title: Does optic nerve head size variation affect circumpapillary retinal nerve fiber layer thickness measurement by optical coherence tomography?
abstract: OBJECTIVE
  To determine the relationship between retinal nerve fiber layer (RNFL) thickness, optic disc size, and image magnification.
  
  
  METHODS
  The cohort consisted of 196 normal eyes of 101 participants in the Advanced Imaging for Glaucoma Study (AIGS), a multicenter, prospective, longitudinal study to develop advanced imaging technologies for glaucoma diagnosis. Scanning laser tomography was used to measure disc size. Optical coherence tomography (OCT) was used to perform circumpapillary RNFL thickness measurements using the standard fixed 3.46-mm nominal scan diameter. A theoretical model of magnification effects was developed to relate RNFL thickness (overall average) with axial length and magnification.
  
  
  RESULTS
  Multivariate regression showed no significant correlation between RNFL thickness and optic disc area (95% confidence interval [CI] = -0.9 to 4.1 μm/mm², P = 0.21). Linear regression showed that RNFL thickness depended significantly on axial length (slope = -3.1 μm/mm, 95% CI = -4.9 to -1.3, P = 0.001) and age (slope = -0.3 μm/y, 95% CI = -0.5 to -0.2, P = 0.0002). The slope values agreed closely with the values predicted by the magnification model.
  
  
  CONCLUSIONS
  There is no significant association between RNFL thickness and optic disc area. Previous publications that showed such an association may have been biased by the effect of axial length on fundus image magnification and, therefore, both measured RNFL thickness and apparent disc area. The true diameter of the circumpapillary OCT scan is larger for a longer eye (more myopic eye), leading to a thinner RNFL measurement. Adjustment of measured RNFL thickness by axial length, in addition to age, may lead to a tighter normative range and improve the detection of RNFL thinning due to glaucoma.
authors: Casey Eye Institute, Oregon Health & Science University, 3375 S.W. Terwilliger Boulevard, Portland, OR 97239-4197, USA. davidhuang@alum.mit.edu
authors: David
authors: D
authors: Huang
authors: Vikas
authors: V
authors: Chopra
authors: Ake Tzu-Hui
authors: AT
authors: Lu
authors: Ou
authors: O
authors: Tan
authors: Brian
authors: B
authors: Francis
authors: Rohit
authors: R
authors: Varma
mesh_terms: D000328:Adult; D000368:Aged; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D009887:Ophthalmoscopy; D009898:Optic Disk / Q000033:anatomy & histology*; D009929:Organ Size; D012165:Retinal Ganglion Cells / Q000166:cytology*; D041623:Tomography, Optical Coherence