The COMET cohort provides unique data on associations of IOP and CCT with various ocular and demographic factors in a large, ethnically diverse population of myopic young adults. Although associations of IOP and CCT have been published with regard to age, gender, refractive status, and AL, many of the previous reports have been limited by smaller sample size, little or no data on ethnic differences, or no evaluation of the potential effects of significant covariates. In addition, although studies may have included individuals with myopia, the refractive error results have frequently been reported in a manner that does not allow the data for myopic or other refractive error categories to be examined independently.
The IOP measured in myopic young adults in the COMET study (15.1 ± 0.1 mm Hg) is similar to that reported in other studies that included subjects of similar ages with a range of refractive errors (14.1 to 16.99 mm Hg).7,20,21 Although no other large study has included only myopic young adults, the current results suggest that on average, IOP is not higher in young adults with myopia than in those with other refractive errors. However, we acknowledge that IOP measured at a single point in time in the current study may not characterize diurnal fluctuations and other variations in IOP over time.
A significant association of IOP with ethnicity was found in the current study. Blacks (15.7 ± 0.3 mm Hg) had higher measured IOP than Hispanics (14.1 ± 0.3 mm Hg) and slightly higher than whites (15.1 ± 0.2 mm Hg). This finding is consistent with the results in the subset of the COMET cohort from one clinical center during the first five years of the study, where mean IOP measured with a different technique (Tonopen) was higher in blacks than in Hispanics and whites.6
Other studies have investigated ethnic differences in IOP. The CLEERE study also reported higher IOP for blacks compared with whites, with mean differences ranging from 0.91 to 1.54 mm Hg (p = 0.001) in 10 to 13 year olds.7 The Barbados Eye Study reported higher IOP in older (≥ 40 years) individuals of African descent than in whites.22 Although the differences in IOP between ethnic groups in the COMET results are small, the findings from COMET and the other studies suggest that IOP is higher in blacks at all ages and not just older ages when primary open angle glaucoma (POAG) is more prevalent.
IOP was not associated with age and gender in the current study, similar to other studies including young adults.20,21,23 However, the limited age range of the participants in the current study may hinder the identification of an age-related relationship. Likewise, IOP was not related to the magnitude of myopia, AL, or VCD. Previous studies of IOP in subjects with a limited range of refractive errors also found no association with magnitude of myopia or AL,24 – 26 while those finding an association included subjects with a wide range of refractive errors.7,27,28 The broad range may have contributed to the significant findings.
The mean CCT in the COMET cohort was 562.4 ± 1.8 μm. In the present study blacks had thinner central corneas than whites, Hispanics, and Asians, with CCT 11.8 and 15.2 μm less in blacks compared with whites and Hispanics, respectively. These ethnic differences are relatively small on average, with large variability. Data on CCT in populations similar in age, ethnicity, and refractive error is not available in the literature. Subjects of an age similar to the COMET cohort, although with a wide range of refractive errors, had a median CCT that was 22-μm thinner in blacks than in the combined white-Hispanic group.11 The COMET results provide new data on CCT and ethnicity in myopic young adults that support the association of thinner CCT in blacks and are consistent with studies conducted in younger9 – 11 and older29 – 36 populations that included hyperopia and myopia.
In COMET, thicker CCT was associated with longer VCD and longer AL. Only one other study has reported a positive relationship between CCT and AL, although this was in older Asian adults (40 to 80 years of age) with a full range of refractive errors, e.g., not limited to myopia.37 However, other investigations in myopic adults,38 young Asian myopic adults,12 and populations of predominately myopic Asian children28,39 have found no relationship between CCT and AL. Several factors, including differences in age or ethnicity, different CCT instrumentation,28,39 and failure to examine interactions between variables12,38 may contribute to the conflicting results. It is important to note that in COMET additional analyses found that the association between CCT and VCD could be attributed mainly to participants wearing contact lenses most or all the time. However, this finding in contact lens wearers cannot be explained by an increase in CCT associated with contact lens use (e.g., corneal edema) because VCD, unlike AL, is not influenced by changes in CCT.
In the present investigation, CCT did not vary with magnitude of myopia. While similar results have been reported in several studies of myopic children and adults,26,38 – 42 CCT has also been found to increase43 or decrease12 with increasing myopic refractive error in Asians. The magnitude of the relationship between CCT and myopia has been small regardless of whether a statistically significant difference was found.
