Indian Journal of Ophthalmology

ORIGINAL ARTICLE
Year
: 2001  |  Volume : 49  |  Issue : 1  |  Page : 19--23

Planimetric optic disc parameters in normal eyes: a population-based study in South India


G Chandra Sekhar1, Kalyani Prasad2, Rakhi Dandona3, Rajesh K John4, Lalit Dandona5,  
1 MD. VST Centre for Glaucoma Care, L.V. Prasad Eye Institute, Hyderabad, India
2 DNB. VST Centre for Glaucoma Care, L.V. Prasad Eye Institute, Hyderabad, India
3 B.Opt. Public Health Ophthalmology Service, L.V. Prasad Eye Institute, Hyderabad, India
4 MSC. Public Health Ophthalmology Service, L.V. Prasad Eye Institute, Hyderabad, India
5 MD, MPH. Public Health Ophthalmology Service, L.V. Prasad Eye Institute, Hyderabad, India

Correspondence Address:
G Chandra Sekhar
L.V. Prasad Eye Institute, L.V. Prasad Marg, Banjara Hills, Hyderabad - 500 034, India

Abstract

Purpose: To study the optic disc parameters of normal eyes in a population-based south Indian study. Methods: One hundred and fifty three subjects from a population-based sample of 1060 included in the Andhra Pradesh Eye Disease Survey (APEDS) were enrolled in the optic disc study. The male-female ratio, the refractive error and distribution of other ocular parameters in the disc study subjects and the APEDS were not significantly different. Magnification corrected morphometry of optic disc photographs obtained by Zeiss telocentric fundus camera was carried out in one randomly chosen eye of each of these 143 subjects. Results: The mean optic disc parameters with the 95% confidence intervals for the distribution were: disc area 3.37 mm2 (2.04 - 4.7), vertical disc diameter 2.12 mm (1.67 - 2.57), vertical cup to disc ratio 0.37 (0.19 -0.55) and neuroretinal rim area 2.8 mm2 (1.76 - 3.84). The disc area, the vertical cup to disc ratio and the rim area showed a normal distribution. The cup to disc ratio correlated with the vertical disc diameter but the association was not strong. Conclusion: The disc area, the vertical cup to disc ratio and the neuroretinal rim area are normally distributed in the South Indian population. The normal optic disc parameters would form a basis for future comparisons in different forms of glaucoma.



How to cite this article:
Sekhar G C, Prasad K, Dandona R, John RK, Dandona L. Planimetric optic disc parameters in normal eyes: a population-based study in South India.Indian J Ophthalmol 2001;49:19-23


How to cite this URL:
Sekhar G C, Prasad K, Dandona R, John RK, Dandona L. Planimetric optic disc parameters in normal eyes: a population-based study in South India. Indian J Ophthalmol [serial online] 2001 [cited 2020 Apr 1 ];49:19-23
Available from: http://www.ijo.in/text.asp?2001/49/1/19/22659


Full Text

Morphological changes in the optic nerve head associated with nerve fibre bundle visual field defects are diagnostic of glaucoma. Increased cup to disc ratio is an important parameter in the assessment of disc damage in glaucoma. The cup to disc ratio is affected not only by glaucomatous damage but also by the absolute disc size.[1, 2] The normal vertical disc diameter is reported to vary from 0.9mm to 2.9mm in diameter.[3] This means that a cup to disc ratio smaller than 0.3:1 could be pathological in a small disc and a cup to disc ratio of even more than 0.5:1 could be physiological in a large disc.[4] Further, the size of the disc is known to be associated with various pathologies of the optic nerve. While small optic nerve heads are more common with optic disc drusen,[5] pseudo papilloedema,[6] and non-arteritic ischaemic optic neuropathy,[7] large optic nerve heads are more common in ocular hypertension,[8] normal tension glaucoma,[9] pits of the optic nerve head[10] and morning glory syndrome.[11]

Normal optic disc parameters have been reported in the Caucasian population.[1, 2, 3] Similar measurements of the optic disc have not been done for the Indian population. This study was undertaken to assess optic disc parameters in a population in southern India.

