Using an anatomically accurate OCT-derived neuroretinal rim parameter
An estimated 79.6 million people will have glaucoma by 2020,1 and the disease already accounts for over 10 million visits to physicians each year.2 Primary open-angle glaucoma is a slowly progressive disease characterized by degeneration of retinal ganglion cells with loss of the retinal nerve fibre layer (RNFL) and thinning of the neuroretinal rim of the optic nerve head. However, new information indicates that current rim measurements lack a solid anatomical foundation and may not accurately reflect rim width.
Current optic disc margin-based neuroretinal rim estimates assume that the clinically visible disc margin is the true anatomic border of the rim tissue from which width, area or volume measurements can be made. Professor Claude Burgoyne, myself, and colleagues3,4,5 recently co-localized stereo optic disc photographs to spectral-domain optical coherence tomography (SD-OCT) and found that:
1) the clinically identified disc margin is rarely a single anatomic entity or an identifiable anatomic junction;
3) rim width measurements made in the plane of the perceived disc margin are inaccurate compared to those that take into account the orientation of the overlying rim tissue.
The scientists developed a new, objective method for measuring the rim that uses the Bruch's Membrane opening (BMO), a logical anatomical outer border of the rim. With the Spectralis SD-OCT (Heidelberg Engineering GmbH, Heidelberg, Germany), a measurement is made from the BMO to the nearest point on the internal limiting membrane (ILM) quantifying the cross section of the nerve fibres exiting the eye also referred to as BMO-based minimum rim width (BMO-MRW).
Other studies by the same group found that BMO-MRW measurements deviated regionally and significantly from conventional rim margin measurement.4 The BMO-MRW measurements are more accurate because they are based on an identifiable border of the rim and take into account its varying trajectory relative to the point of measurement. The investigators also demonstrated that use of the BMO-MRW measurement translates into significantly enhanced diagnostic precision compared to the currently used cSLO or SD OCT-based optic nerve head and RNFL thickness parameters. At 95% specificity, the sensitivity of RNFLT measurements ranged from 31% to 59%, compared to the BMO-MRW measurement, whose sensitivity ranged from 54% to 79%, a significant improvement.
Heidelberg Engineering is currently working with the investigators to develop software for the Spectralis that will employ this new neuroretinal rim analysis.
1. H.A. Quigley and A.T. Broman, Br. J. Ophthalmol., 2006;90(3):262–267.
2. Center for Disease Control and Prevention/National Center for Health Statistics, 2010 & 1995. Viewed April 20, 2013 at http://www.cdc.gov/nchs/data/hus/hus10.pdf
3. B.C. Chauhan et al., Ophthalmology, 2013;120:535–543.
4. A.S. Reis et al., Invest. Ophthalmol. Vis. Sci., 2012;53:1852–1860.
5. A.S. Reis et al., Ophthalmology, 2012;119:738–747.