M. S. Shehzad  ( Benevolent Trust (LRBT) Eye and Cancer Hospital, Lahore. )
K. M. Ashraf  ( Benevolent Trust (LRBT) Eye and Cancer Hospital, Lahore. )
M. Siddique  ( Benevolent Trust (LRBT) Eye and Cancer Hospital, Lahore. )
Z. A. Qazi  ( Benevolent Trust (LRBT) Eye and Cancer Hospital, Lahore. )
M.T.Khan  ( Benevolent Trust (LRBT) Eye and Cancer Hospital, Lahore. )
M.T.Khan  ( Benevolent Trust (LRBT) Eye and Cancer Hospital, Lahore. )

Cup disc ratio measurements are one of the most popular methods not only for early detection but also for monitoring progression of glaucoma. Especially when monitoring the treatment effect on glaucoma, accurate and reproducible measurements of disc are necessary.1 Disc evaluation is important because clinically detectable glaucomatous damage in the optic disc may precede the onset of white on white visual field defects by several years.2-4 Clinical evaluation of optic disc morphology can be made with direct ophthalmoscope, slit lamp biomicroscopy with 78D and 90D and with indirect ophthalmoscope.5
Clinical evaluation of the optic nerve head is notoriously subject to variation between observers though agreement can be substantial given the right conditions. The development of imaging and measurement techniques has brought the prospect of greater objectivity in the measurement of optic disc features, and therefore better agreement between observers. Confocal Scanning Laser Ophthalmoscope is the most advanced method in current use that provides objective, reproducible, quantitative and three-dimensional analysis of the optic disc. Image analysis on the HRT II has been shown to produce optic disc measurements, such as the estimation of cup/disc area ratio, which are comparable to those made by clinician. Heidelberg Retina Tomograph, software version 1.5, Heidelberg Engineering, Germany, uses a Confocal Scanning Laser Ophthalmoscope (Diode 670 nm) to produce a topographic map of optic nerve head. The light reflected from the surface of retina is imaged. Each image is formed by tiny squares called pixels. Totals of 32 images are taken. The first of these 32 is parallel to the surface of the retina, just anterior to blood vessels as they emerge from the optic cup. Subsequent images are acquired by advancing the focal plane posterior to the lamina cribrosa. A plane is chosen as the reference plane from which the parameters of interest can be calculated. Computer converted three-dimensional reconstruction, can be used to evaluate disc damage in glaucoma.6-8
The HRT software computes a set of stereo metric parameters that quantitatively describe the shape of the optic nerve. Only the vertical and horizontal cup disc ratios were taken from these parameters.
Although more objective measurements can be achieved by Heidelberg retina tomograph II, but it does not mean that this is more accurate in estimating disc measurements. This device uses the clinician induced contour line to define disc margin, whereas in slit lamp examination with 90D lens and in stereoscopic photographs, one evaluates the cup disc ratio by observing the three dimensional shape of cup, including the bending of small blood vessels in the optic disc and the disc border is much more delineable then in tomograph.

Thirty glaucomatous patients were selected from glaucoma unit of LRBT Eye and Cancer hospital, Lahore on simple random basis, provided they met the following criteria: age between 20-60 years, spherical Refractive error less then -5.0 or above +5.0 and astigmatic error less than -2.0 or above 2.0, primary open angle on gonioscopy, intraocular pressure more than 22 in at least 3 consecutive times and glaucomatous visual field defects on Humphrey visual field analyser. Normal persons were selected from the staff of LRBT eye hospital, Lahore and relatives of glaucoma patients on simple random basis provided they had intraocular pressure less then 22 in at least three sittings, normal appearance of optic nerve head and normal visual fields.
All subjects underwent full ophthalmological examination including visual acuity, intraocular pressure, gonioscopy, slit lamp examination with 90D lens, dilated fundus examination with direct and indirect ophthalmoscope, slit lamp examination with 90 D super field lens, stereoscopic photography, and HRT II evaluation of both eyes. Two experienced ophthalmologists evaluated the cup disc ratio both in horizontal and vertical meridian on Haag Striet slit lamp with super field 90D lens.
Simultaneous stereoscopic photographs were taken on telemetric fundus camera (Topcon TRC-SS2, Topcon optical Co., Japan) by lateral shifting of camera. Topcon image net system was used for stereo viewing on colour monitor. Both the ophthalmologists were masked to patient diagnosis and all other clinical evaluation. They were asked to evaluate horizontal and vertical cup disc ratio on the basis of stereo viewing on colour monitor with the help of +10 dioptres lens in front of both eyes.
Heidelberg retina tomograph II (HRT II) software version 1.5.0 developed by Heidelberg Instruments, Heidelberg, Germany, is a Confocal scanning Laser Ophthalmoscope for acquisition and analysis of three-dimensional image of posterior segment of eye. It is an upgraded version of laser tomographic scanner. It scans in three dimensions of the space using a Diode laser (670 nm). To acquire the digital image of optic nerve head a laser beam is focused on the retina and periodically deflected by oscillating mirrors so that a 2 dimensional image of retina is scanned sequentially. If a series of optical images is acquired for different positions of focal plane, the result is a layered three-dimensional image referred as laser scanning tomography. At this point, the only manual step in the analysis process was required, the definition of the optic disc margin. A single experienced operator then drew the contour line with the help of a computer mouse system. This manually marked outline of the disc margin was used to determine and calculate the disc area. In case of peripapillary atrophy, beta zone was used for marking the contour line. This beta zone is significantly associated with both visual field and optic disc glaucomatous damage.
Student's t test was applied with the help of computer based statistical program for social sciences (SPSS). This gave the difference between clinical evaluation and HRT II measurements and stereoscopic photographs verses HRT II measurements.