 Materials and Methods



The Andhra Pradesh Eye Disease Study (APEDS) is a population-based epidemiological study in four areas, urban and rural, representative of the Indian State of Andhra Pradesh. Detailed methodology of APEDS is reported elsewhere.[12, 13] This study was approved by the Ethics Committee of the L.V.Prasad Eye Institute, Hyderabad. Expecting to achieve a recruitment rate of 85%, 2954 subjects representative of the population of Hyderabad were sampled for the urban segment of APEDS in Hyderabad city using a stratified, random, cluster, and systematic sampling procedure, of which 2522 subjects underwent a detailed dilated eye examination. As part of this study we wished to get information on the optic disc parameters and obtained disc photographs from 18 November 1996 to 1 February 1997. During this period a total of 1060 subjects were evaluated. Of these 724 subjects were more than 15 years of age, and 509 of them did not have any ocular pathology. From this group of 509 subjects without any ocular pathology, a random selection of 153 subjects was done for the disc study. In these subjects, optic disc photographs were taken with Zeiss telocentric fundus camera (30° view) (Carl Zeiss, Oberkochen, Germany) in one randomly selected eye. Bausch and Lomb keratometer (Bausch and Lomb Ophthalmic Instruments, Rochester, New York, USA) was used to record the corneal topography. Ophthalmic technicians trained in the study procedures performed the photography and keratometry. Patients with refractive error greater than ± 5.0Dsph were excluded.

 Planimetry



The optic disc image obtained on a 35 mm slide is digitized using a Nikon™ cool scan (Nikon Inc., Melville, New York, USA). The software used for the image analysis is developed on a personal computer under Windows environment using Visual C++. The system takes the patient details, scanned image and its resolution, corneal curvature and the refractive error as the inputs. The actual dimensions on the optic nerve are calculated with the formula:

T = pqs

T = actual fundus dimension

p= magnification factor of the camera (1.37 as provided by the manufacturer)

q= magnification factor of the eye

s= measurement on the film

The q value was determined from the corneal curvature and the refractive error by a regression equation from the graphs provided in the publication by Littmann.[14] This is a measure of the magnification of the retinal image produced by the optics of the eye. The value of S is derived by converting the number of pixels into millimeters based on the scan resolution. The disc and the cup outlines were manually drawn by a series of straight lines joining each other as a polygon by one of the authors ( KP) with one-year glaucoma fellowship training. The area of this polygon was calculated as the sum of the areas of the constituent triangles. Optic discs with crescent or gross oblique entry were not seen in this series. The cup was outlined as the central pale depressed area. If there was a discrepancy between the area of pallor (colour cupping) and the site of bending of the vessel (contour cupping), the latter was taken as the true cup margin. The neuroretinal rim was calculated by subtracting the cup area from the disc area. The horizontal and vertical diameters of the optic disc and the optic cup were also measured. The values measured were directly transferred to a Microsoft Access database with open database connectivity.

 Statistical analysis



Descriptive statistics were used to describe each variable. Kolmogrov-Smimov test of normality was used to check if the variables were normally distributed. Pearson's correlation coefficient determined the significance of the correlation between variables.

 Results



Of the 1060 subjects evaluated during the disc study period, 724 subjects were more than 15 years of age, and 509 of them did not have any ocular pathology. From this group of subjects 153 subjects with no ocular pathology were randomly selected for the disc study. Ten subjects were excluded due to poor quality of the photographs, leaving a study population of 143 subjects, with 70 (49%) males and 73 (51%) females. The clinical and demographic parameters of the 143 subjects of the disc study were compared with the reamiaing 508 normal subjects who had no ocular pathology. The data are presented in [Table:1]. As an eye was randomly selected for the disc study, the ocular parametes in the right and the left eye are separately compared in the analysis. There was no statistically significant difference between the two groups for all the parameters.

The planimetry results are detailed in [Table:2]. The optic disc area ranged from 1.95 mm2 to 6.82 mm2 (mean and SD = 3.37 ± 0.68). The optic cup area ranged from 0.12 to 1.89 mm2 (mean and SD = 0.57 ± 0.34 mm2). The (NRR) area ranged from 1.81 to 5.27 mm2 (mean ± SD = 2.8 ± 0.53).

