近距離工作引起的暫時(shí)性與永久性近視中的眼動(dòng)參數(shù)(第二部分)
作者：蔣百川
單位：蔣百川(美國(guó)休斯頓大學(xué)視光學(xué)院)
關(guān)鍵詞：
眼視光學(xué)雜志990223 2 近距工作引起的永久性近視
人們已接受這樣的結(jié)論：兒童的近視發(fā)展主要是由于眼軸的增長(zhǎng)，該增長(zhǎng)的速度無法被角膜和晶狀體屈光力的下降所代償[3，39，40，41]。直到最近，引起LOM的眼參數(shù)方面的機(jī)制才知曉，Kent[42]報(bào)道了對(duì)一病例縱向研究的資料，表明在青少年時(shí)期，角膜屈光力隨近視發(fā)展而增大，Goss和Erickson[43]發(fā)現(xiàn)角膜曲率增加和近視進(jìn)展之間的聯(lián)系在兩條主子午線都是很明顯的，但是，他們沒有檢查隨近視而變化的其他光學(xué)參數(shù)；Adams[44]報(bào)道他自己的角膜曲率保持不變，但近視在進(jìn)展，由此下結(jié)論近視進(jìn)展不是由于角膜變化而是由于眼軸變長(zhǎng)；McBrien和Millodot[45]報(bào)道LOM比正視眼的玻璃體腔深，還發(fā)現(xiàn)LOM前房較深及晶狀體較薄，但兩者的角膜曲率無明顯差別；Grosvenor和Scott[46]對(duì)三組年輕人隨訪三年：EOM(N＝29)、LOM(N＝26)、正視眼(N＝24)測(cè)量屈光狀態(tài)和眼球參數(shù)的變化，他們發(fā)現(xiàn)與屈光變化有關(guān)的眼部成分只有玻璃體腔和眼軸長(zhǎng)度，有趣的是，當(dāng)將兩近視組的度數(shù)進(jìn)行匹配對(duì)照時(shí)，他們的眼部任何成分均無明顯差異，他們從而得出結(jié)論，所有的近視都是軸性的。Jiang和Woessner[47]觀察了一年輕人的屈光不正的進(jìn)展，指出屈光不正變化是由于玻璃體腔的伸長(zhǎng)。McBrien和Adams[11]在兩年中從251名臨床顯微醫(yī)師處收集了大量的屈光與生物統(tǒng)計(jì)學(xué)資料，也作出如下結(jié)論：玻璃體腔伸長(zhǎng)是引起近視的發(fā)生和發(fā)展的主要因素。
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我們相信最近動(dòng)物模型的研究與人類眼動(dòng)文獻(xiàn)的結(jié)合，提供了強(qiáng)有力的依據(jù)，即眼動(dòng)機(jī)制與眼軸增長(zhǎng)有關(guān)。從對(duì)多種動(dòng)物的研究表明，早期發(fā)育的異常視覺經(jīng)歷會(huì)干擾正常的眼各成分間的協(xié)調(diào)而出現(xiàn)屈光不正，目前的動(dòng)物研究動(dòng)向是鑒別影響正視化過程的環(huán)境因素和這些因素的作用機(jī)制.這類研究主要受Wiesel和Raviola[48]經(jīng)典實(shí)驗(yàn)的影響，他們發(fā)現(xiàn)未成熟猴子眼縫合后會(huì)出現(xiàn)眼軸增長(zhǎng)和近視現(xiàn)象(稱為：剝奪性近視)。雖然眼瞼縫合后有許多因素能改變視網(wǎng)膜的像[49，50]，與圖形對(duì)比度減低有關(guān)的降低空間頻率似乎最有可能使眼瞼縫合后誘發(fā)剝奪性近視，但是，僅靠剝奪性近視的研究是不能說明視網(wǎng)膜圖象如何“主動(dòng)地”和“正常地”制約眼球根據(jù)環(huán)境來定焦生長(zhǎng)(如，正常視覺經(jīng)驗(yàn)在正視化中的作用)。雖然有多種方法可以改變視網(wǎng)膜像和可能改變眼球的生長(zhǎng)，但是產(chǎn)生正視化的最可能的視覺反饋是光學(xué)離焦的信號(hào)和幅度。小雞的研究表明視覺依賴(聚焦生長(zhǎng))的適應(yīng)過程確實(shí)出現(xiàn)，配戴不同度數(shù)眼鏡的小雞眼睛能夠生長(zhǎng)以補(bǔ)償誘發(fā)的離焦[51～57]，這種補(bǔ)償主要是通過改變眼球的生長(zhǎng)速率、特別是玻璃體腔的深度來達(dá)到的[57]，而且為造成屈光改變所需的離焦的幅度比原先想象的小，并不大于所估計(jì)的小雞的焦深[58]。對(duì)靈長(zhǎng)類動(dòng)物的研究也表明視覺依賴性“為對(duì)焦而生長(zhǎng)”機(jī)制，如Hung等[59]報(bào)告嬰兒猴子表現(xiàn)出由于正或負(fù)鏡誘發(fā)的光學(xué)離焦所引起的補(bǔ)償性的眼軸生長(zhǎng)。
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形覺剝奪和光學(xué)離焦存在根本差別，光學(xué)離焦時(shí)，如果離焦量不是很大，眼睛能繼續(xù)接受視覺反饋(即視網(wǎng)膜像質(zhì)作為屈光狀態(tài)的誤差信號(hào))，當(dāng)眼球朝合適的聚焦位置生長(zhǎng)，像的質(zhì)量得到了改善，但是，在形覺剝奪的情況下，視覺反饋毫無意義，因?yàn)闊o論眼球如何生長(zhǎng)，視網(wǎng)膜像質(zhì)無法有任何改進(jìn)(即視覺反饋環(huán)是“開放”或固定在某一值)，眼球增長(zhǎng)的兩種類型的明顯區(qū)別是光學(xué)離焦類型要求大腦作為中介的機(jī)制而被牽涉進(jìn)去，而剝奪性近視卻沒有這種要求，因?yàn)榧词箤⒁暽窠?jīng)切斷剝奪性近視也會(huì)出現(xiàn)[60]。在以人為受試者的研究方面，已有通過測(cè)量某些眼動(dòng)及參數(shù)來確定光覺模糊機(jī)制在引起近視中的潛在作用，根據(jù)被測(cè)量參數(shù)差異，這些研究分為兩類：一類測(cè)量調(diào)節(jié)刺激/反應(yīng)函數(shù)，另一類測(cè)量分離性隱斜。
對(duì)不同的屈光組之間的調(diào)節(jié)刺激/反應(yīng)函數(shù)進(jìn)行比較，McBrien和Millodot[61]發(fā)現(xiàn)近距注視(4或5D)時(shí)，LOM的調(diào)節(jié)性反應(yīng)比正視眼低。Rosenfield和Gilmartin[62]用3D近視標(biāo)時(shí)證實(shí)上述結(jié)論。Bullimore等[63]證實(shí)LOM在被動(dòng)注視條件下視近時(shí)調(diào)節(jié)反應(yīng)低，但在主動(dòng)注視條件下就不是這樣。Gwiazda等[46]測(cè)量了正視與近視兒童的調(diào)節(jié)刺激/反應(yīng)函數(shù)，發(fā)現(xiàn)近視兒童對(duì)三種視覺條件中的兩種在視近時(shí)調(diào)節(jié)較差。Abbott等[64]用同樣的方法對(duì)成人作了實(shí)驗(yàn)，他們發(fā)現(xiàn)進(jìn)展性的近視對(duì)負(fù)鏡片誘發(fā)的調(diào)節(jié)要求的反應(yīng)較差，但是穩(wěn)定的近視眼則和正視眼相同。Jiang[37]測(cè)量了雙眼視、單眼視、用Badal光學(xué)系統(tǒng)的單眼視狀態(tài)下的正視眼與LOM調(diào)節(jié)刺激/反應(yīng)函數(shù)。在單眼視和Badal單眼視狀態(tài)下，LOM的平均調(diào)節(jié)刺激/反應(yīng)函數(shù)的坡度比正視眼低，但在雙眼視狀態(tài)下無明顯差別。這些研究給出了初步證據(jù)：近視發(fā)生早期調(diào)節(jié)反應(yīng)減少，但是，在模糊引起的調(diào)節(jié)與近視之間仍存在一個(gè)問題，就是是否其中之一可以誘發(fā)另一個(gè)，或有一共同因素誘發(fā)了兩者[65]。把這些結(jié)論用到調(diào)節(jié)的控制理論上，仍不能清楚解釋屈光組之間不同的調(diào)節(jié)反應(yīng)或坡度是否由他們調(diào)節(jié)控制獲得或調(diào)節(jié)死腔的不同決定，后者常被認(rèn)為是眼的焦深。
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Jiang[66，67]提出了一個(gè)改進(jìn)的靜態(tài)調(diào)節(jié)模型。在該模型中加入了系統(tǒng)增益作為對(duì)于系統(tǒng)的感覺及系統(tǒng)的一種線型模擬(ASG)。模型得出的結(jié)論表明：感覺部分不僅影響調(diào)節(jié)反應(yīng)函數(shù)的坡度，而且會(huì)改變系統(tǒng)對(duì)離焦信號(hào)有效閾值(ET)。為檢驗(yàn)該理論結(jié)果，Jiang比較了13名正視眼與10名LOM的ET值，差異是顯著的。但是沒有發(fā)現(xiàn)兩組在暗焦點(diǎn)或調(diào)節(jié)刺激/反應(yīng)坡度上的明顯差異。分析坡度和ET資料后，他推測(cè)在LOM人員中，對(duì)離焦感覺的敏感性不如正視眼那么高。結(jié)合前面講到的，這突出了光學(xué)離焦在引起屈光不正病因中的重要作用。
