Comparison of different measuring methods in the assessment of the ISNT rule and its variants in a normal population: A cross-sectional study.

PURPOSE

The aim of this study was to compare five different neuroretinal rim (NRR) measurement methods, based on quadrants and NRR widths, in the assessment of the ISNT (inferior (I) > superior (S) > nasal (N) > temporal (T)) rule, and its variants IST (inferior (I) > superior (S) > temporal (S)) rule, IS (inferior (I) > superior (S)) rule and T (temporal is the thinnest) rule in a normal population. Factors influencing compliance with this rule and its variants were also evaluated.

METHODS

Stereoscopic fundus images were analysed through a dichoptic viewing system. Two graders labelled the optic disc and cup, as well as the fovea. Custom-made software automatically determined the limits of the optic disc and cup and examined the ISNT rule and its variants using several NRR measurement methods.

RESULTS

Sixty-nine subjects with normal eyes were enrolled. For the various NRR measuring methods, the percentage of eyes following the rules, that is, validity ranges were 0.0%-15.9% for the ISNT rule, 31.9%-59.4% for the IST rule, 46.4%-59.4% for the IS rule and 50.7%-100.0% for the T rule. Significant intra-measurement agreement ranges were IST (κ = 0.50-0.85), IS (κ = 0.68-1.00) and T (κ = 0.24-0.77). Only the IST and IS rules achieved significant inter-measurement agreement (κ = 0.47-1.00). After multivariate and receiver operating characteristic (ROC) curve analyses, the vertical cup position cup (area under the ROC curve (AUROC) = 0.60-0.96; cut-off = |0.005|) was the most important predictor for virtually all NRR measurement agreements for the ISNT, IST and IS rules. The horizontal cup position (AUROC = 0.50-0.92; cut-off = -0.028 to 0.05) was the most important predictive factor for the majority of the NRR measurement agreements for the T rule.

CONCLUSIONS

Only the IST and IS rules are valid for the same normal subjects. The most important factor affecting the validity of the ISNT rule and its variants was the anatomical cup position. NRR measurement agreements based on NRR quadrants exhibited larger validity and better agreement. The IST and IS rules can be combined with the alternative SIT (superior (S) > inferior (I) > temporal (T)) and SI (superior (S) > inferior (I)) rules to detect almost all normal subjects.