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Editors Selection IGR 9-4

Basic Research: Distribution and SD-OCT

Comment by David Greenfield on:

26314 Analysis of peripapillary retinal nerve fiber distribution in normal young adults, Hong SW; Ahn MD; Kang SH et al., Investigative Ophthalmology and Visual Science, 2010; 51: 3515-3523

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In the present study, Hong et al. (1054) characterize the pattern and distribution of the retinal nerve fiber layer (RNFL) using spectral domain OCT (SDOCT, Carl Zeiss Meditec) among a population of young, mostly male, Korean persons with refractive errors that ranged from -10 to +1.5 diopters (mean -2.5 D). The relationship between three surrogates of globe diameter (axial length, spherical equivalent refractive error, and distance between the center of the optic disc and macula) and peak angle of the RNFL profile were examined.

Important observations were made that have relevance to the clinical interpretation of RNFL thickness measurements. The angle between the peak superior and inferior RNFL arcuate bundles was significantly correlated with each of three measures of globe diameter such that larger eyes were associated with a narrowing of the inter-RNFL peak angle, and smaller eyes were associated with increased distance between the angles of the RNFL peak profiles. Stated differently, eyes with axial myopia and increased distance between the disc and foveola had a significant temporal shift in RNFL distribution that produced nasally located abnormalities on the RNFL pattern deviation map. Conversely, hyperopic eyes with reduced axial length had increased inter-RNFL peak angles that were associated with a nasal shift in RNFL thickness distribution and temporally located abnormalities on the RNFL pattern deviation map.

Determinants of RNFL thickness using time-domain OCT (TDOCT) imaging have been reported and include patient-related factors (age, ethnicity, optic disc area, axial length, and refractive error) and imaging-related factors (signal strength, centration of peripapillary measurement circle, ocular movement, and segmentation of the inner and outer borders of the RNFL). Studies have demonstrated that for every one-unit reduction in SS, the average RNFL thickness is reduced by approximately two microns. The alignment and centration of the peripapillary scanning circle around the optic nerve head will also impact the assessment of RNFL thickness such that sectors imaged closer to the disc margin will be falsely thicker; sectors imaged further from the disc will be falsely thinner. Other factors such as ocular surface disruption, ocular movement, and senile cataract may also contribute to artifact.

In contrast to TDOCT, SDOCT provides RNFL maps that enable clinicians to examine not only a cross-sectional slice of the parapapillary RNFL, but also generate a two-dimensional RNFL distribution map where focal atrophy along the arcuate bundles may be quantified. The present study emphasizes the importance of considering refractive error and axial length when assessing these maps. For example, although myopic eyes have been reported to have diffuse RNFL thinning, the temporal shift of the RNFL arcuate peaks measured using SDOCT in such eyes has not been previously reported and has direct implications on judgment of localized glaucomatous RNFL atrophy as assessed using the pattern deviation maps. Thus, in order to avoid false identification of glaucomatous structural damage among eyes with refractive errors (myopia and hyperopia), it is critical to correct for even low levels of ametropia.

Hong and colleagues have made an important contribution by highlighting the relationship between axial length and refractive error on the pattern and distribution of RNFL thickness. Caution is warranted when interpreting RNFL deviation maps in eyes with moderate hyperopia and myopia.

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