PerspectiveFrom Clinical Examination of the Optic Disc to Clinical Assessment of the Optic Nerve Head: A Paradigm Change
Section snippets
Anatomic Assumptions Underlying the Clinically Visible Optic Disc Margin
The optic disc margin is a clinical landmark that traditionally is defined to be the inner edge of the scleral lip or crescent (Figure 1).15 Within this conceptual framework, the disc margin is assumed to be a single and consistent anatomic structure around the entire ONH and a true outer border of the neuroretinal rim, and therefore the landmark from which the width of the rim can be measured. Current examination methods require identification of the disc margin, whether the examination is
Anatomic Errors in the Current Evaluation of the Neuroretinal Rim
Recent histologic findings in monkey eyes9 and SD OCT findings in human eyes10 (in each case, colocalized to optic disc stereophotographs with the disc margin traced by a glaucomatologist) have revealed 2 new findings that challenge the anatomic assumptions that underlie optic disc margin-based neuroretinal rim evaluation. First, the clinical disc margin rarely is a single anatomic entity, nor are the structures that underlie it consistent in an individual eye. Hence, the structure
Anatomic Rationale for Bruch's Membrane Opening as the Outer Border of the Neuroretinal Rim
The termination of bruch's membrane at the ONH represents the opening through which retinal ganglion cell axons exit the eye to form the choroidal and scleral portions of the neural canal. As such, this anatomic opening, termed Bruch's membrane opening (BMO), is a true outer border of the neural tissues because axons cannot pass through an intact Bruch's membrane to exit the eye. Whether BMO is clinically visible, it is an anatomically accurate landmark from which neuroretinal rim measurements
Geometric Errors in the Current Evaluation of the Neuroretinal Rim
Neuroretinal rim measurement with clinical, photographic, or confocal scanning laser tomographic techniques is made along the 2-dimensional plane of the perceived optic disc margin. However, in a single eye, the orientation of rim tissue varies around the ONH. At one extreme, axons may exit the eye almost parallel to the visual axis, whereas at the other extreme, they may exit the eye almost perpendicular to it, typically in the temporal sector, which can have a shallow sloping rim.20 Hence,
Geometric Rationale for Formulating a Minimum Rim Width Measurement
Because of the varying orientation of the neuroretinal rim relative to BMO, Chen and Povazay and associates and first proposed that the minimum distance from BMO to the internal limiting membrane represents the most geometrically accurate measurement of neuroretinal rim width.11, 12 We subsequently characterized the difference between this rim measurement, termed BMO-minimum rim width (BMO-MRW), and conventional ones13 and its usefulness in the detection of progressive ONH change in
Clinical Examples
The optic disc photograph and 24 colocalized SD OCT radial B-scans around the ONH of each of 30 glaucoma patients and 10 control subjects illustrating (1) that the clinical disc margin is not a single anatomic location, (2) that clinically invisible extensions of Bruch's membrane internal to the disc margin are regionally present in most eyes, and (3) that the dependence of rim width measurement on rim tissue orientation are available online (//ophthalmology.medicine.dal.ca/research/onh.html
Anatomic Variation in Fovea Position Relative to the Optic Nerve Head
In clinical fundus images, the fovea is located below the level of the center of the ONH in most individuals. A recent study on the angle between the fovea and BMO center relative to the horizontal axis defined by the fundus image, termed the fovea-BMO center axis, in 222 patients with ocular hypertension or glaucoma showed that although the mean angle of this axis was −7 degrees (the fovea being 7 degrees below), the range was from −17 degrees to +6 degrees, or 23 degrees (Demirel S, written
Rationale for Regionalization of the Neuroretinal Rim and Peripapillary and Macular Nerve Fiber Layer Relative to the Fovea-Bruch's Membrane Opening Center Axis
Currently, image acquisition and data analysis algorithms report regional data according to the temporal, superior, nasal, and inferior sector positions that are established relative to the fixed horizontal and vertical axes of the image. Hence, for example, the neuroretinal rim width or peripapillary RNFL thickness in a given sector is assumed to refer to precisely the same anatomic location among different persons. However, because the fovea-BMO center axis can vary by as much as 23 degrees,
Rationale for Image Acquisition Relative to the Fovea-Bruch's Membrane Opening Center Axis
Because it is important to regionalize neuroretinal rim width and peripapillary and macular RNFL thickness according to the fovea-BMO center axis, it is reasonable to extend the same logic also to data acquisition. Currently, data acquisition by imaging devices occurs invariably according to a fixed coordinate system for sectors preset in the device.
It is logical, therefore, that the fovea-BMO center axis be determined first in each eye, followed by data acquisition relative to this axis, to
Proposed Paradigm Change for Clinical Assessment of the Optic Nerve Head
Below, we propose a 4-point paradigm change that incorporates the new anatomic insights provided by SD OCT imaging of the ONH into the clinical examination of the optic disc for clinical assessment of the ONH (Figure 6).
1. Optical Coherence Tomography Imaging of the Optic Nerve Head Should not Mimic the Clinical Examination of the Optic Disc.
Because of the fundamental differences between the clinical examination of the optic disc and SD OCT imaging of the ONH, we believe that constraining SD
Summary
From recent findings on ONH anatomy detected with SD OCT, we have argued that the foundation of current clinical optic disc margin-based evaluation of the neuroretinal rim is inaccurate because it lacks a solid anatomic and geometric foundation. We propose a paradigm change from the current clinical examination of the optic disc to the clinical assessment of the ONH that includes SD OCT imaging (Figure 6). The immediate consequence of the new quantitative measures proposed is that our ability
Balwantray C. Chauhan, PhD, is the Mathers Professor of Ophthalmology and Visual Sciences at Dalhousie University. His research centres on alterations in the visual field and optic nerve head in clinical glaucoma, and on the effects of intraocular pressure and endothelin on the neuronal and glial components of the optic nerve in experimental glaucoma. He is Principal Investigator of the Canadian Glaucoma Study, a multi-centre investigation on the risk factors for the progression of open-angle
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Balwantray C. Chauhan, PhD, is the Mathers Professor of Ophthalmology and Visual Sciences at Dalhousie University. His research centres on alterations in the visual field and optic nerve head in clinical glaucoma, and on the effects of intraocular pressure and endothelin on the neuronal and glial components of the optic nerve in experimental glaucoma. He is Principal Investigator of the Canadian Glaucoma Study, a multi-centre investigation on the risk factors for the progression of open-angle glaucoma.
Claude F. Burgoyne, MD, is the Van Buskirk Chair for Ophthalmic Research and Director of the Optic Nerve Head Research Laboratory at Devers Eye Institute in Portland, Oregon. His laboratory studies the effects of aging and experimental glaucoma on the neural and connective tissues of the monkey optic nerve head within post-mortem 3D histomorphometric reconstructions. It is now translating its 3D visualization and quantification capabilities from the monkey to the human optic nerve head with spectral domain optical coherence tomography.