臨床研究也為眼動(dòng)參數(shù)隨近視發(fā)展而變化的理論提供了讓人感興趣的初步證據(jù)。Birnbaom[68]觀察了近視發(fā)展中調(diào)節(jié)與輻輳的臨床測(cè)量值變化，他提出：早期近視可能與低的正相關(guān)調(diào)節(jié)和調(diào)節(jié)幅度以及視近內(nèi)隱斜增大或視近外隱斜減少有關(guān)。Drobe和de Saint-Andre[69]比較了法國(guó)視光學(xué)醫(yī)生測(cè)定的保持正視的和后來發(fā)展為近視的一批兒童和青年的近距隱斜數(shù)據(jù)，發(fā)現(xiàn)兩組的差距有2.9棱鏡度(P＝0.008)而原近視組內(nèi)隱斜更明顯。Gosss和Jackson[70]也發(fā)現(xiàn)發(fā)展為近視組視近時(shí)的內(nèi)隱斜更明顯。這組的兒童中，近視開始前與近視后，視近時(shí)常常表現(xiàn)為內(nèi)隱斜，這結(jié)果與成人中的發(fā)現(xiàn)類似，即發(fā)展成LOM的人在近視前與近視后表現(xiàn)出較高的反應(yīng)性AC/A值[32]。而且，高AC/A值與高分離性近距隱斜有關(guān)[71]。這一發(fā)現(xiàn)揭示了調(diào)節(jié)與輻輳相互作用在近視發(fā)展中的重要作用。同樣的輻輳反應(yīng)，高AC/A值者視近的調(diào)節(jié)量比正�；虻虯C/A者有較大的滯后。而且，調(diào)節(jié)滯后可能在視網(wǎng)膜上產(chǎn)生模糊像質(zhì)，這可以成為誤差信號(hào)引起眼的補(bǔ)償性生長(zhǎng)，而導(dǎo)致近視的發(fā)展。
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3 討論
關(guān)于近視的病因?qū)W說有很多，但還沒有一個(gè)被廣泛接受。但是有一點(diǎn)很明確，就是近距工作是近視發(fā)生相關(guān)的環(huán)境因素之一。在這篇綜述中，我集中闡述了近距工作后眼動(dòng)參數(shù)改變，以及這種變化在近視與正視者之間的差別。眼動(dòng)參數(shù)中，暗焦點(diǎn)被認(rèn)為是調(diào)節(jié)的基態(tài)，持續(xù)近距工作后，暗焦點(diǎn)近移，被稱為調(diào)節(jié)適應(yīng)或近距工作后效應(yīng)。我討論了四種狀態(tài)下的調(diào)節(jié)參數(shù)：即靜態(tài)開環(huán)，靜態(tài)閉環(huán)，動(dòng)態(tài)開環(huán)，動(dòng)態(tài)閉環(huán)。由于調(diào)節(jié)系統(tǒng)可被認(rèn)為是一個(gè)負(fù)反饋環(huán)，在靜態(tài)閉環(huán)狀態(tài)下，如果系統(tǒng)增益很高，暗焦點(diǎn)移動(dòng)對(duì)調(diào)節(jié)反應(yīng)影響很少。但在靜態(tài)和動(dòng)態(tài)開環(huán)狀態(tài)甚至于動(dòng)態(tài)閉環(huán)狀態(tài)，調(diào)節(jié)反應(yīng)表現(xiàn)為調(diào)節(jié)緊張現(xiàn)象。這與調(diào)節(jié)或睫狀肌痙攣有關(guān)。臨床上，睫狀肌痙攣被認(rèn)為是睫狀肌過度的不必要的及不適當(dāng)?shù)氖湛s[20]。這種近距工作引起的異常調(diào)節(jié)痙攣，如果持久，就是假性近視。另一方面，持久性近視主要表現(xiàn)為眼軸的伸長(zhǎng)，動(dòng)物模型及以人為對(duì)象的研究都支持這假設(shè)，即：視網(wǎng)膜成像的精確性或視網(wǎng)膜像質(zhì)對(duì)眼后段的發(fā)育起反饋?zhàn)饔谩Ｉ厦嬷v到的對(duì)近刺激調(diào)節(jié)反應(yīng)的減少，高AC/A值，近距內(nèi)隱斜視，高離焦閾等導(dǎo)致像在視網(wǎng)膜離焦，都支持了這個(gè)假設(shè)。
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很多研究工作已經(jīng)做過，但仍未揭開近視發(fā)展的病因之謎。我們?nèi)圆恢罏槭裁床皇撬械慕喙ぷ髡叨紩?huì)變成近視眼。最近的研究讓我們進(jìn)一步傾向近視發(fā)展的眼動(dòng)力理論的假設(shè)�！霸诿舾袀�(gè)體，發(fā)生屈光不正前，近距工作已引起了眼動(dòng)參數(shù)的改變，這一系列改變顯然可能導(dǎo)致光學(xué)離焦從而成為誘發(fā)近視的補(bǔ)償性變化的潛在因素”[70]。如何將近距工作與近視發(fā)展相聯(lián)系仍有許多不足之處，以至我們無法完全把這理論肯定下來。雖然暫時(shí)性近視不是每個(gè)屈光不正者所必經(jīng)歷的，但可以肯定，暫時(shí)性近視有可能發(fā)展為永久性近視。因此，現(xiàn)在的主要問題是弄清這個(gè)轉(zhuǎn)化過程是如何發(fā)生，這種轉(zhuǎn)變是否與眼動(dòng)參數(shù)有關(guān)。這方面的進(jìn)一步研究顯得非常重要及必要。
感謝：Dr.Harold and Dr Stephen Morse對(duì)形成本文初稿的討論和建議。
作者單位：蔣百川(美國(guó)休斯頓大學(xué)視光學(xué)院)
Oculomotor function in nearwork-induced transiant and permanent myopia(Part Ⅱ)
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Bai-chuanJiang
2 Nearwork-induced permanent myopia
Generally，it is accepted that childhood myopia progression is due to axial elongation，which is not compensated by reductions in corneal and crystalline lens powers[3，39，40，41]. Until recently，the component mechanisms that produce LOM become clear. Kent[70]reported longitudinal data，for one subject，showing that corneal power increased as myopia progressed during the early adult years. Goss and Erickson[43] found that correlations between corneal steepening and myopia progression were significant in both principal meridians. However，they did not check whether other optical components changed with the progress of myopia. Adams[44] reported that his own keratometer findings remained stable as his myopia progressed during the adult years，and concluded that the progression was not due to corneal steepening but due to axial elongation. McBrien and Millodot[45] reported that LOMs had deeper vitreous chambers than emmetropes. Their LOM subjects were also found to have deeper anterior chambers and thinner crystalline lenses，but no differences were found in corneal curvature. Grosvenor and Scott[46] measured the changes in refraction and ocular components over 3 years for three groups of young adults：EOM(n＝29)，LOM(n＝26)，and emmetropes(n＝24). They found that the only ocular components that significantly correlated with changes in refraction were vitreous chamber