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M.C. Bujak, D.B. Yan; Correlation Between Cup–to–disc Ratio Asymmetry and Optic Disc Size in Glaucoma Suspects and Open Angle Glaucoma . Invest. Ophthalmol. Vis. Sci. 2004;45(13):5543.
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Cup to disc asymmetry is considered a characteristic risk factor in the diagnosis of primary open angle glaucoma. However, cup to disc asymmetry (C/Dasym) can occur in the normal physiologic state if the optic canal of one eye is signficantly larger than the fellow eye, but the neuroretinal rim area is equal. Such eyes can be misdiagnosed with glaucoma if differences in optic disc size are not accounted for in the presence of a unilateral enlargement of optic cup. Purpose: To determine the relationship between C/Dasym and the optic disc size for eyes of a given pair in glaucoma suspects and primary open angle glaucoma. Methods: Digital images of the optic nerve head were obtained as part of a standard workup for glaucoma in 116 subjects using a Nikon Coolpix 880 camera attached to a Haag–Streit 900 slit lamp via a Leitz–Wetzlar eyepiece (#519749). A macular laser contact lens with 1.0X magnification factor was used to eliminate magnification variability in the optical system, which could produce artifactual differences in optic disc size measurements. With the observer (MB) blinded to pairing, the vertical disc size was measured using Adobe Photoshop, and the absolute, relative difference in disc size for eyes of a given pair was calculated. Subjects were grouped according to clinically observed vertical C/D ratio (78D slit lamp exam done by DY) into two groups: 1) C/Dasym≥0.15 (n=53) and 2) C/Dasym≤0.1 (n=63). Results: The average, absolute difference in optic disc size was greater (p=0.0025, unpaired, 2–sided t–test) in subjects with C/Dasym≥0.15 (7.4±1.2%) than in subjects with C/Dasym≤0.1 (0.7±1.1%). In the C/Dasym≥0.15 group, 30% of subjects had an absolute difference in disc size >10%; in C/Dasym≤0.1 group, only 14% of subjects had an absolute difference in disc size >10%. This difference was also statistically significant (p=0.001, chi square test). In the C/Dasym≥0.15 group, subjects with absolute difference in disc size >10% had an average mean defect of –2.28dB on Humphrey visual field testing; when the disc size difference was <10%, the mean defect was –3.54dB. This suggests that subjects with disc size asymmetry may be less likely to have glaucoma than subjects without disc size asymmetry. Conclusions: Disc size asymmetry is commonly present in eyes with asymmetric cup size, and should be measured before cup–to–disc asymmetry can be used as an independent risk factor in the diagnosis of POAG.
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