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IOP in Chinese eyes
Ophthalmology Volume 108, Number 8, August 2001 References 1. Shah S. Accurate intraocular pressure measurement—the myth of modern ophthalmology? [editorial]. Ophthalmology 2000;107:1805–7. 2. Mills RP. If intraocular pressure measurement is only an estimate—then what? [editorial]. Ophthalmology 2000;107: 1807– 8. 3. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol (Copenh) 1975; 53:34 – 43.
IOP in Chinese Eyes Dear Editor: Except for epicanthus, there are no known peculiarities to Chinese eyes, hence the wonder of the article “Accuracy of Clinical Estimates of Intraocular Pressure in Chinese Eyes” (Ophthalmology 2000;107:1816 –21). As for accuracy, in 1960 I had pointed to the metaphysical basis of Goldmann’s tonometer,1 and then shown in 400 eyes that its readings are influenced by corneal curvature.2 In a recent study of 513 eyes, we found that a change in axial length of 1 mm changed the tonomerter’s reading by 0.29 units.3 It is well known that myopia is often associated with large elongated globes, which leads to underestimation of tonometric readings, as Sir Stewart Duke-Elder4 put it: “we would expect larger eyes to have lower values of K [coefficient of ocular rigidity] than smaller eyes, just as a container with a capacity of 100 ml, would be easier to distend when 1 ml of fluid was injected into it than a similar container with a capacity of 10 ml.” The researchers at Singapore’s National Eye Center, whence the article originated, have also recently found that childhood myopia in Singapore is four times higher than in the United States, as is also adult myopia in East Asia in general. Racial attitudes in Singapore or the Malayan peninsula aside, the tonometric underestimation the six authors found in their 23 Chinese may well be due to physical rather than ethnic causes. It would therefore be enlightening to see some data of these eyes, such as average axial length, central corneal thickness, and refractive error.
tension” primary open-angle glaucoma,1 and high rates of myopia, with the prevalence in the Chinese population of Singapore being about twice that of a European-derived population.2 Cross-sectional data suggest that prevalence of myopia is greater among younger people and those who live in an urban environment.2,3 Our work suggested that intraocular pressure (IOP) estimates in Chinese people were subject to significant error, both with applanation tonometry and with the Tonopen. We have already described the influence of corneal thickness on variation IOP estimates in Mongolian people.4 The pattern is virtually identical among a population-based cohort of 1,232 Chinese Singaporeans (unpublished data).5 However, the effect seems to be relatively small (0.15– 0.24 mmHg increase in measured IOP per 10 m increase in central corneal thickness). We therefore believe that the corneal thickness does influence IOP estimation, although we did not demonstrate this association or any other association between ocular dimensions and error in our manometric work. This probably reflects the small number of subjects that we examined. The median axial length of the globe (23.74 mm) is not dissimilar to that of a European population, although the range of readings (21.40 –32.15 mm) would suggest a skew to higher axial lengths. The high rates of myopia in our population have led us to question whether the material properties of the corneoscleral coat differ between Chinese and European people. We discussed the issue of determinants of ocular deformability in Asian people and suggested that this is an area that is not well understood. Designating differences as “ethnic” is something that we hope we have not done. We see interethnic differences as most likely being either structural and physiologic and sought to explore this issue of IOP estimation in these terms. We were a little troubled that Dr. Mark had not noticed that the data he asks for (mean axial length, anterior chamber depth, corneal thickness, and curvature) are given in Table 3. PAUL J. FOSTER, FRCS (ED) London, England PAUL T. K. CHEW, FRCOPHTH Singapore
HARRY H. MARK, MD New Haven, Connecticut References 1. Markiewitz HH. The so-called Imbert-Fick law [letter]. Arch Ophthalmol 1960;64:159. 2. Mark HH. Corneal curvature in applanation tonometry. Am J Ophthalmol 1973;76:223– 4. 3. Mark HH, Robbins KP, Mark TL. Axial length in applanation tonometry. In Press. J Cataract Refract Surg. 4. Duke-Elder, S, editor-in-chief. System of Ophthalmology, The Physiology of the Eye and of Vision. Vol. 4. St. Louis: CV Mosby, 1968;272.
Author reply Dear Editor: The eyes of East Asian people do seem to differ fundamentally from European people in a number of ways. The two aspects that have interested us most recently have been the characteristics of glaucoma, with high rates of “normal
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References 1. Shiose Y, Kitazawa Y, Tsukuhara S, et al. Epidemiology of glaucoma in Japan—a nationwide glaucoma survey. Jpn J Ophthalmol 1991;35:133–55. 2. Wong TY, Foster PJ, Hee J, et al. Prevalence and risk factors for refractive errors in adult Chinese in Singapore. Invest Ophthalmol Vis Sci 2000;41:2486 –94. 3. Zhan MZ, Saw SM, Hong RZ, et al. Refractive errors in Singapore and Xiamen, China—a comparative study in school children aged 6 to 7 years. Optom Vis Sci 2000;77:302– 8. 4. Foster PJ, Baasanhu J, Alsbirk PH, et al. Central corneal thickness and intraocular pressure in a Mongolian population. Ophthalmology 1998;105:969 –73. 5. Foster PJ, Oen FT, Machin D, et al. The prevalence of glaucoma in Chinese residents of Singapore. A cross-sectional population survey in Tanjong Pagar district. Arch Ophthalmol 2000;118:1105–11.
