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Abstracts and Posters
"Mechanism
Underlying Eye Elongation in Continuous Light"
[View
Poster in PDF format] (668 kb)
Stephanie
S , Wildsoet CF
School of Optometry, University of California, Berkeley, CA 94720-2020,
USA.
Introduction
High myopia is associated with an increased risk of many sight-threatening
eye diseases due to the excessive eye growth that underlies this condition.
Knowledge of the mechanism underlying this abnormal growth pattern is
important for pharmacological studies that may lead to treatments that
can prevent the development of myopia. Among drugs tested to-date, atropine
has been shown to be effective at inhibiting myopia development (i.e.
excessive eye growth), in both humans and chicks used as an animal model
for myopia. In chicks, atropine treatment of one eye has also been shown
to affect the fellow eye, i.e. there is an apparent yoking of the responses
of the two eyes. However, the mechanisms underlying these.
Discussion
1. Our results showed that the inhibited VC elongation may result from
the repeated injections (once per day for 4 days). There are only two
differences between the injected and fellow eyes for the vehicle-injected
groups: the act of injecting eyes, and the introduction of a small amount
of distilled water into the VC (10L). Since water is likely to have no
significant effect on the growth of VC, other mechanisms
related to the injection is likely to underlie the altered growth of VC.
2. Schmid et al. [2] showed that the atropine inhibited the usual VC elongation
induced with negative lenses and diffusers. Inhibited VC elongation was
also observed in this study, here induced by CL These similar results
suggest that the same mechanism involving muscarinic receptors underlie
all 3 enhanced VC elongation responses.
3. The similarity of VC changes in the fellow and atropine-treated eyes
of the CL groups
suggest that there is a yoking effect. This observation is consistent
with those of
Schmid et al. [1].
Conclusions
1. The exaggerated VC elongation induced by continuous light may involve
the same mechanism as that underlying the myopiainducing effects of negative
lenses and diffusers.
2. Monocular treatment with atropine results in an inhibitory effect on
the VC growth of the fellow eye. To follow up on this yoking effect, further
studies that disconnect the communication between the eyes in the same
animal, e.g. using optic nerve sectioning, will enable us to determine
the mechanism involved.
Photo
of Stephanie Defending her Poster
Photo of Stephanie
and Dr. Wildsoet
"Postural
Variation in Intraocular Pressure and Influence of Refractive Error and
Eye Shape"
Annie Chin, Mary Kwong
Advisor: Christine Wildsoet
School of Optometry, U. Berkeley, CA 94720-2020
Purpose:
The prevalence of myopia has been found to be rising over recent years,
with ethnic Chinese and Japanese communities in Asia being apparently
more susceptible. The purpose of this study is to look for evidence for
a role of intraocular pressure (IOP) in the excessive elongation of the
eye that underlies most myopia. Specifically, we asked whether: 1) there
were refractive error-related differences in postural variations in IOP
that might be linked to the development of myopia, 2) whether postural
variations in IOP show diurnal changes and if so, whether there were associated
refractive error-related differences.
Methods: Twenty-eight Caucasian and 34 Chinese subjects were included
in the study, classified by refractive error as either emmetropes, low-moderate
myopes, and high myopes. Their mean ages were 24.718±3.034 years
for Chinese subjects and 24.286±3.017 years for Caucasian subjects.
IOPs were measured by pneumatonometry in the morning and the evening on
each of two days. Measurements of optical axial length, refractive error,
and central corneal thickness also were obtained.
Results: There were significant differences between IOPs recorded in supine
and upright postures for both AM and PM measurement times (P<0.001),
but there were no statistical significant differences between Chinese
and Caucasian groups, for either the morning (p = 0.6335) or evening (p
= 0.3334) readings (data pooled across refractive error). The mean postural
variation in IOP for this study group was 4.130±2.187 mmHg. However,
there was a significant difference, for the emmetropes, between Causcasian
and Chinese groups, for the morning data (P=0.0051), and high myopes showed
significantly less postural variation in IOP than emmetropes or low myopes
(P=0.009). Postural variations in IOP were shown to be repeatable from
day 1 to day 2.
Conclusions: Changes in posture result in changes to IOP. While there
was no significant difference in the patterns of change exhibited by the
Chinese and Caucasian myopes that could explain the greater susceptibility
of the former group, there was a difference between the equivalent emmetropic
groups that could have predictive significance. Across the refractive
error groups, high myopes displayed the smallest postural change in IOP,
which could reflect altered scleral compliance.
Support:
NEI Summer Training Program (Grant# T35 EY07139; fellowships to AC and
MK) and Vision Service Plan grant to advisor (CFW).
"The
Effect of Colchicine on Ocular Growth and Emmetropization"
[View
Poster in PDF format] (7,987 kb)
Kuo
KW, Ni J, Wildsoet CF
School of Optometry, University of California, Berkeley, CA 94720-2020,
USA.
