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Lee, Shin, and Kee: Discriminating Between Normal and Glaucomatous Eyes Using the Modified ISNT Rule
Original Article

J Korean Ophthalmol Soc 2014;55(1):93-101.

Published online: 7 September 2014

DOI: https://doi.org/10.3341/jkos.2014.55.1.93

Discriminating Between Normal and Glaucomatous Eyes Using the Modified ISNT Rule

Myung Won Lee, MD1, Jong Hoon Shin, MD2, Chang Won Kee, MD, PhD3

1Department of Ophthalmology, Dankook University School of Medicine, Cheonan, Korea

2Department of Ophthalmology, Pusan National University School of Medicine, Busan, Korea

3Department of Ophthalmology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea

Address reprint requests to Chang Won Kee, MD, PhD Department of Ophthalmology, Samsung Medical Center, #81 Irwon-ro, Gangnam-gu, Seoul 135-710, Korea, Tel: 82-2-3410-3569, Fax: 82-2-3410-0074, E-mail: ckee@skku.edu

Received 27 June 201329 July 2013       Accepted 20 November 2013

Copyright © 2014 Korean Ophthalmological Society

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Purpose

To evaluate the diagnostic ability of the modified ISNT rule (disc rim thickness of the smaller of inferior and superior > the larger of nasal and temporal) for normal and glaucomatous eyes compared to the classic ISNT rule (disc rim thickness of inferior > superior > nasal > temporal).

Methods

Color stereo optic disc photographs of 113 normal subjects and 108 open angle glaucoma patients with early and moderate stage were morphometrically evaluated. The classic ISNT rule and the modified ISNT rule were assessed by masked evaluation of disc photographs at the 3, 6, 9 and 12 o’clock positions.

Results

Among normal subjects, 58 of 113 eyes (51.3%) were normal and in open angle glaucoma patients, 104 of 108 eyes (96.3%) were abnormal with the classic ISNT rule. Among normal subjects, 98 of 113 eyes (94.2%) were normal and in open angle glaucoma patients, 102 of 108 eyes (94.4%) were abnormal with the modified ISNT rule. The modified ISNT rule was more accurate than the classic ISNT rule in terms of Cohen’s Kappa analysis used for discriminating between normal and glaucomatous eyes.

Conclusions

The modified ISNT rule is useful for differentiating between normal and glaucomatous optic nerves and easily applied in clinical practice.

Keywords: ISNT rule, Neuroretinal rim, Optic cup, Optic disc, Stereoscopic disc photography

References

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Figure 1.
Clinical assessment of the ISNT rule for a normal optic nerve. The classic ISNT rule is that the neuroretinal rim width is greatest in the order, inferior > superior > nasal > temporal. The modified ISNT rule is that the neuroretinal rim width shows the smaller of inferior and superior is greater than the larger of nasal and temporal.
jkos-55-93f1.tif
Figure 2.
(A) Optic disc having a cup with temporal flat slope, (B) Optic nerve head with steep, punched-out optic cup.
jkos-55-93f2.tif
Table 1.
Agreement of the modified ISNT rule and the classic ISNT rule in normal group and glaucoma group
    Classic ISNT rule
 Modified ISNT rule
    Normal Glaucoma Normal Glaucoma
Diagnosis Normal 58 (51.3%) 55 (48.7%) 98 (86.7%) 15 (13.3%)
  Glaucoma 4 (3.7%) 104 (96.3%) 6 (5.6%) 102 (94.4%)
Table 2.
Comparison of the modified ISNT rule with the classic ISNT rule
  Classic ISNT rule Modified ISNT rule p-value
Sensitivity 0.963 0.944 0.68*
Specificity 0.513 0.867 0.00*
Positive predictive value 0.654 0.872 0.00†
Negative predictive value 0.935 0.942 0.82†
Kappa 0.471 0.887 0.00‡

* Mcnemar’s test

† Bennett’s method

‡ Cohen’s Kappa analysis.

