Xu Yuan, Zhao Wei
C.I.C. Classification Number: R778.1+1 Literature Identification Number: A Article Number: 7237
Abstract: This article presents a literature review on the topics of myopia development, epidemiological studies of myopia, regression of the regulation theory, early-onset myopia and late-onset myopia, continuous near work and myopia, and experimental studies of defocus, suggesting that the occurrence and development of myopia may be related to a variety of factors. We also discuss various attempts to intervene in the development of myopia based on the theory of accommodation, in conjunction with clinical practice. Attempts have been made to change the direction of gaze with prisms in conjunction with ortho-spheres to control patients with myopic progression. Here, our theoretical research and ideas are provided in the hope that more optometrists will be involved. Xu Yuan, Ophthalmology Department, Workshop
Keywords: myopia; control; spherical lens; prism
Development of Myopia and Its Optical Intervention
Xu Yuan, Zhao Wei
From the Department of Ophthalmology, Xijing Hospital, Fourth Military Medical University
710032 Xi’an, Shaanxi Province, China
Abstract: This review summarized literature about development of myopia, epidemiologic study of myopia, recurrence of accommodation theory, early onset myopia (EOP), and the development of myopia. This review summarized literature about development of myopia, epidemiologic study of myopia, recurrence of accommodation theory, early onset myopia (EOM) and late onset myopia (LOM), persistent near work with myopia, and experiment about defocus and so on. Some opinions were discussed considering clinical practice basing attempts to intervene development of myopia by accommodation theory. A method to control development of myopia was attempted, which applied a prism to change direction of visual fixation A method to control development of myopia was attempted, which applied a prism to change direction of visual fixation combined spherical lens.
Key words: myopia, control, spherical lens, prism
Myopia, a phenomenon that has kept pace with human civilization, has caused many inconveniences to people. Myopia is a product of human adaptation to nature, and there are some aspects of low myopia that are not disadvantageous compared to orthopia, such as the adaptation to long-distance work and the convenience of near eye use after presbyopia. If the degree of myopia has been stable, wearing a pair of glasses is not a big deal. However, the continuous increase in myopia has caused many people to develop complications without realizing it. These complications lead to low vision and blindness, second only to cataracts and glaucoma in China. As the incidence of myopia continues to grow in China and as this group of myopic people gets older, the number of people whose complications cause permanent visual impairment is likely to increase dramatically, possibly surpassing cataracts and glaucoma as the number one killer of vision.
1 Studies related to the occurrence and development of myopia
According to the regulatory tension hypothesis in the 1960s, excessive near-eye work during near vision can cause regulatory fatigue, loss of flexibility of the ciliary muscles, and insufficient relaxation during distance vision, hence pseudomyopia; and it was noted that myopia is temporarily shifted away from the near point after sustained near eye use, i.e., myopia worsens [1].Ciuffreda et al [1] (1998) found that myopia is more severe after Vera-Diaz et al[2] (2002) reported that temporary myopia induced by 10 min of 4D accommodation exceeded resting myopia in progressive myopia and even more so in orthopia. Ciuffreda et al[3] (2002) found that continuous reading for 4 h also induced a tendency for transient refractive to myopic conversion in myopia, but not in hyperopia.Wolffsohn et al[4] (2003) found that this phenomenon was more pronounced in yellow individuals with a high prevalence of myopia. This phenomenon was more pronounced in progressive myopia and late onset myopia (LOM) eyes than in resting myopia and early onset myopia (where genetic factors may be more important). This study is direct evidence that conditioning factors can induce myopia in humans and suggests that some individuals may be more sensitive to conditioning load, i.e., susceptible to myopia. So, what characteristics do these individuals exhibit? The normal vision population, especially those who have maintained normal vision despite advanced education, has been the focus of research.
In the past 30 years, myopia research has focused on experimental animal studies, and clinical studies have not received the attention they deserve, and to a certain extent, the differences between experimental animals and humans have been ignored, and the results obtained from animal experiments have not been used in the clinical setting with the expected results. Functional studies (such as temporary myopia caused by persistent near vision), optical index analysis, biochemical tests, and pathological studies (including ultrastructure, immunohistochemistry, molecular pathology, and other means) have gradually become the main direction of myopia research [5].
