Biomicroscopic Measurement of the Optic Disc with a High-Power Positive Lens

Abstract

PURPOSE. To compare the magnification properties of four different indirect double aspheric fundus examination lenses for clinical disc biometry. METHODS. Experimental study in a model eye. The relationship between the true size of a fundus object and its image was calculated for each fundus lens for an ametropic range between Ϫ12.5 and ϩ12.6 D using a slit lamp biomicroscope with adjustable beam length. RESULTS. Equations for determining the correction factor p (degrees per millimeter) were calculated for each fundus lens. The factor can be used in calculations to determine true optic disc size. The total change in magnification of the system from myopia to hyperopia was Ϫ21.1% to ϩ24.0% (60-D lens; Volk Opticals, Mentor, OH), Ϫ12.9% to ϩ16.2% (Volk super 66 stereo fundus lens), Ϫ13.2% to ϩ13.9% (Volk 78-D lens), and Ϫ13.3% to ϩ14.0% (Volk super-field NC lens). When the fundus lens position was altered im relation to the model eye by Ϯ2 mm under myopic conditions, the change in magnification of the system was Ϫ4.3% to ϩ5.7% (60-D lens), Ϫ4.6% to ϩ6.1% (66 stereo fundus lens), Ϫ4.9% to ϩ6.3% (78-D lens), and Ϫ5.9% to ϩ7.8% (super-field NC lens). In the hyperopic condition the change was Ϫ2.7% to ϩ3.6%, Ϫ3.4% to ϩ4.5%, Ϫ3.6% to ϩ4.8%, and Ϫ4.5% to ϩ6.0%. CONCLUSIONS. The study has shown that the use of a single magnification correction value for each fundus lens may not be appropriate. These findings have important implications for the way in which calculations for determining the true optic disc size and other structures of the posterior pole are performed using indirect biomicroscopy. (Invest Ophthalmol Vis Sci. 2001;42:153-157) I n 1953, El Bayadi 1 first examined the fundus with a planoconvex lens of approximately ϩ60 D using the slit lamp biomicroscope, but the technique was not widely accepted because of aberration and difficulty of use. With the introduction of the double aspheric 60-D lens in 1982 (Volk Opticals, Mentor, OH), the technique started to gain popularity for routine stereoscopic examination of the posterior pole. Since then, many attempts have been made to determine the true size of the optic disc with several types of high-power positive lenses using indirect ophthalmoscopy. 2-7 The advantages of this technique for determining the true optic disc size are the immediate availability of the results and the reduced costs in instruments and personnel compared with sophisticated techniques such as computer-based analysis of optic disc photographs (planimetry), scanning laser ophthalmoscopy, video-ophthalmography, and simultaneous stereo optic disc photography with digital photogrammetry. In addition, only a few ophthalmologists have access to this expensive equipment for routine clinical work, and optic disc measurement is usually performed at the slit lamp biomicroscope. The purpose of this study was to compare four widely used high-power positive lenses regarding their magnification over a wide range of ametropia in the center of the image field, by using a slit lamp biomicroscope with adjustable beam length. MATERIALS AND METHODS Four commercially available double aspheric fundus lenses (60-D lens, 66 stereo fundus lens, 78-D lens, and super-field NC lens) manufactured by Volk Optical and a calibrated slit lamp biomicroscope (HaagStreit 900; Bern-Koeniz, Switzerland) were used for this study. All lenses provide a stereoscopic view of the fundus and a wide field of view. In the slit lamp biomicroscopy the observation system of the slit lamp is focused at a finite, short distance. The light from the fundus exits the eye parallel (i.e., from optical infinity); therefore, the slit lamp cannot be focused on the fundus. With the use of a high-power positive lens, a real inverted image of the fundus is formed in front of the slit lamp biomicroscope (toward the observer). For clear imagery, the slit lamp is focused on this image