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Optic disc size important in accurate glaucoma diagnosis, study finds
DENVER – Optic nerve evaluations for glaucoma must include optic disc size evaluations, according to a speaker here at the American Academy of Optometry meeting.
Gregory Nixon, OD, FAAO, also suggested that small optic nerves receive a thorough glaucoma risk assessment to avoid underdetection.
Noting that early detection and intervention for glaucoma is key, Nixon and colleagues conducted a study to determine practitioners’ ability to identify the disease and the influence of optic disc size.
"In this study, we looked to determine the effect of varying optic disc sizes along with the accuracy of identification in determining our subjects," Nixon said.
Researchers obtained optic nerve head images from the practice of study investigators and recruited 261 attendees at an optometric meeting to assess the images.
The participants viewed glaucomatous and normal optic nerve heads that were small, medium and large and answered three related questions, reporting whether or not the optic nerve followed the ISNT rule (the thickest part of a healthy neural rim is located in the inferior quadrant, followed by the superior, nasal and temporal quadrants), if the optic nerve was glaucomatous or if they would typically order optical coherence tomography imaging for further evaluation.
"None of the subject characteristics that we tracked, whether a subject was a student or a practitioner or had years of experience, had a statistical significance on the accuracy of their detection of glaucomatous or non-glaucomatous nerves," Dixon said.
He also showed that the ISNT rule was not an accurate indicator of whether a nerve was glaucomatous or not.
"Less than 9% of the subjects correctly identified the [small optic nerve] as being glaucomatous," Dixon added.
He concluded that optic disc size is important in accurately diagnosing glaucoma.
"Whether done subjectively or with the aid of an objective imaging device, all optic nerve assessments for glaucoma should include optic disc size evaluation," Dixon said. "To avoid the risk of underdetection, a thorough evaluation is warranted to determine glaucomatous risk when small optic nerves are noted." – by Chelsea Frajerman
Disclosure: Nixon has past or present consultant/affiliations with the Allergan academic advisory board, Alcon glaucoma advisory board, Bausch + Lomb advisory board, and the Alcon, Vistakon and VSP speaker’s bureaus.
Perspective
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Glaucoma is characterized by damage to the optic nerve. Optic nerves vary in size and cupping among individuals, making the determination of what is normal vs. abnormal challenging, even for experienced clinicians. In this study, participants were asked to view a series of optic nerve photos and describe the size of the nerve and then determine if the ISNT rule was followed, if the nerve appeared glaucomatous and if an OCT should be ordered.
I think in most clinician’s minds, a glaucomatous optic nerve is one that shows enlarged cupping, particularly in the vertical meridian. However, I am not sure that clinicians always assess the overall size of the optic disc.
As this study demonstrates, large optic discs tend to have large cupping, and that is overestimated as being glaucomatous. Conversely, small optic nerves, which tend to have small cupping, are underestimated. A 0.7 cup triggers an immediate suspicion of glaucoma and initiates further testing. However, 0.3 cupping is felt to be more “normal” and therefore triggers a less frequent response for further testing.
It is crucial for the clinician to incorporate the optic disc size into his or her clinical algorithm to ensure that they are not overlooking a potential glaucoma patient who has small cupping, but a corresponding small disc as well.