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M. J. Sinai, A. S. Khouri, R. D. Fechtner; Is There a Heidelberg Retina Tomograph ISNT Rule?. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3314. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the rank order of optic nerve rim area obtained with Heidelberg Retina Tomograph (HRT) 3 to determine if the ISNT rule applies to optic nerve rim segment areas in a large normal caucasian population.
A subset of the HRT 3 caucasian normative database was analyzed in patients with HRT 3 results in both eyes. This sub-group consists of 280 subjects with normal comprehensive biomicroscopic examination, normal IOP (IOP< 22 mm Hg) and normal automated visual field testing. The right eye was selected for analysis. Neuroretinal rim areas were divided into superior, temporal, inferior and nasal quadrants and ranked by average rim area by location. The percentages of eyes following different patterns of rim area quadrant orders was also determined.
The average rim area by descending order and by location was Inferior, Nasal, Superior then Temporal (INST). The percentage of eyes with thickest rim area by location is shown in the table. Overall 92% of eyes followed one of the following rules. By frequency, the percentage of eyes following the various rules were: NIST (33%), INST (21%), ISNT (18%), NSIT (10%), SINT (6%), and SNIT (3%).
The ISNT rule was coined based on relative neuroretinal rim thickness at the inferior, superior, nasal and temporal poles of the optic nerve. By HRT neuroretinal rim area this rule holds in only 18% of the eyes constituting this normative database. The NIST and INST rules were more common. The vascular structures in the Nasal quadrant likely contributed to this finding. Although the rank order of HRT 3 rim area dose not often follow the ISNT rule, the order of rim area may be helpful for determining optic nerve health. For example, the Temporal rim area was smallest in 92% of eyes.
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