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C.A. Rasmussen, J.C. Peterson, B.A. Hennes, B.T. Gabelt, P.L. Kaufman; Nerve Fiber Layer Measurements Using the GDx VCC in Monkeys . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4824.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the feasibility of using the GDx VCC system with monkeys and to evaluate reproducibility. Methods: One normal eye from each of nine cynomolgus monkeys was studied. Monkeys were anesthetized with a combination of intra–muscular ketamine and medetomidine, followed by inhalation isoflurane. Monkeys were maintained at, or near, surgical anesthetic depth for the scanning procedure. IOP (Goldmann applanation tonometer), corneal curvature (Reichert keratometer) and refraction (Hartinger–coincidence) were measured at each session. Slit lamp exams were performed monthly. A 10mm plano contact lens was applied to ensure adequate corneal hydration. Refraction and curvature measurements were repeated after application of the lens and values entered into the patient focal correction field. A motorized head–holder with remote control was used to make fine adjustments in position to facilitate matching alignment to the baseline image. A GDx1 VCC prototype system (Laser Diagnostic Technology, San Diego, CA) was used to acquire nerve fiber layer (NFL) thickness measurements. Corneal compensation was calculated at baseline and verified by 3 or more compensated macular scans at each time point. A new compensation was calculated if retardance or corneal axis values were variable or if image quality was poor. During each session, 3 to 5 separate peripapillary scans were taken and a mean created. The scan head was realigned between scans to generate discrete images. Ellipse size was kept constant by entering the specific horizontal and vertical radius values for each animal in the edit ellipse field. Intervals during which subjects were scanned 3 or 4 times over 22 to 36 days were used to measure variability for the parameters. Results: In eyes that were consistently clear at the slit lamp exams (no lens opacities, edema, KP etc), GDx VCC parameters of average thickness, superior integral, ellipse average, superior average, and inferior maximum had the lowest coefficient of variation: 3.4, 3.5, 3.5, 4.1, 5.2 % respectively. Ratio parameters superior ratio, inferior ratio and superior/nasal ratio had values of 7.1, 10.7, and 10.2% respectively. Modulation parameter values were the most variable, ranging from 11.6 to 15.7%. Other parameters ranged from 5.4 to 7.9%. Conclusions: Collecting multiple, discrete intrasession scans and creating a mean of those values can help compensate for intersession measurement variation inherent to the GDx1 VCC in monkeys.
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