depth and axial length. Interestingly，when subjects from the two groups of myopes were matched for the amount of myopia，there were no significant differences in any of the ocular components，and they concluded that all myopia is axial in origin. Jiang and Woessner[47]observed a young adult's refractive error development and suggested that her vitreous chamber elongation was responsible for the refractive error change. McBrien and Adams[11] collected refractive and biometric data from 251 clinical microscopists during a 2-year period and concluded that the vitreous chamber elongation caused the onset and/or progress of myopia.
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Recent findings from the animal modeling literature can be integrated with the oculomotor literature in human studies resulting in strong evidence that oculomotor mechanisms are involved in inducing axial-length elongation in certain individuals. In a wide variety of animal species，abnormal visual experience during early development interferes with the normally coordinated growth of ocular components and produces anomalous refractive status. Research on animals is currently directed toward identifying the environmental factors that influence the emmetropization process and the mechanisms by which these factors affect the eye's refractive status. Animal studies on myopia were influenced mainly by the classical experiment of Wiesel and Raviola[48] who found that lid suture in the immature monkey results in an increase in axial length and a myopic eye(called‘deprivation myopia’or'form-deprivation myopia'). Although，many factors could alter the retinal image of the lid sutured eye[49，50]，degraded spatial vision associated with reduced image contrast appears to be the most likely aspect of lid closure that triggers the onset of lid-sutured myopia. However，studies of form deprivation alone do not reveal how，or if，the quality of the retinal image‘a(chǎn)ctively' and ‘normally' regulates the eye to grow to‘be in focus'for its environment(i.e.，the role of normal visual experience in emmetropization). Although there are several ways to change the retinal image and possibly alter the eye's growth，the most plausible visual feedback that can be used by the eye to perform emmetropization is the sign and magnitude of optical defocus. Studies in chicken have shown that a vision-dependent，“grow-to-be-in-focus”，adaptive process is indeed present. Chicken eyes that are fitted with different powered spectacle lenses，can grow to compensate for the induced defocus[51～56]. This compensationis mainly achieved by changing the eye's axial growth rate，especially vitreous chamber depth[57]. Furthermore，the magnitude of the defocus that will result in refractive change is smaller than previously believed，i.e.，not much greater than the estimated depth of focus of the chick eye[58]. Studies in primates have also demonstrated a vision-dependent“grow-to-be-in-focus”mechanism，e.g.，Hung et al.[59] reported that infant monkeys exhibit compensating ocular growth for optical defocus induced by positive or negative lenses.
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There is a fundamental difference between form deprivation and optical defocus. With optical defocus，if the amount of defocus is not very large，the eye can continuously receive visual feedback(i.e.，the quality of the retinal image serves as an error signal for the refractive status). When the eye grows toward the appropriate focus position the quality of the image improves. However，in form deprivation rearing strategies，visual feedback is meaningless because no matter how the eye grows，the retinal image quality is not improved(i.e.，the visual feedback loop is ‘opened'or‘fixed at a constant value'). The striking difference between the two types of axial elongation is that the optical defocus type requires the involvement of abrain-mediated mechanism and the other type(deprivation myopia)does not because it still occurs if the optic nerve is cut[60].
