Magnification-corrected indirect biomicroscopy of the optic nerve head

Background: Proper use of the new Super VitreoFundus and SuperPupil XL non-contact slit-lamp lenses for clinical disc biometry requires knowledge of their comparative magnification. Methods: The optical performance of each fundus lens is described in terms of axial magnification, lens position, and the correction factor p ( in degrees per millimeter) using a calibrated Gull-strand-type model eye adjusted for axial ametropia between - 12.5 D and + 12.6 D. Results: The total change in axial magnification from myopia to hyperopia was - 13.3% to + 15.6% ( Super VitreoFundus lens), and - 13.9% to + 14.1% ( SuperPupil XL lens). When the fundus lens position was altered with respect to the model eye by +/-2 mm under myopic conditions the change in axial magnification was - 5.1% to + 7.7% ( Super VitreoFundus lens), and - 6.5% to + 9.7% (SuperPupil XL lens). In the hyperopic condition the change was - 3.8% to + 5.8%, and - 4.9% to + 7.3%. Both fundus lenses exhibited a linear relationship between p and degree of ametropia of the model eye, while only the Super VitreoFundus lens displayed a constant relationship between p and ametropia of - 5 D to + 5 D. Conclusions: Using the fundus lens correction factor ( p), the clinician may be able to estimate the dimensions of optic nerve head features with sufficient accuracy to allow clinical decisions to be made in the evaluation of patients with diagnosed or suspected glaucoma.