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   Table of Contents      
COMMUNITY OPHTHALMOLOGY
Year : 2005  |  Volume : 53  |  Issue : 4  |  Page : 289-294

Correlation of confocal laser scanning tomography with planimetric photographic measurements of the optic disc in a normal South Indian population: The Vellore eye study


1 Department of Ophthalmology, Christian Medical College, Vellore , Tamil Nadu, India
2 Community Health Department, Christian Medical College, Vellore , Tamil Nadu, India
3 Department of Ophthalmology, Faculty of Clinical Medicine Mannheim, University of Heidelberg, Mannheim, Germany

Correspondence Address:
Ravi Thomas
L.V.Prasad Eye Institute, Banjara Hills, Hyderabad - 500 034, Andhra Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0301-4738.18916

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  Abstract 

Purpose: To compare the confocal laser tomographic scanning evaluation with photographic measurements of the optic nerve head in a South Indian population.
Methods: The prospective comparative clinical non-interventional epidemiologic study included 62 subjects (62 eyes) forming a population-based sample, selected randomly. Mean age was 47.2 ± 8.9 years, mean refractive error measured was -0.17 ± 1.10 diopter (range, - 4.50 to + 2.50 diopter). Confocal laser scanning tomographic images on Heidelberg Retina Tomograph (HRT) and colour optic disc photographs were morphometrically analysed and compared. Main outcome measures were morphologic optic disc parameters.
Results: The optic disc area measurements were significantly smaller (p < 0.001) in the HRT technique than in the photographic method. In contrast, the HRT as compared to the photographic measurements showed significantly (p < 0.001) larger values for the relative width and relative area of the neuroretinal rim. The differences in measurements between both methods were maximum in the nasal part (p < 0.001) of the optic disc and minimum in the temporal disc region.
Conclusion: In normal eyes of South Indians, neuroretinal rim measurements by the HRT and expressed as percentage of disc size measurements are significantly larger than rim measurements on disc photographs.

Keywords: confocal laser scanning tomograhy; Heidelberg retina tomograph; neuroretinal rim; optic disc


How to cite this article:
Thomas R, George R, Muliyil J, Jonas JB. Correlation of confocal laser scanning tomography with planimetric photographic measurements of the optic disc in a normal South Indian population: The Vellore eye study. Indian J Ophthalmol 2005;53:289-94

How to cite this URL:
Thomas R, George R, Muliyil J, Jonas JB. Correlation of confocal laser scanning tomography with planimetric photographic measurements of the optic disc in a normal South Indian population: The Vellore eye study. Indian J Ophthalmol [serial online] 2005 [cited 2020 Aug 13];53:289-94. Available from: http://www.ijo.in/text.asp?2005/53/4/289/18916



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Confocal laser scanning tomography of the optic nerve head is a standard technique in documenting the morphology of the optic disc.[1],[2],[3],[4],[5],[6],[7],[8],[9],[10],[11]The digitised image of the optic nerve head is given in false colours which resemble the natural appearance of the optic nerve head. Any digitising method may change the image of an object. Differences in measurements have been noticed between confocal laser scanning tomographic measurements and planimetric measurements of clinical photographs of the optic nerve head in Caucasians.[12] The morphology of the optic nerve head depends on the ethnic background. [13],[14],[15],[16] No such study has been performed on the Indian population. The aim of the present study was to compare confocal laser scanning tomographic measurements of optic disc with planimetric measurements of clinical photographs of optic disc.


  Methods Top


This prospective comparative clinical non-interventional epidemiologic study included 62 subjects (38 women), which formed a population-based sample selected in a random manner. In a previous study,[17] using a stratified simple random sampling technique, a study population of 1932 persons from 12 random clusters of a defined urban population in Vellore, Tamil Nadu, was identified. Out of this population group, 1521 subjects could be contacted. Of these 1521 subjects, 972 (63.9% of the invited population) persons were examined in the Vellore Eye Survey. All these 972 persons were re-invited for a review examination. A subset of 300 persons previously diagnosed as normal out of these 972 subjects was chosen to be invited for the present study so as to recruit a minimum of 100 control subjects. They were offered free examination and treatment at the hospital. A social worker attempted to contact all selected individuals to give a specific date for examination. In case of non-response the social worker once again tried to contact the selected individual population to fix a fresh appointment date. Of the 300 selected and contacted 110 persons responded. Seventy-five subjects had changed residence and could not be traced, 23 persons could not be contacted, one person was invalid and could not come to the hospital, one person refused the examination, and 90 persons did not come for examination despite repeated requests. Confocal laser scanning tomographic images and colour optic disc photographs of satisfactory quality could be obtained in 62 out of 110 persons (56.4%). Details of the survey methodology and results of the Vellore Eye Study have been published earlier.[17] The methods applied and the study itself adhered to the tenets of the declaration of Helsinki for the use of human subjects in biomedical research. Informed consent was obtained from each subject before enrollment. Mean age was 47.2 ± 8.9 years (median, 47.5 years; range, 32-69 years), mean refractive error measured -0.17 ± 1.10 diopter (median, 0 diopter; range, -4.50 to +2.50 diopter). Only one randomly selected eye (35 right eyes, 27 left eyes) per subject was taken for statistical analysis.

