The influence of axial length on retinal nerve fibre layer thickness and optic-disc size measurements by spectral-domain OCT

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 degrees 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. Discussion 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.