2 Epidemiological studies of myopia
Epidemiological surveys have confirmed that the occurrence of myopia is related to near-eye work. The prevalence of myopia is higher with high near-eye workload, and this phenomenon persists after adjusting for family history [6,7]. The higher prevalence of myopia in the cohort survey was found to be associated with high near-eye workload in those with higher education, while the higher prevalence of myopia in those with high income may be due to their income in relation to their education [8,9]. The difference in myopia prevalence between urban and rural children may also be related to higher near-eye workload in urban areas [10]]. The United Nations Health Organization arranged a population-based survey in China, Chile and Nepal and found that the prevalence of myopia among residents of a district in Beijing increased annually from 5 to 15 years of age, with a greater increase than in Chile and much more than in Nepal [11]. A 2-year follow-up confirmed that this trend persisted [12]. The prevalence of myopia in yellow Asians (China, Japan, and Singapore) has been increasing in recent years, and the prevalence of myopia in both the Japanese and Singaporean adult populations exceeds that of the European and American populations [8,9]. In contrast, the prevalence of myopia in Europe and the United States has remained relatively stable [13]. The vast majority of scholars now accept the idea that the occurrence of myopia is related to both genetics and environment. The mode of inheritance is also mostly considered to be polygenic, i.e., each group of genes acts in a micro-effective and cumulative manner. Recent years have seen higher myopia genetic indices calculated in Western countries based on twin studies or family aggregation analysis, but the role of environmental factors is still acknowledged, especially in Eastern countries [14]. 21st century work in this field has mainly validated the phenomena observed in the 20th century, with little exploration of new and unknown environmental factors [5].
3 The return of the regulation theory
Experimental animal studies in the 20th century confirmed the role of environmental factors in myopia onset, placed emphasis on myopogenic changes dominated by retinal signals, discovered many biochemical substances that influence myopia onset and development, and gave a strong impetus to the study of myopia pathogenesis. However, when applying their results to clinical practice, researchers have not recognized the species differences between humans and experimental animals (especially between chickens and mammals) and the age differences (experimental animals are mostly juveniles, while the high incidence of myopia in humans is in adolescence); they have not understood the differences between the two types of myopia, which are essentially different (form deprivation myopia is extremely rare in humans, and is only occasionally seen in individuals with high juvenile ptosis and refractive media The vast majority of human myopia is closer to out-of-focus myopia), which has also led to some misconceptions. In the past, studies on the mechanism of myopia development denied the role of regulation in myopia development because of the observation that form-deprived myopia could still occur in chickens after severing the optic nerve, destroying the Edinger-Westphal nucleus, and severing the ciliary and choroidal nerves. However, in fact: (1) most human myopia is out-of-focus myopia. Cutting the optic nerve in experimental animals had an inhibitory effect on out-of-focus myopia, indicating the importance of the center in myopia development. The fact that myopia can still occur in experimental animals after disruption of the Edinger-Westphal nucleus only suggests that regulation is not the only mechanism of myopia occurrence, which is not the same as indicating that regulation does not play a role in myopia occurrence under normal conditions. After cutting the ciliary and choroidal nerves, myopia still occurred in chickens with form-deprived myopia, but myopic refraction was significantly reduced, suggesting that the efferent nerve still plays a role in myopia development [14]. The recent findings in experimental animals that conditioning plays a role in refractive changes induced by the double visual plane [15] and the discovery of transient myopia caused by sustained near vision in clinical studies [2,3] have reawakened researchers’ attention to the mechanisms of conditioning.
4 Early-onset myopia and LOM
A recent classification system for myopia is based on age-related myopia prevalence and age of onset. Early-onset myopia is the most common type of myopia, with onset between the ages of 6 and 15 years, and once it occurs, myopia continues to increase until it reaches 20 years of age, when it stabilizes. The distinction between early-onset myopia and LOM is based on the fact that myopia in children stops developing by the age of 16 or 17, when overall physical development has largely ceased.
5 Continuous near work and myopia
There are two types of myopia caused by continuous proximity work: temporary myopia and permanent myopia. Transient myopia is the temporary shifting of the distant point resulting in a refractive state similar to myopia, while a ciliary muscle paralysis examination will reveal the true refractive state to be hyperopic or orthophoric. Many studies of permanent myopia have demonstrated its close association with proximity work, and Zadnik et al [16] suggested that the best model for predicting refractive error should take into account not only the history of refractive error of the parents but also the child’s own proximity work. And it is also of great interest to study the myopia of college students and those working in proximity, as such studies reveal that a large number of proximity jobs are directly related to the development of myopia in adults. Because myopia development in the LOM is closely related to the near-visual environment, studies of persons with myopia caused by proximal work may offer the possibility of investigating the ocular components and oculomotor mechanisms that contribute to myopia development. Because proximity work includes enhanced amounts of accommodation and vergence, it is important to investigate the role of accommodation and vergence and the oculomotor system as a whole in the development of myopia.
