Sunday, May 13, 2012
Review of existing literature on myopia (2012)
Myopia
Prof Ian G Morgan PhD , Prof Kyoko Ohno-Matsui MD , Prof Seang-Mei Saw PhD
The Lancet, Volume 379, Issue 9827, Pages 1739 - 1748, 5 May 2012
The authors of this article searched the Medline and Online Mendelian Inheritance in Man (OMIM) databases for research on myopia. They note that there's is not enough epidemiological support for the idea that intense near work is related to myopia development; that environmental conditions (such as increased study indoors) might play a stronger role than genetics in many cases of school myopia ; that peripheral defocus might be be a consequence rather than a cause of myopia, and that orthokeratology might not work long term. Here are some passages that seemed particularly interesting or informative to me:
From the text:
On myopia development:
"Most children are born hyperopic, with a normal
distribution of refractive errors.7 During the fi rst year or
two after birth, the distribution narrows,8 with a mean in
the hyperopic range of +1–2 dioptres (D). This change
indicates that there is an active process shaping the
distribution of refraction, known as emmetropisation.
After that period, the cornea stabilises,9 but refraction
can become more myopic as axial length can continue to
increase for another two decades. By contrast, lens power
decreases substantially up to the age of about 12 years,10
with slower decreases for most of adult life.9 Myopia
generally develops during the early to middle childhood
years, but significant myopia can also develop in the late
teenage years or early adulthood.11 Axial length is the
most variable factor during development, with the
strongest correlation with refractive status, with longer
eyes more likely to be myopic than shorter eyes.12 Control
of the axial elongation of the eye during development is
thus crucial for achieving normal vision, and therefore is
a primary site for prevention."
"Increased accommodation due to intensive near
work, such as reading and writing, could mediate the
association of myopia with schooling, but epidemiological
support for this idea is not strong. Although Saw and
colleagues31 showed that Singaporean children who read
more than two books per week were more likely to have
higher myopia than those who read less, the Sydney
Myopia Study showed that near work per se was a weak
factor, but that children who read continuously or at a close
distance were more likely to be myopic.32 Results from the
US Orinda Longitudinal Study of Myopia33 showed weak
albeit signifi cant eff ects of increased hours of near work,
and the authors of this study argued that the evidence did
not support a signifi cant effect of near work.27"
"A consistent finding is that children with myopic
parents have a higher prevalence of myopia,33,52,53 but the
relative risk varies substantially, and is lower in locations in which the prevalence of myopia is high, such as in east
Asia. No consistent relation with number of myopic
parents exists. At this stage, the impact of parental
myopia might be evidence of genetic effects. Differences
in family behaviour associated with myopic parents seem
less likely, but cannot be excluded at this time."
"The development of peripheral hyperopia now seems
to be a consequence, rather than a cause of myopia,
because it seems to appear in parallel with the
development of myopia, rather than before.90 Peripheral
hyperopia nevertheless could contribute to myopic
progression, and this perspective does not preclude the
use of localised manipulation of defocus to control
myopia, whatever the developmental mechanisms."
"Preliminary reports have also suggested overnight
orthokeratology contact lenses, which correct refractive
errors by physically flattening the cornea, might also
protect against myopic progression,96 and it has been
proposed that this might due to peripheral myopic
defocus imposed by the distorted cornea. However, the
eff ects might not be permanent and there might be
a rebound of progression rates of the cessation of
orthokeratology treatment."
Link to abstract:
http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(12)60272-4/abstract
Wednesday, May 9, 2012
Moderately Elevated Fluorescent Light Levels Slow Form Deprivation and Minus Lens-Induced Myopia Development in Tree Shrews
Moderately Elevated Fluorescent Light Levels Slow Form Deprivation and Minus Lens-Induced Myopia Development in Tree Shrews
John
T. Siegwart, Jr., Alexander H. Ward1, Thomas T. Norton. Vision Sciences, Genetics and Genomic Sciences,Univ of Alabama at Birmingham, Birmingham, AL.
This abstract is from the ARVO (The Association for Research in Vision and Ophtalmology) website. This study was presented at the May 2012 meeting. Many articles on myopia were presented at that meeting. This study looked at the effect of elevated artificial light levels on young tree shrews with induced myopia and deprivation myopia, and found that the tree shrews exposed to elevated light levels (around 16,000 lux, brighter than a SAD lamp for almost 8 hours) developed induced and deprivation myopia more slowly. Myopia in humans is different, since it generally is not caused by deprivation or induced in a lab, but the studies on time spent outdoors and ocular sun exposure also show that exposure to sunlight is related to lower levels of myopia. This study is interesting because artificial fluorescent light was shown to have an effect too, in animals with induced and deprivation myopia.
From the text:
"Children who spend more time outdoors have a lower prevalence of myopia (Rose
et al., Ophthalmol. 2008) and slower myopic progression (Parssinen and Lyyra,
IOVS, 1993). We examined whether elevated light levels (ELL), produced with
fluorescent bulbs, slow the development of form deprivation myopia (FDM) and
minus lens-induced myopia (LIM) in tree shrews (small diurnal mammals closely
related to primates).
Juvenile tree shrews wore a monocular diffuser to
produce FDM (n=4) or a −5D lens to produce LIM (n=5) starting at 24 days of
visual experience. During treatment, the animals were exposed to ~16,000 lux
for ~7.75 hours per day (~9:15 AM - 5 PM) produced with an array of compact
fluorescent bulbs. The refractive changes in the FDM and LIM animals were
compared to animals from previous studies (FDM, n=6; LIM, n=6) that were
treated in standard light levels of 500 - 1000 lux.
