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The Area of the Neuroretinal Rim of the Optic Nerve in Normal Eyes
The Area of the Neuroretinal Rim of the Optic Nerve in Normal Eyes Robert J. Britton, M.D., Stephen M. Drance, M.D., Michael Schulzer, M.D., Gordon R. Douglas, M.D., and Daniel K. Mawson
We randomly photographed 113 eyes in 113 people without ocular disease. The area of the disk and its neuroretinal rim as well as the width of the disk and cup were measured. The measurements were corrected for magnifica tion of each eye by measuring its refraction and axial length. A linear correlation between disk area and neuroretinal rim area was found (r = .75) as well as a correlation between the disk area and the cup area (r = .83). There was no correlation between age and disk area or rim area. THE DECISION AS TO whether an individual optic disk is normal or abnormal is important in the diagnosis of glaucoma. The classic features of the optic nervehead that enable clinicians to detect early acquired optic nervehead damage in glaucoma suspects are well known. Acquired tissue changes at the optic nervehead may precede localized visual field defects 1,2 while a diffuse loss of axons has also been shown to occur in glaucoma suspects 3 in the absence of localized field defects. Congenitally large optic disks are associated with large cups. The width of the neuroretinal rim has been shown to be more constant and largely independent of disk size. 4 There is a considerable overlap of the cup diameter and, therefore, the cup-to-disk ratio between ac quired enlarged cups and congenitally large cups. The assessment of the optic nerve in glaucoma has recently been enhanced by the use of stereophotography to measure the size of the neuroretinal rim by computerized pla-
Accepted for publication Dec. 29, 1986. From the Department of Ophthalmology (Drs. Brit ton, Drance, and Douglas and Mr. Mawson) and the Department of Statistics and Medicine (Dr. Schulzer), University of British Columbia, Vancouver, Canada. This study was supported by Medical Research Council Grant 68-1578 (Dr. Drance). Reprint requests to S. M. Drance, M.D., Eye Care Centre, 2550 Willow St., Vancouver, BC V5Z 3N9.
nimetry. 5 This has been shown to be a more sensitive method of demonstrating acquired change of the optic nervehead in glaucoma 5,6 than the traditional cup-to-disk ratio, probably because the latter does not take into account the size of the individual disk. Before being able to decide whether a neuro retinal rim area is abnormal, not only the size of the disk but also the physiologic behavior of the disk and neuroretinal rim area with age must be taken into account. Schwartz and Reuling 7 have summarized previous studies that dealt mainly with the cup-to-disk ratio and age, but no measurements of the areas of the disk or of the neuroretinal rim were taken. However, the diameter of the disk has been reported to enlarge with age in normal people, which leads to an enlargement of the cup while the neuro retinal width area remains unchanged. 8 Con versely, decreases in the size of the scleral canal associated with a decrease in cup size have recently been reported in glaucoma pa tients when intraocular pressure was low ered. 9,10 A reduction in the axial length of the eye with increasing age 11 has also been de scribed. Therefore, it might be expected that disk size could decrease with age. 12 We have studied a normal population of all ages over 20 years with various refractive errors to provide the normal ranges for the neuroreti nal rim area.
