Purpose: The ISNT rule for nonglaucomatous eyes suggests that the neuroretinal rim is thickest at the inferior quadrant (I), followed by the superior (S), nasal (N), and temporal (T) quadrants. This study aimed to use Heidelberg Retina Tomograph (HRT III) measurements to assess (a) fulfillment of the ISNT rule and its derivatives in a large normative database and (b) effect of disc size and age on rule fulfillment.
Patients and methods: A multicenter, prospective, cross-sectional study of a Caucasian normative database consisting of 280 subjects with normal comprehensive biomicroscopic examination, intraocular pressure <21 mm Hg, and normal automated visual field testing was conducted. Right eye neuroretinal rim and disc area, measured by HRT III, for each of the 4 quadrants were analyzed. Compliance of the rim area to the ISNT rule (I≥S≥N≥T) and its derivates was determined. Effect of age and disc area on rule compliance was further determined.
Results: Only 18% of normal eyes had rim areas that complied with the ISNT rule; however, a majority complied to IS (77%) and IST (73%) rules. The temporal quadrant had the smallest rim area [(I,S,N)>T] in 91% of patients. The likelihood of ISNT rule violation was increased in larger discs (χ², P=0.003) but was not affected by age.
Conclusions: The ISNT rule does not apply to neuroretinal rim area as measured by HRT, as only 18% of the eyes complied with the ISNT rule in this normative database. Although the ISNT rule may be more applicable to normal eyes with a smaller disc area, the IS and IST rules seem to better represent the normative database.