After 11 days of treatment, ELL reduced FDM
(treated-control eye) by 44% (−3.6 ± 0.1 D vs. −6.4 ± 0.7 D) and LIM by 39%
(−2.9 ± 0.4 D vs. −4.8 ± 0.3 D) (figure). Control-eye refractions remained
hyperopic compared with standard lighting control eyes. When ELL stopped and
form deprivation continued, the myopia progression rate increased to parallel
that of standard light-treated animals but did not “rebound”, suggesting a
permanent saving from the reduced rate during ELL. Two LIM animals continued
lens wear and ELL and fully compensated for the −5 D lens after 16 and 23 days.
Moderately elevated light levels, comparable to
those in the shade on a sunny day, slow the development of FDM and LIM in tree
shrews. These findings are consistent with reports that ELL reduces FDM in
chicks and macaque monkeys (Ashby et al., IOVS 2009; Smith et al., ARVO
E-Abstract 3922, 2011) and LIM in chicks (Ashby & Schaeffel, IOVS 2010).
ELL, from fluorescent bulbs that emit minimal UV radiation, may become a useful
tool to slow the progression of environmentally-induced myopia in children. The minimal UV in the ELL is consistent with a previous finding that vitamin D supplementation did not reduce FDM or LIM in tree shrews (Siegwart et al., ARVO E-Abstract 6298, 2011), and suggests that the effect is not due to a light-induced increase in vitamin D levels."
Link:
An Objective Biomarker Of Ocular Sun Exposure Is Inversely Correlated With Myopia In Young Adults: The Raine Eye Health Study
An Objective Biomarker Of Ocular Sun Exposure Is Inversely Correlated With Myopia In Young Adults: The Raine Eye Health Study
Charlotte M. McKnight, Seyhan Yazar, Justin Sherwin, Hannah Forward, Alex Tan, Terri L. Young, Christopher J. Hammond, Craig Pennell, Minas T. Coroneo, David A. Mackey.
Centre for Ophthalmology & Visual Science, University of Western Australia, Lions Eye Institute, Crawley, Australia; Department of Public Health and Primary Care, University of Cambridge, Cambridge, United Kingdom; Ophthalmology, Duke University Eye Center, Durham, NC; Ophthalmology, King's College London, London, United Kingdom; School of Women's and Infants' Health, University of Western Australia, Subiaco, Australia; Ophthal-Prince of Wales Hosp, Univ of New South Wales, Randwick, Australia; Lions Eye Institute, Perth, Australia.
This abstract is from the ARVO (The Association for Research in Vision and Ophtalmology) website. This cross-sectional study was presented at the May 2012 meeting. Many articles on myopia were presented at that meeting. This one measured ocular sun exposure objectively using
"Several studies have demonstrated an inverse association between outdoor activity
and myopia. Many of these studies have been limited by subjective measurement
of outdoor activity, such as parental or participant recall via questionnaire.
We used conjunctival ultraviolet autofluorescence photography, an objective,
quantitative and reliable method of assessing ocular sun exposure, to
investigate the relationship between outdoor activity, ultraviolet light
exposure and myopia.
This was a cross-sectional study of 1231 young
adults aged 19 to 22 years in the Raine cohort, Western Australia. Ultraviolet
fluorescence images of the interpalpebral conjunctiva (right and left eye,
nasal and temporal regions) were taken using a specially designed camera
system.
Prevalence of myopia decreased with increasing
quartiles of conjunctival autofluorescence, with 32.8% in the lowest quartile,
28.0% in the second quartile, 17.2% in the third quartile and 15.6% in the
highest quartile. Participants in the lowest quartile had 2.6 times the odds of
myopia than those in the highest quartile (95% confidence interval 1.7 to 3.8,
p<0.001). The inverse association between myopia and conjunctival
autofluorescence remained significant after adjustment for age, gender, time
outdoors, educational activity and parental history of myopia (OR 2.5, 95%
confidence interval 1.5 to 4.2, p<0.001).
These findings have implications for our understanding of myopia pathogenesis,
of particular importance given the increasing prevalence of myopia worldwide.
As causality cannot be inferred given the cross-sectional design of this study,
prospective studies looking at conjunctival autofluorescence and development of
myopia are required."
LInk:
Wednesday, May 2, 2012
The Impact of Severity of Parental Myopia on Myopia in Chinese Children.
The Impact of Severity of Parental Myopia on Myopia in Chinese Children.
Source
State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat Sen University, Guangzhou, Guangdong, PR China (FX, MH), and Research School of Biology, College of Medicine, Biology and the Environment, Australian National University, Canberra, Australian Capital Territory, Australia (FX, IGM).
Optom Vis Sci. 2012
This study looked at the prevalence of myopia among 12-15 year olds in relationship to the level of myopia in their parents, in China. The children had one parent who was not myopic and one who either did not have myopia, or had mild, moderate or high myopia. The researchers found that,60-80% of the children who had a parent with mild, moderate of high myopia, also had myopia (the greater percentage was among the children whose parents had high myopia). Interestingly though, 50% of myopic children had two non-myopic parents, and of the children with high myopia, 45.3% also had two non-myopic parents. That seems fairly high for not having a genetic predisposition.
From the abstract:
"CONCLUSIONS.: More severe myopia in one parent results in an increased risk of myopia in the children. However, most highly myopic children did not have a highly myopic parent and also half did not have any reported parental myopia. This suggests that while genetic factors contribute to the development of more severe myopia, environmental factors also contribute to high myopia in children in Guangzhou."
Link:
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