Subjects and Methods The study included 113 eyes of 113 normal people w h o had a visual acuity of 20/20 or better, intraocular pressures less than 21 mm Hg, normal visual fields, absence of ante rior segment abnormality on slit-lamp examina tion, and normal retinal examination including an absence of focal defects on retinal nerve fiber layer photography. The subjects were aged from 20 to 81 years. No exclusions were
©AMERICAN JOURNAL OF OPHTHALMOLOGY 103:497-504, APRIL, 1987
497
498
April, 1987
AMERICAN JOURNAL OF OPHTHALMOLOGY
made on the basis of refractive error. Some of the subjects had a family history of glaucoma. In binocularly normal individuals only one eye was selected for study on a random basis. The axial length of all eyes was measured by ultrasound and the refractive error was deter mined. Stereophotography with a stereoseparator, in both color and black and white, were obtained. Black and white prints were enlarged by a factor of nine and the diameters and areas of the disk, cup, and neuroretinal rim were measured. The color stereophotographs were used to aid in the determination of the contour of the cup and the boundary of the disk. The area of the disk and the area of the cup were determined by computerized planimetry. The measurements of each eye were corrected for the magnification of that eye, using its axial length and refraction, according to a method described by Littmann 13 and using software supplied by Alanko (Oulu, Finland). The reti nal nerve fiber layer was photographed using blue green light on a 60-degree camera. 14 All measurements were made independently by two observers on the same set of photographs. The boundary definitions utilized for the disk, cup, and neuroretinal rim were as fol lows. Disk area—The area of the disk was defined as the area that lies inside the inner circumfer ence of the white scleral lip best identified in the color stereophotographs. Where the scleral rim was obscured by blood vessels or a thick nerve fiber layer it was usually easy to extrapo late between well-identified points. Cup area—The edge of the cup was marked by a contour line indicating the change of slope on the edge of the neuroretinal rim. Where a vessel followed the cup edge it was not includ ed in the estimation of the cup. If a vessel was isolated within the cup and not attached to nerve fiber tissue it was obviously part of the cup and not the rim. Topographic neuroretinal rim area—The rim area was calculated by subtracting the cup area from the disk area. Cup and disk diameters—On each set of photographs horizontal and vertical diameters were measured at the 0, 90, 180, and 270-degree meridians. Where the scleral rim was obscured by blood vessels or a thick nerve fiber layer an attempt was made to extrapolate between easi ly recognizable points. The mean of the vertical and horizontal linear measurements was used in the analysis.
Results Of the 113 individuals studied, 44 were men and 69 were women. There was no statistical difference in the measurements between the sexes with the exception of axial length, which confirmed that male eyes were slightly longer. The mean axial length of the male eyes was 23.96 ± 1.23 mm. The mean axial length of the female eyes was 23.23 ± 1.24 mm (P = .0003). The mean age was 51.1 ± 16.8 years. The mean spherical equivalent was +0.16 ± 2.49 diopters (Table 1). Correlation analyses were performed be tween pairs of corresponding measurements in both eyes of 87 subjects. The correlation coeffi cient between the disk areas in two eyes was .89 (P = .00000); between the cup areas of the two eyes, .84 (P = .00000); and between the topo graphic neuroretinal rim areas of the two eyes, .78 (P = .00000). To study the correlations between different variables, one eye was selected at random from each subject. Based on these measurements the mean disk area (± S.D.) of the 113 eyes measured 2.102 ± 0.50 mm 2 and the mean topographic neuroretinal rim area was 1.65 ± 0 . 3 0 mm 2 (Table 1). A scatter plot of the topographic neuroretinal rim area plotted against the optic disk area (Fig. 1) shows a linear relationship between them with a correlation coefficient of .75 (P = .00000095). We also calculated the mean neuroretinal rim areas for different sizes of the disk (Fig. 2) and the 95% confidence inter vals for the neuroretinal rim areas of a random
TABLE 1 QUANTITATIVE FINDINGS OF NORMAL OPTIC DISKS MEAN
Age Axial length Spherical equivalent Disk area Topographical neuroretinal rim area Vertical disk diameter Vertical cup diameter Horizontal disk diameter Horizontal cup diameter
S.D.
51.1 yrs 23.51 mm +0.16 D 2.102 mm2
16.8 yrs 1.28 mm 2.49 D 0.50 mm2
1.65 mm2 1.66 mm 0.70 mm 1.57 mm 0.66 mm
0.30 mm2 0.20 mm 0.29 mm 0.17 mm 0.25 mm
Normal Neuroretinal Rim
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correlation was found between the disk area and the spherical equivalent, axial length, or age (Table 2). There was an excellent correlation in the disk measurements between the two observers (r = .98, P = .0000049) (Fig. 6). There was also excellent agreement in the cup area meas-
