title: The effect of optic disc diameter on vertical cup to disc ratio percentiles in a population based cohort: the Blue Mountains Eye Study.
abstract: OBJECTIVE
  The 97.5th percentile for vertical cup to disc ratio (VCDR) has been proposed as a useful tool to assist in the diagnosis of glaucoma in population studies. Previous reports of VCDR percentiles have either not been adjusted for disc size or have been calculated by regression analysis from small hospital based cohorts. The authors' aim was to generate VCDR percentiles in a large, population based sample.
  
  
  METHODS
  Data were collected from 3654 individuals, aged 49 years or older, living in the Blue Mountains, west of Sydney. Vertical disc diameter and VCDR were determined by planimetry from stereo optic disc photographs. The distribution of VCDR and percentiles (95th, 97.5th, 99th) were calculated.
  
  
  RESULTS
  6678 eyes were included in the analysis. Median cup to disc ratio, 95th, 97.5th, and 99th percentile increased with vertical optic disc diameter in a linear fashion. An increase of 0.2 in median VCDR (0.35 to 0.55) was observed between small (1.1-1.3 mm) and large (1.8-2.0 mm) optic discs. An equivalent increase of 0.2 (0.59 to 0.74) was observed for the 97.5th percentile from small to large discs.
  
  
  CONCLUSIONS
  VCDR percentiles for a "normal" population, adjusted for vertical optic disc diameter are presented. One quarter of all discs fell within the small or large disc categories highlighting the importance for estimating optic disc size. These data may assist in the diagnosis of glaucoma in clinical practice as well as providing a normative database. Sole use of VCDR percentile cut offs in defining glaucoma cases in population surveys requires further validation.
authors.affiliation: Centre for Vision Research, Department of Ophthalmology, Westmead Millennium and Save Sight Institutes, University of Sydney, Australia.
authors.forename: J G
authors.initials: JG
authors.lastname: Crowston
authors.forename: C R
authors.initials: CR
authors.lastname: Hopley
authors.forename: P R
authors.initials: PR
authors.lastname: Healey
authors.forename: A
authors.initials: A
authors.lastname: Lee
authors.forename: P
authors.initials: P
authors.lastname: Mitchell
mesh_terms: D005654:Fundus Oculi; D005902:Glaucoma, Open-Angle / Q000175:diagnosis; D006801:Humans; D016014:Linear Models; D008875:Middle Aged; D009898:Optic Disk / Q000033:anatomy & histology*; D012016:Reference Values; D012160:Retina / Q000033:anatomy & histology; D014065:Tonometry, Ocular / Q000379:methods; D014794:Visual Fields