Why what you have been taught about the optic disc may not be entirely true

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Clinicians evaluate neuroretinal rim health according to the appearance of the optic disc, the clinically visible surface of the optic nerve head (ONH). Recent anatomic findings with optical coherence tomography challenge the basis and accuracy of current rim evaluation for 3 reasons: (1) The DM is rarely a single anatomical structure or an identifiable junction, such as the inner edge of border tissue. In most eyes it corresponds variably to multiple anatomical structures. (2) In some regions of all ONHs, Bruch’s membrane extends internally beyond the DM (towards the centre of the ONH) and is both clinically and photographically invisible. Since in these areas the outer border of the rim is the termination of Bruch’s membrane and not the more external DM, the rim is narrower than that with clinical or photographic evaluation. (3) Because current rim width measurements are made in a fixed plane without reference to the orientation of the rim tissue, for the same number of axons, the rim width will be greater in cases where the orientation of the rim tissue is more horizontal (for example in the temporal sector of tilted optic discs) compared to when it is more perpendicular. This presentation will review and interpret ONH anatomy detected with optical coherence tomography pertaining to optic disc examination and demonstrate why a paradigm change for clinical assessment of the optic nerve head is now necessary.

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