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Discussion A generalized enlargement of the cup is one of the early signs of glaucomatous damage to the optic nerve and denotes a reduction in the neuroretinal rim, which almost certainly re flects a loss of axons. The presence of a large cup in an individual optic nerve may indicate such damage. However, Teal, Morin, and McCulloch15 and Bengtsson 4 have shown that there is a strong correlation between large disks and large cups in normals. Bengtsson 4 noted the large variation in disk size among normal subjects. In his 1,322 normal subjects there was a 2.5 times difference between the smallest and largest disk diameters. He also noted that the linear width of the neuroretinal rim was inde pendent of the diameter of the disk and was, therefore, more likely to be clinically useful. The present study confirms the great varia
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bility of disk sizes and the lack of relationship of the linear disk diameter to the linear neuro retinal rim width. That the linear rim width is uncorrelated with disk diameter, and that cup diameter is highly correlated with disk diame ter (r =.83 P = .0000) leads, on a purely geomet ric basis, to a correlation between the areas. The larger disks, therefore, have larger cups as well as larger neuroretinal rim areas. Our previous work with 33 normal subjects showed that the mean neuroretinal rim area (± S.D.) was 1.40 ± 0.156 mm 2 . This meant that 97.5% of normal eyes should have a neuro retinal rim area larger than 1.09 mm 2 . 6 How-
TABLE 2 CORRELATIONS OF DISK AREA WITH REFRACTION, AXIAL LENGTH, AND AGE
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ever, 27% of glaucoma patients with early but reproducible field defects still had a neuro retinal rim within these normal values. 12 In the present study, we found a mean neuroretinal rim area of 1.65 ± 0.30 mm 2 . The 95% predic tion intervals of the smallest neuroretinal rim areas for varying sizes of optic nerves show that for small disks the lower limit could be as low as 0.80 mm 2 while in large disks (3 mm2) the limit would be 1.36 mm 2 . These variations would alter the ability to detect early neuro retinal rim diminutions. The differences in the means of the neuroreti nal rim areas in normal subjects in our previous study and the present studies indicates that different observers, while showing excellent correlations in estimating the disk area and the area of the cup, do show greater variation in the assessment of the area of the rim. Not only is the judgment of the edge of the cup impor tant, but the way in which the large blood ves sels are measured in relationship to the cup and rim accounts for considerable differences. Bengtsson states that when he estimated "the extent of cupping not only the differences in color between the cup and the rim but also other factors such as deflections of vessels were
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taken into account." He looked at pallor and less at topography in describing and measur ing the cup. Standardization is, therefore, important in defining the cup. We have devel oped a disk analyzer to perform this task by computer (unpublished data). The results cor relate highly with stereophotogrammetric tech niques. 16 If these measurements are done man ually, each laboratory must establish its own normal values. It is easier to identify the disk margin than the edge of the cup, even in eyes with various patterns of peripapillary choroidal atrophy. Our finding that disk size corrected for magnifi cation is not correlated with axial length was unexpected. We expected no correlation or a negative correlation in the direct measure ments from the photographs, but the correc tion for magnification should have established the expected positive correlation between axial length and disk size. In 1951, Ishii16 measured disks in postmortem specimens on Japanese eyes. The mean vertical diameter was 1.80 mm and the mean horizontal diameter was 1.62 mm. The only measurements on North American populations 17 showed the mean verti cal diameter to be 1.86 mm and a mean horizon-
504
AMERICAN JOURNAL OF OPHTHALMOLOGY
tal d i a m e t e r of 1.75 m m . C a l c u l a t i n g t h e a r e a s of the optic n e r v e from t h e s e d i a m e t e r s g a v e a n optic disk area of 2.10 m m 2 in t h e J a p a n e s e s t u d y a n d 2.56 m m 2 in the N o r t h A m e r i c a n s t u d y . O u r o w n s t u d i e s s h o w a m e a n d i s k area of 2.10 mm 2 , w h i c h s h o w s t h a t t h e magnifica tion corrections s e e n a p p r o p r i a t e . T h e n o m o g r a m s for e x p e c t e d n e u r o r e t i n a l rim area given t h e area of a n i n d i v i d u a l disk s h o u l d p r o v e helpful clinically in t h e early r e c o g n i t i o n of g l a u c o m a t o u s n e u r o r e t i n a l d a m a g e . H o w e v e r , t h e v a l u e s u s e d m u s t b e locally e s t a b l i s h e d for the e x a m i n e r or t h e l a b o r a t o r y t h a t m a n u a l l y m e a s u r e s a n d d e l i n e a t e s t h e disk and cup margins.
References 1. Motolko, M , and Drance, S. M.: Features of the optic disc in preglaucomatous eyes. Arch. Ophthalmol. 99:1992, 1981. 2. Pederson, J. E., and Anderson, D. R.: The mode of progressive disc cupping in ocular hyper tension and glaucoma. Arch. Ophthalmol. 98:490, 1980. 3. Airaksinen, P. J., Drance, S. M., Douglas, G. R., Mawson, D. K., and Nieminen, H.: Diffuse and localized nerve fibre loss in glaucoma. Am. J. Ophthalmol. 98:566, 1984. 4. Bengtsson, B.: The variation and covariation of cup and disc diameters. Acta Ophthalmol. 54:804, 1976. 5. Balazsi, A. G., Drance, S. M., Schulzer, M , and Douglas, G. R.: Neuroretinal rim area in sus pected glaucoma and early chronic open angle glau coma. Arch. Ophthalmol. 102:1011, 1984.
