**Purpose.**:
To evaluate the magnitude of Visual Field Index (VFI) change attributable to change in the estimation algorithm from the pattern deviation probability plot (PDPP) to the total deviation probability plot (TDPP) when the mean deviation (MD) crosses −20 decibels (dB).

**Methods.**:
In a retrospective study, 37 stable glaucoma eyes in which MD of the VFs crossed −20 dB were identified. For each eye, a pair of VFs was selected so that one VF of the pair had a MD better than but close to −20 dB and the other had a MD worse than but again close to −20 dB. The change in VFI in the VF pairs and its associations with the number of points in probability plots with normal threshold sensitivities were evaluated. Similar pairs of VFs from 28 stable glaucoma eyes where the MD crossed −10 dB were chosen as controls.

**Results.**:
The change in VFI in VF pairs when the MD crossed −20 dB ranged from 3% to 33% (median: 15%), while the change when MD crossed −10 dB ranged from 1% to 8% (median: 4%). Difference in the number of points with normal threshold sensitivities in PDPP when MD was better than −20 dB compared to those in TDPP when MD crossed −20 dB significantly influenced the VFI change (*R* ^{2} = 0.65). Considering the eccentricity of these points further explained the VFI change (*R* ^{2} = 0.81).

**Conclusions.**:
The decrease in VFI when MD crosses −20 dB can be highly variable. This has to be considered with the use of VFI in clinical and research settings.

^{ 1 }In brief, VFI is the aggregate percentage of visual function for a given field at each point where the visual thresholds are estimated. VFI is calculated from the pattern deviation probability plot (PDPP) in eyes with a mean deviation (MD) better than −20 decibels (dB) and from the total deviation probability plot (TDPP) in eyes with a MD worse than −20 dB. The central points have more weight than the peripheral points. The VFI can range from 100% (normal visual field) to 0% (perimetrically blind field).

^{ 1 }The VFI has been shown to be less susceptible than the MD to the effects of cataract or diffuse media opacities.

^{ 1,2 }VFI is intended for use in calculating rates of progression and staging glaucomatous functional damage.

^{ 3–13 }However, little is known about the behavior of VFI when the estimation of VFI changes from PDPP to TDPP as the MD crosses the −20 dB threshold. Bengtsson and Heijl in their original article on VFI mentioned that “shifting from pattern deviation probabilities to total deviation probabilities for identification of depressed points is likely to result in a slight stepwise worsening of VFI near MD values of −20 dB.”

^{ 1 }The purpose of the present study was to evaluate the magnitude of change in VFI that can occur when the MD crosses −20 dB and the factors that accounted for this change.

^{ 8 }The collection of data was approved by the ethics committee of L V Prasad Eye Institute, and written informed consent obtained from all subjects. All methods adhered to the tenets of the Declaration of Helsinki for research involving human subjects. From the database, eyes of patients in whom the MD of the VFs crossed the −20 dB mark were identified. For each eye, a pair of VFs was selected so that one VF of the pair had a MD better than but very close to −20 dB and the other had a MD worse than but again very close to −20 dB. Both VFs of the pair had to be performed with the 24–2 standard strategy of the Swedish interactive threshold algorithm. If multiple VFs were eligible to be included in the pair, then the two VFs that performed closest to each other in time were selected. As a control cohort, we selected similar pairs of VFs from eyes of patients in whom the MD crossed −10 dB. The clinical impression noted in the medical records of all these eyes by the treating physician was that the glaucoma was stable, and the change in VFs was judged as fluctuations. The GPA classification in the control eyes, in addition, was “no progression detected.”

*P*> 0.05) in the PDPP of the VF with MD better than −20 dB and in the TDPP of the VF with MD worse than −20 dB (hereafter called normal points). As the estimation of VFI is also dependent on the eccentricity of the points in the VF, we also separately calculated the number of normal points in each of the five zones of the PDPP and TDPP of the VF pairs (Fig. 1), with zone 1 being the innermost and zone 5 the outermost.

**Figure 1.**

**Figure 1.**

^{ 14 }Statistical analyses were performed using Stata (version 11.2; StataCorp, College Station, TX). A

*P*value of ≤0.05 was considered statistically significant.

*P*< 0.001, Wilcoxon rank-sum test).

