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R. Rosado, D.K. Wallace; Comparison of Different Powers of Condensing Lenses for Estimation of Zone I Border in Retinopathy of Prematurity . Invest. Ophthalmol. Vis. Sci. 2003;44(13):591.
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Purpose: Zone I is defined by the area of a circle centered on the optic nerve with a radius equal to twice the nerve-macula distance. It can be difficult to accurately determine the border between Zone I and Zone II, particularly in the nasal retina where the macula cannot be viewed simultaneously. It has been suggested that the diameter of a 25D or 28D condensing lens can be useful to estimate this border. Using this technique, the optic nerve is viewed at one edge of the lens and the extent of zone I is seen at the opposite edge of the lens. Our purpose was to determine which condensing lens most accurately estimates the extent of zone I when using this technique. Methods: During routine inpatient screening exams, video indirect ophthaloscopy was performed on 4 premature infants using a 20D, 28D, and 30D lens. 12 digital images were then captured from the videotape, representing one with each lens from the right eye of each infant. The following values were determined: (1) optic disc diameter (dd), (2) nerve-macula distance (NMD), or distance from the center of the optic nerve to the center of macula, and (3) lens diameter (LD), or diameter of the total view of the retina within the lens. The "target distance&rsquotdbl; (TD) was calculated using the formula TD = (2 x NMD) + (0.5 x dd) to account for the entire optic nerve (instead of half) being visible in the field of view. Results: The mean target distance and mean lens diameter expressed in optic disc diameters (dd) was 8.8dd and 8.1dd for the 20D lens, 8.3dd and 8.3dd for the 28D lens, and 7.9dd and 9.6dd for the 30D lens. With the 20D lens, the target distance (TD) exceeded the lens diameter (LD) in all 4 patients. With the 28D lens, LD was greater than TD in 2 patients and less than TD in 2 others. With the 30D lens, LD exceeded TD in all 4 patients. The average difference between the target distance and the lens diameter for each infant was 0.8dd for the 20D lens, 0.3dd for the 28D lens, and 1.7dd for the 30D lens. Conclusion: The 28D diopter condensing lens is the most accurate of the three tested in estimating the border between Zone I and Zone II in infants with retinopathy of prematurity. The area of retina viewed by a 20D lens is too small and that with a 30D lens is too large for these lenses to be useful in this technique.
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