Regulation and vergence are fundamental elements of the oculomotor system’s mechanism of near work, and their combination produces a clear image of monocular vision in the binocular state. Based on Westheimer’s pioneering research, feedback control theory was used to produce models that describe the static and dynamic responses of the accommodation and vergence systems. The basic feature of all models is that regulation initiated by fuzzy imaging and vergence initiated by lavish opening are controlled by two negative feedback loops, and the interaction between the two is represented by the crossover of the two feedbacks at the control output. In this way, modulation control can trigger a convergence response (modulated convergence or AC), and conversely, convergence control can trigger a modulation response (convergence modulation or CA). The acquisition of AC or CA is represented by AC/A or CA/C, respectively. Either the modulation system or the vergence system can be separated by opening the feedback loop, e.g. covering a glance can open the vergence feedback loop, or using a pinhole pupil to open the modulation feedback loop. The control of the convergence system (VCG) can be estimated from the amplitude of the stimulus/response function in the open-loop convergence state and from the gaze lavish opening curve in the open-loop conditioning state; whereas conditioning and convergence measured in the dark is the physiological resting state.
Temporary myopia caused by near work is considered to be a temporary shift of the far point after sustained near eye use. This myopia is considered to be the first stage of refractive error. Unlike true myopia, pseudomyopia is a temporary myopia in which the far point is continuously shifted closer, and pseudomyopia is a myopic refractive error when examined without ciliary muscle paralysis. However, pseudomyopia is reversible because the refraction of pseudomyopia is orthoopic when examined after ciliary muscle paralysis, suggesting that the temporary myopia is caused by an adjustment spasm or ciliary muscle spasm.
Temporary myopia due to close work has been extensively studied in the laboratory. Since 1960-1970 when the dark focus was identified as a marker of regulatory rest, the initial approach to speculate on the oculomotor/ocular component was proposed: a person with a dark focus that is too far away (low refraction) will exert more oculomotor force to focus on the myopic marker during near work than a person with a close dark focus. However, similar studies have had different results, with some finding that the dark focus of the LOM is significantly more distant than that of the orthoptic eye, while others have concluded that there is no significant difference between the two [17].The single study by Gilinartin and Bullimore highlights the diversity of findings. In their study, the dark focus values were the same for the ortho- and LOM groups, and the results seem to be such that although the dark focus of a given individual is relatively stable, inward proximity work causes an inward shift of the dark focus that persists for some time until the proximity work stops, and this shift is referred to as a modulatory adaptation or modulatory lag. Since regulatory hysteresis refers to a prolonged and sustained increase in ciliary muscle tone, it is conceivable that this hysteresis is associated with myopia triggered by proximity work [17,18]. However, studies of accommodative adaptation between refractive groups have shown different results. It has been reported that the LOM group showed greater accommodative adaptation than other types of refractive groups, but other studies did not find differences between the ortho-optic and LOM groups. Although all of these studies were designed to identify differences in dark focus across refractive groups, because they were cross-sectional or retrospective, it is not possible to determine whether the differences in dark focus observed from these subjects were the cause of myopia occurrence or the result of myopia produced by myopia [19-21]. These same problems exist for the comparison of differences in oculomotor parameters between refractive groups cited in the following paragraphs, where Jiang conducted a longitudinal study and found lower dark focus values in LOM than in orthoptic eyes. However, this low dark focus was altered when a person developed myopia to permanent myopia. Instead, orthoptics with high dark focus values were at risk of developing myopia. It is clear that a distant dark focus is evidence of regulatory spasm or lag and can lead to the development of myopia [22].
McBrien and Millodot recorded the dark focus regression process after sustained near work in the ortho- and LOM groups, and they could not determine the regression rate in the ortho-group because there was little change in dark focus after near work in these subjects, whereas the LOM group had a significant dark focus shift after near work and no significant regression process in dark focus for 15 min after the end of near work [ Rosenfield and Gilmartin found no significant difference between the ortho- and LOM groups after relatively short periods (15, 30, and 45 s) of near-visual (3D) gaze [23,24], however, Gilmartin and Bullimore reported a significantly lower dark focus regression rate in the LOM group after 20 min of gaze on the same near-visual subject than in the ortho-visual group However, Gilmartin and Bullimore reported that the dark focus regression rate was significantly lower in the LOM group than in the orthogonal group after 20 min of gaze on the same near visual standard, and this difference was even more pronounced if the near visual standard was increased to 5 D [24,25]. Strang et al. also showed that the dark focus regression was slower in the LOM group after adaptation [26]. In summary, these studies all suggest that the time constant for dark focus regression after adaptation is longer in the LOM group than in the ortho-optic group when sustained close work or in situations requiring high accommodation. It is possible that the vagal inhibition of the ciliary muscle was weaker in the LOM group than in the orthoptic group. If this phenomenon existed before the onset of myopia, it would be a precursor to triggering accommodation hysteresis in near working conditions [27].