CCT was not associated with age in COMET, as might be anticipated with the narrow age range of COMET participants, nor with gender, which is consistent with some studies38,41,44 but not others.11,39
In COMET thinner corneas were found to be associated with lower IOP; hence the measured IOP may underestimate the true IOP in individuals with thinner CCT. Doughty showed this same positive association in a meta-analysis of non-glaucomatous eyes.8 When the current unadjusted data were analyzed by ethnicity, the association between IOP and CCT were significant for blacks (r = 0.29; p = 0.002), Asians (r = 0.47; p = 0.008), and whites (r = 0.24; p = 0.002). These results are consistent with results of other studies, which reported similar findings in children and in older adults where refractive error was not limited to myopia.23,28,32,39,44 – 48 The present study did not find a significant relationship between IOP and CCT for Hispanics, in contrast to previous reports.32,49 Differences in the populations, such as age and type of refractive error, may have contributed to these conflicting findings of the association between IOP and CCT in Hispanics.
The higher IOP of the blacks in the COMET cohort, combined with their thinner CCT, is an important finding in the current study and has implications for the clinical care of these patients. It is known that increased IOP is a risk factor for POAG,50 – 52 as is African descent50,53,54 and thinner CCT.51,55 Myopia, regardless of magnitude,56 is also frequently linked to elevated risk of glaucoma in children as young as 10 years of age and adults up to 40 years of age,53,57 as well as adults 40 years or older.58 – 63 In addition, glaucoma is more prevalent at an earlier age in blacks than whites54 and its diagnosis may be delayed in blacks.32 Lastly, the assessment of the true IOP may be more difficult in blacks with thinner corneas who undergo refractive surgery.64,65 All these risk factors suggest that early routine assessment of IOP and CCT should be considered in young myopic blacks.
CCT is a critical factor in determining the suitability of an eye for keratorefractive surgery procedures.13,14 Given that many myopic young adults seek such procedures, the COMET results suggest that the thinner corneas in blacks should be taken into consideration, along with other criteria when determining their eligibility for refractive surgery. A minimum thickness of the residual corneal bed after refractive surgery is necessary to reduce the risk of corneal ectasia.38,66
The mean IOP in this group of young adults with myopia was slightly, but significantly, higher in blacks than whites or Hispanics. CCT was also significantly related to ethnicity, with corneas thinner in blacks than those of whites, Hispanics, or Asians. Thinner CCT in myopic blacks should be considered along with other criteria when determining suitability for corneal refractive surgery in young adulthood. A modest, but significant positive relationship between CCT and IOP suggests that the ethnic differences in IOP may be even greater than those observed; measured IOP may underestimate true IOP to a greater extent in blacks than in other ethnic groups especially following refractive surgery. Given the risk factors for POAG (e.g., higher IOP, thinner CCT, myopia, and African descent), these findings suggest that examination of myopic blacks should begin at a young age and include both IOP and CCT.
Karen D. Fern
505 J. Davis Armistead Building
This work was supported by National Eye Institute, National Institute of Health, NEI/NIH grants EY11756, EY11754, EY11805, EY11752, EY11740, and EY11755.
Presented in part at the Association for Research in Vision and Ophthalmology Meeting in Ft. Lauderdale, FL on May 3, 2010.
1. Gwiazda J, Hyman L, Hussein M, Everett D, Norton TT, Kurtz D, Leske MC, Manny R, Marsh-Tootle W, Scheiman M. A randomized clinical trial of progressive addition lenses versus single vision lenses on the progression of myopia in children. Invest Ophthalmol Vis Sci 2003;44:1492–500.
2. Friedman B. Stress upon the ocular coats: effects of scleral curvature, scleral thickness, and intra-ocular pressure. Eye Ear Nose Throat Mon 1966;45:59–66.
3. Pruett RC. Progressive myopia and intraocular pressure: what is the linkage? A literature review. Acta Ophthalmol Suppl 1988;185:117–27.
4. Tokoro T, Funata M, Akazawa Y. Influence of intraocular pressure on axial elongation. J Ocul Pharmacol 1990;6:285–91.
5. Quinn GE, Berlin JA, Young TL, Ziylan S, Stone RA. Association of intraocular pressure and myopia in children. Ophthalmology 1995;102:180–5.
6. Manny RE, Deng L, Crossnoe C, Gwiazda J. IOP, myopic progression and axial length in a COMET subgroup. Optom Vis Sci 2008;85:97–105.
