Evaluation of the "IS" Rule to Differentiate Glaucomatous Eyes From Normal

Purpose: To compare the accuracy of the "ISNT" rule [neural rim width of inferior(I) >= superior(S) >= nasal(N) >= temporal(T) regions] and the abbreviated variant, the "IS" rule (inferior >= superior regions) to differentiate normal from glaucomatous eyes. Materials and Methods: Medical records of patients who were evaluated in 2011, had glaucomatous optic neuropathy and visual field defects, on glaucoma treatment, and had stereoscopic optic disc photographs were reviewed. Optic discs with focal complete loss of neural rim or long axis rotated > 30 degrees from vertical meridian, and patients with >= 5D of myopia or any retinal pathology or nonglaucomatous optic neuropathy were excluded. One eye per patient was randomly enrolled. Normal control eyes were also included. Rim widths were measured with an image processing program (ImageJ, National Institutes of Health) in a masked manner. The sensitivity and specificity of the ISNT rule, the IS rule, and cup-to-disc ratio (CDR) were compared. Results: A total of 134 glaucoma and 110 normal eyes were enrolled. The mean CDRs of the glaucoma and normal eyes were 0.65 +/- 0.13 and 0.39 +/- 0.15, respectively. Sensitivities of the ISNT and IS rules were 85% and 41%, respectively, whereas specificities were 46% and 85%, respectively. Application of the IS rule in eyes with larger CDR (> 0.57) increased the specificity of the IS rule to 93% while keeping the sensitivity at 41%. When ISNT or IS rule and CDR > 0.57 were combined in differentiating normal from glaucomatous eyes for the entire sample, specificities approached 90% and 99%, respectively. Conclusions: The ISNT rule alone has a high sensitivity but relatively low specificity. Application of the IS rule in eyes with increased CDR yields a much higher specificity for differentiating normal from more advanced glaucomatous eyes. A combination of different features of the optic disc (increase of CDR and ISNT or IS rule) improves the specificity of optic disc evaluation for glaucoma.