Studies regarding muscles and accommodation

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=7223834&dopt=Abstract

Am J Optom Physiol Opt. 1980 Dec;57(12):902-14. Related
Articles, Links

Mechanical considerations in myopia: relative effects of
accommodation, convergence, intraocular pressure, and the extraocular
muscles.

Greene PR.

Axial myopia is a common ocular problem characterized by the fact
that the posterior sclera has apparently stretched out of shape over a
period of several years. The debate persists as to whether myopia is
an inherited or acquired disorder. During the last few years, several
new laboratory techniques have surfaced which can create large amounts
of myopia in normal experimental animals. In an attempt to find a
rational mechanical explanation common to both human and laboratory
myopia, this report examines the stresses experienced by the posterior
sclera as a result of accommodation, convergence, vitreous pressure,
and the extraocular muscles. The conclusion is that convergence and,
more generally, the tension in the extraocular muscles are
mechanically more important than accommodation because of the sizable
increase in vitreous pressure. The oblique muscles, because of their
attachment sites at the back of the globe near the optic nerve
entrance port, have the capability of producing local stress
concentrations which may be very important in understanding
pathological myopia. Suggestions are offered for future experiments in
terms of muscle surgery, the use of prisms, diagnostic techniques, and
animal models.

PMID: 7223834 [PubMed - indexed for MEDLINE]


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=6423942&dopt=Abstract


Med Hypotheses. 1984 Jan;13(1):115-8. Related Articles, Links

Model of visual focussing involving extraocular muscles and the
causes of myopia and glaucoma.

Doonan B.

The classical model of visual focus has increased curvature of the
lens for accommodation and flattening of the lens for far vision, with
both determined by ciliary muscle tension. This new model proposes
that change in the focal length is a second modulator of acuity. This
model also proposes that the extraocular muscles are the more
important neuromuscular entities for both lens and focal length
changes. At accommodation there would be relaxation of the recti
muscles and contraction of the superior oblique muscle. This increases
focal length while allowing the ciliary muscle ring to contract,
releasing tension on the lens capsule. Far object focus is due to
contraction of the recti muscles. These pull on the sclera, stretching
the zonule fibers taut to give flattening tension of the lens capsule.
The myopic eye, in addition to a highly curved lens, also has longer
focal length and is misaligned in the orbit. Ageing glaucoma is
hypothesized as due to: 1. excessive curvature of an ageing lens; 2.
chronic contraction of the recti in compensatory effort to flatten the
lens and shorten the focal length; 3. the overcontracted recti pull to
excess, stretching the inner tissue and distorting the Schlemm canal
and its access mesh area to inhibit efflux of aqueous fluid.

PMID: 6423942 [PubMed - indexed for MEDLINE]


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8736261&dopt=Abstract


Vision Res. 1996 Apr;36(7):1023-36. Related Articles, Links

Effects on the compensatory responses to positive and negative
lenses of intermittent lens wear and ciliary nerve section in chicks.

Schmid KL, Wildsoet CF.

Centre for Eye Research, Queensland University of Technology and
Vision, Brisbane, Australia.

This study examined the ocular compensation to lens-induced
defocus in chick and the effect of interrupting lens wear on a daily
basis. Eyes fitted with +10 D lenses at hatching compensated rapidly,
with almost complete compensation after 4 days of lens wear; they had
decreased vitreous chamber depth compared to normal eyes and were thus
hyperopic when the lenses were removed. In contrast, adaptation to the
-10 D lenses was much slower, was still incomplete after 9 days of
lens wear, and in this case, eyes had increased vitreous chamber depth
and were myopic without the lenses. Adaptation improved when lens wear
was delayed until 7 days after hatching. The effect of interrupting
lens wear by periods of normal vision varied with the sign of the
lenses worn. Hyperopia was always seen in response to +10 D lenses,
although the magnitude of the response decreased as the duration of
lens wear was decreased. In contrast, even brief periods of normal
vision, i.e., 3 hr, prevented the development of myopia in response to
the -10 D lenses; this apparent sensitivity to normal vision is
similar to that reported for form-deprivation myopia. Ciliary nerve
section used here to eliminate accommodation did not alter these
response patterns.

PMID: 8736261 [PubMed - indexed for MEDLINE]



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More research needs to be done...
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Kory

 

You will notice this article is 23 years old. Following this line of
thought, it was hypothesized that children with excessive convergence
(esophoria at near) would be more likely to develop myopia due to the
accommodative stress of near work. Several well designed, (randomized,
control group, blind investigatior) studies were done using glasses to
eliminate the convergence and accommodative stress at near for children with
esophoria. These studies did not find significant differences in the rate
of myopia development or progression in treated vs control group, so this
hypothesis is mostly defunct.

So? A hypothesis without any supporting evidence.

This experiment and many like it were done to study the normal
emmetropization mechanism; they are not studies about the development or
treatment of myopia, but studies about how the infant eye directs its growth
to reduce/eliminate any refractive error present at birth. A successful
mechanism to reduce myopia must delay growth of axial length or induce
flatter lens development whereas a successful mechanism to reduce hyperopia
must speed up growth of axial length or induce steeper lens development.
This means that the system somehow recognizes the difference between myopic
and hyperopic retinal defocus.

What is interesting is the difference in reaction to brief periods of normal
vision. The +10 lens, which simulates a chick born with 10 diopters of
myopia, always induces quick and full reduction with no diminishing of
effect with brief periods of normal vision. This makes good biological
evolutionary sense; a chick born with myopia will have blurred distance
vision and clear near vision so the successful mechanism, once it recognizes
myopic defocus, should not be deterred by the normally occurring periods of
clear near vision. The system should work fast, as a chick normally grows
quickly and with an eye that is already relatively too long at birth, the
mechanism must rapidly slow growth.

The -10 lens, which simulates a chick born with 10 diopters of hyperopia
showed a slower reduction and loss of effect with brief periods of normal
vision. This also makes good biological evolutionary sense; a chick born
with hyperopia will have a relatively too short eye: as the chick is growing
rapidly, the eye may naturally grow to the right length without interference
by the mechanism, so it is natural that the mechanism to speed growth is
slower and more cautious. A chick born with hyperopia will have blurred
distance vision and blurred near vision so the successful mechanism, once it
recognizes hyperopic defocus, should have the ability to react to abnormally
occurring periods of clear vision. These periods of clear vision serve as a
signal to the mechanism that hyperopia is not the cause of the periods of
blurred vision and thus stop the eye growth alterations.

I should emphasize that these studies are about the way a normally
developing eye reacts to refractive error present at birth. They do not, in
any way, support the idea that refractive error is caused by or increased by
the use of corrective lenses on eyes that have refractive error past the age
of infancy.

Dr Judy

 

Unfortunately, neither myopia or imperfect sight in general is defunct.

They are soaring every day more.