轴向长度回归是否可能？

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轴向长度回归是否可能？这是一个有趣的postfrom the Myopia Profile Facebook community discussing a reduced axial length measurement in a child who was prescribed bifocal spectacle lenses.

1st axl regression

什么可以解释轴向长度的减少？

Generally one would expect axial length to elongate with age, especially since even visually normal children exhibit axial elongation of around 0.1mm per year during emmetropization.1This case suggests axial length regression of 1.6mm measured over 4 months. What reasons did the commenters suggest?

2nd axl regression

A dive into the literature does suggest that axial length regression may occur, albeit at smaller degrees than what is described in this case.

1。Response to defocus and choroidal thickening

Animal studies have shown evidence that axial length adaptations can occur in response to defocus. Zhu et al (as quoted by TA, above) showed that the eyes of the chick, tree shrew, marmoset, and rhesus macaque can shorten axially when presented with myopic defocus or when recovering from myopia induced by either hyperopic defocus or form-deprivation. Calculations and comparisons to untreated eyes indicated that this could not simply be explained by A-scan ultrasound measurement error.2

In humans, a study in young adults showed that axial length increased (+7 ± 5 μm) and decreased (−8 ± 9 μm) over one hour when hyperopic defocus and myopic defocus were induced respectively.3这是用干涉仪器测量的，该仪器的准确性比A-Scan超声检查高10倍。4轴向长度变化是短暂的，在去焦焦后20分钟内恢复。假设这些轴向长度的暂时变化是在脉络膜水平上发生的。

In children, a combination of myopic defocus plus 0.3% atropine increased choroidal thickness by 21μm.5The mechanism of these choroidal thickness changes is unknown, with a presumption of blood flow changes not demonstrated on MRI imaging.6

As an executive bifocal lens has a large inferior portion of relative positive power, which would cast myopic defocus on the superior retina and potentially influence axial length. When looking at the small - 10-20 micron or 0.01 to 0.02mm - changes measured in human studies, the change of 1.6mm reported in this case doesn't appear to align.

2。Other reductions in axial length - atropine, exercise and violet light

尽管这些原因不一定与这种情况有关，但尽管研究缩短轴向长度，但它们是一个有趣的旅程。首先，最近在台湾进行的一项研究显示，轴向长度回归（0.016毫米）和前腔室加深（0.058mm），一周使用0.125％阿托品，这可能是由于环椎式效应导致了触角折射的。7Secondly, axial length has been shown to temporarily reduce by around 0.02mm after moderate exercise in both emmetropic and myopic young adults.8

一项有趣的案例研究声称，紫罗兰色向一只眼睛的镜子发射眼镜导致轴向长度减小。作者将紫色的光描述为360-400nm-紫外线和几乎可见光的范围 - 并被称为“被大多数奇观镜头阻塞”。这个孩子4岁，R（OD）-5.75和L（OS）-9.75与Amblyopia。右眼每天6小时修补，两年中显示0.85mm和1.00d的近视进展。左眼戴着紫罗兰色的发射眼镜，没有斑点，并且在两年内显示出0.20mm的轴向长度和1.88D近视。作为一个案例研究，两只眼睛都接受了不同的治疗方法，结果很有趣，但要得出的结论受到限制。9Axial length was measured by an interferometry instrument in each of these studies.

3。Orthokeratology can transiently reduce axial length

轴向长度可能会缩短（正常角）（确定）。在一项交叉研究中，一只眼睛戴着标准气体可渗透的“天”镜头，另一只眼睛在一夜之间穿着，26名儿童的OK-EYES在头六个月内平均缩短了0.02mm，然后平均0.04mm在第二个六个月中，在GP-To-Ok中缩短了眼睛。10Another OK study found that a 26um (0.026mm) shortening after commencing OK wear was 70% explained by central corneal thinning and choroidal thickening, but it was transient and returned to baseline within a month.11This shouldn't affect your clinical measurement of axial length in orthokeratology when comparing baseline measurements to that after six or twelve months of treatment.

总体而言，由于各种刺激引起的轴向长度减小的观察结果为20微米或0.02mm。那么，访问之间的测量值1.6毫米最可能的原因是什么是什么呢？

最可能的原因 - 测量错误

2nd-axl-regression-cropped PC

PC's comment is right on track here. There are two approaches to axial length measurement - A-scan ultrasonography (which contacts the cornea) and optical biometry or interferometry (non-contact). TheIMI临床试验和仪器报告4explains that ultrasonography has test-retest repeatability of around ±0.2 to ±0.3 mm, whereas interferometry is ±0.04 mm. In addition, applanation ultrasound can reduce axial length by around 0.3mm due to corneal compression.12

Take multiple measurements.大多数研究采用5个轴向长度（有时长达10个）的度量，并将平均值作为最终读数。

Ensure proper alignment.离轴测量将引入错误的选择ical biometry this has been shown as up to 0.03mm13在超声测量中可能会更高。确保患者的固定和对齐方式是正确的测量。

Take care in higher myopes.A-scan contact ultrasound can show more inaccuracy in higher myopes (over 26mm),13据推测，由于视网膜形状更大，甚至是后葡萄球菌的存在。这种情况符合高敏部的标准，两种测量值都超过27mm。相比之下，干涉仪在正常和高度近视的眼睛上似乎相当。14

Read more about different methods and instruments inChoosing an instrument to measure axial length。

带回家消息：

  1. Small reductions in axial length can occur due to a variety of influences, from optical defocus to exercise, atropine and orthokeratology treatment. These are usually in the order of around 20 microns or 0.02mm, and can be transient. Larger apparent reductions in axial length than this could most likely be due to measurement error.
  2. Repeatability and accuracy of axial length measurement can be ensured through using the most accurate optical interferometry instruments, taking repeated measures and ensuring correct patient alignment.
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关于康妮

Connie Ganis a clinical optometrist from Kedah, Malaysia, who provides comprehensive vision care for children and runs the myopia management service in her clinical practice.

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About Kimberley

金伯利NGU是来自澳大利亚珀斯的临床验光师，在澳大利亚和新加坡曾在患者教育计划方面拥有经验。

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

References

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  2. Zhu X, McBrien NA, Smith EL, Troilo D, Wallman J. Eyes in various species can shorten to compensate for myopic defocus. Investigative ophthalmology & visual science. 2013 Apr 1;54(4):2634-44.（关联）
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  4. Wolffsohn IMI 2019
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  12. Trivedi RH，威尔逊我。轴向长度测量通过与白内障的小儿眼中的接触和浸入技术进行测量。眼科。2011年3月; 118（3）：498-502。（（link）
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