Six questions on axial length measurement in myopia management

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轴向长度（AXL）已被很好地确定为研究近视的进展和控制的研究。它在评估近视变性的风险状况方面也有明确的措施；当轴向长度达到并延长26毫米以上时，后极并发症的风险和潜在的视力障碍就更高。1,2For a look at the clinical use of single versus repeated measures of AXL, and the approach of the International Myopia Institute, read our blogAxial Length Measurement – A Clinical Necessity?Measuring AXL in clinic requires specialty equipment not found in most primary eye care practices. So how crucial is it? Is it required for practicing myopia control?

问题1：轴向长度与折射的关系如何？

There is the simple formula you may have held as truth, that 1mm=3.00D. Easy? Not quite. A variety of papers have shown that this relationship is complex. In the three year MiSight study,1随着孩子的年龄增长，这种相关性变得更加牢固 - 这意味着这种关系在年轻人中的变化更大。在整个三年的研究中，他们发现轴向长度的变化0.1mm，对应于治疗组和对照组的近视的0.24D变化，比率为2.40d/mm。在新发表的眨眼研究中2this ratio was 1.44D/mm in the high add (+2.50) multifocal contact lens wearing group, 1.55D/mm in the medium add (+1.50) group and 1.63D/mm in the single vision CL group. While we don’t know if those ratios are statistically different, it highlights how much we still need to learn about this ratio. A big part of the complexity is due to the variability in measurement of both refraction and axial length. Axial length measurement is considered the gold standard in the research context when investigating myopia control strategies, and is becoming increasingly important in the clinical setting where reducing axial elongation to reduce lifelong eye health risk is the key goal.3

However even in a research setting, the International Myopia Institute agrees that refractive error should be used in conjunction with axial length measurement to evaluate success of treatments.4They noted that subjective refraction can be more variable, however axial length is not the only ocular component to change throughout childhood - the cornea and crystalline lens are also key anatomical sites of refractive change and contribution. Refraction provides a summary measurement of all ocular structures, and is routinely and universally measured for all myopes. Despite this, in the case of orthokeratology when the refraction is intentionally altered, the axial length may be the only parameter that can be used as a gauge of myopia control outcomes. Also, since the refraction-to-axial-length ratio seems to come unstuck with low dose atropine – where the refractive outcomes outshine the minimal control of axial length5– knowledge of axial length progression provides the primary measure of myopia control success.

Question 2: Can we predict future myopia from AXL changes?

Yes, you can, but refraction may be more accurate and simpler – more on that shortly. On axial length, the Comparison of Ocular Component Growth Curves Among Refractive Error Groups in Children paper6测量了6和14岁的737名儿童的眼部成分和轴向长度，每人至少评估了3次。通过这些数据，作者评估了最终结果是远视，艾米特罗皮症或近视的参数和生长模式之间的关系。

Children who became myopic had a similar axial length at age 6 to children who stayed emmetropic, although it quickly showed escalation over time (see the graph below). This highlights the importance of early intervention. Patients who were hyperopic and emmetropic had similar models of growth. The authors developed formulas for predicting changes with age for the children who stayed emmetropic, and compared this to the other refractive error groups for significant differences.

AXL 2

Growth curve for axial length, using the best model derived from emmetropic data and applying it to the other three refractive groups. Comparison of Ocular Component Growth Curves among Refractive Error Groups in Children, Jones et Al. 2005 - Figure 8

Your patient is 5 years and 11 months of age (5.91 years old - the age must include months too. This child is +0.50D and has an axial length of 22.35mm. They are emmetropic and under ten, so their axial length should equal = 20.189+(1.258*Ln(5.91)) = 22.42mm. This data suggests that the axial length is appropriate for this child's age.

When measuring the axial length as a predictive tool, you can theoretically use this equation as a prognostic opportunity that may help guide the initiation and proactivity of your myopia treatment. Interestingly, Rozema et al7found that in Singaporean children, myopia onset seemed to occur at approximately the same mean axial length, although with large variability: 24.08±0.67mm in boys and 23.69±0.69mm in girls. Future myopes had a higher axial growth rate in the years before onset than children who stayed emmetropic.

轴向长度金标准的发作预测是吗？卡拉·扎德尼克（Karla Zadnik）和对种族和屈光误差的长达11年合作纵向评估（CLEERE）研究的合着者评估了Cleere的数据，并发现简单的折射率是未来近视的最佳预测指标。8Children aged six with less than +0.75D were at greatest risk of future myopia, even after adjustment for all other factors.

