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Ralf P. Tornow, Robert Lämmer, Folkert K. Horn, Delia Bendschneider, Christian Y. Mardin; A New Parameter To Describe The Retinal Nerve Fiber Layer (RNFL): The Cross Sectional Area Of Peripapillary RNFL. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3663.
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© ARVO (1962-2015); The Authors (2016-present)
It was shown that the mean peripapillary retinal nerve fibre layer (RNFL) thickness decreases with increasing axial length of the eye. However, the diameter of the circle around the optic nerve head depends on the optics of the eye (e.g. refractive power, axial length). In eyes with long axial length the diameter of the circle is larger compared to eyes with lower axial length. To calculate the total amount of nerve fibres, the different length of the circle between eyes has to be taken into consideration. In other words, it is more reliable to compare the cross sectional area (csa) of the RNFL. RNFL-csa should be independent of the circle diameter as was shown for monkey eyes (Patel NB, IOVS 2010, ARVO E-Abstract 2724). The purpose of this study was to calculate the cross sectional area (mean RNFL thickness * length of the circle circumfence) of the RNFL in normal subjects and to measure how mean RNFL thickness and RNFL-csa depend on axial length.
In 165 normal subjects axial length (IOL-master, Carl Zeiss Meditec, Jena, Germany) and mean perpapillary RNFL thickness (Spectralis HRA+OCT, Heidelberg Engineering, Germany) were measured. Mean RNFL thickness, the diameter d of the peripapillary circular scan, the length of the circle circumfence (d * Pi) and the cross sectional area (mean RNFL thickness * length of the circle circumfence, see figure) of the RNFL bundles were calculated from parameters exported from the Spectralis OCT.
The mean axial length (± SD) was (24.0 ± 1.2) mm, range 21.4 to 28.3 mm. The mean RNFL thickness (± SD) was (96.0 ± 9.1) µm. There is a correlation of mean RNFL thickness with axial length (R = -0.243, p = 0.002). The mean RNFL cross sectional area (± SD) was (1.067 ± 0.102) mm². There is no correlation of the RNFL-csa with the axial length.
The lack of correlation of the parameter RNFL cross sectional area with axial length supports the assumption that the cross sectional area does not change in the proximity of the optic nerve head. However, this assumption has to be confirmed with RNFL measurements of different circle diameters in the same subject.
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