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Bryan Paul Jones, Jonathan Kahn; Retinal Nerve Fiber Layer Thinning on Optical Coherence Tomography in Eyes with Small Disc Sizes. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):4566.
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Cup-to-disc ratio measurement by slit lamp biomicroscopy may be insufficiently sensitive to detect glaucomatous damage in eyes with smaller disc sizes. We conducted a cross-sectional study comparing clinical assessments of cup-to-disc ratio, disc height and disc width with areas of thinning on ocular coherence tomography (OCT) of the retinal nerve fiber layer (RNFL).
We included a random sample of fourty-two eyes in twenty-five patients. One doctor carefully measured the cup-to-disc ratio, disc height and disc width of dilated eyes by slit lamp biomicroscopy before performing OCT RNFL using a Zeiss OCT. Eyes with and without glaucoma or suspicion thereof were included. Eyes were excluded if the OCT examination had a quality rating of less than 7/10. Patients were considered glaucoma suspects if cup-to-disc ratio was greater than 0.5 and/or OCT results demonstrated generalized or focal RNFL thinning. For each cup-to-disc ratio, we compared the number of patients with full RNFL to those with possible glaucomatous thinning.
Of the eyes with a clinical cup-to-disc ratio of 0.4, four of seven (57%) had borderline or overt segmental RNFL thinning on OCT. Two of nine (22%) eyes with a cup-to-disc ratio of 0.5 had similar RNFL thinning on OCT. All thirteen eyes with cup-to-disc ratios of 0.3 or less had a full RNFL on OCT. Half of the 0.4-0.5 C/D eyes with thinning on OCT had disc heights of less than 1.6mm, while all of the 0.4-0.5 C/D eyes without thinning were greater than 1.6mm in disc height. Of the 0.4 to 0.5 C/D eyes, eyes with RNFL thinning on OCT had a median disc height of 1.7mm, versus 1.9mm in eyes without thinning.
Over one third of eyes with a cup-to-disc ratio from 0.4 to 0.5 had focal thinning on OCT RNFL, typically in the setting of a small disc. Since patients with cup-to-disc ratios of 0.5 or less are typically not flagged as glaucoma suspects, standard clinical practice may be missing a substantial number of patients deserving further glaucoma workup. If corroborated by a larger cross-sectional or prospective cohort study, small disc size should be considered in assessing glaucoma risk and could lead to earlier detection and treatment of glaucoma in patients with small discs.
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