Studies regarding muscles and accommodation

Discussion in 'Optometry Archives' started by Kory Postma, Aug 1, 2003.

  1. Kory Postma

    Kory Postma Guest

    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

    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]

    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]

    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]

    More research needs to be done...

    Kory Postma, Aug 1, 2003
  2. Kory Postma

    Dr Judy Guest

    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
    Dr Judy, Aug 2, 2003
  3. Kory Postma

    Rishigg Guest

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

    They are soaring every day more.
    Rishigg, Aug 2, 2003
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