Optic disc size in ocular hypertension

GC Sekhar; R Nagarajan; TJ Naduvilath; L Dandona; KM Rao; VD Rao

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ORIGINAL ARTICLE

Year : 1999 | Volume : 47 | Issue : 4 | Page : 229-231

Optic disc size in ocular hypertension

GC Sekhar, R Nagarajan, TJ Naduvilath, L Dandona, KM Rao, VD Rao
VST Center for Glaucoma Care, L.V. Prasad Eye Institute, Hyderabad, India

Correspondence Address:
G C Sekhar
VST Center for Glaucoma Care, L.V. Prasad Eye Institute, Hyderabad
India

Source of Support: None, Conflict of Interest: None

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PMID: 10892478

Abstract

Purpose: To study the optic disc size in eyes with ocular hypertension (OHT) in comparison to primary open-angle glaucoma (POAG) and normals.
Methods: Optic disc photographs obtained with the Nidek 3dx NM camera were digitized (Nikon^™ coolscan) and disc area calculated using Littmann correction in a randomly chosen eye of 28 OHT, 42 POAG and 30 normal subjects. OHT was defined as increased intraocular pressure with no disc or field changes suggestive of glaucoma with open angles.
Results: The optic disc area in OHT was 9.47 ± 1.09 mm2; 12.27 ± 2.87mm² in POAG; and 12.11 ± 2.83 mm² in normal individuals.
Conclusion: Using magnification corrected morphometry and the criteria for OHT diagnosis, the optic disc area in OHT was significantly smaller (p<0.0001) in POAG and normals.

Keywords: Ocular hypertension, optic disc size, primary open angle glaucoma, Littmann correction

How to cite this article:
Sekhar G C, Nagarajan R, Naduvilath T J, Dandona L, Rao K M, Rao V D. Optic disc size in ocular hypertension. Indian J Ophthalmol 1999;47:229-31

How to cite this URL:
Sekhar G C, Nagarajan R, Naduvilath T J, Dandona L, Rao K M, Rao V D. Optic disc size in ocular hypertension. Indian J Ophthalmol [serial online] 1999 [cited 2023 Nov 28];47:229-31. Available from: https://journals.lww.com/ijo/pages/default.aspx/text.asp?1999/47/4/229/14907

VDD IS VERTICAL DISC DIAMETER; HDD IS HORIZONTAL DISC DIAMETER.

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OHT IS OCULAR HYPERTENSION; POAG IS PRIMARY OPEN ANGLE GLAUMA; VDD IS VERTICAL DISC DIAMETER; HDD IS HORIZONTAL DISC DIAMETER.

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Ocular hypertension (OHT) is a condition in which high intraocular pressure (IOP) is not accompanied by the optic disc or visual field changes suggestive of glaucoma.[1],[2] Various studies have emphasised that few patients with OHT evolve to primary open-angle glaucoma (POAG).[3-8] There have been reports that the optic disc size is large in eyes with normal-tension glaucoma.[9] Differences in the relationship of IOP to the neuroretinal rim area in blacks as opposed to whites have been documented;[10] since the optic disc size is large in blacks as compared to whites,[11] it is possible that larger discs are more susceptible to glaucoma. This study examines the hypothesis that optic disc size is smaller in patients with OHT as compared to normal subjects and patients with POAG.

Materials and Methods

Forty two patients with POAG, 26 with OHT and 30 normal subjects were enrolled in the study. A complete ophthalmic examination including assessment of refractive error, slitlamp evaluation, applanation tonometry, gonioscopy, and stereo-disc evaluation with Volk 78 D lens was carried out in all subjects.

All patients underwent keratometry, axial length determination by A-scan biometry and automated perimetry with the Humphrey visual field analyser using 30-2 program, and the disc photography with Nidek 3dx NM. In 23 eyes disc photography was done with the Zeiss fundus camera. Only one randomly chosen eye of each subject was included in the study.

POAG was defined as glaucomatous disc and visual field changes with raised or normal IOP and open angles. OHT was defined as IOP of more than 21 mm Hg on applanation tonometry with normal visual field and disc morphology. Normal controls had normal discs, normal visual fields, and intraocular pressure of 21 mm Hg.

The optic disc image obtained on a 35 mm colour slide was digitised using a Nikon^™ coolscan. Planimetry was carried out using software developed by us which included Littmann correction. The magnification factor for the Nidek camera was taken as 2.6 and that for the Ziess camera was 1.37. The q value was determined from the axial length or corneal curvature, and the refractive error by a regression equation from the graphs provided by Littmann.[12] This is a measure of the magnification of the retinal image produced by the optics of the eye. The horizontal and the vertical diameters were measured. The optic disc margin was manually outlined by a series of straight lines joining each other in a polygon. The area of the polygon was calculated as the sum of the areas of the constituent triangles. The mean and standard deviation of each of these parameters for the three groups were compared by Analysis of Variance (ANOVA).