The mean horizontal disc diameter was 1.94 ± 0.20 mm (range = 1.50 - 2.82 mm) and mean vertical disc diameter was 2.12 ± 0.23 mm (range = 1.57 - 3.11 mm). Cup to disc ratio by diameter ranged horizontally from 0.18 - 0.67 (mean 0.39 ± 0.09) and vertically from 0.19 -0.63 (mean 0.39 ± 0.09).

The disc area, the vertical cup to disc ratio and the rim area showed a normal distribution [Figure:1]-[Figure:3]. There was a positive correlation between vertical disc diameter and the vertical cup to disc ratio [Figure:4].

The coefficient of variation of the parameters was estimated. The disc area and the rim area had a coefficient of variation of 20% and 19% respectively, whereas the cup area and the cup to disc ratio had a coefficient of variation of 60% and 50%.

 Discussion



The subjects enrolled in this study were part of the population-based APEDS study. Comparison of the demographic, refractive and other ocular parameters of the study subjects and the APEDS population shows no statistically significant difference between the two groups for any of the parameters. The optic nerve head parameters of the present study are tabulated along with the results from similar studies in literature in [Table:3]. It is seen that the disc area in the current study is the largest, larger than the disc area in the African-American population reported in the Baltimore study.[15] A possible reason for this could be ethnic variation. But an equally important likely explanation is measurement variations related to the different techniques used in the various studies. The imaging systems generally used are conventional photography and the confocal scanning laser ophthalmoscopy (CSO) systems. In both these systems the margin of the disc (CSO), or the disc and the cup (conventional photography) have to be manually drawn to compute the measurements. This introduces an element of subjectivity and measurement variability into these systems. This variability along with possible measurement variations secondary to the differences in the technologies involved could confound the comparisons between studies. No two studies quoted in the above table have used the same technique. The methodology was image analysis of the stereo optic disc photograph manually in Jonas's study;[1] computer image analysis by Rodenstock in Caprioli[3] study and stereo image computerised planimetry in Britton study.[2] The Baltimore study used the Topcon Imagenet.[15]

A linear correlation between the disc size and the cup to disc ratio has been reported earlier.[16] The increase in the cup to disc ratio for each millimeter increase in the disc diameter was 0.27 in the Blue Mountain study.[15] In the current study, though there was a correlation between the disc size and the cup to disc ratio, the association was not strong enough to calculate a regression equation to estimate one from the other. Realization of the correlation between disc size and the cup to disc ratio is important in the clinical assessment of glaucomatous optic nerve head changes. Optic nerve heads with small discs can have significant visual field damage before a significant cupping develops. Conversely, a large disc may have an ominous looking disc cupping without any visual field damage.

The coefficients of variation for the parameters involving the cup measurements in the present study were very large. A possible reason for this is an error in marking the cup outline. Though a trained specialist did these, they were not marked simultaneously while viewing the original disc photograph. An ideal way of doing this would be to view the disc photograph stereoscopically and mark the disc and the cup outlines. This was not possible in the current study, as the disc photographs were not stereoscopic. Our current practice is to digitize stereo photographs obtained by Nidek stereo camera (Nidek Aichi, Japan) and use these images for the planimetry.

Optic disc size variation is associated with different disc pathologies. Small optic nerve heads are associated with optic disc drusen,[5] pseudo papilloedema[6] and non-arteritic ischemic optic neuropathy.[7] Pits of the optic nerve head[10] and morning glory syndrome[11] are more common in large optic nerve heads. As regards glaucoma and the optic nerve parameters; ocular hypertension is more common in small optic nerve heads[8] and normal tension glaucoma[9] in large optic nerve heads. To correlate these in a given population we must have an estimate of the optic disc parameters in the normal population for comparison. The disc area and the disc diameters with their 95% confidence limits reported here would be useful for future studies aimed at elucidating the possible relationship of the disc size to various optic nerve head pathologies.

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