IOP in Chinese Eyes Dear Editor: Except for epicanthus, there are no known peculiarities to Chinese eyes, hence the wonder of the article “Accuracy of Clinical Estimates of Intraocular Pressure in Chinese Eyes” (Ophthalmology 2000;107:1816 –21). As for accuracy, in 1960 I had pointed to the metaphysical basis of Goldmann’s tonometer,1 and then shown in 400 eyes that its readings are influenced by corneal curvature.2 In a recent study of 513 eyes, we found that a change in axial length of 1 mm changed the tonomerter’s reading by 0.29 units.3 It is well known that myopia is often associated with large elongated globes, which leads to underestimation of tonometric readings, as Sir Stewart Duke-Elder4 put it: “we would expect larger eyes to have lower values of K [coefficient of ocular rigidity] than smaller eyes, just as a container with a capacity of 100 ml, would be easier to distend when 1 ml of fluid was injected into it than a similar container with a capacity of 10 ml.” The researchers at Singapore’s National Eye Center, whence the article originated, have also recently found that childhood myopia in Singapore is four times higher than in the United States, as is also adult myopia in East Asia in general. Racial attitudes in Singapore or the Malayan peninsula aside, the tonometric underestimation the six authors found in their 23 Chinese may well be due to physical rather than ethnic causes. It would therefore be enlightening to see some data of these eyes, such as average axial length, central corneal thickness, and refractive error.
tension” primary open-angle glaucoma,1 and high rates of myopia, with the prevalence in the Chinese population of Singapore being about twice that of a European-derived population.2 Cross-sectional data suggest that prevalence of myopia is greater among younger people and those who live in an urban environment.2,3 Our work suggested that intraocular pressure (IOP) estimates in Chinese people were subject to significant error, both with applanation tonometry and with the Tonopen. We have already described the influence of corneal thickness on variation IOP estimates in Mongolian people.4 The pattern is virtually identical among a population-based cohort of 1,232 Chinese Singaporeans (unpublished data).5 However, the effect seems to be relatively small (0.15– 0.24 mmHg increase in measured IOP per 10 m increase in central corneal thickness). We therefore believe that the corneal thickness does influence IOP estimation, although we did not demonstrate this association or any other association between ocular dimensions and error in our manometric work. This probably reflects the small number of subjects that we examined. The median axial length of the globe (23.74 mm) is not dissimilar to that of a European population, although the range of readings (21.40 –32.15 mm) would suggest a skew to higher axial lengths. The high rates of myopia in our population have led us to question whether the material properties of the corneoscleral coat differ between Chinese and European people. We discussed the issue of determinants of ocular deformability in Asian people and suggested that this is an area that is not well understood. Designating differences as “ethnic” is something that we hope we have not done. We see interethnic differences as most likely being either structural and physiologic and sought to explore this issue of IOP estimation in these terms. We were a little troubled that Dr. Mark had not noticed that the data he asks for (mean axial length, anterior chamber depth, corneal thickness, and curvature) are given in Table 3. PAUL J. FOSTER, FRCS (ED) London, England PAUL T. K. CHEW, FRCOPHTH Singapore
HARRY H. MARK, MD New Haven, Connecticut References 1. Markiewitz HH. The so-called Imbert-Fick law [letter]. Arch Ophthalmol 1960;64:159. 2. Mark HH. Corneal curvature in applanation tonometry. Am J Ophthalmol 1973;76:223– 4. 3. Mark HH, Robbins KP, Mark TL. Axial length in applanation tonometry. In Press. J Cataract Refract Surg. 4. Duke-Elder, S, editor-in-chief. System of Ophthalmology, The Physiology of the Eye and of Vision. Vol. 4. St. Louis: CV Mosby, 1968;272.
Author reply Dear Editor: The eyes of East Asian people do seem to differ fundamentally from European people in a number of ways. The two aspects that have interested us most recently have been the characteristics of glaucoma, with high rates of “normal
1366
References 1. Shiose Y, Kitazawa Y, Tsukuhara S, et al. Epidemiology of glaucoma in Japan—a nationwide glaucoma survey. Jpn J Ophthalmol 1991;35:133–55. 2. Wong TY, Foster PJ, Hee J, et al. Prevalence and risk factors for refractive errors in adult Chinese in Singapore. Invest Ophthalmol Vis Sci 2000;41:2486 –94. 3. Zhan MZ, Saw SM, Hong RZ, et al. Refractive errors in Singapore and Xiamen, China—a comparative study in school children aged 6 to 7 years. Optom Vis Sci 2000;77:302– 8. 4. Foster PJ, Baasanhu J, Alsbirk PH, et al. Central corneal thickness and intraocular pressure in a Mongolian population. Ophthalmology 1998;105:969 –73. 5. Foster PJ, Oen FT, Machin D, et al. The prevalence of glaucoma in Chinese residents of Singapore. A cross-sectional population survey in Tanjong Pagar district. Arch Ophthalmol 2000;118:1105–11.