Active emmetropization describes the process by which young eyes regulate
their growth to attain optimal vision. This study was conducted to determine
whether this regulation is controlled locally within the eye or through
a feedback system involving higher neural centers. Previous studies offer
support for the local regulation theory, having shown that both optic
nerve section and blockade of retinal ganglion cell action potentials
have little effect upon emmetropization. Colchicine, a cytotoxin that
affects microtubule polymerization, has been reported to destroy most
of the retinal ganglion cells when injected into chick eyes at hatching.
We have used this effect of colchicine as an alternative model to test
the local regulation theory. Colchicine-injected eyes were tested for
their ability to respond to various visual manipulations (+10 D, - 10D
lenses, and occluder), imposed using Velcro rings for attachment. We found
that colchicine led to increased vitreous chamber depth and corneal flattening
in both treated and untreated eyes. The lack of significant differences
across treatment groups for measured parameters except refractive error
suggest that colchicine disrupts the process of emmetropization. This
seems especially true for the occluder group, in which colchicine-injected
eyes were little different from their fellow eyes. Nonetheless, for the
positive and negative lens groups, colchicine injected eyes showed appropriate
directional responses for axial length, choroidal thickness, and refractive
error, indicating that although impaired, the mechanisms underlying emmetropization
are still functional. These results suggest ocular growth regulation involves
both local circuits and feedback from higher neural centers.
"Does
Nystagmus Cause Astigmatism? The effect of periodic eye movements on intraocular
pressure and/or corneal astigmatism"
[View
Poster in PDF format] (3,787 kb)
Gloria
Chiu, Wildsoet CF
School of Optometry, University of California, Berkeley, CA 94720-2020,
USA.
Background. Nystagmus refers to involuntary
periodic eye movements and is seen most commonly in people with monochromatism,
albinism, and reduced central vision. These conditions are associated
with high astigmatism. While nystagmus has not been causally linked to
astigmatism, the periodic eye movements are likely to exert a massaging
effect on the cornea by the eyelids. A consequence of this massaging effect
could be a lowering of the pressure in the eyes, thereby contributing
to distortion in the cornea, leading to astigmatism. Purpose.
The purpose of this project was to investigate whether regular periodic
eye movements decrease intraocular pressure (IOP) and/or cause corneal
distortions. Methods. Measurements were taken
from 20 subjects (10 female; 10 male) before and after three tasks: (1)
following a target with one’s eyes for 15 minutes, (2) reading for
60 minutes, and (3) walking outdoors for 60 minutes. Ocular changes were
evaluated with an aberrometer (cylinder, Zernike polynomials, higher order
Rms), corneal topographer (corneal distortions), and non-contact tonometer
(IOP). Results. While statistical analysis revealed
significant changes in IOP and astigmatism when related to gender, ethnicity,
eye, and measurement time (pre and post-task), they did not support a
causal link between periodic eye movements (simulated nystagmus in this
experimental design) and IOP or between periodic eye movements and corneal
astigmatism. In relation to IOP and corneal astigmatism, an increase in
astigmatism in the against-the-rule direction correlated with an increase
in IOP in the right eye (p=0.0398). Conclusions. These findings do not
support our prediction that periodic eye movements may lead to a decrease
in IOP, making the cornea more susceptible to eyelid molding effects,
thereby contributing to corneal astigmatism. It is yet to be determined
why some individuals and ethnic groups are more prone to high astigmatism.
"The Role of the Isthmo-Optic Nucleus in the
Regulation of Eye Growth"
[View
poster in PDF format] (7,070 kb)
Cindy
S. Hwang, Howard H. Ellenberger, Christine F. Wildsoet
School of Optometry, University of California, Berkeley, CA 94720-2020,
USA
Studies involving optic nerve section in young chicks report hyperopia
to the eyes that underwent the surgery (Wildsoet and Wallman, 1995). This
result suggests that there is a global (brain) control mechanism
for eye growth regulation. A recent related study reported that lesions
to the isthmo-optic nucleus (ION) also result in hyperopia, in this case
in the eye ipsilateral to the surgery (Erichesen et al, 2002). The latter
result raises the possibility that the isthmo-optic nucleus which project
to the retina via the isthmo-optic tract (IOT) is important in regulating
eye growth and also offers a possible explanation for the hperopia observed
in ONS eyes. Specifically, projecting fibers from the ION to the retina
will be lesioned in ONS. The purpose of this study was to further investigate
the role of the ION in regulating eye growth. Mechanical lesions were
made to the IOT at the level of the optic tectum of five day old White
Leghorn chicks (Gallus gallus domesticus) and eye growth was subsequently
monitored for 9 days using high frequency A-scan ultrasonography. Some
chicks were left untreated while others underwent sham surgery as controls.
At the end of the monitoring period, chicks were perfused with1X PBS,
followed by 4 % formalin after being injected with Euthasol and the fixed
brains were later sectioned at 25?m and stained with Fluoro-Jade to confirm
the success of the IOT lesioning procedure. This study found no effect
of lesions to the IOT on eye growth in young chicks. Both eyes of the
birds that underwent the lesioning surgery had similar growth patterns
and these patterns were also similar to those of untreated control birds.
The findings of this study do not support a role for the ION in the regulation
of normal eye growth.
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