Perspective
From Clinical Examination of the Optic Disc to Clinical Assessment of the Optic Nerve Head: A Paradigm Change

https://doi.org/10.1016/j.ajo.2013.04.016Get rights and content

Purpose

To review and interpret the anatomy of the optic nerve head (ONH) detected with spectral-domain optical coherence tomography (SD OCT) pertaining to the clinical examination of the optic disc and to propose that a paradigm change for clinical assessment of the ONH is necessary.

Design

Perspective.

Methods

Presently, the clinician evaluates neuroretinal rim health according to the appearance of the optic disc, the clinically visible surface of the ONH. Recent anatomic findings with SD OCT have challenged the basis and accuracy of current rim evaluation. We demonstrate why incorporation of SD OCT imaging of the ONH into the clinical examination of the disc is required.

Results

Disc margin-based rim evaluation lacks a solid anatomic basis and results in variably inaccurate measurements for 2 reasons. First, the clinically visible disc margin is an unreliable outer border of rim tissue because of clinically and photographically invisible extensions of Bruch's membrane. Second, rim tissue orientation is not considered in width measurements. We propose alternative anatomically and geometrically accurate SD OCT-based approaches for rim assessment that have enhanced detection of glaucoma. We also argue for new data acquisition and analysis strategies with SD OCT that account for the large interindividual variability in the angle between the fovea and ONH.

Conclusions

We propose a 4-point paradigm change for clinical assessment of the ONH that is anchored to the eye-specific anatomy and geometry of the ONH and fovea. Our approach is designed to enhance the accuracy and consistency of rim width, as well as of peripapillary and macular intraretinal thickness measurements.

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

References (28)

  • E.J. Lee et al.

    Visualization of the lamina cribrosa using enhanced depth imaging spectral-domain optical coherence tomography

    Am J Ophthalmol

    (2011)
  • N.G. Strouthidis et al.

    Comparison of clinical and spectral domain optical coherence tomography optic disc margin anatomy

    Invest Ophthalmol Vis Sci

    (2009)
  • T.C. Chen

    Spectral domain optical coherence tomography in glaucoma: qualitative and quantitative analysis of the optic nerve head and retinal nerve fiber layer (an AOS thesis)

    Trans Am Ophthalmol Soc

    (2009)
  • B. Povazay et al.

    Minimum distance mapping using three-dimensional optical coherence tomography for glaucoma diagnosis

    J Biomed Opt

    (2007)
  • Cited by (226)

    • Measures of disease activity in glaucoma

      2022, Biosensors and Bioelectronics
    View all citing articles on Scopus

    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.

    View full text