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客观评价眼轴长度的概念和测量方法

Definition and measurement of axial length: Clinical and research perspective

来源期刊: 眼科学报 | 2021年12月 第36卷 第12期 947-950 发布时间:2021-12 收稿时间:2023/5/4 8:58:01 阅读量:6325
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关键词:
近视眼轴长度超声光学
myopia axial length ultrasound optical coherence
DOI:
10.3978/j.issn.1000-4432.2021.05.05
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近视防控已经上升为国家战略,眼轴长度是近视防控的重要建档参数。眼轴长度的测量有多种方法,受到年龄、性别、脉络膜厚度等因素的影响。目前,眼轴长度的定义尚未统一,建议建立标准化的定义和方法学,以指导近视防控的临床和科研工作。
The prevention and control of myopia has become a national strategy, and the axial length is an important parameter in the documentation of myopia prevention and control. This review summarized the measurement technology and factors associated with axial length. Axial length can be measured in various ways and can be influenced by age, gender, choroidal thickness, and other factors. A standardized definition of axial length is warranted to be established for clinical and scientific purposes.
近视已经成为全球重大公共卫生问题。据估计,2050年全球近视患者将达到47.58亿,约占世界人口的一半[1]。我国青少年儿童近视问题尤其严峻,广州一年级小学生的近视患病率为12.0%,到七年级初中生近视患病率攀升至67.4%[2]。随着近视患病率持续攀升,青少年儿童近视防控已经成为国家战略。国家卫生健康委员会要求从幼儿园开始为儿童建立屈光发育档案,确保一人一档,随学籍变化实时转移。
眼轴长度是眼科学的一个经典名称,也是常用的临床眼生物学测量参数。眼轴在一定程度上反映了眼球的发育状况,也能为双眼视觉状态评估提供一定的参考。轴性近视是最常见的近视类型,眼轴增长是驱动近视发生进展的主要因素[3]。随着屈光档案建设工作在全国铺开,除视力、屈光度数外,眼轴长度测量在很多地方也成为必查项目。因此,眼轴这一名词也在各种媒体报道中广泛使用。值得关注的是,眼轴长度其实有其科学内涵的,在临床研究和公众科普时必须深刻理解其内涵,本文对此进行讨论。

1 眼轴长度的定义

传统上,以眼球前面角膜的正中点为前极,后面巩膜的正中点为后极,连结前、后两极的直线为眼外轴(眼球前后径)。从角膜前表面至视网膜(视网膜内的确切位置因测量技术而异)的连线为眼内轴,眼内轴的距离统称为眼轴。在近视研究中,眼轴长度通常作为临床试验的主要结局指标之一[4]。国际近视研究学会(International Myopia Institute,IMI)近视防控研究白皮书[5-6]提出:近视防控研究应该主要招募轴性近视,并且建议对接受任何近视干预的患者,至少每6个月测量1次眼轴。
眼轴变化是近视发生发展的重要标志,在广州双生子眼病研究中发现眼轴增长会在近视起病前1年增大。在IMI近视研究指南中,建议近视防控的研究应该包括以下三类结局指标:1 )主要结局指标包括屈光度和眼轴长度;2 )次要结局指标包括患者报告结局和治疗依从性;3 )探索性结局指标包括周边屈光度、调节、眼位、瞳孔、户外活动时间等[5-6]。鉴于眼轴长度在临床和科研中的重要地位,有必要对眼轴概念明确以下几个方面的表述。1 )眼外轴:连结前、后两极的直线为眼外轴(眼球前后径),笔者建议细分为“解剖学眼轴”。2 )眼内轴:自角膜内面前极至视网膜内面后极的连线。3 )超声测量获得的眼轴:从角膜前表面到内界膜的距离。4 )光学测量获得的眼轴:自角膜前表面至视网膜色素上皮层的长度,笔者建议细分为“视光学眼轴”。5 )未来包括脉络膜在内的眼轴:脉络膜变化引起眼轴长度的变化。但是最近的研究[7]发现:中央角膜变薄和脉络膜增厚并不能完全解释O K镜治疗后眼轴长度的变化,有待进一步研究。

2 眼轴长度的测量方法

目前,眼轴长度的测量主要基于超声或者光学测量方法。超声眼生物学测量是一种接触式测量方法,包括直接测量法(接触法)和间接测量法(浸润法),测量的眼轴长度是角膜顶点到视网膜内界膜的声学距离。A超费用低、技术成熟,在很长一段时间被认为是眼轴长度测量的金标准,测量精确度可以达到100~120 μm。接触法操作简单、受检者不适感轻,但重复性相对较差,并且测量时探头可能压迫角膜导致测量值偏低。浸润法超声波探头不接触角膜,而是在角膜和探头之间使用耦合液以防止角膜压陷,重复性好,但是受检者不适感强,而且操作复杂、依赖检查者经验。两种A超眼轴长度测量值存在差异,接触式测量值要低于浸润式测量值(表1 )。此外,超声测量获得的结果不一定是视轴,例如部分高度近视患者存在后极部葡萄肿,往往会造成比较大的测量误差[7-8]

表1 接触式与浸润式A 超测量眼轴长度的对比
Table 1 Comparison of contact type and immersion type A-ultrasound measuring axial length