All subjects underwent a standardised ophthalmic examination including keratometry, refractometry, measurement of visual acuity, applanation tonometry, slit lamp biomicroscopy of anterior segment of the eye, gonioscopy, ophthalmoscopy, and sonographic determination of the anterior chamber depth and axial length of the globe. Confocal laser scanning tomography was performed using the Heidelberg Retina Tomograph (HRT 2; Heidelberg Engineering, Dossenheim, Germany), for the optic nerve head by an experienced examiner (RG). To obtain the horizontal and vertical diameters of the optic disc and optic cup and the width of the neuroretinal rim in the various disc regions, we took the printouts of the confocal laser scanning tomographic images and measured the disc and cup diameters and the rim width on the print-outs using a ruler. We corrected these measurements for the magnification of the printouts by calculating the magnification factor as ratio of disc area measurements (as given by the tomographic examination) to the disc area measurements (as obtained by measuring the disc area on the print-outs using the ruler). The method has already been described in detail.[16]

Using a conventional fundus camera (Carl Zeiss, Oberkochen, Germany; camera model number FF450), 30° sequential stereo colour optic disc photographs were also taken after mydriasis for all subjects. The optic disc slides were projected in a scale of 1-15. The outlines of the optic cup, optic disc, and peripapillary scleral ring were plotted on paper and morphometrically analysed. To obtain values in absolute size units, i.e. millimeter or square millimeter, the ocular and photographic magnification was corrected using the Littmann method.[18] The optic cup was defined on the basis of contour and not of pallor. The border of the optic disc was identical with the inner side of the peripapillary scleral ring.[19]


  Results Top


Optic disc

Mean area of the optic disc was significantly ( p < 0.001) smaller in the confocal laser scanning tomographic measurements (2.24 ± 0.53 mm2, mean ± SD) than in the planimetric measurements of the clinical photographs (2.58 ± 0.67 mm2) [Table - 1]. The mean difference in optic disc area between both techniques was 0.33 ± 0.51 mm2 (range, -1.44 to 1.30 mm2). Expressed as percentage of the disc area as measured by the HRT, it was 16.4 ± 22.0% (range, -39 to 78%). For both techniques, the area of the optic disc was statistically independent of age ( p > 0.50), refractive error ( p > 0.35), axial length of the eye ( p > 0.50), and anterior chamber depth ( p > 0.45). For both methods, there was a slight difference in optic disc area between females and males. This difference, however, did not reach the level of statistical significance ( p = 0.54 and 0.60).

The shape of the optic disc was for both methods slightly vertical oval, with the vertical disc diameter being about 3-6% longer than the horizontal disc diameter [Table - 1].

Neuroretinal rim

The area of the neuroretinal rim did not differ significantly ( p = 0.33) between the confocal laser scanning tomographic technique (1.64 ± 0.28 mm2) and in the planimetric measurements of the clinical photographs (1.60 ± 0.38 mm2). Expressed as percentage of the disc area, the neuroretinal rim area was significantly ( p < 0.001) larger in the confocal laser scanning tomographic measurements than in the planimetry measurements of the disc photographs [Table - 1].

For both techniques, the area of the neuroretinal rim was significantly and positively correlated with the size of the optic disc. It was statistically independent of age ( p > 0.25), gender ( p > 0.60), refractive error ( p > 0.30), axial length of the globe ( p > 0.35), and depth of the anterior chamber ( p > 0.25). As difference between both techniques, the rim was significantly and positively correlated with the area of the optic cup in the assessment of the clinical photographs (correlation coefficient r = 0.47) However, it was statistically independent of optic cup area in the confocal laser scanning tomographic examinations ( p = 0.44).