Mean horizontal and vertical cup/disc ratio measurement were different between the two observers and there was a moderate agreement among the clinical evaluation and stereoscopic photographs and with Heidelberg Retina tomograph II. Difference between the Heidelberg assessment and stereoscopic photographs measurements varied by the ophthalmologists.
Interestingly difference of more than 0.2 between the clinical observation and HRT II findings was found in 14 observations by each ophthalmologist. For horizontal cup disc ratio the difference of more than 0.2 was found in 13.33% by ophthalmologist A and 15% by ophthalmologist B. For vertical cup disc ratio the same difference was 10% by ophthalmologist A and 8.33% by ophthalmologist B.

Table 1. Mean difference between stereoscopic photographic evaluation and automated analysis by HRT II.

Cup/Disc ratio Ophthalmogist A Ophthalmogist B Vertical -0.08±0.02 0.05±0.03 P<0.75 P<0.03 Horizantal -0.15±0.03 -.0.06±0.03 P<0.001 P<0.031

Mean ± standard deviation for HRT II assessment of vertical cup disc ratio in normal subjects was 0.243 ± 0.284 and horizontal cup disc ratio was 0.256 ± 0.175. In glaucoma assessment the cup disc ratio was 0.564 ±0.324 and for horizontal cup disc ratio was 0.529 ± 0.246. Mean difference between the photographic and HJRT II assessment are presented in Table 1. Mean difference between the clinical assessment and HRT II assessment are presented in Table 2.

Table 2. Mean difference between clinical evaluation of cup disc ratio and automated analysis by HRT I I.

Cup/Disc ratio Ophthalmogist A Ophthalmogist B Horizantal -0.15±0.05 0.25±0.03 P Value 0.001 P Value 0.076 Vertical -0.11±0.03 -0.09±0.03 P Value 0.001 P Value 0.001

Difference between photographic and Heidelberg assessment were larger in estimating horizontal then vertical cup disc ratio and also varied by the two ophthalmologists.

Table 3. Difference of more than 0.2 between clinical evaluation by ophthalmologist and HRT II.

Ophthamologist Cup/disc Ratio Discrepancy of more than 2(numbers of cases) Percentage A Horizantal 6 10 Vertical 8 13.33 B Horizantal 5 8.33 Vertical 9 15

Measurements of mean ± standard deviations of horizontal cup disc ratio were smaller in Heidelberg assessment than clinical or photographic measurements. No significant difference was found in vertical cup disc ratio and in mean ± standard deviations of glaucomatous verses normal cup disc ratio.

The optic disc analysis is the only objective method for detection of early glaucoma.11 The purpose of our study was to find out the reliability of Heidelberg retina tomograph by comparison with the conventional clinical techniques for cup disc ratio measurements.
In our study, it was found that clinical measurement of cup disc ratio by the two ophthalmologists was in the close range of tomographic measurements. However when represented statistically, it was found that there was significant difference between the 2 values in vertical and highly significant difference in horizontal cup disc ratio. No significant difference was found in glaucomatous verses normal optic disc in regard to their cup disc ratio measurements.
Measurements by stereoscopic photographs analysis through Image net system showed better correlation with automated tomograph than with clinical observations. Vertical measurements by one observer showed no significant difference and other one showed significant difference from automated technique. However, the horizontal measurements showed significant to highly significant differences by the two observers from automated technique. Therefore, the agreement between the observers and automated technique was better in vertical cup disc ratio than horizontal. Heidelberg measurements were smaller than clinical or photographic measurements in 74% cases and were larger in 26% cases. Various studies have compared the HRT measurements with stereoscopic methods. Zangwill et al studied the agreement between clinician measurements and the scanning laser ophthalmoscope when measuring vertical and horizontal e scanning laser ophthalmoscope when measuring vertical and horizontal cup-disc ratios. They found agreement was moderate to substantial for vertical cup-disc ratios and fair to moderate for horizontal cup-disc ratios in three glaucoma experts.6 Hatch et al recently found that agreement between the planimetric method and HRT when measuring area cup-disc ratios was moderate to substantial among the observers.12 In some other studies, HRT measurements of cup disc ratio were found to be significantly smaller than photographic techniques13 and measurements of absolute rim area and rim to disc area were found significantly larger with the HRT compared with planimetry of disc photographs.14 These findings are comparable with the results of the present study.
There are several explanations to these differences. Clinician measures the cup disc ratio by first considering the disc margin followed by the cup margin from the slope of cup and small vessels angulations at the periphery of cup.15 In clinical, photographic and automated techniques optic disc border has to be outlined manually. Since the optic disc margin can better be detected clinically and on stereoscopic disc photographs than on HRT II images, a different outlining of the optic disc may partially explain the discrepancy between the two methods. HRT II uses operators defined disc margin (contour line) and disc area is total area enclosed by the contour line while the cup area (mm2) as the area enclosed by the contour line and located beneath the reference plane (reference plane is an arbitrary plane 0.32 mm below the reference ring).
There was better agreement for vertical cup disc ratio then horizontal in both comparisons. Reason of this difference is that there is difficulty in measurements of horizontal cup disc ratio because of obscuration by blood vessels. Blood vessel trunk present at the cup area may be the reason of another finding that tomographic measurements were smaller then the clinical. Another reason that can explain the discrepancy among the different technique is that the tomograph measures parts of the central retinal vessel trunk as if it belongs to the neuroretinal rim even if there is no nerve fibre tissue beneath the vessels. It does not distinguish between different types of tissue. Since the retinal vessels of the optic nerve head cross the reference plane, they are partially included in the measurement of the neuroretinal rim area. In contrast, clinically and photographically retinal vessels are considered to belong to the neuroretinal rim only, if neuroretinal rim tissue with retinal ganglion axons is underlying them. Other researchers showed similar results.16,17

