Glaucoma mystery: Focus on deep optic nerve structures

January 9, 2020

Optical coherence tomography (OCT) of the rim and peripapillary retina can aid in the diagnosis of glaucoma and its progression. 

Optical coherence tomography (OCT) of the rim and peripapillary retina can aid in the diagnosis of glaucoma and its progression. 

This article was reviewed by Claude F. Burgoyne, MD

Current optical coherence tomography (OCT) devices provide imaging of the rim and peripapillary retinal nerve fiber layer (RNFL) to aid the diagnosis of glaucoma. Future generations of the technology should be able to visualize the deeper structures of the optic nerve head (ONH) and reveal existing damage that currently is masked, with the goals of earlier diagnosis and intervention. 

The need for the expanded capabilities of OCT is apparent upon a review of the literature.  “Clinicians are highly variable in their assessments of the disc margin and rim width,” said Claude F. Burgoyne, MD, the Van Buskirk Chair for Ophthalmic Research, and director, Optic Nerve Head Research Laboratory, Devers Eye Institute, Portland, OR.

Related: OCT finds niche in glaucoma diagnosis, management 

In a study published by Dr. Burgoyne and colleagues (Am J Ophthalmol 2018;192:65-76), five glaucoma specialists evaluated 214 photographs, and the authors reported the wide variability of the disc margin and rim width.

In a follow-up study (Am J Ophthalmol 2019;199:28-43), the authors used a Spectralis device (Heidelberg Engineering) to evaluate a subset comprised of 151 of the 214 photographs in which OCT images were performed on the same day and evaluated how well and consistently suspicious rim tissue could be detected by the OCT instrument.

Another study performed by Jean-Claude Mwanza, MD, and colleagues (Am J Ophthalmol 2017;184:183-4) used a Cirrus instrument (Carl Zeiss Meditec) to evaluate the cup-to-disc ratios obtained by the instrument and clinicians.

“All three studies found great disparities and inconsistencies in the estimates of the rim widths,” Dr. Burgoyne said, underscoring the need for better detection.

Related: OCT providing physicians with improved view of ocular surface 
Different machines: different capabilities

The Spectralis gathers information about the fovea and the ONH and uses the axis between the foveal centroid and the Bruch’s membrane opening (BMO) centroid to acquire and parameterize regionalized data on all subsequent images of a particular eye, he explained.

The current scan patterns of the Spectralis include a combination of radial B-scans and peripapillary circular scans, a posterior pole grid scan, and high-resolution vertical and macular scans.

“The strength of acquiring radial B-scans allows each to be repeated up to 100 times, which improves the signal:noise ratio and image quality,” Dr. Burgoyne said.

This is in contrast to acquiring a grid, which the Spectralis can do, and as most other instruments do, which provide higher density but the scans cannot be repeated as many times and so the demonstrate lower signal to noise ratios.

Related: Budding technology could be an OCT analysis game-changer 

Acquisition and parameterization relative to  the foveal-to-BMO (FoBMO) axis facilitates reduction of the cyclotorsional effects.

The Spectralis uses BMO as the measurement for estimating minimum rim width, defined as the minimal distance to the internal limiting membrane in all B-scans. The measurements are done within 24 radial B-scans and the data are displayed either in six Garway-Heath sectors, 12 clock-hour sectors or in a TSNIT graph. The Spectralis also shows the actual anatomy. 

“We believe this is important for clinicians to improve their clinical examinations by viewing the anatomy in that way,” Dr. Burgoyne said.

The progression analysis of the minimum rim width and the peripapillary RNFL using the Spectralis is plotted over time relative to the effects of aging in the cross-sectional database.

Angiography is currently available in this instrument, but does not yet include data analysis.

Related: Angiography boosts OCT in glaucoma 

The Cirrus acquires a grid of A-scans of the ONH within which BMO is segmented and used as the orientation for peripapillary retinal circle scans that are extracted from the grid, Dr. Burgoyne explained.

Dr. Burgoyne showed that the minimal rim measurement, when evaluated three-dimensionally, can provide an estimate of the minimal rim area.

This instrument shows the data obtained for the optic disc parameters, cup-to-disc ratio, and the peripapillary RNFL thickness for individual eyes in tables, Garway-Heath sectors, and graphs; the peripapillary RNFL thicknesses also can be shown in deviation plots. The instrument also provides glaucoma progression analysis for the ONH and peripapillary RNFL. Angiography of the ONH is available and FDA approval of the indices is pending.

Related: Managing glaucoma with OCTA

The Avanti (Optovue) also acquires a grid scan, and rim measurements are made relative to the Bruch’s membrane opening. The Avanti reports the ONH parameters in a manner similar to the other machines, but it also provides the angiography data (AngioAnalytics), which is the only instrument to date that has received FDA approval for both structure and angiography.

The Swept Source (Topcon) also acquires grids, the sizes of which are based on operator preference. Disc detection points are determined based on the edge points of the retinal pigment epithelium, rather than using the Bruch’s membrane opening, to define the disc and establish a base plane, and a circle scan is extracted from the A-scan grid. 

However, he explained, the rim width and cup volume are measured relative to or within a reference plane above the base plane, which is similar to how measurements were done in confocal scanning laser tomography. The Swept Source includes a normative database, which is pending FDA approval.

Related: Reshaping medical treatment of glaucoma management 

Current status
Dr. Burgoyne pointed out that the findings in the current literature regarding glaucoma detection are “very encouraging.”

“We are now achieving greater than 95% specificity and detecting glaucoma (defined by the presence of glaucomatous visual field loss) with from 80% to 95% sensitivity, depending on the severity of the visual field loss,” he said. “Moderate to advanced disease, i.e., that which exceeds -4 to -6 decibels is detected with greater than 95% sensitivity.” 

This is important because it may allow OCT structural data to be used to structurally define glaucoma at an early to moderate level of visual field loss.

“Both the minimal rim width and peripapillary RNFL thickness are being used and their performance is similar,” he explained. “However, using them together fosters the eventual integration of macular data. OCT angiography may be helpful, but is still early in development.”

Related: OCTA: Potential early-warning system for glaucoma 

Conclusion
​Dr. Burgoyne predicted that ONH anatomy will be integrated with the nerve fiber layer and  the macula to increase detection of early glaucoma.

“This will help clinicians improve their examination,” he concluded. “Deep imaging targets will emerge on the instruments in the near future. Deep learning is being applied to improve the performance of the current algorithms that will open a new phase of development.”

Read more glaucoma content here 

Claude F. Burgoyne, MD
E: cfburgoyne@deverseye.org
Dr. Burgoyne receives unrestricted research support from and is an unpaid consultant to Heidelberg Engineering. He is also NIH 401 funded to study the issues presented in this talk. Dr. Burgoyne disclosed that the information about each instrument was provided by the specific manufacturer and should be confirmed by each.