub-capsular cataract compared to an emmetrope.(2)
Limitations and future research
This is a robust analysis taking in the data of more than 15,000 individuals, across multiple studies. The key area for future research is to measure how myopia control interventions for today's generation influence their rates of pathology and vision impairment in the future. This is a very long-term question, as our current generation of children with access to myopia management will need to age another 50-60 years before their outcomes could be compared to the ages analyzed in this paper. To this point, the authors recognized that their calculated risk estimates will become overstated with adoption of (successful) myopia strategies to slow myopia progression.
Race specific studies: The authors made predictions of varying prevalence for different countries, but these were based on values originally gathered from Europeans and they were aware this may not be directly applicable to other ethnicities. They were able to allow for this by using reported prevalence estimates for each country. At the time of publishing (2016), Tideman et al were unaware of any likely ethnic differences and felt confident they were making feasible predictions. Similar research across different ethnicities would help determine whether this observation holds true.
Changes in prevalence: The study revealed a steady shift from hyperopia to myopia when they looked at the birth decades of the participants, particularly from 1920 onward, with a higher myopic prevalence in the younger group (25yrs to 60yrs). This suggested that the population had been getting more myopic with time and that the chance of impairment with age was increasing alongside it. The authors predicted that by 2055 there would be a 2 to 3-fold increase risk of visual problems in Europe, a 3 to 5-fold chance in Singapore and a 3 to 6-fold in the Republic of Korea. This was based on their findings where the last study to be included in their analysis was from 2012. It is possible that the likelihood of myopic pathologies could have increased beyond their expectations since then. Further follow-on studies could verify if this has changed significantly.
Myopia Progession
Accommodation Is Unrelated To Myopia Progression In Chinese Myopic Children
Summary
Although the relationship between accommodation and myopia has been widely investigated, disagreement remains on whether accommodative lag (under-accommodation) plays a part in myopia progression. The proposed mechanism being that the resultant hyperopic defocus stimulates axial length growth towards reduce the lag in focus. A contributing factor towards this conjecture is differences in adopted methodology, which the authors of the set out to address.
By implementing analysis across a continuous range of accommodation between 0 and 6.00D, and including distance accommodation facility, the authors found no association between both measurement conditions leading them to conclude that lag of accommodation has no influence on rate of myopia progression.
This study adds weight to the argument that accommodation lag has no influence on myopia progression, however, even though complex and thorough analysis was conducted, there remain many differences to real world viewing conditions, meaning the jury is still out on whether or not myopia progression is influenced by accommodation lag.
Clinical relevance
- An interesting observation from this study was that a faster myopia progression rate at mean study cohort age (0.61D/yr) was found than previously reported by Donovan et al (0.48D/yr).1 Leading the authors to suggest that more attention should be given to younger myopes whose myopia progresses rapidly and thus more likely to become high myopes in later years.
- Results from this study indicate that accommodation lag has no influence on rate of myopia progression, at least in a Chinese population. However, the results should not be considered as definitive due to differences between study measurement protocols and real world conditions.
Limitations and future research
- Accommodation lag, while measured using an accurate system and carefully analysed, does not necessarily replicate actual accommodation lag under normal conditions:
-
- Accommodation lag was measured under monocular conditions, meaning that blur was the only stimulus to accommodation – this does not match real world viewing where binocular fusion also acts as a stimulus to accommodation.
- Accommodation was measured dynamically – the near fixation target focus demand was being continuously moved during the measurement process. This does not replicate static focus stimulus distance, i.e. when reading a book.
- Refraction was subjective maximum plus and did not include cycloplegia, which is the current gold standard in a research setting.
- This study involved Chinese children only due to the geographic setting so outcomes cannot be necessarily extrapolated to be representative of children from other ethnicities or countries.
- Most myopia progression data were retrospective, meaning that any potential additional influential factors may not necessarily be similar across all participants. The authors argue that applying a Gompertz function to the data helps alleviate against this potential for error.
- The authors were aware that different times spent concentrating at near wasn’t measured and therefore a conclusion on effect of accommodation-induced hyperopic defocus can’t be deduced.
- Measurements were conducted in a clinical environment with controlled lighting and deliberately chosen working distances doesn’t necessarily translate to how children might read at home.
Prevalence of Myopia
Prevalence Of Myopia Among Disadvantaged Australian Schoolchildren
Summary
This retrospective cross sectional analysis of disadvantaged Australian school children reveals:
-
- Lower overall prevalence of myopia than emmetropia and hyperopia
- Higher prevalence of myopia in older children
- No change in overall myopia prevalence between 2014 and 2018, but double the prevalence when compared to study data from 1976
Clinical relevance
- The children included in this study were predominantly from rural areas which in comparison to children living in urban areas conceivably means:
- Greater time spent outdoors
- Less time spent on digital devices
- Lower expectations from education
- This research only captures data from a snapshot in time, however it reveals that while prevalence of myopia was higher in older compared to younger children as expected, myopia prevalence remained low, with most older children (14 & 15 yrs) being emmetropic.
- Study outcomes add further weight to the suggestion that environmental factors such as outdoor lifestyle and early age educational pressures may have an impact on prevalence of myopia.
Limitations and future research
- Non cycloplegic retinoscopy refraction was undertaken, which could potentially bias towards slightly more myopic refractions
- The authors indicate that the measured lower prevalence of myopia, compared to that reported in studies on other Australian locations, suggests the need for including questions on socioeconomic status, education and outdoor activity in myopia prevalence studies.
