Measuring the whole eye in myopia

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轴向长度（AXL）已被很好地确定为研究环境中近视的进展和控制的关键测量。它被认为是理解近视控制治疗功效的黄金标准，并且是临床指标，检测近视进展的敏感性可能比折射高10倍。1AXL also appears to be the key risk factor for lifelong myopia pathology; more so than refraction.2However, most eye care practitioners don’t routinely measure AXL in clinical practice, mainly due to lack of access to the instrumentation and its expense. How well does AXL represent what’s happening with the young myopic eye? What else is important in measuring the whole eye in myopia?

轴向长度比折射重要吗？

The average axial length of an emmetropic eye is 16.5 mm at birth3并增加到成年后的23.5毫米。4简而言之，更长的眼睛面临着近视相关病理和导致视力障碍的终生风险。虽然与近视的每一个局势相关的病理风险都会增加，但5the ‘line in the sand’ for AXL appears to be around 26mm.

In an analysis of more than 15,000 Dutch individuals by Tideman et al,到75岁时，轴向长度为26mm或更长时间与三分之一的视力障碍相关。轴向长度为30mm或更长时间，与90％的视力障碍频率有关。相同的分析发现，尽管轴向长度与球形等效之间存在很强的相关性 - 折射解释了轴向长度变化的70％ - 当两者都在风险模型中考虑时，轴向长度保持了与视觉障碍的显着关联，而同时又保持了显着关联。球形当量没有。2

In this paper, 26mm or greater axial length delineated significant increase in vision impairment risk, and 26mm was equivalent to around 5D of myopia.2The low-to-moderate myope, though, cannot be presumed to have an axial length below 26mm, as variation exists due to cornea and crystalline lens power in the individual.

近视的轴向长度变化

As an absolute measure, the Singapore Cohort Study of the Risk Factors for Myopia (SCORM) found that myopia onset related closely to axial length across different ages of onset, being 24.08±0.67mm in boys and 23.69±0.69mm in girls.6A single measurement of AXL is also a stronger indicator for disease risk in myopes of all ages than refraction.2For example, an annual retinal examination through dilated pupils may not be as necessary for a 4D myope with a 24.5mm axial length as it is for a 4D myope with a 26mm axial length.

As a repeated measure, axial length increases by around 0.1mm per year in emmetropic children according to the large scale Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study where children aged 6 to 14 years at baseline were followed for up to 10 years. By comparison, children who became myopic showed axial growth of more than 0.3mm in the year just prior to myopia onset, and then around 0.2 to 0.3mm absolute change in the years thereafter.7

轴向长度生长图

The likely future of judging the success of a myopia control strategy based on axial length will fall to percentile growth charts. Varying by age, gender and ethnicity, growth charts for axial length have already been published for Dutch children8and Chinese children.9Authors of the Dutch percentile growth charts emphasised that axial lengths which are on the 75th百分位或更高的近视风险高，因此视力障碍的风险更大。这些相同的作者最近报道了利用75％百分位数差异者来确定将接受高剂量阿托品治疗的个体（0.5％）。每六个月随访时，轴向长度进行测量并绘制在生长图上，并降低了表明成功治疗的个体的百分位数。作者报道说，绘制了“轴向长度百分位数减少的可视化[是]巨大的刺激，使患者坚持治疗。”10

In developing these growth charts, the authors highlighted that more datasets would require additional analysis to ensure robustness, with gender and ethnicity both requiring specific consideration.8、9

Does corneal curvature change in myopia?

克莱尔（Cleere）的研究比较了在发病前5年和5年后近5年变成近视的孩子。对角膜曲率的分析发现，膜型和肌关皮之间的差异很小 - 变成近视的孩子的角膜略陡峭，但小于0.25d。在Emmetropic和“成为毛皮”儿童的十年后，角膜力量的变化很小，而不必少于圆点。11

So if the corneal curvature isn’t changing much in myopia, why measure it? The key reason is to delineate progressive myopia which could be due to corneal steepening and potential early keratoconus. Increasing astigmatism can also be a trigger to monitor for keratoconus risk.

The Northern Ireland Childhood Errors of Refraction (NICER) Study quantified rates of astigmatism (>1.00DC) in white school children and found a similar prevalence across age groups from 6-to-7 years up to 15-to-16 years of around 18%. The prevalence in two groups who were 6- to 7-years old and 12- to 13-years-old at baseline was stable when measured again three years later, although 10-17% lost their astigmatism and around 10% became astigmatic who weren’t before.12另一个在澳大利亚childr横断面研究en of diverse ethnicities also suggested that astigmatism was stable between 6 and 12 years of age, with 88% of corneal astigmatism being with-the-rule.13

Astigmatism progression in myopia is NOT normal

This indicates that progression of childhood astigmatism isnottypical, as it is for childhood myopia progression. The NICER Study found that less than 3% in both age cohort groups showed an increase of>三年来的散光症的1d。随着规则的散光增加（基线时12至13岁）与折射球形分量的近视移位较弱相关，每1d近视的Cyl均增加了0.3D。

Data from the Singapore Cohort Study Of the Risk Factors for Myopia (SCORM) Study undertaken in 7-to 9-year-old children (Chinese, Malay and Asian Indian ethnicity) found 12% had astigmatism of 1D or more. With-the-rule progression per year was 0.01D over three years, higher in Chinese children of 0.02D/yr; the oblique component of astigmatism did not progress. Myopic children had a higher prevalence of astigmatism at baseline, but still only averaged 0.06D of WTR astigmatic progression per year.14

The sum total of this data is that astigmatism progression in myopic children aged 6-12 years is NOT typical – progression of 0.50DC or more over three years is unusual and measurement of keratometry and/or corneal topography is useful in these cases to rule out corneal ectasia.

Crystalline lens changes in myopia

The CLEERE study reported that the crystalline lens thickness of children who became myopic became significantly thinner than emmetropes one year before myopia onset, with the resultant reduced lens power difference persisting thereafter to five years after onset.11This reduction in lens power indicates an attempt at compensating for increasing axial length.

Will we still need refraction measures in myopia?

是的当然！折射提供了所有眼睛结构的摘要测量，并常规和普遍测量所有近视。折射是近视对父母和年轻患者的可见结果和功能影响，因此将仍然是临床图片和沟通过程的重要组成部分。188足彩比分直播虽然激光干涉技术的轴向长度测量可以比折射高出10倍，但国际近视研究所同意应与轴向长度测量结合使用折射误差来评估治疗的成功。1

如果你没有在practi测量轴向长度ce, you could consider collaborating with a colleague primary eye care practitioner and/or local ophthalmologist as best suits your patient base and mode of practice. In the meantime, for guidance on how to gauge success on the basis of short-term changes in refraction, the blogGauging success in myopia managementdetails how to balance analysis of scientific outcomes with clinical communication.

Take home messages

  • Axial length appears to be the stronger risk factor than refraction for future myopia-associated vision impairment, hence the key measure to control in myopia management
  • 使用干涉法技术测量轴向长度测量可以是检测近视的变化的准确性10倍 - 但我们仍然总是需要折射作为临床图片的一部分
  • 轴向长度的增长图已在某些人群中发表，并正在进一步开发，这可能是判断个人的轴向长度如何在预期规范上变化的未来
  • 角膜散光不典型的是近视时期的发展。如果您注意到这一点，则需要进一步评估角膜的角膜测定法和/或地形。

Further reading

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About Kate

Dr Kate Gifford是澳大利亚布里斯班的临床验光师，研究人员，同伴教育者和专业领导者，也是Myopia个人资料的联合创始人。

This educational content is brought to you thanks to unrestricted educational grant from

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