Optic disc margin should be abandoned

Using a clinician-defined disc margin to assess neuroretinal rim should be abandoned in favour of using Bruch’s membrane opening-minimum rim width (BMO-MRW) measurements performed with spectral-domain optical coherence tomography (SD-OCT), said Nick Strouthidis MBBS, MD, PhD, FRCS, FRCOphth, FRANZCO at a Glaucoma Day session at the XXXIII Congress of the ESCRS in Barcelona.

“The disc margin defines the outermost limit of the neuroretinal rim. Accurate, reproducible measurement of its location is essential for the quantification of changes in the optic nerve head in glaucoma. However, the disc margin as defined by the clinician does not correspond to any consistent anatomical landmark and therefore is essentially a meaningless theoretical construct. Therefore, quantifying change based on the clinician’s definition of the disc margin is error-prone,” said Dr Strouthidis, Moorfields Eye Hospital, London, UK.

When using optic disc stereophotographs, clinicians define the disc margin as the inner aspect of Elschnig’s ring, a white reflective ring internal to the termination of the retinal pigment epithelium. However, studies show that different clinicians viewing the same stereophotograph will invariably mark its location differently.

Moreover, studies involving optic disc stereophotographs co-localised with SD-OCT-based delineations of the neuroretinal rim’s dimensions have failed to identify a consistent anatomical basis for the disc margin as defined by clinicians. The outer limit of the disc’s neural tissue as defined by SD-OCT is the innermost edge of Bruch’s membrane, the BMO, which may be invisible by ophthalmoscopy or in stereophotographs. It represents the narrowest aperture through which the neural tissue passes and is easily and reliably imaged using SD-OCT.

The advent of SD-OCT represents a true turning point in the imaging of the optic nerve head, Dr Strouthidis noted. Time-domain OCT had poor neuroretinal rim penetration. SD-OCT is faster and has better penetration and is therefore is much more capable of imaging the finer neuroretinal tissue structures.

He added that to provide a precise and consistent demarcation of the neuroretinal rim, it should be defined as being where the distance is shortest between the BMO and the internal limiting membrane, that is, the BMO-MRW.

The BMO-MRW parameter is potentially better for phenotyping glaucoma suspects than OCT peripapillary retinal nerve fibre layer thickness or Heidelberg retinal tomography (HRT Heidelberg Engineering) Moorfields regression analysis. It is the first optic disc parameter to change in the primate model of glaucoma. BMO-MRW measurements are now included in the optic nerve head parameters of Heidelberg Engineering’s Glaucoma Module Premium Edition (GMPE) for the Spectralis.

Nick Strouthidis: nick.strouthidis@moorfields.nhs.uk

Declaration of Interest: Nick Strouthidis received lecture fees and travel expenses from Heidelberg Engineering