7. Manny RE, Mitchell GL, Cotter SA, Jones-Jordan LA, Kleinstein RN, Mutti DO, Twelker JD, Zadnik K. Intraocular pressure, ethnicity, and refractive error. Optom Vis Sci 2011;88:1445–53.
8. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol 2000;44:367–408.
9. Dai E, Gunderson CA. Pediatric central corneal thickness variation among major ethnic populations. J AAPOS 2006;10:22–5.
10. Haider KM, Mickler C, Oliver D, Moya FJ, Cruz OA, Davitt BV. Age and racial variation in central corneal thickness of preschool and school-aged children. J Pediatr Ophthalmol Strabismus 2008;45:227–33.
11. Bradfield YS, Melia BM, Repka MX, Kaminski BM, Davitt BV, Johnson DA, Kraker RT, Manny RE, Matta NS, Weise KK, Schloff S. Central corneal thickness in children. Arch Ophthalmol 2011;129:1132–8.
12. Chang SW, Tsai IL, Hu FR, Lin LL, Shih YF. The cornea in young myopic adults. Br J Ophthalmol 2001;85:916–20.
13. American Academy of Ophthalmology Refractive Management/Intervention Panel. Preferred Practice Pattern Guidelines. Refractive Errors & Refractive Surgery. San Francisco, CA: American Academy of Ophthalmology; 2007. Available at: http://www.aao.org/ppp
. Accessed October 10, 2011.
14. Ehlers N, Hjortdal J. Corneal thickness: measurement and implications. Exp Eye Res 2004;78:543–8.
15. Hyman L, Gwiazda J, Marsh-Tootle WL, Norton TT, Hussein M. The Correction of Myopia Evaluation Trial (COMET): design and general baseline characteristics. Control Clin Trials 2001;22:573–92.
16. Gwiazda J, Marsh-Tootle WL, Hyman L, Hussein M, Norton TT, COMET Study Group. Baseline refractive and ocular component measures of children enrolled in the correction of myopia evaluation trial. Invest Ophthalmol Vis Sci 2002;43:314–21.
17. Gwiazda JE, Hyman L, Norton TT, Hussein ME, Marsh-Tootle W, Manny R, Wang Y, Everett D, COMET Study Group. Accommodation and related risk factors associated with myopia progression and their interaction with treatment in COMET children. Invest Ophthalmol Vis Sci 2004;45:2143–51.
18. Hyman L, Gwiazda J, Hussein M, Norton TT, Wang Y, Marsh-Tootle W, Everett D. Relationship of age, sex, and ethnicity with myopia progression and axial elongation in the correction of myopia evaluation trial. Arch Ophthalmol 2005;123:977–87.
19. Schmidt TA. The clinical application of the Goldmann applanation tonometer. Am J Ophthalmol 1960;49:967–78.
20. Puell-Marin MC, Romero-Martin M, Dominguez-Carmona M. Intraocular pressure in 528 university students: effect of refractive error [published erratum appears in J Am Optom Assoc 1997;68):756]. J Am Optom Assoc 1997;68:657–62.
21. Hashemi H, Kashi AH, Fotouhi A, Mohammad K. Distribution of intraocular pressure in healthy Iranian individuals: the Tehran Eye Study. Br J Ophthalmol 2005;89:652–7.
22. Leske MC, Connell AM, Wu SY, Hyman L, Schachat AP. Distribution of intraocular pressure. The Barbados Eye Study. Arch Ophthalmol 1997;115:1051–7.
23. Gelaw Y, Kollmann M, Irungu NM, Ilako DR. The influence of central corneal thickness on intraocular pressure measured by Goldmann applanation tonometry among selected Ethiopian communities. J Glaucoma 2010;19:514–8.
24. Goss DA, Caffey TW. Clinical findings before the onset of myopia in youth: 5. Intraocular pressure. Optom Vis Sci 1999;76:286–91.
25. Lee AJ, Saw SM, Gazzard G, Cheng A, Tan DT. Intraocular pressure associations with refractive error and axial length in children. Br J Ophthalmol 2004;88:5–7.
26. Jiang Z, Shen M, Mao G, Chen D, Wang J, Qu J, Lu F. Association between corneal biomechanical properties and myopia in Chinese subjects. Eye (Lond) 2011;25:1083–9.
