Researchers recently suggested the ISNT and IST (I=inferior, S=superior, N=nasal, T=temporal) rules are useful and simple tools to differentiate normal from glaucomatous eyes when taking neuroretinal rim width measurements by optic disc photography. The translation of these rules to retinal nerve fiber layer (RNFL) thickness by spectral-domain OCT (SD-OCT), however, was of limited value.

This population-based study of 940 controls and 93 patients with glaucoma included neuroretinal rim width measurements on monoscopic optic disc photographs and RNFL thickness measurements with SD-OCT.

The investigators found that the sensitivity of the ISNT rule with optic disc photographs was 94.1% with a specificity of 49.2%. With the IST rule, they noted that the sensitivity decreased to 69.9% while the specificity increased to 87.0%.

They added that all diagnostic indicators were somewhat lower using RNFL thickness, with sensitivities of 79.4% and 50.0% and specificities of 34.1% and 64.9% for the ISNT and IST rules, respectively. 

Maupin E, Baudin F, Arnould L, et al. Accuracy of the ISNT rule and its variants for differentiating glaucomatous from normal eyes in a population-based study. Clin Sci. January 20, 2020. [Epub ahead of print].