title: The influence of axial length on retinal nerve fibre layer thickness and optic-disc size measurements by spectral-domain OCT.
abstract: BACKGROUND
  To evaluate the influence of axial length on measurements of the retinal nerve fibre layer (RNFL) thickness and optic nerve head (ONH) parameters in healthy subjects.
  
  
  METHODS
  Using Cirrus HD-OCT, RNFL thickness and ONH parameters (disc and rim area) were measured in 15 short (<22.5 mm), 15 medium (22.51-25.5 mm) and 15 long (>25.51 mm) eyes.
  
  
  RESULTS
  The mean axial length was 21.5 ± 0.5 mm in short eyes, 24.1 ± 0.8 mm in medium eyes and 26.6 ± 1.0 mm in long eyes. The RNFL thickness decreased with longer axial lengths in the superior (r=-0.52, r(2)=0.27, p=0.0003), inferior (r=-0.72, r(2)=0.52, p<0.0001), nasal (r=-0.60, r(2)=0.37, p<0.0001) and temporal (r=-0.30, r(2)=0.09, p=0.0485) quadrants, as well as in the 360° mean measurement (r=-0.69, r(2)=0.48, p<0.0001). The optic-disc area (r=-0.74, r(2)=0.54, p<0.0001) and rim area (r=-0.41, r(2)=0.17, p=0.0051) decreased with longer axial lengths. Correcting for axial length-induced ocular magnification by means of the Littmann formula resolved the relationship between axial length and both RNFL thickness and ONH area.
  
  
  CONCLUSIONS
  Axial length influences measurements of RNFL thickness and ONH parameters in healthy subjects. Caution is recommended when comparing the measured values of myopic and hyperopic eyes with the normative database of the instrument.
authors.affiliation: GB Bietti Eye Foundation-IRCCS, Via Livenza 3, Rome 00100, Italy. giacomo.savini@alice.it
authors.forename: Giacomo
authors.initials: G
authors.lastname: Savini
authors.forename: Piero
authors.initials: P
authors.lastname: Barboni
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Parisi
authors.forename: Michele
authors.initials: M
authors.lastname: Carbonelli
mesh_terms: D000328:Adult; D058453:Emmetropia; D006801:Humans; D006956:Hyperopia / Q000473:pathology*; D016014:Linear Models; D008875:Middle Aged; D009216:Myopia / Q000473:pathology*; D009412:Nerve Fibers; D009898:Optic Disk / Q000166:cytology*; D012016:Reference Values; D041623:Tomography, Optical Coherence / Q000379:methods* / Q000592:standards*