1. Schumann JS, Muhammad RL, Echelman D, et al. Optic nerve head and nerve fibre imaging. In: Albert DM, Jakobiec FA (editors).Principles and practice of ophthalmology. 2nd ed. Philadelphia: W B Saunders 2000, pp. 2641-43.

2. Kamal DS, Garway-Heath DF, Hitching RA, et al. Use of sequential Heidelberg retina tomograph images to identify changes at the optic disc in ocular hypertensive patients at risk of developing glaucoma. Br J Ophthalmol 2000;84:993-8.

3. Zeyen TG, Caprioli J. Progression of disc and field damage in early glaucoma. Arch Ophthalmol 1993;111:62-5.

4. Caprioli J, Prum B, Zeyen T. Comparison of methods to evaluate the optic nerve head and the nerve fiber layer for glaucomatous damage. Am J Ophthalmol 1996;121:659-67.

5. Kronfeld PC. History of glaucoma. In: Tasman W, Jaeger EA. Duanes Ophthalmology CD-Rom edition. 1997;Qurey (group 3041).

6. Zangwill L,Irak I, Berry CC, et al. Effect of cataract and pupil size on image quality with confocal scanning laster ophthalmoscopy. Arch Ophthalmol 1997;115:983-90.

7. Caproili J, Prum B, Zeyten T. Comparison of methods to evaluate the optic nerve head and nerve fibre layer for glaucomatous changes. Am J Ophthalmol 1996;121:659-67.

8. Elkington AR, Frank HJ, Greaney MJ. Clinical optics, third edition, Oxford: Blackwell Science Ltd., 1999, p. 227.

9. King AJW, Bolton N, Aspinall P, et al. Measurement of peripapillary retinal nerve fiber layer volume in glaucoma. Am J Ophthalmol 2000;129:599-607.

10. Wendy VH, John GF, Edward EE, et al. Laser scanning tomography of optic nerve head in ocular hypertension and glaucoma. Br J Ophthalmol 1997;81:871-6.

11. Miglior S, Albe E, Guareschi M, et al. Intra observer and inter observer reproducibility in the evaluation of optic disc stereometric parameters by Heidelberg retina tomograph. Ophthalmology 2002;109:1072-7.

12. Hatch WV, Trope GE, Buys YM, et al. Agreement in assessing glaucomatous disc in a clinical teaching setting with stereoscopic disc photograph, planimetry and laser scanning tomography.J Glaucoma 1999;8:99-104.

13. Spencer AF, Sadiq SA, Pawson P, et al. Vertical optic disk diameter: discrepancy between planimetric and SLO measurements. Invest Ophthalmol Vis Sci 1995;36:796-803.

14. Jonas JB, Mardin CY, Grundler AE. Comparison of measurements of neuroretinal rim area between confocal laser scanning tomography and planimetry of photographs. Br J Ophthalmol 1998;82:362-6.

15. Albert D, Jost B, Jonas JB, et al. Comparison between tomographic scanning evaluation and photographic measurement of the neuroretinal rim. Am J Ophthalmol 1996;121:494-501.

16. Jonas JB, Mardin CY, Grundler AE. Comparison of measurements of neuroretnal rim area between confocal laser scanning tomography and planimetry of photograph. Br J Ophthalmol 1998;82:362-6.

17. Dichtl A, Jonas JB, Mardin CY. Comparison between tomographic scanning evaluation and photographic measurement of the neuroretnal rim. Am J Ophthalmol 1996;1121:494-501.