Table 3.
Patient demographics and ocular findings by diagnosis using the modified ISNT rule and results of univariate analysis (normal group, n = 113)
Characteristic Modified ISNT rule
 p-value
Normal (n = 98) Glaucoma (n = 15)
Age (years) 51.0 ± 13.2 49.3 ± 11.6 1.00*
Range 19 to 77 29 to 66  
Sex (no, %)     0.11†
  Female 46 (47) 11 (73)  
  Male 52 (53) 4 (27)  
  Refractive error (spherical equivalent) (diopters) -1.65 ± 2.20 -1.48 ± 2.20 1.00*
  Range -8.00 to 3.00 -5.75 to 1.50  
  Intraocular pressure (mm Hg) 14.9 ± 3.0 14.0 ± 2.6 0.50*
  Range 9 to 23 10 to 18  
Average visual field measures      
  Mean deviation (dB) -0.11 -0.41 0.73*
  Range -6.88 to 3.22 -3.02 to 2.55  
  Central corneal thickness (mm) 532.2 ± 36.7 522.8 ± 38.4 0.98*
  Range 454 to 623 427 to 576  
Diameter      
  Horizontal (mm) 1.77 ± 0.26 1.80 ± 0.21 1.00*
  Range 0.96 to 2.55 1.43 to 2.23  
  Vertical (mm) 1.82 ± 0.20 1.82 ± 0.19 1.00*
  Range 1.25 to 2.28 1.48 to 2.12  
CD ratio      
  Horizontal 0.69 ± 0.10 0.66 ± 0.10 0.23*
  Range 0.39 to 0.88 0.51 to 0.77  
  Vertical 0.54 ± 0.11 0.60 ± 0.10 0.16*
  Range 0.27 to 0.71 0.44 to 0.74  
Cup shape (no, %)     1.00†
  Punched-out cup 68 (69) 10 (67)  
  Cup with temporal flat slopes 30 (31) 5 (33)  
Rim thickness      
  Inferior (mm) 0.43 ± 0.09 0.38 ± 0.10 0.26*
  Range 0.26 to 0.67 0.23 to 0.55  
  Superior (mm) 0.41 ± 0.08 0.34 ± 0.06 0.01*
  Range 0.26 to 0.59 0.24 to 0.46  
  Nasal (mm) 0.31 ± 0.07 0.37 ± 0.06 0.01*
  Range 0.17 to 0.52 0.27 to 0.47  
  Temporal (mm) 0.22 ± 0.09 0.24 ± 0.07 1.00*
  Range 0 to 0.41 0.15 to 0.37  
  Diameter vertical / diameter horizontal 1.04 ± 0.09 1.01 ± 0.09 0.78*
  Range 0.83 to 1.30 0.89 to 1.21  

Values are presented as mean ± SD.

* Mann-Whitney test

† Fisher exact test.

Table 4.
Patient demographics and ocular findings by diagnosis using the modified ISNT rule and results of univariate analysis (glaucoma group, n = 108)
Characteristic Modified ISNT rule
 p-value
Normal (n = 6) Glaucoma (n = 102)
Age (years) 54.5 ± 6.4 58.2 ± 13.6 0.87*
Range 44 to 63 29 to 88  
Sex (no, %)     0.40†
  Female 4 (67) 37 (36)  
  Male 2 (33) 65 (64)  
  Refractive error (spherical equivalent), diopters -0.25 ± 0.50 -1.79 ± 2.90 0.64*
  Range -0.75 to 0.50 -13.25 to 2.00  
  Intraocular pressure (mm Hg) 16.2 ± 2.6 17.9 ± 4.4 0.65*
  Range 14 to 21 10 to 37  
Average visual field measures      
  Mean deviation (dB) -3.91 -6.32 0.88*
  Range -9.92 to -1.59 -29.19 to 1.94  
  Central corneal thickness (mm) 516.2 ± 31.9 521.2 ± 35.2 1.00*
  Range 455 to 535 418 to 616  
Diameter      
  Horizontal (mm) 1.88 ± 0.23 1.77 ± 0.25 0.49*
  Range 1.63 to 2.17 1.19 to 2.72  
  Vertical (mm) 1.97 ± 0.18 1.85 ± 0.20 0.22*
  Range 1.70 to 2.21 1.43 to 2.58  
CD ratio      
  Horizontal 0.81 ± 0.10 0.74 ± 0.07 0.03*
  Range 0.65 to 0.87 0.56 to 0.88  
  Vertical 0.75 ± 0.08 0.76 ± 0.06 1.00*
  Range 0.60 to 0.82 0.57 to 0.90  
Cup shape (no, %)     1.00†
  Punched-out cup 3 (50) 73 (72)  
  Cup with temporal flat slopes 3 (50) 29 (28)  
Rim thickness      
  Inferior (mm) 0.25 ± 0.11 0.20 ± 0.08 0.30*
  Range 0.17 to 0.44 0.08 to 0.45  
  Superior (mm) 0.24 ± 0.07 0.24 ± 0.07 1.00*
  Range 0.14 to 0.34 0.05 to 0.40  
  Nasal (mm) 0.18 ± 0.07 0.25 ± 0.07 0.04*
  Range 0.12 to 0.29 0.11 to 0.48  
  Temporal (mm) 0.17 ± 0.08 0.21 ± 0.06 0.26*
  Range 0.10 to 0.32 0.07 to 0.38  
  Diameter vertical / diameter horizontal 1.05 ± 0.05 1.06 ± 0.13 1.00*
  Range 1.00 to 1.12 0.79 to 1.48  

