Application of the ISNT Rule to Neuroretinal Rim Thickness Determined Using Cirrus HD Optical Coherence Tomography

Abstract

The study investigated the applicability of the inferior≥superior≥nasal≥temporal neuroretinal rim thickness (ISNT) rule to rim thickness measured using Cirrus high-definition spectral-domain optical coherence tomography (Cirrus HD-OCT) in healthy eyes and eyes with glaucoma. Rim thicknesses of 4 quadrants were investigated using the rim thickness curve of Cirrus HD-OCT for 40 eyes with mild glaucoma (mean deviation ≥ -6 dB), 40 eyes with moderate to advanced glaucoma (mean deviation < -6 dB), and 80 healthy eyes (control group). The relative amount of rim loss in each quadrant in the glaucoma group compared with the control group was calculated. The ISNT rule was considered intact if there was a gradual decrease in rim thickness in the order of inferior≥superior≥nasal≥temporal quadrants. The ISNT rule was intact in 13%, 18%, and 8% of eyes in the control, mild, and moderate to advanced glaucoma groups, respectively, and the distribution was not significantly different among the 3 groups (P>0.05). Relative rim loss in the inferior quadrant of the moderate to advanced glaucoma group (63.8%) was greater than that of the mild glaucoma group (53.6%; P=0.010), whereas rim loss in other quadrants was not significantly different between the 2 groups (P>0.05). The ISNT rule with rim thickness measured using Cirrus HD-OCT has limited clinical value in the diagnosis of glaucoma. There is a topographic difference in inferior rim loss according to the stages of glaucomatous damage.