27. Tomlinson A, Phillips CI. Applanation tension and axial length of the eyeball. Br J Ophthalmol 1970;54:548–53.
28. Song Y, Congdon N, Li L, Zhou Z, Choi K, Lam DS, Pang CP, Xie Z, Liu X, Sharma A, Chen W, Zhang M. Corneal hysteresis and axial length among Chinese secondary school children: the Xichang Pediatric Refractive Error Study (X-PRES) report no. 4. Am J Ophthalmol 2008;145:819–26.
29. Brandt JD, Beiser JA, Kass MA, Gordon MO. The Ocular Hypertension Treatment Study (OHTS) Group. Central corneal thickness in the Ocular Hypertension Treatment Study (OHTS). Ophthalmology 2001;108:1779–88.
30. La Rosa FA, Gross RL, Orengo-Nania S. Central corneal thickness of Caucasians and African Americans in glaucomatous and nonglaucomatous populations. Arch Ophthalmol 2001;119:23–7.
31. Nemesure B, Wu SY, Hennis A, Leske MC. Corneal thickness and intraocular pressure in the Barbados eye studies. Arch Ophthalmol 2003;121:240–4.
32. Shimmyo M, Ross AJ, Moy A, Mostafavi R. Intraocular pressure, Goldmann applanation tension, corneal thickness, and corneal curvature in Caucasians, Asians, Hispanics, and African Americans. Am J Ophthalmol 2003;136:603–13.
33. Aghaian E, Choe JE, Lin S, Stamper RL. Central corneal thickness of Caucasians, Chinese, Hispanics, Filipinos, African Americans, and Japanese in a glaucoma clinic. Ophthalmology 2004;111:2211–9.
34. Semes L, Shaikh A, McGwin G, Bartlett JD. The relationship among race, iris color, central corneal thickness, and intraocular pressure. Optom Vis Sci 2006;83:512–5.
35. Leite MT, Alencar LM, Gore C, Weinreb RN, Sample PA, Zangwill LM, Medeiros FA. Comparison of corneal biomechanical properties between healthy blacks and whites using the ocular response analyzer. Am J Ophthalmol 2010;150:163–8 e1.
36. Racette L, Liebmann JM, Girkin CA, Zangwill LM, Jain S, Becerra LM, Medeiros FA, Bowd C, Weinreb RN, Boden C, Sample PA. African Descent and Glaucoma Evaluation Study (ADAGES): III. Ancestry differences in visual function in healthy eyes. Arch Ophthalmol 2010;128:551–9.
37. Su DH, Wong TY, Foster PJ, Tay WT, Saw SM, Aung T. Central corneal thickness and its associations with ocular and systemic factors: the Singapore Malay Eye Study. Am J Ophthalmol 2009;147:709–16.
38. Price FW Jr., Koller DL, Price MO. Central corneal pachymetry in patients undergoing laser in situ keratomileusis. Ophthalmology 1999;106:2216–20.
39. Tong L, Saw SM, Siak JK, Gazzard G, Tan D. Corneal thickness determination and correlates in Singaporean schoolchildren. Invest Ophthalmol Vis Sci 2004;45:4004–9.
40. Fam HB, How AC, Baskaran M, Lim KL, Chan YH, Aung T. Central corneal thickness and its relationship to myopia in Chinese adults. Br J Ophthalmol 2006;90:1451–3.
41. Altinok A, Sen E, Yazici A, Aksakal FN, Oncul H, Koklu G. Factors influencing central corneal thickness in a Turkish population. Curr Eye Res 2007;32:413–9.
42. Al-Mezaine HS, Al-Obeidan S, Kangave D, Sadaawy A, Wehaib TA, Al-Amro SA. The relationship between central corneal thickness and degree of myopia among Saudi adults. Int Ophthalmol 2009;29:373–8.
43. Kunert KS, Bhartiya P, Tandon R, Dada T, Christian H, Vajpayee RB. Central corneal thickness in Indian patients undergoing LASIK for myopia. J Refract Surg 2003;19:378–9.
44. Cho P, Lam C. Factors affecting the central corneal thickness of Hong Kong-Chinese. Curr Eye Res 1999;18:368–74.
45. Xu L, Zhang H, Wang YX, Jonas JB. Central corneal thickness and glaucoma in adult Chinese: the Beijing Eye Study. J Glaucoma 2008;17:647–53.
46. Hikoya A, Sato M, Tsuzuki K, Koide YM, Asaoka R, Hotta Y. Central corneal thickness in Japanese children. Jpn J Ophthalmol 2009;53:7–11.
