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.
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