FP18 : Effect of Ocular Torsion on the Followability of Isnt Rule By the Rnfl Using SD -OCT

Dr. Paaraj Dave D13919, Dr. Jitendra Nenumal Jethani, Dr. Juhi Shah


Ocular torsion is measured by calculating the disc-foveal angle (DFA) formed by a horizontal line extending from optic disc center and a line joining the center of the disc to the fovea. 1, 2 There are no studies on the variation in the followability of the ISNT rule based on the location of the fovea as assessed by the DFA measurement which is the purpose of the current study.


This was a prospective, cross sectional study which included consecutive 64 eyes of 64 normal subjects. All subjects were of age > 18 years, best corrected visual acuity 20/40 or better and a refractive error within ±5 Diopter (D) sphere and ±3 D cylinder. The followability of ISNT rule on RNFL examination and the disc-foveal angle (DFA) measured from the infra-red image of the RNFL printout was noted. The ISNT rule was considered violated only if the difference in the RNFL measurement between the quadrants was > 10 microns


The ISNT rule was followed by 20/32 (62.5%) and 28/32 (87.5%) eyes with DFA less than and greater than 6 degrees respectively (p=0.02). None of the eyes with a DFA of less than 3 degrees followed the ISNT rule. A moderate to strong correlation was seen between the DFA and inferior-superior quadrant RNFL thickness difference (ISRD) (r=0.6). The mean DFA and ISRD were 7.4±3.9 degrees and 7.8±11.9 microns respectively.


Study confirms the importance of foveal location in the measurement of RNFL on SD-OCT. There are several limitations in our study. The young age of the subjects included in our study may limit the generalization of our findings to an older age group. Also, our study involved Indian eyes only.

To conclude, ISRD and the followability of the ISNT rule for RNFL are dependent on the location of the fovea relative to the optic disc. The greater the DFA, the greater is the ISRD and better is the followability of the ISNT rule on the RNFL. The lesser the DFA, the lesser is the ISRD and lesser is the followability of the ISNT rule on the RNFL.


  • Herzau V, Joos-Kratsch E. Objective and subjective evaluation of cyclovergence and cyclofusion. Doc Ophthalmol 1984;58(1):85-90.
  • Madigan WP Jr, Katz NN. Ocular torsion-direct measurement with indirect ophthalmoscope and protractor. J Pediatr Ophthalmol Strabismus 1992;29(3):171-4.


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