**Figure 2.**

**Figure 2.**

**Table 1.**

**Table 1.**

VF with MD Better Than −20 dB | VF with MD Worse Than −20 dB | |

Fixation losses (%) | 0 (0, 6) | 0 (0, 6) |

False-positive responses (%) | 1 (0, 2) | 1 (0, 2) |

False-negative responses (%) | 0 (0, 6) | 0 (0, 0) |

MD (dB) | −19.56 (−19.76, −19.08) | −20.82 (−21.31, −20.32) |

PSD (dB) | 10.80 (9.87, 12.26) | 10.41 (9.08, 11.34) |

VFI (%) | 51 (47, 54) | 35 (33, 39) |

*P*= 0.01). However, change in MD explained only 16% of the variability of the VFI change (coefficient of determination,

*R*

^{2}= 0.16).

*P*= 0.93) with the difference in the number of normal points between PDPP and TDPP of the VF pairs.

**Figure 3.**

**Figure 3.**

**Table 2.**

**Table 2.**

Number of Normal Points in PDPP of VF with MD Better Than −20 dB | Number of Normal Points in TDPP of VF with MD Worse Than −20 dB | Difference in the Number of Normal Points between PDPP of VF with MD Better Than −20 dB and TDPP of VF with MD Worse Than −20 dB | ||||

Median | Range | Median | Range | Median | Range | |

Zone 1 | 2 (2, 3) | 0–4 | 0 (0, 0) | 0–1 | 2 (2, 3) | 0–4 |

Zone 2 | 4 (3, 6) | 0–8 | 0 (0, 0) | 0–2 | 4 (3, 5) | 0–8 |

Zone 3 | 4 (3, 6) | 0–10 | 0 (0, 0) | 0–2 | 4 (3, 6) | 0–10 |

Zone 4 | 6 (4, 7) | 0–8 | 0 (0, 0) | 0–4 | 5 (3, 7) | 0–8 |

Zone 5 | 0 (0, 0) | 0–2 | 0 (0, 0) | 0–0 | 0 (0, 0) | 0–2 |

Entire plot | 16 (13, 19) | 8–26 | 0 (0, 1) | 0–6 | 16 (12, 19) | 7–26 |

*P*> 0.2 for all comparisons). The regression formula derived from Table 3 to account for the change in VFI could be thus written as −6.49 + 2.67* MD change + 1.97* difference in the number of normal points in zone 1 + 1.49* difference in the number of normal points in zone 2 + 0.78* difference in the number of normal points in zone 3 + 0.66* difference in the number of normal points in zone 4.

**Table 3.**

**Table 3.**

Coefficient | 95% Confidence Interval | P Value | |

MD change | 2.67 | 1.65 to 3.68 | <0.001 |

Difference in the number of normal points in zone 1 | 1.97 | 1.25 to 2.69 | <0.001 |

Difference in the number of normal points in zone 2 | 1.49 | 0.94 to 2.04 | <0.001 |

Difference in the number of normal points in zone 3 | 0.78 | 0.30 to 1.26 | 0.002 |

Difference in the number of normal points in zone 4 | 0.66 | 0.14 to 1.19 | 0.02 |

Difference in the number of normal points in zone 5 | 0.45 | −1.97 to 2.87 | 0.71 |

Intercept | −6.49 | −11.31 to −1.66 | 0.01 |

**Figure 4.**

**Figure 4.**

**Figure 5.**

**Figure 5.**

^{ 15–18 }PSD is less affected by media opacities, but has the disadvantage that it falsely improves as the severity of VF loss increases.

^{ 1 }VFI was meant to address some of the limitations of MD and PSD; and since its introduction, VFI has been used extensively to quantify the amount of VF loss in clinical studies.

^{ 3–13 }The results of our study show that a significantly steep step in the VFI scale can occur when MD crosses the −20 dB mark and the VFI estimation strategy changes from PDPP to TDPP.

^{ 19 }the possibility that this change in MD was true progression was difficult to rule out with certainty because of the retrospective nature of the study. Therefore in models investigating the factors responsible for the change in VFI, we also included the MD change as a variable to account for the change in VFI that occurred because of a possible true progression. Though statistically significant, the association between MD change and VFI change in the VF pairs was weak, with the change in MD explaining only 16% of the variability of VFI change. The other possible limitation is that we evaluated only the points that had threshold sensitivities within the normal limits on the probability plots. The reason was that VFI values at these points are considered 100%, so the influence of these points on the VFI estimation is supposed to be significant. Considering the actual threshold sensitivities at the remaining points (points depressed at probability values of less than 5% on the probability plots) would have increased the ability to explain the VFI change. However, we did not evaluate this because it would have been too complicated for use in clinical practice.

^{ 19 }demonstrated that VFI, because of its dependence on PDPP, had a ceiling effect that reduced its sensitivity to change in early glaucoma. This, along with our results, which show a significant variability in VFI scale at a MD of −20 dB, indicates that the utility of VFI may be limited at either end of the spectrum of glaucoma severity.

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