A recent study suggests that applying the ISNT rule aided by an optical coherence tomography (OCT) metric is more effective at distinguishing between healthy and glaucomatous optic discs than using disc photographs. The latter method is fraught with variability because it relies on subjective assessment and sometimes-differing criteria between clinicians.

Researchers analyzed 124 healthy eyes and 136 eyes with normal tension glaucoma, measuring neuroretinal rim (NRR) thickness at the superior, inferior, nasal and temporal sides of the optic disc using two-dimensional disc photographs. To provide an objective comparison of different methods, the team also used OCT to measure Bruch’s membrane opening minimum rim width (BMO-MRW) at the same regions.  

They found that the specificity of “violation of the ISNT rule” for diagnosing glaucoma was higher with BMO-MRW (66.3%) than with disc photographs (42.2%), although the difference in sensitivity between methods was not significant (91.7% and 86.5%, respectively). Compared with eyes with distinguishable NRR (179), the study notes that eyes with indistinguishable rims on disc photographs (81) had a longer axial length, more negative refractive error and a higher tilt ratio. The diagnostic ability of the ISNT rule violation using BMO-MRW was not significantly different between eyes with and without distinguishable neuroretinal rim tissue.

The study concludes “the ISNT rule can be applied using BMO-MRW even when NRR is indistinguishable on disc photographs, such as in myopic tilted discs.”