Applicability of ISNT and IST rules to the retinal nerve fibre layer using spectral domain optical coherence tomography in early glaucoma

Abstract

Even though useful during ophthalmoscopy, ISNT and IST rules by themselves don't clearly distinguish normal eyes from those with glaucoma when applied to the quadrant values on RNFL on SD-OCT examination.

The ISNT Rule: How Often Does It Apply to Disc Photographs and Retinal Nerve Fiber Layer Measurements in the Normal Population?

“…When previous histologic studies 10, 11 and OCT studies 16, 27 also used mean quadrant RNFL thickness values, they also demonstrated that the ISNT rule was valid (Table 1). However, when examining individual patients, we found that only 43.8% of subjects in our normal population met the ISNT rule (Table 2), which is similar to reported rates of 42% to 55%.…”

“…However, when examining individual patients, we found that only 43.8% of subjects in our normal population met the ISNT rule (Table 2), which is similar to reported rates of 42% to 55%. 16, 17, 27 This highlights that when applying a rule to an aggregate population, the rule may no longer be valid for the individual due to individual variability. In contrast to our findings for disc photos, we found that the sole reason that nasal RNFL thickness deviated from the ISNT rule was that the nasal quadrant was thinner than the temporal quadrant in 42.9% of normal subjects (Figure 4), which was a surprising finding and not previously reported by other studies.…”

“…Therefore, many investigators have sought to determine whether the ISNT rule, either applied to the neuroretinal rim disc photos 12–15 or to RNFL thickness measurements, 16, 17 is useful in the diagnosis of glaucoma or not. However, the optic disc photo ISNT rule studies are conflicting, with some finding the ISNT rule and its variants clinically useful, 13, 14 while others have not.…”

“…12, 15 In contrast, RNFL ISNT rule studies based on OCT findings are in uniform agreement, stating that the ISNT rule and its variants were not helpful in the diagnosis of glaucoma. 16, 17 Some have hypothesized that the ISNT rule is not easily generalizable to the individual, because the initial studies were derived from mean values. 8, 10, 11 Therefore, some of the limitations of the ISNT rule may stem from the fact that it is unclear what percentage of individual normal eyes follow the ISNT rule.…”

“…Thirdly, in the context of the ISNT rule and its variants, this study will assess how much agreement there is between disc photo neuroretinal rim assessments and RNFL thickness measurements. Although past ISNT rule studies have investigated the neuroretinal rim 13, 14, 18–23 and the RNFL 16 separately, none of these studies have evaluated these counterparts together in one study. The only other study 17 that investigated the ISNT rule for both the neuroretinal rim and the RNFL used confocal scanning laser ophthalmoscopy (CSLO) and time domain OCT for its measurements of RNFL thickness.…”