6 Experimental studies on defocus
McFadden et al. gave guinea pigs concave lenses for 6 weeks to cause a relatively significant change in their eye axis length [28]. Ouyang Chaohu et al. also observed that guinea pigs exhibited different changes in eye axis and refractive error over time using different degrees of convex and concave lenses [29,30].
7 Attempts to intervene in the development of myopia based on the regulation theory
Based on the regulation theory, there have been numerous attempts by many scholars at home and abroad in addressing the effects of regulation on myopia.
7.1 Progressive multifocal lenses (PAL) Leung JT, Brown B (reported in 1999) conducted a 2-year study in Hong Kong with a small sample (N = 168) with progressive multifocal lenses in an attempt to improve the clarity of imaging on the retina, (because progressive multifocal lenses can provide continuous clear vision in the far, middle and near regions). The study of Chinese children with progressive lenses for 2 years resulted in significantly less myopia progression and eye axis increase than children with monovision lenses. Children aged 9 to 12 years were selected and randomly grouped for follow-up every 6 months. It is believed that progressive lenses can alleviate myopic progression [31].
In 1998, the National Eye Institute led a study of progressive lenses to control myopia in nearly 400 American myopic children using a randomized, double-blind method in conjunction with four institutions of higher learning, including the New England College of Optometry and the University of Houston College of Optometry, in contrast to single-light lenses [32,33]. A similar experimental design was used to study progressive lenses for myopia control in 300 Chinese myopic children at Wenzhou Medical College and Tongren Hospital in Beijing and Shanghai Five Glaucoma Hospital in China during the same period. The same conclusion that progressive lenses can alleviate myopia progression was obtained.
For PAL used for myopia control in adolescents, various manufacturers in the market currently use a fixed downward additive degree (ADD), mostly +1.50 D. Foreign studies have used different ADDs, but the results mostly use 1.50 to 2.00 D as more scientific.
The control of myopic progression by PAL remains controversial because there are still contradictory results in different countries and regional populations, and in some regions researchers have found that the use of PAL can delay some myopic progression in myopic progressives with internal obliquity [34]; while for most general adolescent myopia, there is not much effect.
PAL is primarily optically altering accommodation and changing oculomotor parameters such as vergence, so its use is limited, and moreover, it has been suggested that PAL may negatively affect visual development such as stereopsis in children.
7.2 Myopic regression lenses A type of myopic regression lenses tried in China at the same time as the student progressive multifocal lenses, taking into account the effect of pooling on accommodation, obtained a recent effect of improving visual acuity by adding a bottom-facing prism to the near fog vision therapy [35,36]. The lure of this effect led to the development of a variety of myopic regression products in an attempt to get patients to remove their glasses, but it was not to be and these recent improvements in vision were quickly overwhelmed by the later development of myopia that failed to achieve myopic regression. It has been proven that myopia cannot be controlled without clear distance vision. Would such regression lenses for myopia control make a difference? Since the astigmatism of each pair of glasses is not taken into account, it is impossible to provide clear gaze. In addition, the causes of myopia development vary, and no long-term observations have been seen so far on whether abandoning the use of trigeminal lenses with the bottom facing inward will aggravate the effect of exotropia and make myopia develop more rapidly in myopic patients with exotropia.
7.3 Reading Additional Lenses Guided by the above theory of accommodation, we envisioned that common features might be found in people with undergraduate education or higher who still maintain normal vision. After investigating various refractive parameters in more than 100 such normal-sighted people and comparing them with a large number of myopic patients [37], no significant differences were found other than the known differences in refractive error and degree of exotropia, and data from a larger sample may be needed. As seen in a large number of myopic investigations, many progressive myopia, exhibit varying degrees of fatigue symptoms, most of these fatigue symptoms appear after prolonged reading, and each of these fatigue symptoms may find some obvious causative factors. So, can addressing these triggers play a role in limiting the progression of myopia? By chance, we added bottom-down 5△ trigeminal lenses to the original +1.50DS reading glasses for a patient with visual fatigue for whom we could not find a specific cause, and the symptoms of visual fatigue were relieved. Inspired by this phenomenon, a reading add-on mirror was designed for improving reading fatigue symptoms, and satisfactory results were obtained. In the observation [38], it was found that this reading add-on mirror also provided some relief in some patients with progressive myopia.