Tideman and co-authors in 20189试图开发一个百分位增长图，以评估12,000名儿童的轴向长度，折射率和其他生长参数。他们发现，轴向长度并不是未来近视的完美预测指标，轴向长度增加，预测未来的近视时间仅为50％。该图确实为高肌关注提供了一定程度的预测，但在预测近视较低或评估治疗功效方面的有效性较差。作者建议，增长图可能是预测近视较高并确定有发育近视风险的儿童的工具，但不一定是跟踪近视增长或比较近视治疗的疗效，而无需添加更多数据集以提高准确性。

Question 3: So if I’m going to measure axial length, how?

有一个轴向的可变性勒的问题ngth measurements, both between visits and between different instruments. Most present day studies employ non-contact, interferometry measurement instruments rather than the older generation A-scan applanation ultrasound technique. The IMI Clinical Trials and Instrumentation report4解释说，超声检查的分辨率限制为约0.30d，而干涉测量值的分辨率约为0.03d，这使得该技术的数量级更好，以衡量近视进展。将其与折射相比 - 环长式自动置换的可重复性为±0.21d。10

评估轴向长度的研究进行多次测量，并在报告中使用平均值。需要多少个测量？一些研究测量了10次1others measured 5 times.2,8The IMI Clinical Trials paper does not make a specific recommendation on this.4

If you are looking at a particular instrument, search the literature and/or ask the company if data has been published on repeatability of that instrument.

Question 4: Is axial length measurement crucial in orthokeratology and atropine treatment?

When the refractive error is intentionally altered to plano or nearabouts, the efficacy of orthokeratology for myopia control can be monitored via axial length measurement, without having to “wash out” the patient from the OrthoK to measure their full refraction. “Wash out” also requires ensuring the cornea has returned to topographical baseline to appropriately judge refraction change, presenting inconvenience to the patient and additional chair time for the practitioner. Clinicians may measure the refraction with the lens-on-eye, but since OK lenses can warp with time this could also prove inaccurate. Research has suggested that OrthoK's induced corneal changes will have minimal influence on repeated axial length measures, when comparing pre- and post-OK.11

There is a similar complication with atropine treatment. When it comes to axial length, the one year LAMP study5发现控制效应为12％，29％和51％，分别为0.01％，0.025％和0.05％的浓度，但折射率的27％，42％和66％的折射率分别更令人印象深刻。相比之下，三年的误导研究表明，轴向和屈光控制之间存在很强的相关性，相似的百分比刚刚超过50％。1虽然比较整个研究的百分比是有问题的（这是对另一个或两个或三个博客的讨论！），但阿托品折射率和轴向控制之间的明显不匹配也表明了轴向长度测量的价值。

Question 5: Will axial length dictate my treatment plan?

它可以决定所需的水平的积极主动性。A child who is -3.00D and 25mm may need more proactive treatment and closer monitoring than the same -3.00D who is 24mm. We’re still learning if axial length is a better predictor of ocular disease risk than refraction – there are more prevalence studies available which have measured myopia by dioptres rather than axial length.3One large-scale, multicentre analysis has indicated that axial length is more closely related to myopic complications than refractive error.12

Axial length can also determine other necessary eye health monitoring. For example, it is wise to schedule an annual fundus examination through dilated pupils for a patient if their axial length is over 26mm, regardless of their refraction, due to the increased risk of pathology. Here the data is striking – among people aged 75 years or older, cumulative risk of vision impairment or blindness from any cause increases from 3.8% in eyes less than 26mm to 25% in eyes longer than 26mm; and to more than 90% in eyes longer than 30mm.12

Question 6: Can I practice myopia control without measuring axial length?

毫无疑问，轴向长度的测量将为近视进展和控制提供更准确的指标 - 如上所述，光学生物测定法的准确性可能比折射高5-10倍。4,10This is why axial length is a required measurement for treatment comparisons in myopia control research, especially to enable evaluation across different studies and patient groups. At this stage, however, it is not a requirement to safely and effectively practice myopia management. In the International Myopia Institute Clinical Management Guidelines, axial length measurement was included as a ‘standard procedure’ but with the caveat that there is currently no established criteria for normal or accelerated axial elongation in a given individual.13

As accessibility to axial length measurement technology becomes easier, and more evidence is developed to support normative and typical myopic patterns for axial length growth across a variety of ethnicities and populations, axial length measurement will become increasingly important in myopia prediction and management. Keep an eye on the research and available equipment, because axial length measurement may become standard of care in our comprehensive myopia management of the future.

Further reading:

卡桑德拉·海恩斯（Cassandra Haines）BIO image 2019_white background

关于Cassandra

卡桑德拉·海恩斯（Cassandra Haines）is a clinical optometrist, researcher and writer with a background in policy and advocacy from Adelaide, Australia. She has a keen interest in children's vision and myopia control.

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References

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