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There have been some human studies that looked at the potential role of the optical blur mechanism in producing myopia by examining a specific oculomotor parameter. According to the parameter being measured，these studies can be divided into two categories. In one，the accommodative stimulus/response function was measured and in the other，the dissociated near phoria was measured.
The accommodative stimulus/response function has been compared in different refractive groups. McBrien and Millodot[61] found that the accommodative response for near targets(4 and 5 D)is lower in LOM versus in emmetropes. With a 3 D target，Rosenfield and Gilmartin[62] confirmed the above findings. Bullimore et al.[63] confirmed that LOMs have a lower response for near targets under a passive viewing condition，but not under an active condition. Gwiazda et al.[46] measured accommodative stimulus/response functions for emmetropic and myopic children and found that myopic children accommodated less to high accommodative demands in their two of three viewing conditions. Abbott et al[64]. applied the same method on young adults. They found that progressing myopes had reduced accommodative response to negative lens-induced accommodative demands，but the accommodative responses in stable myopes were same as in emmetropes. Jiang[37] measured the accommodative stimulus/response functions of emmetropes and LOMs under binocular，monocular，and monocular with a Badal optical system viewing conditions. The mean slope of the function of the LOMs was lower than the mean slope of the emmetropes in the monocular and the Badal conditions，but there was no difference in the binocular condition. These studies provided preliminary evidence that reduced accommodative responses occurred during the onset of myopia. However，a remaining question regarding the correlation between blur-driven accommodation and myopia is whether one could cause the other，or whether a common factor influences both[65]. Applying these results to the control theory model of accommodation，it is still not very clear whether the difference in accommodative responses and/or slope between refractive groups is caused by the difference in their accommodative controller's gain or in the accommodative dead-space，the latter usually being thought of as the depth-of-focus of the eye.
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Jiang[66，67] suggested a modified model of static accommodation，in which an accommodative sensory gain(ASG)was added as a linear operator to simulate the sensory part of the system. Results derived from the model showed that the sensory part not only affected the slope of the accommodative response function but also increased the system's effective threshold(ET)to the defocus signal. To test the theoretical result，Jiang compared calculated values of ET between 13 emmetropic and 10 LOMs. This difference in ET between the two groups was significant. However，no significant difference was found in the dark-focus or the accommodative stimulus/response slopes between the groups. From analyzing the data of the slope and ET，he speculated that the sensory system in LOM subjects might be less sensitive to defocus than that of emmetropic subjects. This，combined with the information provided by the studies described above，emphasizes the role of optical defocus in inducing refractive error change.
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Clinical studies have also provided interesting preliminary evidence of oculomotor changes accompanying myopia development. Birnbaum[68] observed differences in clinical measures of accommodation and vergence during myopia development. He suggested that“incipient myopia”can be associated with lower positive relative accommodation and amplitude of accommodation，and greater esophoria or less exophoria at near. Drobe and de Saint-Andre[69] compared near phorias measured by French optometrists in children and young adults who become myopic and who remained emmetropic. They found that the difference between the two groups was 2.9 prism diopters(P＝0.08)with the pre-myopic group having more esophoria. Goss and Jackson[70] found that their became-myopic group had a more convergent(eso)phoria at near. In the children of this group，the near phoria showed an eso shift with time，beginning before and continuing after the onset of myopia. Their results are similar to the finding that young adults who developed LOM showed higher response AC/A ratios before and continuing after the onset of myopia[32]. In addition，the higher AC/A ratios are related to the higher dissociated near phorias[71]. This finding reveals the important role of the interaction between accommodation and vergence in the development of myopia. For the same vergence response，a subject with higher AC/A ratio has bigger lag in accommodative response to the near target than someone with a normal or low AC/A ratio. Furthermore，the accommodative lag potentially creates a blurred image on the retina that may serve as an error signal for the compensative growth of the eye that could lead to the development of myopia.
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3 Discussion
Many hypotheses of myopia etiology have been suggested but none has been widely accepted. But，it is certain that one of environmental factors associated with myopia prevalence is nearwork. In this review，I concentrated on work which revealed how the oculomotor functions are altered by nearwork and whether these effects differ between emmetropes and myopes. Among oculomotor parameters，dark-focus has been identified as the tonic posture of accommodation. After sustained nearwork，dark-focus shifts to near，which is called accommodative adaptation or nearwork aftereffect. I have discussed the changes of accommodative parameters under 4 conditions，i.e.，static open-loop，static closed-loop，dynamic open-loop，and dynamic closed-loop. Because accommodation system can be thought of as a negative feedback control system，under the static closed-loop condition，dark-focus shift has a little effect on the accommodative response(i.e.，the output of this system)if the gain of the system is high. But，under the static and dynamic open-loop conditions and even in the dynamic closed-loop condition，the accommodative response shows a hysteresis phenomenon which is related to accommodative or ciliary spasm. In the clinic，ciliary spasm is defined as an excessive，unnecessary and inappropriate contraction of the ciliary muscle(Borish，1970). This abnormal status of accommodative spasm associated with nearwork can be thought as a transient myopia and pseudomyopia if it becomes continuous. On the other hand，permanent myopia mainly shows axial elongation of the eye. Both animal model studies and work on human subjects lead to a hypothesis in which the accuracy of the retinal image and/or retinal image quality provides feedback for growth of the posterior segment of the eye. The reduced accommodative response to near stimuli，the high AC/A ratio，the near esophoria，and the high defocus threshold of the accommodative system described above result in a condition of a defocused image on the retina and support this hypothesis.
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Much work has been done，but not enough yet to solve the mystery. We still face the puzzle why not all people who perform nearwork become myopic. Recent studies led us to further hypothesize an oculomotor theory of myopia development：“Near work causes changes in the oculomotor characteristics of susceptible individuals which begin even before the development of refractive error. This series of changes in oculomotor function apparently results in optical defocus，which has the potential to induce compensatory changes resulting in myopia”[69]. There still are gaps in bridging the relationship between nearwork and the development of myopia that prevent our complete formulation of the theory. Although transient myopia is not a necessary step for a person to develop refractive error，it is true that transient or pseudo-myopia can become permanent myopia. Therefore，a major question is how this change occurs. Is the change related to oculomotor functions？Further research in this area appears to be very critical and necessary.
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Acknowledgments The author is grateful to Dr. Harold Bedel and Dr. Stephen Morse for their helpful comments on the manuscript.
