title: Behavior of visual field index in advanced glaucoma.
abstract: OBJECTIVE
  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.
authors.affiliation: VST Glaucoma Centre, L V Prasad Eye Institute, Banjara Hills, Hyderabad, India. harsha@lvpei.org
authors.forename: Harsha L
authors.initials: HL
authors.lastname: Rao
authors.forename: Sirisha
authors.initials: S
authors.lastname: Senthil
authors.forename: Nikhil S
authors.initials: NS
authors.lastname: Choudhari
authors.forename: Anil K
authors.initials: AK
authors.lastname: Mandal
authors.forename: Chandra S
authors.initials: CS
authors.lastname: Garudadri
mesh_terms: D000368:Aged; D018450:Disease Progression; D005260:Female; D005901:Glaucoma / Q000175:diagnosis / Q000503:physiopathology*; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D011336:Probability; D012189:Retrospective Studies; D014792:Visual Acuity; D058609:Visual Field Tests; D014794:Visual Fields / Q000502:physiology*