Visual field progression may be analyzed by either ‘event-’ or ‘trend-’based methods

Event analysis: is change from baseline greater than a predefined threshold; the threshold is based on test retest variability (according to level of damage).Trend analysis: determines the rate of change over time; the significance is determined by the variability of the measurement and the magnitude of change.

10. Both event and trend analyses are needed, largely for different time points in the follow-up during clinical care.
11. In general, event-based methods are used early in the follow-up, when few VFs are available for serial analysis.
    Comment: progression by an event criterion usually requires confirmation on at least two further occasions to be sufficiently sure that progression has truly occurred.
    Comment: confirmation of progression should usually be made on a separate occasion (patients have ‘off days’).
    Comment: When interpreting VF progression that is confirmed by an ‘event’ method, the clinician should look at:
    – the baseline fields, to ensure they are reliable and appropriate for the analysis
    – the estimated rate of progression and the confidence of the estimate;
    – the severity of the visual loss in terms of impending impairment;
    – the risk factors for progression.
12. In general, rate-based analyses are used later in the follow-up, when a greater number of VFs is available over a sufficient period of time to measure the rate of progression.
    Comment: a rate of progression in the first two years is a rough estimate (wide range of possible rates around the central estimate); in most patients it takes longer to obtain a reliable estimate of the rate of progression.
    Comment: trend (regression) analysis provides an estimate of the rate of progression and a measure of the reliability of the estimate; the reliability of the estimate is judged from the confidence limit.
    Comment: clinicians should consider other clinical measures of progression and risk of progression when interpreting this information (these data provide the ‘prior probability’ for progression).
13. When progression is identified, the clinician should ensure that the progression is consistent with glaucoma and not related to some other cause.

Measure the rate of visual field progression

14. Clinicians should aim to measure the rate of VF progression.
    Comment: Estimating the rate of progression is invaluable for guiding therapeutic decisions and estimating the likelihood of visual impairment during the patient’s lifetime.
15. In the absence of significant changes in therapy, the rate of progression of suitable global indices (MD or VFI, but not PSD or LV) is linear in treated glaucoma eyes, except at the most advanced stages.
16. As a linear model for progression is acceptable, trends may be extrapolated to predict future loss if there is no change in therapy, over appropriate intervals.
17. Both local and global metrics are needed for assessment of progression.
    Comment: Rates are most often measured on ‘global’ parameters, such as mean deviation, mean defect or visual field index. However, focal progression (such as paracentral) may be missed by a global index.
18. Total Deviation based methods are more sensitive to cataract than Pattern Deviation based methods. However, by eliminating or reducing the component of diffuse visual field loss, Pattern Deviation based methods may underestimate progression rates.
19. Use available software support.
    Comment: Subjective judgment of VF print-outs is unreliable and agreement among clinicians is poor. Statistical analysis, either in the perimeter software or stand-alone software, is advantageous to reliably identify and measure progressive VF change.

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