Values are presented as mean ± SD.

* Mann-Whitney test

† Fisher exact test.

Table 5.
Results of multivariate analysis using logistic regression analysis (normal group)
  Odds ratio 97.5% Confidence limits p-value*
Age 0.99 0.92-1.08 1.00
Male (ref: female) 0.66 0.11-3.94 1.00
Refractive error (spherical equivalent) 0.94 0.58-1.53 1.00
IOP > 18 mm Hg (ref: IOP ≤ 18 mm Hg) 0.67 0.07-6.78 1.00
Mean deviation 1.00 0.64-1.58 1.00
Central corneal thickness 1.01 0.98-1.03 1.00
Cup shape 2.89 0.37-22.37 0.49
Punched-out cups (ref: Cups with temporal flat slopes)      
Horizontal diameter (ref: ≤1.7 mm)      
  1.7-1.8 mm 0.44 0.03-5.76 0.95
  >1.8 mm 0.19 0.01-3.45 0.39
Vertical diameter (ref: ≤1.7 mm)      
  1.7-1.8 mm 0.79 0.04-15.50 1.00
  >1.8 mm 1.73 0.08-36.17 1.00
Vertical diameter / horizontal diameter 0.40 0.09-1.73 0.32
Horizontal CD ratio 0.06† 0.01-0.42 0.00
Vertical CD ratio 15.55† 2.24-108.01 0.00

* Logistic regression with Firth's penalized ML method

† For each increase of 0.1 in CD ratio.

Table 6.
Results of multivariate analysis using logistic regression analysis (glaucoma group)
  Odds Ratio 97.5% Confidence Limits p-value*
Age 1.04 0.97-1.11 0.44
Male (ref: female) 1.72 0.39-7.69 0.83
Refractive error (spherical equivalent) 0.86 0.62-1.18 0.56
IOP > 18 mm Hg (ref: IOP ≤ 18 mm Hg) 1.62 0.32-8.15 1.00
Mean deviation 0.99 0.84-1.17 1.00
Central corneal thickness 1.00 0.98-1.02 1.00
Cup shape 0.78 0.14-4.32 1.00
Punched-out cups (ref: Cups with temporal flat slopes)      
Horizontal diameter (ref: ≤1.7 mm)      
  1.7-1.8 mm 1.63 0.19-14.03 1.00
  >1.8 mm 1.13 0.1-12.73 1.00
Vertical diameter (ref: ≤1.7 mm)      
  1.7-1.8 mm 1.14 0.06-20.32 1.00
  >1.8 mm 0.68 0.04-11.58 1.00
Vertical diameter / horizontal diameter 0.88 0.33-2.39 1.00
Horizontal CD ratio 0.25† 0.05-1.27 0.19
Vertical CD ratio 3.48† 0.66-18.28 0.11

* Logistic regression with Firth's penalized ML method

† For each increase of 0.1 in CD ratio.

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