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Editors Selection IGR 17-3

Structure Measure: Diagnostic accuracy of the INST rule

Comment by Jost Jonas on:

49107 The accuracy of the inferior>superior>nasal>temporal neuroretinal rim area rule for diagnosing glaucomatous optic disc damage, Morgan JE; Bourtsoukli I; Rajkumar KN et al., Ophthalmology, 2012; 119: 723-730

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In their study on the validity of the ISNT- (Inferior-Superior-Nasal- Temporal)-rule, Morgan and colleagues determined the accuracy with which the optic disc could be diagnosed as normal or glaucomatous. They used stereoscopic optic disc photographs. For the ISNT rule to be obeyed, the three Boolean comparisons of the neuroretinal rim area width, Inferior >Superior, Superior > Nasal, and Nasal >Temporal, had to be true. If any of the comparisons returned false, the rule was considered not to have been obeyed. The authors found low positive likelihood ratios and concluded that the ISNT rule had limited utility in the diagnosis of openangle glaucoma. The limitations of the study were, firstly, that the ISNT rule may be helpful mainly for the differentiation of normal optic discs from optic discs with early glaucoma. As soon as the rim width in the temporal disc region is markedly reduced, eyes with medium advanced glaucoma can fulfill the ISNT rule (again), although the neuroretinal rim width would have been markedly decreased. It indicates that the assessment of the ISNT rule would have been useful only for the 26 patients with mild visual field loss out of the whole group of 78 glaucoma patients in the study group. Secondly, in many normal eyes, the rim is wider inferior than superiorly, and the difference in the physiological rim width between the superior disc region and the nasal disc region is marginal. The most important part of the ISNT rule is that the smallest part of the rim should be in the temporal disc region i.e., the 'T' in the ISNT rule). ( The authors may be encouraged to re-perform their statistical analysis by including only normal eyes and eyes with early glaucoma and by concentrating on the letter 'T' in the ISNT-rule.

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