References
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From College of Optometry，University of Houston，USA
(收稿：1998-11-15), 百拇醫(yī)藥
單位：蔣百川(美國(guó)休斯頓大學(xué)視光學(xué)院)
關(guān)鍵詞：
眼視光學(xué)雜志990223 2 近距工作引起的永久性近視
人們已接受這樣的結(jié)論：兒童的近視發(fā)展主要是由于眼軸的增長(zhǎng)，該增長(zhǎng)的速度無法被角膜和晶狀體屈光力的下降所代償[3，39，40，41]。直到最近，引起LOM的眼參數(shù)方面的機(jī)制才知曉，Kent[42]報(bào)道了對(duì)一病例縱向研究的資料，表明在青少年時(shí)期，角膜屈光力隨近視發(fā)展而增大，Goss和Erickson[43]發(fā)現(xiàn)角膜曲率增加和近視進(jìn)展之間的聯(lián)系在兩條主子午線都是很明顯的，但是，他們沒有檢查隨近視而變化的其他光學(xué)參數(shù)；Adams[44]報(bào)道他自己的角膜曲率保持不變，但近視在進(jìn)展，由此下結(jié)論近視進(jìn)展不是由于角膜變化而是由于眼軸變長(zhǎng)；McBrien和Millodot[45]報(bào)道LOM比正視眼的玻璃體腔深，還發(fā)現(xiàn)LOM前房較深及晶狀體較薄，但兩者的角膜曲率無明顯差別；Grosvenor和Scott[46]對(duì)三組年輕人隨訪三年：EOM(N＝29)、LOM(N＝26)、正視眼(N＝24)測(cè)量屈光狀態(tài)和眼球參數(shù)的變化，他們發(fā)現(xiàn)與屈光變化有關(guān)的眼部成分只有玻璃體腔和眼軸長(zhǎng)度，有趣的是，當(dāng)將兩近視組的度數(shù)進(jìn)行匹配對(duì)照時(shí)，他們的眼部任何成分均無明顯差異，他們從而得出結(jié)論，所有的近視都是軸性的。Jiang和Woessner[47]觀察了一年輕人的屈光不正的進(jìn)展，指出屈光不正變化是由于玻璃體腔的伸長(zhǎng)。McBrien和Adams[11]在兩年中從251名臨床顯微醫(yī)師處收集了大量的屈光與生物統(tǒng)計(jì)學(xué)資料，也作出如下結(jié)論：玻璃體腔伸長(zhǎng)是引起近視的發(fā)生和發(fā)展的主要因素。
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我們相信最近動(dòng)物模型的研究與人類眼動(dòng)文獻(xiàn)的結(jié)合，提供了強(qiáng)有力的依據(jù)，即眼動(dòng)機(jī)制與眼軸增長(zhǎng)有關(guān)。從對(duì)多種動(dòng)物的研究表明，早期發(fā)育的異常視覺經(jīng)歷會(huì)干擾正常的眼各成分間的協(xié)調(diào)而出現(xiàn)屈光不正，目前的動(dòng)物研究動(dòng)向是鑒別影響正視化過程的環(huán)境因素和這些因素的作用機(jī)制.這類研究主要受Wiesel和Raviola[48]經(jīng)典實(shí)驗(yàn)的影響，他們發(fā)現(xiàn)未成熟猴子眼縫合后會(huì)出現(xiàn)眼軸增長(zhǎng)和近視現(xiàn)象(稱為：剝奪性近視)。雖然眼瞼縫合后有許多因素能改變視網(wǎng)膜的像[49，50]，與圖形對(duì)比度減低有關(guān)的降低空間頻率似乎最有可能使眼瞼縫合后誘發(fā)剝奪性近視，但是，僅靠剝奪性近視的研究是不能說明視網(wǎng)膜圖象如何“主動(dòng)地”和“正常地”制約眼球根據(jù)環(huán)境來定焦生長(zhǎng)(如，正常視覺經(jīng)驗(yàn)在正視化中的作用)。雖然有多種方法可以改變視網(wǎng)膜像和可能改變眼球的生長(zhǎng)，但是產(chǎn)生正視化的最可能的視覺反饋是光學(xué)離焦的信號(hào)和幅度。小雞的研究表明視覺依賴(聚焦生長(zhǎng))的適應(yīng)過程確實(shí)出現(xiàn)，配戴不同度數(shù)眼鏡的小雞眼睛能夠生長(zhǎng)以補(bǔ)償誘發(fā)的離焦[51～57]，這種補(bǔ)償主要是通過改變眼球的生長(zhǎng)速率、特別是玻璃體腔的深度來達(dá)到的[57]，而且為造成屈光改變所需的離焦的幅度比原先想象的小，并不大于所估計(jì)的小雞的焦深[58]。對(duì)靈長(zhǎng)類動(dòng)物的研究也表明視覺依賴性“為對(duì)焦而生長(zhǎng)”機(jī)制，如Hung等[59]報(bào)告嬰兒猴子表現(xiàn)出由于正或負(fù)鏡誘發(fā)的光學(xué)離焦所引起的補(bǔ)償性的眼軸生長(zhǎng)。
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形覺剝奪和光學(xué)離焦存在根本差別，光學(xué)離焦時(shí)，如果離焦量不是很大，眼睛能繼續(xù)接受視覺反饋(即視網(wǎng)膜像質(zhì)作為屈光狀態(tài)的誤差信號(hào))，當(dāng)眼球朝合適的聚焦位置生長(zhǎng)，像的質(zhì)量得到了改善，但是，在形覺剝奪的情況下，視覺反饋毫無意義，因?yàn)闊o論眼球如何生長(zhǎng)，視網(wǎng)膜像質(zhì)無法有任何改進(jìn)(即視覺反饋環(huán)是“開放”或固定在某一值)，眼球增長(zhǎng)的兩種類型的明顯區(qū)別是光學(xué)離焦類型要求大腦作為中介的機(jī)制而被牽涉進(jìn)去，而剝奪性近視卻沒有這種要求，因?yàn)榧词箤⒁暽窠?jīng)切斷剝奪性近視也會(huì)出現(xiàn)[60]。在以人為受試者的研究方面，已有通過測(cè)量某些眼動(dòng)及參數(shù)來確定光覺模糊機(jī)制在引起近視中的潛在作用，根據(jù)被測(cè)量參數(shù)差異，這些研究分為兩類：一類測(cè)量調(diào)節(jié)刺激/反應(yīng)函數(shù)，另一類測(cè)量分離性隱斜。
對(duì)不同的屈光組之間的調(diào)節(jié)刺激/反應(yīng)函數(shù)進(jìn)行比較，McBrien和Millodot[61]發(fā)現(xiàn)近距注視(4或5D)時(shí)，LOM的調(diào)節(jié)性反應(yīng)比正視眼低。Rosenfield和Gilmartin[62]用3D近視標(biāo)時(shí)證實(shí)上述結(jié)論。Bullimore等[63]證實(shí)LOM在被動(dòng)注視條件下視近時(shí)調(diào)節(jié)反應(yīng)低，但在主動(dòng)注視條件下就不是這樣。Gwiazda等[46]測(cè)量了正視與近視兒童的調(diào)節(jié)刺激/反應(yīng)函數(shù)，發(fā)現(xiàn)近視兒童對(duì)三種視覺條件中的兩種在視近時(shí)調(diào)節(jié)較差。Abbott等[64]用同樣的方法對(duì)成人作了實(shí)驗(yàn)，他們發(fā)現(xiàn)進(jìn)展性的近視對(duì)負(fù)鏡片誘發(fā)的調(diào)節(jié)要求的反應(yīng)較差，但是穩(wěn)定的近視眼則和正視眼相同。Jiang[37]測(cè)量了雙眼視、單眼視、用Badal光學(xué)系統(tǒng)的單眼視狀態(tài)下的正視眼與LOM調(diào)節(jié)刺激/反應(yīng)函數(shù)。在單眼視和Badal單眼視狀態(tài)下，LOM的平均調(diào)節(jié)刺激/反應(yīng)函數(shù)的坡度比正視眼低，但在雙眼視狀態(tài)下無明顯差別。這些研究給出了初步證據(jù)：近視發(fā)生早期調(diào)節(jié)反應(yīng)減少，但是，在模糊引起的調(diào)節(jié)與近視之間仍存在一個(gè)問題，就是是否其中之一可以誘發(fā)另一個(gè)，或有一共同因素誘發(fā)了兩者[65]。把這些結(jié)論用到調(diào)節(jié)的控制理論上，仍不能清楚解釋屈光組之間不同的調(diào)節(jié)反應(yīng)或坡度是否由他們調(diào)節(jié)控制獲得或調(diào)節(jié)死腔的不同決定，后者常被認(rèn)為是眼的焦深。
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Jiang[66，67]提出了一個(gè)改進(jìn)的靜態(tài)調(diào)節(jié)模型。在該模型中加入了系統(tǒng)增益作為對(duì)于系統(tǒng)的感覺及系統(tǒng)的一種線型模擬(ASG)。模型得出的結(jié)論表明：感覺部分不僅影響調(diào)節(jié)反應(yīng)函數(shù)的坡度，而且會(huì)改變系統(tǒng)對(duì)離焦信號(hào)有效閾值(ET)。為檢驗(yàn)該理論結(jié)果，Jiang比較了13名正視眼與10名LOM的ET值，差異是顯著的。但是沒有發(fā)現(xiàn)兩組在暗焦點(diǎn)或調(diào)節(jié)刺激/反應(yīng)坡度上的明顯差異。分析坡度和ET資料后，他推測(cè)在LOM人員中，對(duì)離焦感覺的敏感性不如正視眼那么高。結(jié)合前面講到的，這突出了光學(xué)離焦在引起屈光不正病因中的重要作用。