Studies as early as the 1960s have suggested that reducing myopia or increasing hyperopia during close reading can effectively prevent myopia onset or slow down myopia progression, but past research data have some design flaws, such as reasonable control group design, adequate longitudinal observations, standardization of standardized and uniform testing instruments or standardization of testing devices. There is no conclusive data to prove that this method can prevent the occurrence of myopia or slow its progression. However, it is worthy of further research as a way to relax the regulation of close reading, based on the “regulation theory” of myopia onset and development. After searching, no such attempts to control the progression of myopia with a bottom-facing tricorder have been seen.
Under normal conditions, accommodation increases in both eyes as the point of gaze moves from far to near, while the eyes turn inward. Correct focusing is a prerequisite for maintaining normal visual function in both eyes. Those with over-assembly often experience eye discomfort and headache after a short period of reading, and occasional blurred or double vision when working at close range. Overconcentration is basically non-retentive in distance gaze, but internal retentive in near gaze, with increased AC/A ratio (usually greater than 6△). Reading addition (+1.50Ds) can drastically alter the gaze and alleviate the symptoms caused by hyperconvergence. In this case, the clinical finding is that myopic progression is significantly less than before with this add-on, i.e., it is most effective in the case of internal obliquity.
For most myopic eyes with exotropia, the use of a trigeminal lens with +1.50 DS on the nasal side has not only improved the fatigue symptoms that many myopic eyes tend to experience after prolonged reading, but has also alleviated the progression of myopia and improved the naked eye vision in some patients. However, this measure did not show the desired effect after long-term use. On the contrary, some patients even showed accelerated progression of myopia after abandoning its use.
As for the patients found clinically without obvious eye position abnormalities who still showed rapid progression of myopia, observation of their reading habits revealed that some patients’ gaze habits were not excessively low, but rather sitting very straight at the request of their parents and teachers, but with their eyes excessively turned downward. As shown in Figure 1.
Figure 1 Downward turning of the eyeballs while reading
When reading for a long time in this posture, there is an increased chance of visual fatigue. The analysis has the following factors: First, the role of the lower eyelid. When gazing downward, the pressure of the lower lid on the cornea distorts the lower cornea, increasing the refractive power in this area. This may be manifested in the aberration as an upper concave and lower convex higher order aberration. Second, there may be enhanced accommodation due to the influence of the inferior rectus muscle as well as the internal rectus muscle in both eyes. When the eyes gaze downward, the action of the internal rectus muscle is habitually increased due to the continuous impulse of both lower rectus muscles, and the impulse of the internal rectus muscle matches the impulse of the accommodation, thus increasing the accommodation. Third, the wise-difference that arises after the direction of gaze deviates from the optical axis. Again, this affects the quality of vision as a higher order aberration and produces fatigue.
Under normal circumstances optometry only measures the refractive state under horizontal direct vision, while when gazing below, it may have an effect on the refractive power of the human eye for the above reasons. In order to allow these patients to read without having to lower their heads excessively (low head movement is good for improving the direction of gaze, but it tends to reduce the reading distance), and not to change the refractive state after looking up and down, for this reason, we tried a bottom-facing trigonometry. As in Figure 2.
Figure 2 The role of the prism
The downward gaze is assisted by attaching a trigeminal prism to the reading lens to reduce the excessive downward rotation of the eye. This reading add-on made of an ortho-spherical mirror with a bottom-facing trigonoscope provides some relief from the symptoms of reading fatigue, thus providing control of progression in these patients who do not have significant eye position abnormalities and have progressive myopia.
Can this approach be tried for most of the myopic progressives who exhibit occult exotropia? Theoretically, this method will not change the assembly and dispersion of the eyes after continued use, and certainly will not rebound after abandonment, and should be worth a try.
8 Summary
The optical mechanism of myopia is well understood, but its etiology, natural history, and the best methods of prevention are not yet fully understood. The developmental mechanism of myopia has become more complicated by the involvement of regulatory factors that are once again being recognized, oculomotor parameters and cryptopia. Since the causes of myopia are complex and there are many factors that trigger the progression of myopia, it is impossible to use one method to solve all problems, and the indications for each method need a lot of practice to confirm. In the clinical practice, different methods have been implemented for patients with various conditions of myopia progression, and preliminary results have been obtained, which still need a large number of controlled observations and longitudinal studies over a long period of time. Here, our theoretical studies and ideas are offered in the hope that more optometrists will be involved in finding the best myopic eye control program in a large number of practices.
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Received: 2008-09-28 Revision date: 2008-12-24
Author: Xu Yuan (1959- ) Deputy Chief Physician, Department of Optometry, Xijing Hospital, Fourth Military Medical University, China E-mail: [email protected] Tel: 029-84775376