April, 1987
6. Airaksinen, P. J., Drance, S. M., and Schulzer, M.: Neuroretinal rim area in early glaucoma. Am. J. Ophthalmol. 99:1, 1985. 7. Schwartz, T. J., and Reuling, F. H.: Acquired cupping of the optic nervehead in normotensive eyes. Br. J. Ophthalmol. 59:216, 1975. 8. Bengtsson, B.: The alteration and asymmetry of cup and disc diameters. Acta Ophthalmol. 58:726, 1980. 9. Kessing, S. V., and Gregersen, E.: The distend ed disc in early stages of congenital glaucoma. Acta Ophthalmol. 55:431, 1977. 10. Greenidy, K. C., Spaeth, G. L., and Traverso, C. E.: Changes in appearance of the optic disc associ ated with lowering of intraocular pressure. Ophthal mology 92:897, 1985. 11. Leighton, D. A., and Tomlinson, A.: Changes in axial length and other dimensions of the eyeball with increasing age. Acta Ophthalmol. 50:815, 1972. 12. Tomlinson, A., and Phillips, C. I.: Ratio of optic cup to optic disc. Br. J. Ophthalmol. 53:765, 1969. 13. Littmann, H.: Zur bestimmung der wahren Grossen eines Objektes auf dem Hintergrund des lebenden Auges. Klin. Monatsbl. Augenheilkd. 180:286, 1982. 14. Mawson, D. K., and Nieminen, H.: Retinal nerve fiber layer photography in glaucoma. J. Oph thalmol. Photo. 1:8-10, 1985. 15. Teal, P. K., Morin, J. D., and McCulloch, C : Assessment of the normal disc. Trans. Am. Ophthal mol. Soc. 70:164, 1972. 16. Ishii, K.: Optic disc diameter in Japanese eyes. Acta. Soc. Ophthalmol. Jpn. 55:242, 1951. 17. Straatsma, B. R., Foos, R. Y., and Spencer, L. M.: The retina topography and clinical correla tion. New Orleans Academy of Ophthalmology: Symposium on Retina and Retinal Surgery. St. Louis, C. V. Mosby, 1969, 1-26.
We randomly photographed 113 eyes in 113 people without ocular disease. The area of the disk and its neuroretinal rim as well as the width of the disk and cup were measured. The measurements were corrected for magnifica tion of each eye by measuring its refraction and axial length. A linear correlation between disk area and neuroretinal rim area was found (r = .75) as well as a correlation between the disk area and the cup area (r = .83). There was no correlation between age and disk area or rim area. THE DECISION AS TO whether an individual optic disk is normal or abnormal is important in the diagnosis of glaucoma. The classic features of the optic nervehead that enable clinicians to detect early acquired optic nervehead damage in glaucoma suspects are well known. Acquired tissue changes at the optic nervehead may precede localized visual field defects 1,2 while a diffuse loss of axons has also been shown to occur in glaucoma suspects 3 in the absence of localized field defects. Congenitally large optic disks are associated with large cups. The width of the neuroretinal rim has been shown to be more constant and largely independent of disk size. 4 There is a considerable overlap of the cup diameter and, therefore, the cup-to-disk ratio between ac quired enlarged cups and congenitally large cups. The assessment of the optic nerve in glaucoma has recently been enhanced by the use of stereophotography to measure the size of the neuroretinal rim by computerized pla-
Accepted for publication Dec. 29, 1986. From the Department of Ophthalmology (Drs. Brit ton, Drance, and Douglas and Mr. Mawson) and the Department of Statistics and Medicine (Dr. Schulzer), University of British Columbia, Vancouver, Canada. This study was supported by Medical Research Council Grant 68-1578 (Dr. Drance). Reprint requests to S. M. Drance, M.D., Eye Care Centre, 2550 Willow St., Vancouver, BC V5Z 3N9.