臨床研究也為眼動(dòng)參數(shù)隨近視發(fā)展而變化的理論提供了讓人感興趣的初步證據(jù)。Birnbaom[68]觀察了近視發(fā)展中調(diào)節(jié)與輻輳的臨床測(cè)量值變化，他提出：早期近視可能與低的正相關(guān)調(diào)節(jié)和調(diào)節(jié)幅度以及視近內(nèi)隱斜增大或視近外隱斜減少有關(guān)。Drobe和de Saint-Andre[69]比較了法國(guó)視光學(xué)醫(yī)生測(cè)定的保持正視的和后來發(fā)展為近視的一批兒童和青年的近距隱斜數(shù)據(jù)，發(fā)現(xiàn)兩組的差距有2.9棱鏡度(P＝0.008)而原近視組內(nèi)隱斜更明顯。Gosss和Jackson[70]也發(fā)現(xiàn)發(fā)展為近視組視近時(shí)的內(nèi)隱斜更明顯。這組的兒童中，近視開始前與近視后，視近時(shí)常常表現(xiàn)為內(nèi)隱斜，這結(jié)果與成人中的發(fā)現(xiàn)類似，即發(fā)展成LOM的人在近視前與近視后表現(xiàn)出較高的反應(yīng)性AC/A值[32]。而且，高AC/A值與高分離性近距隱斜有關(guān)[71]。這一發(fā)現(xiàn)揭示了調(diào)節(jié)與輻輳相互作用在近視發(fā)展中的重要作用。同樣的輻輳反應(yīng)，高AC/A值者視近的調(diào)節(jié)量比正�；虻虯C/A者有較大的滯后。而且，調(diào)節(jié)滯后可能在視網(wǎng)膜上產(chǎn)生模糊像質(zhì)，這可以成為誤差信號(hào)引起眼的補(bǔ)償性生長(zhǎng)，而導(dǎo)致近視的發(fā)展。
, 百拇醫(yī)藥
3 討論
關(guān)于近視的病因?qū)W說有很多，但還沒有一個(gè)被廣泛接受。但是有一點(diǎn)很明確，就是近距工作是近視發(fā)生相關(guān)的環(huán)境因素之一。在這篇綜述中，我集中闡述了近距工作后眼動(dòng)參數(shù)改變，以及這種變化在近視與正視者之間的差別。眼動(dòng)參數(shù)中，暗焦點(diǎn)被認(rèn)為是調(diào)節(jié)的基態(tài)，持續(xù)近距工作后，暗焦點(diǎn)近移，被稱為調(diào)節(jié)適應(yīng)或近距工作后效應(yīng)。我討論了四種狀態(tài)下的調(diào)節(jié)參數(shù)：即靜態(tài)開環(huán)，靜態(tài)閉環(huán)，動(dòng)態(tài)開環(huán)，動(dòng)態(tài)閉環(huán)。由于調(diào)節(jié)系統(tǒng)可被認(rèn)為是一個(gè)負(fù)反饋環(huán)，在靜態(tài)閉環(huán)狀態(tài)下，如果系統(tǒng)增益很高，暗焦點(diǎn)移動(dòng)對(duì)調(diào)節(jié)反應(yīng)影響很少。但在靜態(tài)和動(dòng)態(tài)開環(huán)狀態(tài)甚至于動(dòng)態(tài)閉環(huán)狀態(tài)，調(diào)節(jié)反應(yīng)表現(xiàn)為調(diào)節(jié)緊張現(xiàn)象。這與調(diào)節(jié)或睫狀肌痙攣有關(guān)。臨床上，睫狀肌痙攣被認(rèn)為是睫狀肌過度的不必要的及不適當(dāng)?shù)氖湛s[20]。這種近距工作引起的異常調(diào)節(jié)痙攣，如果持久，就是假性近視。另一方面，持久性近視主要表現(xiàn)為眼軸的伸長(zhǎng)，動(dòng)物模型及以人為對(duì)象的研究都支持這假設(shè)，即：視網(wǎng)膜成像的精確性或視網(wǎng)膜像質(zhì)對(duì)眼后段的發(fā)育起反饋?zhàn)饔谩Ｉ厦嬷v到的對(duì)近刺激調(diào)節(jié)反應(yīng)的減少，高AC/A值，近距內(nèi)隱斜視，高離焦閾等導(dǎo)致像在視網(wǎng)膜離焦，都支持了這個(gè)假設(shè)。
, 百拇醫(yī)藥
很多研究工作已經(jīng)做過，但仍未揭開近視發(fā)展的病因之謎。我們?nèi)圆恢罏槭裁床皇撬械慕喙ぷ髡叨紩?huì)變成近視眼。最近的研究讓我們進(jìn)一步傾向近視發(fā)展的眼動(dòng)力理論的假設(shè)�！霸诿舾袀�(gè)體，發(fā)生屈光不正前，近距工作已引起了眼動(dòng)參數(shù)的改變，這一系列改變顯然可能導(dǎo)致光學(xué)離焦從而成為誘發(fā)近視的補(bǔ)償性變化的潛在因素”[70]。如何將近距工作與近視發(fā)展相聯(lián)系仍有許多不足之處，以至我們無法完全把這理論肯定下來。雖然暫時(shí)性近視不是每個(gè)屈光不正者所必經(jīng)歷的，但可以肯定，暫時(shí)性近視有可能發(fā)展為永久性近視。因此，現(xiàn)在的主要問題是弄清這個(gè)轉(zhuǎn)化過程是如何發(fā)生，這種轉(zhuǎn)變是否與眼動(dòng)參數(shù)有關(guān)。這方面的進(jìn)一步研究顯得非常重要及必要。
感謝：Dr.Harold and Dr Stephen Morse對(duì)形成本文初稿的討論和建議。
作者單位：蔣百川(美國(guó)休斯頓大學(xué)視光學(xué)院)
Oculomotor function in nearwork-induced transiant and permanent myopia(Part Ⅱ)
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Bai-chuanJiang
2 Nearwork-induced permanent myopia
Generally，it is accepted that childhood myopia progression is due to axial elongation，which is not compensated by reductions in corneal and crystalline lens powers[3，39，40，41]. Until recently，the component mechanisms that produce LOM become clear. Kent[70]reported longitudinal data，for one subject，showing that corneal power increased as myopia progressed during the early adult years. Goss and Erickson[43] found that correlations between corneal steepening and myopia progression were significant in both principal meridians. However，they did not check whether other optical components changed with the progress of myopia. Adams[44] reported that his own keratometer findings remained stable as his myopia progressed during the adult years，and concluded that the progression was not due to corneal steepening but due to axial elongation. McBrien and Millodot[45] reported that LOMs had deeper vitreous chambers than emmetropes. Their LOM subjects were also found to have deeper anterior chambers and thinner crystalline lenses，but no differences were found in corneal curvature. Grosvenor and Scott[46] measured the changes in refraction and ocular components over 3 years for three groups of young adults：EOM(n＝29)，LOM(n＝26)，and emmetropes(n＝24). They found that the only ocular components that significantly correlated with changes in refraction were vitreous chamber