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

Purpose: To determine what percentage of normal eyes follow the ISNT rule, and whether ISNT rule variants may be more generalizable to the normal population.

Design: Cross-sectional study.

Methods: Setting: Institutional setting.

Study population: Total of 110 normal subjects.

Observation procedures: Neuroretinal rim assessments from disc photographs and retinal nerve fiber layer (RNFL) thickness measurements from spectral-domain optical coherence tomography.

Main outcome measures: The percentages of subjects that obeyed the ISNT rule and its variants.

Results: The ISNT rule is only valid for 37.0% of disc photograph rim assessments and 43.8% of RNFL measurements. Deviation of the nasal sector from the expected ISNT pattern was a major cause for the ISNT rule not being obeyed for both rim and RNFL assessments. Specifically, 10.9% of subjects had wider nasal rims than the inferior rims, 29.4% had wider nasal rims than the superior rims, 14.7% had narrower nasal rims than the temporal rims, and 42.9% had thinner nasal RNFLs compared to the temporal quadrant. Exclusion of the nasal quadrant from the ISNT rule significantly increased the validity of ISNT variant rules, with 70.9% and 76.4% of disc photographs following the IST rule and the IS rule, respectively. Similarly, for RNFL thickness, 70.9% and 71.8% of patients followed the IST and IS rule, respectively.

Conclusions: The ISNT rule is only valid for about a third of disc photographs and less than half of RNFL measurements in normal patients. ISNT rule variants, such as the IST and IS rule, may be considered, as they are valid in more than 70% of patients.