Study to look at large differences in refractive error in identical twins

Discussion in 'Optometry Archives' started by andrewedwardjudd, Mar 21, 2005.

  1. Studies amongst true identical twins with large population numbers (250
    twins) show that many of the twins have large refractive error

    Studying differences in identical twins is therefore a fantastic
    opportunity to look at why these twins may have differences that are:

    1. Created by certain specific environmental/behavioural conditions or

    2. Are random or chaotic and have no known statistically correlating
    single item or items.

    To my knowledge no such study has ever been done before.

    Meanwhile a well known existing Twin myopia study has said "we are
    already planning to see whether the MZ discordant twins have had a
    different childhood environment in some way"

    andrewedwardjudd, Mar 21, 2005
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  2. andrewedwardjudd

    otisbrown Guest

    Dear Andrew,
    Let us suppose that one twin had a refractive status of +1/4 and +1/2
    diopter, while
    the other twin had a refractive status of -1/2 and -3/4 diopter.

    Question: Is that a "large" difference in "refractive error" or a
    small difference?


    otisbrown, Mar 21, 2005
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  3. Your maximum value of differentness is 1.25D

    Generally less than 3 is low myopia
    3 to 6 is moderate
    6 and over is high

    The study results were as follows after combining astigmatism to
    simplify analysis.

    Exact match for every twin would produce a straight line for the MZ and
    DZ groups.

    I am not a maths person so i printed the graphs off and figured it out
    with a ruler.

    You can see that for normal sighted (twin2)twins there are two minus 6
    (twin1) twins and many minus 3 twins.

    For several -5 region (twin2) twins there are several +1 region twins

    So the biggest differences are moderate to high and quite clearly
    commonly present in MZ twins

    andrewedwardjudd, Mar 21, 2005
  4. andrewedwardjudd

    Dr Judy Guest

    When I look at the graph, I see classic straight line data. No biological
    measurement has exact measures, but it is obvious from the graph that the MZ
    twins have much more similar refractive error than the DZ twins.

    Thanks for the link to the actual study, here is the conclusion; about 90%
    of the refractive error can be explained by inherited factors, the rest
    attributed to environment:

    "RESULTS. For the continuous spectrum of myopia/hyperopia, a model
    specifying additive genetic and unique environmental factors showed the best
    fit to the data, yielding a heritability of 84% to 86% (95% confidence
    interval [CI], 81%-89%). If myopia and hyperopia ( -0.5 D and 0.5 D,
    respectively) were treated as binary traits, the heritability was 90% (95%
    CI, 81%-95%) for myopia and 89% (95% CI, 81%-94%) for hyperopia. For total
    and corneal astigmatism, modeling showed dominant genetic effects are
    important; dominant genetic effects accounted for 47% to 49% of the variance
    of total astigmatism (95% CI, 37%-55%) and 42% to 61% of corneal astigmatism
    variance (95% CI, 8%-71%), with additive genetic factors accounting for 1%
    to 4% and 4% to 18%, respectively (95% CIs, 0%-13% and 0%-60%,

    CONCLUSIONS. Genetic effects are of major importance in myopia/hyperopia;
    astigmatism appears to be dominantly inherited."
    Dr Judy, Mar 22, 2005
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