nimetry. 5 This has been shown to be a more sensitive method of demonstrating acquired change of the optic nervehead in glaucoma 5,6 than the traditional cup-to-disk ratio, probably because the latter does not take into account the size of the individual disk. Before being able to decide whether a neuro retinal rim area is abnormal, not only the size of the disk but also the physiologic behavior of the disk and neuroretinal rim area with age must be taken into account. Schwartz and Reuling 7 have summarized previous studies that dealt mainly with the cup-to-disk ratio and age, but no measurements of the areas of the disk or of the neuroretinal rim were taken. However, the diameter of the disk has been reported to enlarge with age in normal people, which leads to an enlargement of the cup while the neuro retinal width area remains unchanged. 8 Con versely, decreases in the size of the scleral canal associated with a decrease in cup size have recently been reported in glaucoma pa tients when intraocular pressure was low ered. 9,10 A reduction in the axial length of the eye with increasing age 11 has also been de scribed. Therefore, it might be expected that disk size could decrease with age. 12 We have studied a normal population of all ages over 20 years with various refractive errors to provide the normal ranges for the neuroreti nal rim area.
Subjects and Methods The study included 113 eyes of 113 normal people w h o had a visual acuity of 20/20 or better, intraocular pressures less than 21 mm Hg, normal visual fields, absence of ante rior segment abnormality on slit-lamp examina tion, and normal retinal examination including an absence of focal defects on retinal nerve fiber layer photography. The subjects were aged from 20 to 81 years. No exclusions were
©AMERICAN JOURNAL OF OPHTHALMOLOGY 103:497-504, APRIL, 1987
497
498
April, 1987
AMERICAN JOURNAL OF OPHTHALMOLOGY
made on the basis of refractive error. Some of the subjects had a family history of glaucoma. In binocularly normal individuals only one eye was selected for study on a random basis. The axial length of all eyes was measured by ultrasound and the refractive error was deter mined. Stereophotography with a stereoseparator, in both color and black and white, were obtained. Black and white prints were enlarged by a factor of nine and the diameters and areas of the disk, cup, and neuroretinal rim were measured. The color stereophotographs were used to aid in the determination of the contour of the cup and the boundary of the disk. The area of the disk and the area of the cup were determined by computerized planimetry. The measurements of each eye were corrected for the magnification of that eye, using its axial length and refraction, according to a method described by Littmann 13 and using software supplied by Alanko (Oulu, Finland). The reti nal nerve fiber layer was photographed using blue green light on a 60-degree camera. 14 All measurements were made independently by two observers on the same set of photographs. The boundary definitions utilized for the disk, cup, and neuroretinal rim were as fol lows. Disk area—The area of the disk was defined as the area that lies inside the inner circumfer ence of the white scleral lip best identified in the color stereophotographs. Where the scleral rim was obscured by blood vessels or a thick nerve fiber layer it was usually easy to extrapo late between well-identified points. Cup area—The edge of the cup was marked by a contour line indicating the change of slope on the edge of the neuroretinal rim. Where a vessel followed the cup edge it was not includ ed in the estimation of the cup. If a vessel was isolated within the cup and not attached to nerve fiber tissue it was obviously part of the cup and not the rim. Topographic neuroretinal rim area—The rim area was calculated by subtracting the cup area from the disk area. Cup and disk diameters—On each set of photographs horizontal and vertical diameters were measured at the 0, 90, 180, and 270-degree meridians. Where the scleral rim was obscured by blood vessels or a thick nerve fiber layer an attempt was made to extrapolate between easi ly recognizable points. The mean of the vertical and horizontal linear measurements was used in the analysis.
Results Of the 113 individuals studied, 44 were men and 69 were women. There was no statistical difference in the measurements between the sexes with the exception of axial length, which confirmed that male eyes were slightly longer. The mean axial length of the male eyes was 23.96 ± 1.23 mm. The mean axial length of the female eyes was 23.23 ± 1.24 mm (P = .0003). The mean age was 51.1 ± 16.8 years. The mean spherical equivalent was +0.16 ± 2.49 diopters (Table 1). Correlation analyses were performed be tween pairs of corresponding measurements in both eyes of 87 subjects. The correlation coeffi cient between the disk areas in two eyes was .89 (P = .00000); between the cup areas of the two eyes, .84 (P = .00000); and between the topo graphic neuroretinal rim areas of the two eyes, .78 (P = .00000). To study the correlations between different variables, one eye was selected at random from each subject. Based on these measurements the mean disk area (± S.D.) of the 113 eyes measured 2.102 ± 0.50 mm 2 and the mean topographic neuroretinal rim area was 1.65 ± 0 . 3 0 mm 2 (Table 1). A scatter plot of the topographic neuroretinal rim area plotted against the optic disk area (Fig. 1) shows a linear relationship between them with a correlation coefficient of .75 (P = .00000095). We also calculated the mean neuroretinal rim areas for different sizes of the disk (Fig. 2) and the 95% confidence inter vals for the neuroretinal rim areas of a random
TABLE 1 QUANTITATIVE FINDINGS OF NORMAL OPTIC DISKS MEAN
Age Axial length Spherical equivalent Disk area Topographical neuroretinal rim area Vertical disk diameter Vertical cup diameter Horizontal disk diameter Horizontal cup diameter
S.D.
