The "ISN'T rule" in healthy participant optic nerve head by confocal scanning laser ophthalmoscopy

In normal patients the rim had a characteristic configuration with the rim width being broadest in the inferior disc region, followed by the superior, the nasal, and the temporal sector. The aim of this study was to assess whether the rim shape as measured by the Heidelberg Retina Tomograph 3 (HRT3) followed the ISN'T rule or any other specific distribution pattern.

This is a prospectively planned cross-sectional study. Two hundred and twenty-six healthy eyes were recruited into this study. All the patients who were included had normal ocular examination, an intraocular pressure less than 21 mm Hg and a normal visual field by the Humphrey Field Analyzer (program 24-2, Swedish Interactive Threshold Analysis standard). For each patient, the optic nerve head was morphometrically evaluated by using the HRT3. All data were analyzed by the analysis of variance test and Tukey's Multiple Comparison Test.

When the rim area and rim volume were considered, a statistically significant difference (P<0.001) was found between the temporal rim area and superior, nasal, and inferior rim area. No significant difference was found among the superior, nasal, and inferior rim area. A similar result was found for the cup area. The temporal cup shape measure was significantly (P<0.001) different from the other disc sectors.

In healthy participants the rim shape as measured by the HRT did not follow the ISN'T rule, except that the smallest rim part was located in the temporal disc region. The thickness of the rim was similar in the superior and the inferior sectors by HRT.