The neuroretinal rim width measurements did not vary significantly between the two techniques in the temporal disc region ( p = 0.71). The confocal laser scanning tomographic determinations of the neuroretinal rim width were significantly larger than the photographic planimetric measurements in the inferior disc region ( p < 0.001), the nasal disc region ( p < 0.001), and the superior disc region ( p < 0.001) [Table - 1]. Expressed as percentage of the horizontal disc diameter, the confocal laser scanning tomographic measurements were significantly ( p < 0.001) larger than the measurements on the optic disc photographs in the inferior disc region ( p < 0.001), the nasal disc region ( p < 0.001), and the superior disc region ( p < 0.001) [Table - 1]. In the temporal horizontal disc sector, the HRT measurements and the photographic data did not vary significantly [ p = 0.69]. Correspondingly, the ratios of inferior-to-temporal rim width and superior-to-temporal rim width were significantly ( p < 0.001) higher for the confocal laser scanning tomographic technique than for the photographic method [Table - 1].

On the photographs for all eyes included in the study, the smallest part of the rim was located in the temporal horizontal disc sector covering 600 from the 2 o'clock position to the 4 o'clock position in left eyes, and from the 8 o'clock position to the 10 o'clock position in right eyes. On the printouts of the confocal scanning laser tomographs, the smallest part of the rim was located in all but two (96.8%) eyes in the temporal horizontal disc sector. The widest rim part was located in all but two (96.8%) eyes in the nasal disc sector (nasal 1200) on the print-outs of the confocal scanning laser tomographs, while on the photographs, the widest part of the rim was usually located in the inferior disc sector (900 wide; tilted 300 to the temporal side) or the superior disc sector (900 wide; tilted 300 to the temporal side). Correspondingly, the difference in the relative rim area between the two methods was more marked in the nasal disc sector than in any other disc sector [Table - 1].

Optic cup

Mean area of the optic cup was significantly ( p < 0.001) smaller in the confocal laser scanning tomographic measurements (0.60 ± 0.42 mm2) than in the planimetric measurements of the clinical photographs (0.97 ± 0.40 mm2) [Table - 1]. Expressed as percentage of the disc area, the optic cup area covered a significantly ( p < 0.001) larger proportion of the optic disc when examined by the photographic technique (37.0 ± 8.1%) than that of measured by confocal laser scanning tomography (24.7 ± 12.7%) [Table - 1].

For both methods, the area of the optic cup was significantly ( p < 0.001) and positively correlated with the size of the optic disc. For both techniques, the area of the optic cup was independent of age ( p > 0.602), refractive error ( p > 0.50), gender ( p > 0.30), axial length of the globe ( p > 0.30), and depth of the anterior chamber ( p > 0.40). For both methods, the horizontal optic cup diameter was significantly ( p < 0.05) longer than the vertical cup diameter indicating a horizontally oval shape of the optic cup.

Cup/disc ratios.

As the optic cup area measurements were significantly larger in the photographic technique than that of confocal laser scanning tomographic technique, the horizontal and vertical cup/disc diameter ratio were also significantly larger ( p < 0.001) in the photographic method [Table - 1]. Additionally, the ratio of the horizontal-to-vertical cup/disc diameter ratios was slightly, however not significantly ( p = 0.08) larger in the photographic technique [Table - 1].

For both techniques, the horizontal cup/disc diameter ratio was significantly ( p < 0.05) larger than the vertical cup/disc diameter ratio. Consequently, the mean value of the quotient of the horizontal to vertical cup/disc diameter ratio was higher than 1.0 for both techniques. The horizontal and the vertical cup/disc diameter ratios were significantly correlated with the optic disc area ( p < 0.005) for both techniques. For both techniques, the quotient of horizontal to vertical cup/disc diameter ratio was statistically independent of the optic disc size ( p > 0.25).

The cup/disc area ratio (the equivalent of the optic cup area expressed as percentage of the optic disc area) was significantly larger for the photographic technique than for the confocal laser scanning tomographic method. The cup/disc diameter ratio, the cup/disc area ratio was highly significant ( p <0.001) and positively correlated with the optic disc area for both techniques. It was independent of age ( p > 0.35), refractive error ( p > 0.90), and gender ( p > 0.20).


  Discussion Top


The optic nerve head is the bottleneck of the whole visual afferent pathway, and it is, therefore, of great importance for pathogenesis and detection of glaucoma and other optic neuropathies. For the quantitative examination of the optic nerve head, confocal laser scanning tomography has increasingly been employed. The confocal laser tomographic scanning technique is reproducible [20],[21],[22],[23],[24],[25],[26],[27] and is a relatively new method however, it should correlate well with other clinical techniques. This was the purpose of the present study addressing the question whether optic nerve head measurements as obtained by HRT differ from optic nerve head values as obtained by optic disc photographic measurements in the South Indian population.

