A new view on optic disc margin

Halifax, Nova Scotia-Researchers comparing stereo photographs and spectral-domain optical coherence tomography (SD-OCT) in glaucoma patients found the device could detect structures invisible to ophthalmoscopy, calling into question clinical estimates of the optic disc margin.

Clinicians appear to have long overestimated the size of the optic disc and, thus, underestimated the cup-to-disc ratio, a key sign of glaucoma's severity, according to the study, published online in the journal Ophthalmology. (Ophthalmology. 2012;.119:738-747)

Structures not always visible

The study involved 30 glaucoma patients and 10 age-matched controls. The glaucoma patients were further classified according to three types of disc damage: focal, diffuse, and sclerotic. The primary objectives of the study were to characterize border tissue orientation; identify the deep optic nerve head structures detected with SD-OCT that co-localize with the clinically visible optic disc margin; and determine the frequency of SD-OCT-detected Bruch's membrane extending beyond the clinically visible disc margin.

Study bottom line

"The bottom line is that what we observe as the scleral ring or the margin of the disc might be overestimated," said Leo P. Semes, OD, FAAO, professor at the University of Alabama, Birmingham, School of Optometry, who intends to add the study to his glaucoma course. "And then consequently, we're overestimating the rim tissue."

As if that were not enough cause for concern, another clinical implication emerges from the study. "I think it also might help us explain in some cases why the visual field looks worse than the disc appears," Dr. Semes said. "They're telling us what you see isn't always what you get."

Key anatomic assessments

The authors reported that they believe this is the first published study using SD-OCT optic nerve head data matched to clinical photographs to identify structures underlying the optic disc margin in glaucoma patients.

The study authors noted that the oblique nature of the optic disc border tissue was important. Internally oblique border tissue extends to the anterior scleral opening internally to fuse with Bruch's membrane, in effect hiding the underlying sclera. Externally oblique border tissue extends externally to the anterior scleral opening to fuse with Bruch's membrane, making a portion of the border, and sometimes the underlying sclera, clinically visible. As regions of internally oblique tissue transition to externally oblique tissue, areas of vertical or non-oblique border tissue may develop.

The most common configuration was internally oblique, most often seen in the superior and nasal quadrants, and frequently accompanied by Bruch's membrane overhang.