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TVCS meeting
on 30 March 2000 at Whittington Hospital, Highgate
Myopia is the
state of refraction in which parallel rays of light are brought
to focus in front of the retina of a resting eye. It is measured
by power in diopters of the correcting lens needed to focus light
on the retina.
Following an
experience of unexpected deterioration in his vision, Dr Alan
Rubin explored the epidemiology and hypothesised causes of Myopia,
with a view to finding whether near work activity such as microscopy
influences the prevalence of the condition.
Population studies
show a wide variation in the prevalence of myopia and provide evidence
that there is a link with the length of time spent in education
and myopia. A study looking at heredity and environment in the same
children shows that parental history explains significantly more
variance than near work activity.
However, twin
studies show a higher concordance rate for monozygotic twins with
concordant close-work habits than for monozygotic twins with discordant
close work habits. Putting these studies together suggests that
both genetic factors and close work are involved in the development
of myopia.
A study of Orthodox
school boys in Israel show the prevalence as 81.3% compared to 27.4%
among their sisters or in boys from general schools. A possible
explanation may be that many hours daily are spent reading, often
swaying as they study texts. Arguably, this visual experience of
continual accommodation is a similar visual experience to microscopy.
Experimental work show that 30% monkeys raised in laboratories become
myopic, and chicks made to wear negativ lenses become myopic.
The use of ultrasound
demonstrates that increased length of the eye is the main cause
of myopia. In the newborn, the average length is 18mm. By age 6
it grows by 5mm, and the prevalence of myopia is only 2%. By 15
it grows only an additional 1mm, and the prevalence of myopia is
15%. Parallel rays of light should come to focus on the retina with
accommodation completely relaxed. Increased growth is compensated
for by reduction in the refractive power of the cornea and the lens.
Failure of this compensation results in myopia.
A possible mechanism
is that accommodation may produce an increase in vitreous chamber
pressure. Steady continual accommodation produces an increase in
vitreous chamber depth.
A study of
251 'Clinical Microscopists' showed 71% of subjects were myopic.
- 49% reported
onset or progression after initial entry into clinical microscopy.
- 54% report
sore eyes,
- 37% report
headaches,
- 28% report
blurred distant vision.
The major structural
difference was difference in vitreous chamber depth, with which
myopic changes are associated. Genetic differences, occupational
differences, and differences in the way that the eye responds to
occupational tasks might account for the fact that some microscopists
suffer worsening myopia and others do not.
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