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Editors Selection IGR 12-3
Structure Measurement: The iSN'T rule with SLO
Comment by Nathan Radcliffe on:
46814 The "iSN'T rule" in healthy participant optic nerve head by confocal scanning laser ophthalmoscopy, Iester M; Bertolotto M; Recupero SM et al., Journal of Glaucoma, 2011; 20: 350-354
Planimetric evaluations of non-glaucomatous eyes have shown that in healthy optic nerves, the inferior neuroretinal rim is typically thickest, followed by the superior, nasal and temporal rim, respectively.1 Based on an analysis of optic nerve photographs by Harizman and colleagues, the ISNT Rule (inferior neuroretinal rim thickness ≥ superior ≥ nasal ≥ temporal) was intact in 79% of normal patients but in only 28% of those with glaucoma.2 Iester et al. (1172) evaluated the ISNT rule using confocal laser scanning ophthalmoscopy in 113 healthy participants with normal intraocular pressure and perimetry.
As with any imaging technology, guidelines derived from one technique should be revalidated on each new instrument or imaging platform prior to clinical implementation
Rim area and volume were significantly thinner only when comparing the temporal region to the nasal, superior and inferior regions, which were all similar to each other. Considering the rim area, the ISNT rule was only respected in 12% of participants, with the inferior rim thicker than the superior rim in 59%. There are several explanations for the discrepancy between confocal laser scanning ophthalmoscopy and planimetric evaluations of the ISNT rule. Papillary blood vessels are included in rim volume and area measurements but can be discounted in clinical assessment of photographs, so the use of topographic analysis may affect the observed differences between optic nerve regions through the inclusion of blood vessels into those regions. Furthermore, the use of a reference plane 50 microns below the retinal surface to divide the neuroretinal rim and optic disc cup could be expected to differ from clinical assessment. This study demonstrates that as with any imaging technology, guidelines derived from one technique (e.g., photographic assessment of the ISNT rule) should be revalidated on each new instrument or imaging platform prior to clinical implementation.
References
- Wang Y, Xu L, Jonas JB. Shape of the neuroretinal rim and its correlations with ocular and general parameters in adult chinese: the beijing eye study. Am J Ophthalmol. 2007; 144: 462-464.
- Harizman N, Oliveira C, Chiang A, Tello C, Marmor M, Ritch R, Liebmann JM. The ISNT rule and differentiation of normal from glaucomatous eyes. Arch Ophthalmol. 2006; 124: 1579-1583.
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