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Practical recommendations for measuring rates of visual field change in glaucoma
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  • Published on:
    Authors' reply

    We thank Dr. Carkeet for his comments on our paper,1 which is now over 15 years old.

    As discussed with Dr. Carkeet in personal correspondence recently, the discrepancy between his results and ours occurred because we simplified the 1 exam/year and 3 exam/years conditions by linearly scaling the outputs from the 2 exam/year condition. We repeated the simulations under the conditions Dr. Carkeet has outlined, and we agree with the result. The simulations yield approximately the same time required to detect the various rates of change for 2 exams per year, and slightly different values for 1 and 3 exams per year. He has pointed out discrepancies in the 1 and 3 exam per year conditions which appear large only in extreme conditions and are not realistic in clinical practice, for example, detecting a -0.25 dB/y change with high variability, where we estimated 30 years and Dr. Carkeet estimated 18 years.

    In the final analysis simulations are only simulations that can be made with conditions assumed to reflect reality. The precision with which these estimates is made can be low. Ultimately, the message in our paper was that it takes a long time to detect a small amount of change if visual field results are variable and the testing frequency is low.

    Our paper has been used to inform guidelines from various organizations and is based on one of the key messages of the paper, i.e., that ruling out fast progression (worse -2 dB/y or worse) requires 6 visual...

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    Conflict of Interest:
    None declared.
  • Published on:
    Calculation errors overemphasise the value of increasing visual field test frequency.
    • Andrew Carkeet, Senior Lecturer QUT School of Optometry and Vision Science

    Chauhan and co-workers [1] have provided Table 1, showing times taken to detect significant field progression with 80% power, based on a number of modelling parameters: frequency of examinations, rate of field progression, intrasession variability of field assessment. They have also provided Table 2 showing the number of annual eye examinations required to detect different total visual field changes, for different time periods, and for moderate variability. I have checked the calculations of Chauhan and co-workers, using Monte Carlo modelling, assuming a one-tailed significance value of 0.025. Of the 36 outcome values in Table 1, 33 are incorrect. Of the 12 outcome values in Table 2, 11 are incorrect.

    Chauhan and co-workers have made 2 main errors in their calculations for Table 1. The first is in applying their estimates of power. The curves shown in Figure 2 (statistical power plotted against number of field examinations) are appropriate for the case of 2 field examinations per year, but Chauhan and co-workers appear to have incorrectly also used them for the cases of 1 examination per year and 3 examinations per year. Separate sets of curves should have been calculated for those conditions. The effect on Table 1 is that the time taken to detect a field change is incorrectly reported as being inversely proportional to the number of examinations per year. This anomalous relationship was commented on by Albert Alm in his 2008 Rapid Response, “Is a field every 4...

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    Conflict of Interest:
    None declared.
  • Published on:
    Authors' Reply
    • Balwantray C Chauhan
    • Other Contributors:
      • David F. Garway-Heath, Francisco J. Goñi, Luca Rossetti, Boel Bengtsson, Ananth C Viswanathan, and Anders Heijl

    Dear Editor

    We thank Dr. Alm for his interest and comment on our paper entitled “Practical recommendations for measuring rates of visual field change in glaucoma.” We agree that the standard error of slope estimates is dependent on the number of examinations and duration of follow-up. However, these two parameters are not interchangeable. As pointed out correctly by Dr. Alm, the same number of examinations ov...

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    Conflict of Interest:
    None declared.
  • Published on:
    Is a field every 4 month a significant improvement over a field every 6 months?

    Dear Editors

    How often should we do visual fields in the first 2 years? Chauhan and co-workers [1] recommend 3 visual fields per year. It will have an 80% power of detecting a rate of loss of 2 dB/year in an eye with moderate variability. Is this a significant improvement over 2 fields per year? In order to answer that we should look at the efficacy of increasing the frequency of field examinations versus prolongin...

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    Conflict of Interest:
    None declared.