Most myopia develops during the school years and stabilizes in the teenage years. Nonetheless, a number of individuals will show myopic changes after entering college. This may manifest as an increase in myopia in a previously myopic subject—adult myopia progression—or the onset of myopia in a previously emmetropic or hyperopic individual—adult-onset myopia. The National Research Council Committee on Vision Working Group on Myopia Prevalence and Progression reviewed over 500 articles on myopia. On the basis of the studies reviewed, the report concluded that up to 40% of low hyperopes and emmetropes entering college and military academies are likely to become myopic by the age of 25 years. Conversely, in populations in which college graduates are excluded, less than 10% of individuals become myopic as adults.
There have been a number of reports of myopia progression in adulthood. Waring et al., for example, reported a mean myopic shift of –0.65 D across 10 years in the fellow eye of 47 Prospective Evaluation of Radial Keratotomy (PERK) study patients who elected not to undergo radial keratotomy on their second eye. Adams and McBrien found that 50% of clinical microscopists reported significant myopia progression since joining the profession. A number of studies, including our own, have documented myopia progression in subjects in their thirties. This agrees with eye care practitioners’ descriptions of adult myopia progression anecdotally associated with professional or graduate school, increasing computer use, or both.
None of the aforementioned studies has demonstrated a compelling relationship between adult myopia progression and near work. Clinicians may tell their patients that their adult myopia progression is related to their computer use. Nonetheless, there is little evidence to support this assertion. Rather, the association has been based on the occupation and education levels of different groups of subjects.
We describe the design and baseline characteristics of a 5-year observational study of myopia progression in adults with detailed measures of near work and other risk factors. At study end, subjects will be categorized into those whose myopia progressed and those whose refractive error was stable. The two groups will be analyzed with respect to near work-related risk factors. In particular, two broad categories of risk factors will be assessed: the proportion of time a subject spends reading or performing other forms of near work and selected characteristics of the subjects’ ocular accommodation and vergence.
The rationale for studying near work-related risk factors is based on the clinic and research community’s belief that near work causes adult myopic progression, reports of myopic changes in occupations involving large amounts of near work, and reports of an association between myopic progression and hours of near work in university students.
The rationale for studying accommodation-related risk factors is the numerous publications implicating accommodation and vergence in the etiology of myopia. A number of researchers have hypothesized that underaccommodation, or accommodative lag, induces myopia in humans by a similar mechanism to that which produces experimental myopia in animals. This hypothesis is supported by studies in children and adults showing greater accommodative lag in myopes compared with emmetropes. Subsequent studies have shown that accommodative lag is greater in children and adults. whose myopia is increasing than in those whose myopia is stable.
Studies have also linked the interaction between accommodation and convergence—usually characterized by the accommodative convergence/accommodation (AC/A) ratio—to the etiology of myopia. Cross-sectional studies in children and adults have found that higher AC/A ratios are associated with myopia. Jiang found that adults whose myopia developed or progressed over a 2- to 3-year period had significantly higher response AC/A ratios than those whose refractive error was stable. Likewise, Mutti et al. reported that a high response AC/A ratio is a significant risk factor for the onset of myopia in children.
DISCUSSION
This article presents the baseline findings from the SPAN. In comparing our results with previous studies, it is important to remember that all of the subjects in SPAN are myopes and the analyses use refractive error (degree of myopia) rather than the presence or absence of myopia. This is particularly germane when considering the impact of family history on the refractive status of the SPAN cohort. It is not surprising that so many subjects report myopic parents and siblings, but it is surprising that myopia in a parent, particularly the subject’s mother, is related to the degree of myopia. Although the degree of myopia is not significantly associated with the reported refractive status of the subject’s father, there is a trend for higher levels in those with a myopic father.
As reported by previous researchers, the degree of myopia is related to the age of onset. In the SPAN cohort, each 1-year increase in the age at which the subject began wearing spectacles is associated with –0.16 D less myopia and early-onset myopes are significantly more myopic than late-onset myopes. Further analysis demonstrates that that the number of myopic parents is significantly related to the age of myopia onset. For example, twice as many late-onset myopes (32.1%) report no parental history of myopia as early-onset myopes (16.6%). Given that both age of onset and parental history are significantly related to the degree of myopia in our cohort, we performed a multivariate analysis to simultaneously assess the affect of these variables. In the multivariate model, age of onset (early vs. late) was significant related to degree of myopia, but parental history was no longer significant. Thus, any impact of parental refractive error on the level of myopia is mediated by the age of onset of myopia. In other words, those subjects with myopic parents have higher levels of myopia because they developed myopia at an earlier age.
