Optical Coherence Tomographic Optic Nerve Head Morphology in Myopia III: The Exposed Neural Canal Region in Healthy Eyes—Implications for High Myopia

Abstract

PurposeTo determine the prevalence and magnitude of OCT exposed neural canal (ENC), externally oblique choroidal border tissue (EOCBT), and exposed scleral flange (ESF) regions in 362 non-highly myopic (spherical equivalent -6.00 to 5.75 diopters) eyes of 362 healthy subjects. DesignCross-sectional study MethodsAfter OCT optic nerve head (ONH) imaging, Bruch's membrane opening (BMO), the anterior scleral canal opening (ASCO), and the scleral flange opening (SFO) were manually segmented. BMO, ASCO and SFO points were projected to BMO reference plane. The direction and magnitude of BMO/ASCO offset as well as the magnitude of ENC, EOCBT and ESF was calculated within 30° sectors relative to the Foveal-BMO axis. Hi-ESF eyes demonstrated an ESF ≥ 100 µms in at least one sector. Sectoral perineural canal choroidal thickness (pNC-CT) was measured and correlations between the magnitude of sectoral ESF and proportional pNC-CT were assessed. ResultsSeventy-three Hi-ESF (20.2%) and 289 Non-Hi-ESF eyes (79.8%) were identified. BMO/ASCO offset, as well as ENC, EOCBT and ESF prevalence and magnitude were greatest inferior temporally where pNC-CT was thinnest. Among Hi-ESF eyes, the magnitude of each ENC region correlated with BMO/ASCO offset magnitude and the sectors with the longest ESF correlated with the sectors with proportionally thinnest pNC-CT. ConclusionsONH BMO/ASCO offset, either as a cause or result of ONH neural canal remodeling, corresponds with the sectoral location of maximum ESF and minimum pNC-CT in non-highly myopic eyes. Longitudinal studies to characterize the development and clinical implications of ENC Hi-ESF regions in non-highly myopic and highly myopic eyes are indicated.