title: Nonlinear behavior of certain optic nerve head parameters and their determinants in normal subjects.
abstract: OBJECTIVE
  To investigate the relationship between the absolute area of certain optic disc parameters and a number of ocular dimensional characteristics and demographic factors in young, healthy subjects.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  One hundred twenty-one subjects, aged 16.5 to 35.4 years participated, encompassing a wide range of refractive error from +4 diopters (D) to -25.75 D. One eye from each subject was randomly selected for examination.
  
  
  METHODS
  Absolute size of the optic disc parameters: optic disc area (DA), neuroretinal rim area (NRA), optic cup area (CA), and peripapillary atrophy area (PA).
  
  
  METHODS
  Absolute sizes of optic disc parameters were determined from digitized photographic color transparencies, taking into consideration the magnification of the fundus camera and human eye. Relationships between the absolute size of the optic disc features and axial length, crystalline lens thickness, anterior chamber depth, ocular refraction, front surface keratometry, age, and gender were examined.
  
  
  RESULTS
  Axial length, lens thickness, front surface keratometry, and age demonstrated positive associations with all optic disc parameters, and anterior chamber depth and ocular refraction demonstrated negative associations in all cases. Multiple regression analysis revealed that the associations observed univariately for age, anterior chamber depth, lens thickness, front surface keratometry, and ocular refraction were confounded by axial length for all disc parameters. After taking logs, DA and NRA exhibit an exponential relationship with axial length, whereas PA and CA exhibit a linear relationship with axial length. DA and NRA increase by approximately 3% per millimeter increase in axial length at 20 mm and up to 40% for DA and 50% for NRA at an axial length of 35 mm. PA and CA are estimated to increase by 26% (10%-44%) and 10% (5%-16%), respectively, per millimeter increase in axial length. Females have approximately 5% smaller DA and 7% smaller NRA than males, but these effects are not statistically significant (P > 0.2). From the multiple regression analysis, females have 31% larger optic cup areas (1%-57%, P = 0.03) and 24% smaller PA (42% smaller to 5% larger P = 0.06) than males, but these effects are of borderline statistical significance at 5%.
  
  
  CONCLUSIONS
  Of the ocular biometric factors considered, axial length seems to be the most important predictor of the absolute area of the optic disc parameters. No association exists between any disc parameter and age in this sample of subjects less than 40 years of age. Females exhibited smaller values for DA, NRA, and PA than did males, differences that were not statistically significant, and larger CA (P = 0.03).
authors.affiliation: Wolfson Institute of Preventive Medicine, St. Bartholomew's and The Royal London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, England, UK.
authors.forename: A R
authors.initials: AR
authors.lastname: Rudnicka
authors.forename: C
authors.initials: C
authors.lastname: Frost
authors.forename: C G
authors.initials: CG
authors.lastname: Owen
authors.forename: D F
authors.initials: DF
authors.lastname: Edgar
mesh_terms: D000293:Adolescent; D000328:Adult; D001699:Biometry / Q000379:methods; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D017711:Nonlinear Dynamics; D009898:Optic Disk / Q000033:anatomy & histology*; D010781:Photography; D011897:Random Allocation; D012016:Reference Values; D012737:Sex Factors