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光学生物测量是一种非接触式光学方法,测量的眼轴长度是角膜前表面到视网膜色素上皮层间的光学距离。IOL-Master 500和Lenstar LS 900是常用仪器,基于部分相干干涉技术(partial coherence interferometry,PCI)进行测量,使用波长为780 nm的光源。相比于超声法,非接触测量避免了对角膜的压迫和泪膜的破坏。扫频光源光学相干断层扫描技术(swept source optical coherence tomography,SS-OCT)采用1 055 nm或1 060 nm的光源,穿透能力更强、扫描速度更快,目前基于此技术的商业化仪器有OA-2000和IOL -Master 700。SS-OCT技术同时显示黄斑中心凹,测得的眼轴长度精度更高、重复性更好[11]。因此,光学生物测量仪器在近视临床研究中更为常用。值得注意的是,受检者注视状态、眼动及泪膜会影响到光学路径的补偿,从而产生数微米的误差。由于光学测量包括神经视网膜,而且不存在角膜接触和压陷,测得的眼轴长度比超声法要长。
由于测量技术的限制,早期近视研究采用超声法,近年的研究则主要采用光学法。例如,ATOM-1研究采用A超测量眼轴长度,ATOM-2研究则应用光学生物测量仪测量眼轴长度[12-13]。总的来说,光学测量法测得的眼轴长度比超声法要长,两者相关性较好,然基于光学原理的眼轴测量方法其简单便捷非接触的特征明显优于A超[14]

3 眼轴长度测量的影响因素

人的视觉系统在出生时尚没有完成,在正视化过程中眼轴长度发生变化。既往研究[15]表明:正常足月产婴儿出生时眼轴长度约为16~17 mm,1岁时快速增长到20~21 mm,随后缓慢增长,3岁时约为22 mm,到6岁时达到23 mm,随后以每年0.1 mm的速度增长直至约14周岁。眼轴增长与遗传、环境、发育、营养等各种因素相关。在深圳的流行病学研究[16]发现:3~7岁儿童眼轴长度为(22.51±0.69) mm,与年龄、性别相关,并且受到身高和体重的影响。此外,北京眼病研究发现,中国人群中更长的眼轴与角膜厚度更厚、角膜曲率半径更大、前房深度更深、晶体厚度更厚独立相关。因此,临床评估眼轴长度时应该考虑这些因素,并且从动态的角度来看。
脉络膜厚度的改变也会影响眼轴长度测量。目前的研究[17-18]已证明脉络膜的病变和近视眼的发生发展有很大关系。脉络膜的厚薄可以影响视网色素上皮层(retinal pigment epithelium,RPE)的位置,从而直接影响眼轴长度测量值。脉络膜增厚时推动RPE前移,导致眼轴测量值变小;脉络膜变薄时RPE后移,导致眼轴偏长。脉络膜厚度受到多种因素的影响,包括吸烟、饮酒、运动、睡眠、咖啡、饮酒、头位、眼压,甚至昼夜节律等。正常人黄斑中心凹下脉络膜厚度的日夜波动可达25.9~103 μm,这些脉络膜厚度的变化,会直接影响光学生物测量的眼轴结果。每天中午1 2点时的眼轴最长,晚上2 1点时眼轴最短,变化幅度达到25~45 μm[19-22]。因此,未来的眼轴长度测量应考虑脉络膜厚度的因素。
目前的观点认为:调节时除前房变浅及晶状体变厚外,随之而来的是眼内充血、眼内压升高、玻璃体腔内压力增加,对巩膜产生压力,引起眼轴暂时性延长。Shum等[23]报道18~23岁青年使用3.00 D与8.00 D调节时,眼轴(解剖学眼轴)分别延长0.05 mm、0.09 mm。此作用是暂时性的,但若不及时解除,则日久后会损害巩膜的弹性,使巩膜在延伸后不易恢复,造成永久性的眼轴延长并发生近视。治疗假性近视能使眼轴“缩短”(但这个量非常有限)。已经固定的“解剖学眼轴”(传统概念的眼轴)是无法缩短的。
角膜塑形镜通过对角膜中心产生正压、周边产生负压,引起角膜中心上皮细胞向周边移动,进而改变屈光度,延缓近视进展。测量眼轴长度的改变是评价角膜塑形镜防控近视效果的重要指标,相比于屈光度更为重要[24]。在评估角膜塑形镜近视治疗效果的过程中,应该常规进行眼轴长度的测量,并建议采用光学测量法。

4 结语

目前眼轴长度的概念存在多种定义,需要进一步明确细分,不宜笼统地称为“眼轴”。在临床上,需要明确测量的前后极,所代表的具体距离。在研究上,需要明确测量的具体方法,并考虑眼轴长度的影响因素。建议建立标准化的定义和方法学,以指导近视防控的临床和科研工作。

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    主管:中华人民共和国教育部
    主办:中山大学
    承办:中山大学中山眼科中心
    主编:林浩添
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    主办:中山大学
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  • Eye Science

    主管:中华人民共和国教育部
    主办:中山大学
    承办:中山大学中山眼科中心
    主编:林浩添
    主管:中华人民共和国教育部
    主办:中山大学
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