51.1 yrs 23.51 mm +0.16 D 2.102 mm2
16.8 yrs 1.28 mm 2.49 D 0.50 mm2
1.65 mm2 1.66 mm 0.70 mm 1.57 mm 0.66 mm
0.30 mm2 0.20 mm 0.29 mm 0.17 mm 0.25 mm
Normal Neuroretinal Rim
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Discussion A generalized enlargement of the cup is one of the early signs of glaucomatous damage to the optic nerve and denotes a reduction in the neuroretinal rim, which almost certainly re flects a loss of axons. The presence of a large cup in an individual optic nerve may indicate such damage. However, Teal, Morin, and McCulloch15 and Bengtsson 4 have shown that there is a strong correlation between large disks and large cups in normals. Bengtsson 4 noted the large variation in disk size among normal subjects. In his 1,322 normal subjects there was a 2.5 times difference between the smallest and largest disk diameters. He also noted that the linear width of the neuroretinal rim was inde pendent of the diameter of the disk and was, therefore, more likely to be clinically useful. The present study confirms the great varia
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bility of disk sizes and the lack of relationship of the linear disk diameter to the linear neuro retinal rim width. That the linear rim width is uncorrelated with disk diameter, and that cup diameter is highly correlated with disk diame ter (r =.83 P = .0000) leads, on a purely geomet ric basis, to a correlation between the areas. The larger disks, therefore, have larger cups as well as larger neuroretinal rim areas. Our previous work with 33 normal subjects showed that the mean neuroretinal rim area (± S.D.) was 1.40 ± 0.156 mm 2 . This meant that 97.5% of normal eyes should have a neuro retinal rim area larger than 1.09 mm 2 . 6 How-
TABLE 2 CORRELATIONS OF DISK AREA WITH REFRACTION, AXIAL LENGTH, AND AGE
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ever, 27% of glaucoma patients with early but reproducible field defects still had a neuro retinal rim within these normal values. 12 In the present study, we found a mean neuroretinal rim area of 1.65 ± 0.30 mm 2 . The 95% predic tion intervals of the smallest neuroretinal rim areas for varying sizes of optic nerves show that for small disks the lower limit could be as low as 0.80 mm 2 while in large disks (3 mm2) the limit would be 1.36 mm 2 . These variations would alter the ability to detect early neuro retinal rim diminutions. The differences in the means of the neuroreti nal rim areas in normal subjects in our previous study and the present studies indicates that different observers, while showing excellent correlations in estimating the disk area and the area of the cup, do show greater variation in the assessment of the area of the rim. Not only is the judgment of the edge of the cup impor tant, but the way in which the large blood ves sels are measured in relationship to the cup and rim accounts for considerable differences. Bengtsson states that when he estimated "the extent of cupping not only the differences in color between the cup and the rim but also other factors such as deflections of vessels were
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taken into account." He looked at pallor and less at topography in describing and measur ing the cup. Standardization is, therefore, important in defining the cup. We have devel oped a disk analyzer to perform this task by computer (unpublished data). The results cor relate highly with stereophotogrammetric tech niques. 16 If these measurements are done man ually, each laboratory must establish its own normal values. It is easier to identify the disk margin than the edge of the cup, even in eyes with various patterns of peripapillary choroidal atrophy. Our finding that disk size corrected for magnifi cation is not correlated with axial length was unexpected. We expected no correlation or a negative correlation in the direct measure ments from the photographs, but the correc tion for magnification should have established the expected positive correlation between axial length and disk size. In 1951, Ishii16 measured disks in postmortem specimens on Japanese eyes. The mean vertical diameter was 1.80 mm and the mean horizontal diameter was 1.62 mm. The only measurements on North American populations 17 showed the mean verti cal diameter to be 1.86 mm and a mean horizon-
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AMERICAN JOURNAL OF OPHTHALMOLOGY
tal d i a m e t e r of 1.75 m m . C a l c u l a t i n g t h e a r e a s of the optic n e r v e from t h e s e d i a m e t e r s g a v e a n optic disk area of 2.10 m m 2 in t h e J a p a n e s e s t u d y a n d 2.56 m m 2 in the N o r t h A m e r i c a n s t u d y . O u r o w n s t u d i e s s h o w a m e a n d i s k area of 2.10 mm 2 , w h i c h s h o w s t h a t t h e magnifica tion corrections s e e n a p p r o p r i a t e . T h e n o m o g r a m s for e x p e c t e d n e u r o r e t i n a l rim area given t h e area of a n i n d i v i d u a l disk s h o u l d p r o v e helpful clinically in t h e early r e c o g n i t i o n of g l a u c o m a t o u s n e u r o r e t i n a l d a m a g e . H o w e v e r , t h e v a l u e s u s e d m u s t b e locally e s t a b l i s h e d for the e x a m i n e r or t h e l a b o r a t o r y t h a t m a n u a l l y m e a s u r e s a n d d e l i n e a t e s t h e disk and cup margins.