Results

The mean, standard deviation and 95% confidence limits of the vertical and horizontal disc diameters and the disc area are shown in [Table - 1]. The ANOVA shows the difference in all the parameters for the three groups to be significant. The post-hoc analysis following the ANOVA revealed that for the disc area the difference between OHT and POAG as well as normal subjects was significant at p<0.0001. The horizontal disc diameter was significantly smaller in OHT as compared to both FOAG (p<0.0036) and normal subjects (p<0.0001). Similarly, the vertical disc diameter also was significantly smaller in OHT as compared to POAG (p<0.014) and normal subjects (p<0.0003). There was no statistically significant difference for any of the parameters between the POAG and normal subjects.

Though the disc parameters were significantly smaller in the OHT group as compared to the POAG and the normal subjects, the absolute values for all the parameters were abnormally high. To understand the reason for this we looked at the disc parameters in 16 normal and 7 ocular hypertensive eyes that had disc photographs with both Nidek and Zeiss cameras. The mean and standard deviation (SD) of horizontal and vertical disc diameters and the disc area for these 23 eyes are shown in [Table - 2]. The diameters were larger by a factor of 1.8 and the area was greater by a factor of 3.3 with the Nidek camera as compared to the Zeiss camera.

Discussion

Definition of OHT

Epidemiological studies[3][4][5][6][7][8] have firmly established the presence of a population with raised IOP without visual damage. This group of eyes with ocular hypertension is characterised by an IOP higher than the statistical mean plus two standard deviations of the population distribution of IOP, with no optic nerve head or visual field changes suggestive of glaucoma with gonioscopically open angles.

A series of editorials debated the need to treat these patients but agreed on the definition as raised IOP with open angle and normal optic disc and visual field.[12][13][14] Over time, in an effort to more closely identify the subset of OHT patients who are at risk of developing a field loss, patients with ophthalmoscopic features of disc damage with normal visual fields have been categorised as OHT by some authors[15][16][17][18][19][20] while others still use normal disc appearance as a part of the definition of OHT.[21] Since it is known that optic nerve head and nerve fibre layer changes precede visual field damage,[22][23][24][25] we have included in our OHT cohort only eyes with raised IOP with no disc changes suggestive of glaucoma and normal visual fields.

The OHT conversion rate to POAG is known to be very small.[3][4][5][6][7][8] This is probably because OHT comprises eyes that belong to more than one subgroup. One group would consist of eyes that have no damage and probably are not at risk of developing damage in future, some of these eyes in fact may have a falsely recorded high IOP that is secondary to increased corneal thickness. The other group of eyes may have a tendency to develop future damage or have visual damage that cannot be identified by conventional automated perimetry; these could be termed 'pre-perimetric' glaucoma. This latter group may have subtle optic nerve head changes either at the time of evaluation or would develop these changes in due course depending on the time lag between the optic nerve head anatomical changes and the appearance of visual field changes on standard automated perimetry. Because OHT is defined as eyes with no damage we feel that the latter group of "pre-perimetric" glaucomatous eyes should not be included in OHT.

Study data

Using magnification-corrected morphometry and defining OHT as increased IOP with no disc or field changes suggestive of glaucoma, we found that the optic nerve head area (in mm²) in OHT (9.47, 95% CI = 9.04 -9.9) is significantly smaller (p<0.0001) than in both normals (12.11, 95% CI = 11.09 - 13.14) and POAG (12.77, 95% CI - 11.91 - 13.63). Optic disc size was found to be smaller in OHT in another study, but did not reach statistical significance.[21] In this study OHT eyes did not have disc changes suggestive of glaucoma. This brings up the possibility that small discs are resistant to IOP induced damage or that in these eyes visual field changes take a longer time to manifest. The findings that smaller discs are more resistant to IOP-related damage is indirectly supported by the data from the Baltimore Eye Survey. This study showed that blacks have larger optic discs as compared to whites,[11] and that the decrease in neuroretinal rim area in relation to increasing IOP is greater in blacks as compared to whites.[10] While this difference could be related to racial factors, the disc size could be a contributing factor.

Study limitations

The abnormally large measurements recorded with the Nidek camera as opposed to the Zeiss is a concern with the data. An error in the magnification factor of 2.6 was considered but the manufacturer refuted this possibility. This error will not affect the results because the study seeks to compare the optic nerve head size in OHT with that in POAG and normals. Since the measurement in all groups is subjected to the same factor the relative size in OHT as compared to POAG and normals is still significant.

The other limitation of this study could be a bias in the sample towards specifically taking small optic discs into the OHT group as eyes with increased cup-disc ratio, raised IOP and no visual field changes were not included in the cohort of OHT. On the other hand studies that include eyes with disc changes suggestive of glaucoma with no field changes into OHT could be including 'pre-perimetric' glaucomas into OHT.

It would be useful to include in OHT eyes with cup-to-disc ratios normal for the size of the disc in the given eye. To the best of our knowledge such data have not been reported. In conclusion, the data from the present study show that the optic disc size in OHT is smaller than that in POAG and normals.

Acknowledgement

This study was supported in part by a. grant from Hyderabad Eye Research Foundation, Hyderabad, India.

References

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