|
 Back to Myopia Index
Previous Page Next Page
3.2 Accommodation and Near Work
In principle, results and experiences about accommodation (that is, changes in lens shape for near
work) are related to artificially negative-lens-induced myopia, and myopia which was induced by
forcing the eye to permanent near focus (see section 3.3).
3.2.1 Experiences and Results
3.2.1.1 General Experiences and Results
In general the influence of near work alone is not easy to isolate, as extensive near work means extensive
in-door work (often at low levels of illumination, see section 3.7.2), which is mostly connected
with a potential lack of vitamin D because of a lack of exposure to sunlight (see section
3.16.3). Moreover, children who are interested very much in reading often have a more introverted
personality (see section 3.13.2) and are less interested in physical activities (see section
4.2.3.8 on the impact on the blood sugar level).
There are numerous indications that people, who are doing extensive near work, i.e., who accommodate
extensively, are more often myopic. Some observations are:
The appearance of myopia at people in USA and Greenland coincides with the introduction
of universal schooling,112 and the rate of myopia of school children in Berlin depended very strongly on the level of the school they visited: the more advanced the type of
school, the higher the rate of myopia. Rates were between 55 and 35%113.
The progression of myopia of school kids is slower during summer holidays114, 115, 116.
Comparisons between myopia rates in 1882 and 1964 showed that the rate is very different
for individual professions68 (students in the 30% range, unskilled workers in the 2 to 3%
range): While the total rate of myopia was increasing, the myopia rate for all the individual
professions except unskilled work was decreasing. Explanation: The overall increase in
myopia, according to these results, can be largely attributed to a changed ratio of job distributions
from unskilled work towards professional and office work.
Highly elevated rates of myopia were found for craftspeople of various professions, who
have to do extensive near work, e.g. for typesetters (in year 1930117), tailoring (in the years
1953118 and 1961119). Up to 77% of the persons in these professions were found to be myopic.
A test with monkeys (already in 1961)120, whose visual space was restricted to an average
of fifteen inches distance, showed that all of them developed some myopia.
There are indications that extended accommodation can create a chronic spasm of the
ciliary muscle, and experiments on monkeys showed that this stretching will begin after
spasms for 2 to 4 months so spastic myopia and axial length myopia exist at the same
time121.
Note:
A permanent load can damage any muscle, not only the ocular muscles122, 123.
A study of university students showed a strong relationship between reading and other
near work with myopia was found, however no relationship between myopia and working
on video display terminals (VDT) or watching TV124. Watching TV at close distance however
was promoting myopia125, 126.
Note:
These results appear to be contradictory. As will be shown in section 3.3 the effect of VDTs
and TVs can be expected to depend largely on the quality of the displayed images. Due to the
limited image resolution of TVs and VDTs (LCD screens are better) a watching of these
screens from near distance will create a blur image on the retina, with the potential consequences
as described in section 3.3.
For Chinese schoolchildren a correlation of myopia with the reading of books was
found127, in Finland a relation between myopia progression and time spent on reading/
close work and on reading distance was found128.
It was measured (with partial coherence interferometry) that the eye generally elongates
during accommodation, with this explanations by Drexler et al.26: "
by the accommodation-
induced contraction of the ciliary muscle, which results in forward and inward pulling
of the choroids, thus decreasing the circumference of the sclera, and leads to an elongation
of the axial eye length." and by Shum et al.27: "
the elongation was more pronounced
in emmetropes than in myopes."
Notes:
- Is it possible that the difference is a structural weakness of the myopic eye, i.e. that the
myopic eye shows some kind of "memory effect" or hysteresis after this stretching?
- Elongation means stretching of scleral tissue, and stretching of scleral fibroblasts is
changing significantly the gene expression of these fibroblasts129. This means, accommodation
has a direct impact of the biochemistry of the eye; this builds a link between mechanical
and biochemical models of myopia, and a link between functional and structural
models of myopia as well.
Rosenfield et al. stated130 that "...results demonstrate that myopes are less sensitive to the
presence of blur, and may at least partially explain why previous reports have demonstrated
a larger lag of accommodation...". On the other hand, the compensatory eye
growth in experimental myopia (see section 3.3) was always in the right direction, even in
the presence of very poor images131.
Rosenfield et al. stated132 that "... findings do not support the proposal that the development
of myopia in young adults is accompanied by a reduced accommodative response
during near work."
Harb et al. stated133: "Within subjects, accommodative lags significantly increased with
closer reading distances, however there was no significant relationship between lag and
refractive state. The variability in the accommodative response significantly increased
with closer reading distances ... Increased lags and the variability in accommodation at
higher accommodative demands suggest that an increase in overall blur at closer reading
distances might be related to the development of refractive state."
In similar experiments, it was found that generally at a reading distance of 30 cm no perfectly
focusing accommodation is performed134. The resulting degradation of the image
quality can contribute to the development of myopia.
Myopic children have less tonic accommodation (i.e. dark focus accommodation) than
normal, i.e. emmetropic children135, 136, 137. After near work, however, the tonic accommodation
of myopes increases more than for emmetropes. A large shift in tonic accommodation
after near work was found to be typical for periods of acquisition and progression of
myopia (some kind of accommodative hysteresis)138.
Very contrary to these results is the statement by Kushner139: "Overcorrecting minus lens
therapy for intermittent exotropia [see section 3.4] does not appear to cause myopia."
Note:
A potential explanation is that the risk is not so much originating from the excess accommodation
caused by the negative lens, but by the increased esophoria caused by the negative
lenses (esophoria means that the axes of the two eyes are adjusted too much inwards
when accommodating). Esophoria is frequently associated with myopia (see section 3.4.1
about phoria).
Near work, i.e. accommodation was shown to increase the temperature of the anterior
segment of the eyeball, and to cause a hyperproduction of intraocular fluid140.
Note:
This hyperproduction of intraocular fluid can increase the intraocular pressure; and an elevated
ocular pressure was often found to correlate with myopia (see section 3.6.2). About the
impact of the temperature on myopia see section 3.10).
Stress in connection with near work was made responsible for the development of myopia:
It was stated that stress induces distant-accommodation, which conflicts with the actual
task near-accommodation. This conflict was claimed to cause either a spasm of the
ciliary muscle, or phoria141 (phoria means the exes of the eye are not adjusted appropriately;
see section 3.4 about phoria and section 3.13 about mental issues).
A helpful support of accommodation by illumination was found: see section 3.7.2.
Obviously near-work induced transient myopia (NITM) is increased by mental stress: see
section 3.13.1.
Summary as worded in one paper by Schaeffel et al.66: "The current theory is that lag of accommodation
during reading shifts the image plane behind the retina and thereby stimulates retinal
neurons to release growth promoting factors that enhance scleral growth. This is thought to be
mechanistically equivalent to the effects of a negative lens in animal experiments."
Note:
The word "growth" appears to be somehow misleading, as at least for higher grades of myopia -
there is no growth of structurally intact tissue (which is normally meant by the word growth), but a
stretching of degraded tissue (see section 3.3.2).
Contact Us
Complete the form below to contact us. If you would like a reply, please include your name and email address.
|
|