References 1. Motolko, M , and Drance, S. M.: Features of the optic disc in preglaucomatous eyes. Arch. Ophthalmol. 99:1992, 1981. 2. Pederson, J. E., and Anderson, D. R.: The mode of progressive disc cupping in ocular hyper tension and glaucoma. Arch. Ophthalmol. 98:490, 1980. 3. Airaksinen, P. J., Drance, S. M., Douglas, G. R., Mawson, D. K., and Nieminen, H.: Diffuse and localized nerve fibre loss in glaucoma. Am. J. Ophthalmol. 98:566, 1984. 4. Bengtsson, B.: The variation and covariation of cup and disc diameters. Acta Ophthalmol. 54:804, 1976. 5. Balazsi, A. G., Drance, S. M., Schulzer, M , and Douglas, G. R.: Neuroretinal rim area in sus pected glaucoma and early chronic open angle glau coma. Arch. Ophthalmol. 102:1011, 1984.
April, 1987
6. Airaksinen, P. J., Drance, S. M., and Schulzer, M.: Neuroretinal rim area in early glaucoma. Am. J. Ophthalmol. 99:1, 1985. 7. Schwartz, T. J., and Reuling, F. H.: Acquired cupping of the optic nervehead in normotensive eyes. Br. J. Ophthalmol. 59:216, 1975. 8. Bengtsson, B.: The alteration and asymmetry of cup and disc diameters. Acta Ophthalmol. 58:726, 1980. 9. Kessing, S. V., and Gregersen, E.: The distend ed disc in early stages of congenital glaucoma. Acta Ophthalmol. 55:431, 1977. 10. Greenidy, K. C., Spaeth, G. L., and Traverso, C. E.: Changes in appearance of the optic disc associ ated with lowering of intraocular pressure. Ophthal mology 92:897, 1985. 11. Leighton, D. A., and Tomlinson, A.: Changes in axial length and other dimensions of the eyeball with increasing age. Acta Ophthalmol. 50:815, 1972. 12. Tomlinson, A., and Phillips, C. I.: Ratio of optic cup to optic disc. Br. J. Ophthalmol. 53:765, 1969. 13. Littmann, H.: Zur bestimmung der wahren Grossen eines Objektes auf dem Hintergrund des lebenden Auges. Klin. Monatsbl. Augenheilkd. 180:286, 1982. 14. Mawson, D. K., and Nieminen, H.: Retinal nerve fiber layer photography in glaucoma. J. Oph thalmol. Photo. 1:8-10, 1985. 15. Teal, P. K., Morin, J. D., and McCulloch, C : Assessment of the normal disc. Trans. Am. Ophthal mol. Soc. 70:164, 1972. 16. Ishii, K.: Optic disc diameter in Japanese eyes. Acta. Soc. Ophthalmol. Jpn. 55:242, 1951. 17. Straatsma, B. R., Foos, R. Y., and Spencer, L. M.: The retina topography and clinical correla tion. New Orleans Academy of Ophthalmology: Symposium on Retina and Retinal Surgery. St. Louis, C. V. Mosby, 1969, 1-26.