Accuracy of the ISNT rule and its variants for differentiating glaucomatous from normal eyes in a population-based study

Aims To evaluate the accuracy of the ISNT rule (I=inferior, S=superior, N=nasal, T=temporal) and its variants with neuroretinal rim width and retinal nerve fibre layer (RNFL) thickness measurements differentiating normal from glaucomatous eyes. Methods The diagnosis accuracy of the ISNT rule and its variants was evaluated in a population-based study. Neuroretinal rim widths were measured on monoscopic optic disc photographs with an image-processing program. RNFL thickness measurements were obtained with spectral-domain optical coherence tomography (SD-OCT). Results In this study including 940 normal subjects and 93 patients with glaucoma, the sensitivity of the ISNT rule with optic disc photographs was 94.1% (95% CI 90.2 to 98.1), whereas its specificity was 49.2% (46.9 to 51.6). When using the IST rule, the sensitivity decreased to 69.9% (62.1 to 77.6) with a higher specificity, 87.0% (85.3 to 88.6). All the diagnosis indicators were somewhat lower for the different rules using RNFL thickness: the sensitivity of the ISNT rule was 79.4% (72.6 to 86.2) and its specificity was 34.1% (31.9 to 36.4). With the IST rule, the sensitivity decreased to 50.0% (41.6 to 58.4) while the specificity increased to 64.9% (62.7 to 67.2). Conclusions The ISNT and IST rules applied to neuroretinal rim width measurement by optic disc photographs are useful and simple tools for differentiating normal from glaucomatous eyes. The translation of these rules to RNFL thickness by SD-OCT is of limited value.