Validity of Rim Area Measurements by Different Reference Planes

Abstract

Reference plane description of the neuroretinal rim in scanning laser tomography should correctly represent optic nerve morphology. We evaluated how well rim area analysis by different reference planes agreed with the appearance of rim area in disc images. Three expert observers subjectively and repeatedly analyzed rim area in Heidelberg Retina Tomograph (HRT) images so that each optic disc was measured six times in 100 eyes, 50 normal and 50 glaucoma. Rim area was evaluated globally and in 30 degree sectors. Agreement between rim appearance, as subjectively analyzed, and objective analysis by an experimental reference plane, the standard reference plane, and a reference plane fixed 320 microm below the reference ring was assessed in HRT images. Subjective analysis of rim area in HRT images was consistent between expert observers. Their analysis of rim appearance agreed more closely with experimental reference plane analysis than analysis by the standard or 320-microm reference planes; this was true globally and in every region of the nerve (P = 0.000). The experimental reference plane yielded higher estimates of rim area than did the standard or 320-microm reference planes. There was closer correspondence between the appearance of the neuroretinal rim in images and description by the experimental reference plane compared with description by the standard and 320-microm reference planes.