Optic Disc

The optic disc area measured significantly smaller by HRT than that of obtained by the optic disc photographs. Confirming previous studies,[16],[28],[29],[30] this finding suggests that the method to correct the magnification of the fundus structures by the optic media of the eye and the camera differ between the technique as incorporated in the HRT and the technique as applied in the morphometry of optic disc photographs. It confirms previous studies, which have shown that the algorithm of the HRT is closer to the real dimensions than the photographic technique.[31],[32] It means clinically, that if a patient moves to another city attending different ophthalmologists with a different techniques to measure the optic nerve head, the optic nerve head measurements can not be compared without correction.

Neuroretinal rim

The neuroretinal rim as the intrapapillary equivalent of the optic nerve fibers is one of the main targets of the morphologic evaluation of the optic nerve. Its size, expressed as raw data of the measurements, did not vary significantly between the two techniques examined [Table - 1]. Expressed as percentage of the optic disc size, however, the rim area measurements were significantly ( p < 0.001) larger for the confocal laser scanning tomographic technique than for the photographic technique.

The difference between both techniques was even more marked, when the width measurements of the neuroretinal rim in the inferior, superior and nasal optic disc regions were compared. Since the percentage of rim area on disc area is a relative measure, it is independent of the magnification by the optic media of the eye and camera. Reason for the discrepancy between the two techniques can, therefore, not be differences in the correction of the magnification of fundus structures, but it may be the algorithm of the HRT. In contrast to the clinical assessment of the optic nerve head, in HRT the retinal vessel trunk, as soon as it is higher than the reference level, is taken as neuroretinal rim, irrespective of whether there is underlying neuroretinal rim or not. It leads to an enlargement of the neuroretinal rim primarily in the nasal sector of the optic disc. In confocal laser scanning tomography, the widest part of the rim is, therefore, usually located in the nasal disc region, in contrast to the clinical assessment of the optic nerve head with the widest part of the neuroretinal rim usually located in the inferior or superior disc region.[33] Corresponding to the differences in the definition of the neuroretinal rim between the two techniques, in the temporal region of the optic nerve head with only few and small blood vessels, the determinations by confocal laser scanning tomography and the measurements on the photographs agree well with each other [Table - 1]. It means clinically, that the so-called ISNT (inferior - superior - nasal temporal) rule describing the physiologic shape of the neuroretinal rim with its widest part in the inferior disc region, followed by the superior, the nasal, and the temporal disc region in decreasing order, cannot fully be transferred to the confocal laser scanning tomographic examinations of the optic nerve head. The main part of the ISNT rule, however, that the narrowest rim should be located in the temporal horizontal 600 of the optic disc is valid for both techniques. The question arises whether a modified formulation of the ISNT rule can be incorporated into the algorithm of the HRT to further refine the morphometric optic nerve head analysis by the HRT.

Similar differences in neuroretinal rim area measurements between both techniques have been reported for Caucasians [16],[28],[29],[30] suggesting that the results of the confocal laser scanning tomographic examinations of the optic nerve head can be transferred from Caucasians to South Indians and vice versa.

Both techniques did not differ in most of the correlations between the optic disc parameters, with the exception of the relationship between neuroretinal rim area and area of the optic cup. In the clinical assessment of the optic disc, the neuroretinal rim was highly significantly correlated with the optic cup area indicating that the larger the optic cup was, the larger was the neuroretinal rim. In the confocal laser scanning tomography, however, area of the optic cup and area of the neuroretinal rim were statistically independent of each other. Again, reason may be the difference in the definition of the neuroretinal rim between both techniques.

Optic cup

In absolute measurements, the optic cup values were significantly larger when measured by assessment of the photographs than that of determined by confocal laser scanning tomography [Table - 1]. It partially corresponds to the larger optic disc area measurements in the morphometric examination of the clinical photographs compared with the confocal laser scanning tomographic technique, since the area measurements of the neuroretinal rim did not vary between the two techniques [Table - 1]. Another reason for the discrepancy between both techniques in the measurements of the optic cup may be the difference in the definition of the neuroretinal rim, as the relative measurements of the optic cup area when expressed as percentage of optic disc area were significantly larger in the photographic technique. Correspondingly, the horizontal and vertical cup/disc diameter ratios and the cup/disc area ratio were significantly higher in photographic measurements of the optic nerve head.

In conclusion, confocal laser scanning tomographic examinations of the optic nerve head differ from photographic morphometric examinations of the optic nerve head. The correction of the magnification by the optic media of the eye and camera may be a systematic source of difference, and the definition of the neuroretinal rim may be a variable source of difference. The latter leads to differences in the applicability of the ISNT rule as well as in the relationship between optic cup area and neuroretinal rim area. These differences in the normals should be kept in mind while evaluating a glaucoma patient on HRT.





 
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    Tables

  [Table - 1]


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