47. Nangia V, Jonas JB, Sinha A, Matin A, Kulkarni M. Central corneal thickness and its association with ocular and general parameters in Indians: the Central India Eye and Medical Study. Ophthalmology 2010;117:705–10.
48. Vijaya L, George R, Arvind H, Ve Ramesh S, Baskaran M, Raju P, Asokan R, Velumuri L. Central corneal thickness in adult South Indians: the Chennai Glaucoma Study. Ophthalmology 2010;117:700–4.
49. Hahn S, Azen S, Ying-Lai M, Varma R. Central corneal thickness in Latinos. Invest Ophthalmol Vis Sci 2003;44:1508–12.
50. Leske MC, Connell AM, Wu SY, Hyman LG, Schachat AP. Risk factors for open-angle glaucoma. The Barbados Eye Study. Arch Ophthalmol 1995;113:918–24.
51. Gordon MO, Beiser JA, Brandt JD, Heuer DK, Higginbotham EJ, Johnson CA, Keltner JL, Miller JP, Parrish RK II, Wilson MR, Kass MA. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol 2002;120:714–20.
52. Miglior S, Pfeiffer N, Torri V, Zeyen T, Cunha-Vaz J, Adamsons I. Predictive factors for open-angle glaucoma among patients with ocular hypertension in the European Glaucoma Prevention Study. Ophthalmology 2007;114:3–9.
53. Lotufo D, Ritch R, Szmyd L Jr., Burris JE. Juvenile glaucoma, race, and refraction. JAMA 1989;261:249–52.
54. Tielsch JM, Sommer A, Katz J, Royall RM, Quigley HA, Javitt J. Racial variations in the prevalence of primary open-angle glaucoma. The Baltimore Eye Survey. JAMA 1991;266:369–74.
55. Leske MC, Wu SY, Hennis A, Honkanen R, Nemesure B. Risk factors for incident open-angle glaucoma: the Barbados Eye Studies. Ophthalmology 2008;115:85–93.
56. Marcus MW, de Vries MM, Junoy Montolio FG, Jansonius NM. Myopia as a risk factor for open-angle glaucoma: a systematic review and meta-analysis. Ophthalmology 2011;118:1989–94.
57. Ko YC, Liu CJ, Chou JC, Chen MR, Hsu WM, Liu JH. Comparisons of risk factors and visual field changes between juvenile-onset and late-onset primary open-angle glaucoma. Ophthalmologica 2002;216:27–32.
58. Mitchell P, Hourihan F, Sandbach J, Wang JJ. The relationship between glaucoma and myopia: the Blue Mountains Eye Study. Ophthalmology 1999;106:2010–15.
59. Ramakrishnan R, Nirmalan PK, Krishnadas R, Thulasiraj RD, Tielsch JM, Katz J, Friedman DS, Robin AL. Glaucoma in a rural population of southern India: the Aravind comprehensive eye survey. Ophthalmology 2003;110:1484–90.
60. Wong TY, Klein BE, Klein R, Knudtson M, Lee KE. Refractive errors, intraocular pressure, and glaucoma in a white population. Ophthalmology 2003;110:211–7.
61. Suzuki Y, Iwase A, Araie M, Yamamoto T, Abe H, Shirato S, Kuwayama Y, Mishima HK, Shimizu H, Tomita G, Inoue Y, Kitazawa Y. Risk factors for open-angle glaucoma in a Japanese population: the Tajimi Study. Ophthalmology 2006;113:1613–7.
62. Kuzin AA, Varma R, Reddy HS, Torres M, Azen SP. Ocular biometry and open-angle glaucoma: the Los Angeles Latino Eye Study. Ophthalmology 2010;117:1713–9.
63. Xu L, You QS, Jonas JB. Refractive error, ocular and general parameters and ophthalmic diseases. The Beijing Eye Study. Graefes Arch Clin Exp Ophthalmol 2010;248:721–9.
64. Chatterjee A, Shah S, Bessant DA, Naroo SA, Doyle SJ. Reduction in intraocular pressure after excimer laser photorefractive keratectomy. Correlation with pretreatment myopia. Ophthalmology 1997;104:355–9.
65. Emara B, Probst LE, Tingey DP, Kennedy DW, Willms LJ, Machat J. Correlation of intraocular pressure and central corneal thickness in normal myopic eyes and after laser in situ keratomileusis. J Cataract Refract Surg 1998;24:1320–5.
66. Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology 2008;115:37–50.