depth and axial length. Interestingly，when subjects from the two groups of myopes were matched for the amount of myopia，there were no significant differences in any of the ocular components，and they concluded that all myopia is axial in origin. Jiang and Woessner[47]observed a young adult's refractive error development and suggested that her vitreous chamber elongation was responsible for the refractive error change. McBrien and Adams[11] collected refractive and biometric data from 251 clinical microscopists during a 2-year period and concluded that the vitreous chamber elongation caused the onset and/or progress of myopia.
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Recent findings from the animal modeling literature can be integrated with the oculomotor literature in human studies resulting in strong evidence that oculomotor mechanisms are involved in inducing axial-length elongation in certain individuals. In a wide variety of animal species，abnormal visual experience during early development interferes with the normally coordinated growth of ocular components and produces anomalous refractive status. Research on animals is currently directed toward identifying the environmental factors that influence the emmetropization process and the mechanisms by which these factors affect the eye's refractive status. Animal studies on myopia were influenced mainly by the classical experiment of Wiesel and Raviola[48] who found that lid suture in the immature monkey results in an increase in axial length and a myopic eye(called‘deprivation myopia’or'form-deprivation myopia'). Although，many factors could alter the retinal image of the lid sutured eye[49，50]，degraded spatial vision associated with reduced image contrast appears to be the most likely aspect of lid closure that triggers the onset of lid-sutured myopia. However，studies of form deprivation alone do not reveal how，or if，the quality of the retinal image‘a(chǎn)ctively' and ‘normally' regulates the eye to grow to‘be in focus'for its environment(i.e.，the role of normal visual experience in emmetropization). Although there are several ways to change the retinal image and possibly alter the eye's growth，the most plausible visual feedback that can be used by the eye to perform emmetropization is the sign and magnitude of optical defocus. Studies in chicken have shown that a vision-dependent，“grow-to-be-in-focus”，adaptive process is indeed present. Chicken eyes that are fitted with different powered spectacle lenses，can grow to compensate for the induced defocus[51～56]. This compensationis mainly achieved by changing the eye's axial growth rate，especially vitreous chamber depth[57]. Furthermore，the magnitude of the defocus that will result in refractive change is smaller than previously believed，i.e.，not much greater than the estimated depth of focus of the chick eye[58]. Studies in primates have also demonstrated a vision-dependent“grow-to-be-in-focus”mechanism，e.g.，Hung et al.[59] reported that infant monkeys exhibit compensating ocular growth for optical defocus induced by positive or negative lenses.
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There is a fundamental difference between form deprivation and optical defocus. With optical defocus，if the amount of defocus is not very large，the eye can continuously receive visual feedback(i.e.，the quality of the retinal image serves as an error signal for the refractive status). When the eye grows toward the appropriate focus position the quality of the image improves. However，in form deprivation rearing strategies，visual feedback is meaningless because no matter how the eye grows，the retinal image quality is not improved(i.e.，the visual feedback loop is ‘opened'or‘fixed at a constant value'). The striking difference between the two types of axial elongation is that the optical defocus type requires the involvement of abrain-mediated mechanism and the other type(deprivation myopia)does not because it still occurs if the optic nerve is cut[60].
