Background To report the applicability of ISNT (inferior>superior>nasal>temporal) and IST (inferior>superior>temporal) rules on the retinal nerve fibre layer (RNFL) using spectral domain optical coherence tomography (SD-OCT) for detecting early glaucoma.
Methods A prospective, cross-sectional study which included 80 eyes of 80 normal subjects and 76 eyes of 76 patients with early glaucoma by Hodapp-Anderson-Parrish classification. All subjects were of age more than 18 years, best corrected visual acuity 20/40 or better and a refractive error within ±5 dioptres (D) sphere and ±3 D cylinder. Control subjects had a normal ocular examination, intraocular pressure <22 mm Hg, no past history of high intraocular pressure, no family history of glaucoma, normal optic disc morphology and visual field. All eyes underwent SD-OCT examination for RNFL analysis. The sensitivity, specificity and likelihood ratio for violation of ISNT and IST rules was calculated for early glaucoma diagnosis.
Results The ISNT rule was followed by 44 (55%) normal and 28 (36.84%) early glaucoma eyes. The IST rule was followed by 48 (60%) normal and 40 (52.63%) early glaucoma eyes. The sensitivity/specificity for violation of ISNT and IST rules for early glaucoma diagnosis was 63.2%/55% and 47.4%/60% respectively. The positive/negative likelihood ratio for ISNT and IST rules was 1.4/0.67 and 1.2/0.88, respectively, for diagnosing early glaucoma.
Conclusions Even though useful during ophthalmoscopy, ISNT and IST rules by themselves don't clearly distinguish normal eyes from those with glaucoma when applied to the quadrant values on RNFL on SD-OCT examination.