The degree of myopia is higher among contact lens wearers than nonwearers, but this is probably the result of the cosmetic and functional limitations of higher-powered spectacle lenses or having spent more years as a myope and thus having greater opportunity to begin contact lens wear. As would be expected, degree of myopia was associated with increased axial length. Although less compelling, the association with anterior chamber depth and corneal power has been reported previously.
The response rate for the ESM was impressive and bodes well for the characterization of near activity in the cohort. On average, subjects spend approximately one-third of their time engaged in near activity (34.1%) with computer use contributing just over half of the activity (18.9%). Despite the cohort having been recruited from university faculty and staff, the range of near activity is quite broad (0–67.3%).
Risk Factors for Progression
The goal of SPAN is to determine the risk factors associated with adult myopia progression. The primary risk factors to be evaluated are performing near work for a greater proportion of the day, performing near tasks at a close distance, a high response AC/A ratio, and a high accommodative lag. The first two near work-related risk factors were chosen based on the longstanding, but largely unsubstantiated, assertion that adult myopia progression is associated with high levels of near activity. Of the previous studies, few examined near work as a potential risk factor for adult myopia progression, and only Kinge et al. reported an association between near work and myopia progression (r = 0.25). Reading at a closer distance has been proposed as a potential risk factor in children. An informal survey at the Eighth International Conference on Myopia in 2000 found that 31 of 47 of meeting presenters (66%) felt that environmental factors were primarily responsible for adult myopia progression, and an additional nine (19%) felt that the progression was the result of an interaction between environmental and genetic factors.
The hypothesis that increased accommodative lag is a risk factor for myopia progression is supported by both animal myopia research and accommodative studies in humans. The neonatal animal eye can compensate for refractive errors induced by convex or concave lenses. A minus lens placed in front of the cornea shifts the focal plane posteriorly. In an emmetropic eye, this results in hyperopic defocus unless the eye accommodates. In young chicks, tree shrews, and monkeys, the eye compensates over a period of days or weeks by increasing its axial growth rate until the retina has shifted to this modified focal plane. A recent report suggests that the mechanism may still be active in adolescent monkeys.
In humans, underaccommodation to a near target—accommodative lag—results in hyperopic defocus similar to that produced by a minus lens in the animal studies. A number of researchers have hypothesized that this accommodative lag induces myopia in humans by a mechanism similar to that which produces experimental myopia in animals. Studies in children and adults reported greater accommodative lag in myopes compared with emmetropes. Subsequent studies have shown that accommodative lag is greater in children and adults whose myopia is increasing compared with subjects whose myopia is stable.
Studies have also linked the interaction between accommodation and convergence to the etiology of myopia. This interaction is usually characterized by the AC/A ratio, measured as the change in convergence (or phoria) induced by a change in accommodation. Cross-sectional studies in children and adults found that higher AC/A ratios were associated with myopia. Of particular interest is a small prospective study that found that adults whose myopia developed or progressed over a 2- to 3-year period had significantly higher response AC/A ratios than those whose refractive error was stable. Likewise, it has been reported that a high response AC/A ratio is a significant risk factor for the onset of myopia in children.
The ability of SPAN to successfully identify significant risk factors depends in part on there being a broad distribution of the relevant variables in the study population. In this regard, it is important to note that there is a broad range in the proportion of near activity undertaken by the subjects. Likewise, other primary risk factors—accommodative lag and AC/A ratio—along with secondary factors like phoria show similar broad distributions. A detailed analysis of the ESM data is the subject of a future manuscript.
Public Health Significance
Myopic progression in adults is of increasing clinical interest as increasing numbers of patients undergo refractive surgery, e.g., LASIK, to correct their myopia. Adult myopic changes affect the long-term patient satisfaction with such procedures. For example, a 25 year old who is rendered emmetropic by LASIK, but whose myopia then progresses by a diopter over the next decade, will evolve into a 35-year-old –1.00-D myope (although such a refractive error may be desirable in a presbyope). Javitt and Chiang analyzed the cost-effectiveness of excimer laser photorefractive keratectomy and concluded that over a 20-year period, it was a less expensive investment than either daily wear or extended-wear soft contact lenses. Their analyses were based, however, on the premise that there were no long-term refractive changes in the postsurgery patient and that the vast majority of patients remained “glasses-free.” If patients shift in a myopic direction, then clearly refractive surgery may be a less cost-effective alternative than proposed by Javitt and Chiang.
No comments:
Post a Comment