Evaluation of the “IS” Rule to Differentiate Glaucomatous Eyes From Normal

Abstract

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.

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

“…In a normal population of 92 Chinese subjects, Wang et al 19 found that 9 out of 92 subjects (10%) had a nasal rim that was the widest compared to all the other rims, while Harizman et al 13 also reported in their study that 5 out of 66 normal subjects (7.6%) had nasal rim that was thicker than the inferior rim. Additionally, we also found that the nasal rim was wider than the superior rim in 29.4% of subjects and that the nasal rim was narrower than the temporal rim in 14.7% of subjects; and these percentages are consistent with previously reported rates for normal subjects of 28% – 36% 14, 19 and 14% – 29%, 14, 19 respectively. A possible reason for the high variability in nasal rim order and hence the low fulfillment rate of the ISNT rule in our study is that, although the central retinal vessel was not considered as part of the neuroretinal rim during disc assessments, there often is partial obscuration of the nasal rim by these large retinal vessels, which would make evaluation of the nasal rim width ranking more variable.…”

“…In particular, for disc photo assessments, having only 37% of disc photos obey the ISNT rule is lower than previously reported rates of 52% to 79% 13, 14, 19 in studies that also evaluated the neuroretinal rim width using disc photographs in the normal population. By examining the relationship between the nasal quadrant and the other quadrants with respect to the expected trend of the ISNT rule, we found that a main reason that the ISNT rule was not valid was because of considerable variation in the rim order of the nasal quadrant, in which 10.9% of subjects had a rim width that was wider nasally than inferiorly (Figure 3).…”

“…In general, previous studies have also reported similar rates, with 79.4% validity for the IST rule 17 and 85% validity for the IS rule. 14 A population based study conducted on Indians found that the temporal rim was thinnest in 79.8% of subjects. 23 Therefore, our study and the literature seem to indicate that the ISNT rule should be at least changed to the IST rule, if not to the IST/IS/T thinnest rule; because all of these variant rules are valid over 70% of time compared to the ISNT rule which is valid only 37% of the time.…”

“…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.…”

“…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.…”