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There have been some human studies that looked at the potential role of the optical blur mechanism in producing myopia by examining a specific oculomotor parameter. According to the parameter being measured，these studies can be divided into two categories. In one，the accommodative stimulus/response function was measured and in the other，the dissociated near phoria was measured.
The accommodative stimulus/response function has been compared in different refractive groups. McBrien and Millodot[61] found that the accommodative response for near targets(4 and 5 D)is lower in LOM versus in emmetropes. With a 3 D target，Rosenfield and Gilmartin[62] confirmed the above findings. Bullimore et al.[63] confirmed that LOMs have a lower response for near targets under a passive viewing condition，but not under an active condition. Gwiazda et al.[46] measured accommodative stimulus/response functions for emmetropic and myopic children and found that myopic children accommodated less to high accommodative demands in their two of three viewing conditions. Abbott et al[64]. applied the same method on young adults. They found that progressing myopes had reduced accommodative response to negative lens-induced accommodative demands，but the accommodative responses in stable myopes were same as in emmetropes. Jiang[37] measured the accommodative stimulus/response functions of emmetropes and LOMs under binocular，monocular，and monocular with a Badal optical system viewing conditions. The mean slope of the function of the LOMs was lower than the mean slope of the emmetropes in the monocular and the Badal conditions，but there was no difference in the binocular condition. These studies provided preliminary evidence that reduced accommodative responses occurred during the onset of myopia. However，a remaining question regarding the correlation between blur-driven accommodation and myopia is whether one could cause the other，or whether a common factor influences both[65]. Applying these results to the control theory model of accommodation，it is still not very clear whether the difference in accommodative responses and/or slope between refractive groups is caused by the difference in their accommodative controller's gain or in the accommodative dead-space，the latter usually being thought of as the depth-of-focus of the eye.
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Jiang[66，67] suggested a modified model of static accommodation，in which an accommodative sensory gain(ASG)was added as a linear operator to simulate the sensory part of the system. Results derived from the model showed that the sensory part not only affected the slope of the accommodative response function but also increased the system's effective threshold(ET)to the defocus signal. To test the theoretical result，Jiang compared calculated values of ET between 13 emmetropic and 10 LOMs. This difference in ET between the two groups was significant. However，no significant difference was found in the dark-focus or the accommodative stimulus/response slopes between the groups. From analyzing the data of the slope and ET，he speculated that the sensory system in LOM subjects might be less sensitive to defocus than that of emmetropic subjects. This，combined with the information provided by the studies described above，emphasizes the role of optical defocus in inducing refractive error change.
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Clinical studies have also provided interesting preliminary evidence of oculomotor changes accompanying myopia development. Birnbaum[68] observed differences in clinical measures of accommodation and vergence during myopia development. He suggested that“incipient myopia”can be associated with lower positive relative accommodation and amplitude of accommodation，and greater esophoria or less exophoria at near. Drobe and de Saint-Andre[69] compared near phorias measured by French optometrists in children and young adults who become myopic and who remained emmetropic. They found that the difference between the two groups was 2.9 prism diopters(P＝0.08)with the pre-myopic group having more esophoria. Goss and Jackson[70] found that their became-myopic group had a more convergent(eso)phoria at near. In the children of this group，the near phoria showed an eso shift with time，beginning before and continuing after the onset of myopia. Their results are similar to the finding that young adults who developed LOM showed higher response AC/A ratios before and continuing after the onset of myopia[32]. In addition，the higher AC/A ratios are related to the higher dissociated near phorias[71]. This finding reveals the important role of the interaction between accommodation and vergence in the development of myopia. For the same vergence response，a subject with higher AC/A ratio has bigger lag in accommodative response to the near target than someone with a normal or low AC/A ratio. Furthermore，the accommodative lag potentially creates a blurred image on the retina that may serve as an error signal for the compensative growth of the eye that could lead to the development of myopia.
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3 Discussion
Many hypotheses of myopia etiology have been suggested but none has been widely accepted. But，it is certain that one of environmental factors associated with myopia prevalence is nearwork. In this review，I concentrated on work which revealed how the oculomotor functions are altered by nearwork and whether these effects differ between emmetropes and myopes. Among oculomotor parameters，dark-focus has been identified as the tonic posture of accommodation. After sustained nearwork，dark-focus shifts to near，which is called accommodative adaptation or nearwork aftereffect. I have discussed the changes of accommodative parameters under 4 conditions，i.e.，static open-loop，static closed-loop，dynamic open-loop，and dynamic closed-loop. Because accommodation system can be thought of as a negative feedback control system，under the static closed-loop condition，dark-focus shift has a little effect on the accommodative response(i.e.，the output of this system)if the gain of the system is high. But，under the static and dynamic open-loop conditions and even in the dynamic closed-loop condition，the accommodative response shows a hysteresis phenomenon which is related to accommodative or ciliary spasm. In the clinic，ciliary spasm is defined as an excessive，unnecessary and inappropriate contraction of the ciliary muscle(Borish，1970). This abnormal status of accommodative spasm associated with nearwork can be thought as a transient myopia and pseudomyopia if it becomes continuous. On the other hand，permanent myopia mainly shows axial elongation of the eye. Both animal model studies and work on human subjects lead to a hypothesis in which the accuracy of the retinal image and/or retinal image quality provides feedback for growth of the posterior segment of the eye. The reduced accommodative response to near stimuli，the high AC/A ratio，the near esophoria，and the high defocus threshold of the accommodative system described above result in a condition of a defocused image on the retina and support this hypothesis.
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Much work has been done，but not enough yet to solve the mystery. We still face the puzzle why not all people who perform nearwork become myopic. Recent studies led us to further hypothesize an oculomotor theory of myopia development：“Near work causes changes in the oculomotor characteristics of susceptible individuals which begin even before the development of refractive error. This series of changes in oculomotor function apparently results in optical defocus，which has the potential to induce compensatory changes resulting in myopia”[69]. There still are gaps in bridging the relationship between nearwork and the development of myopia that prevent our complete formulation of the theory. Although transient myopia is not a necessary step for a person to develop refractive error，it is true that transient or pseudo-myopia can become permanent myopia. Therefore，a major question is how this change occurs. Is the change related to oculomotor functions？Further research in this area appears to be very critical and necessary.
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Acknowledgments The author is grateful to Dr. Harold Bedel and Dr. Stephen Morse for their helpful comments on the manuscript.
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From College of Optometry，University of Houston，USA
(收稿：1998-11-15), 百拇醫(yī)藥