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Central Corneal Thickness as a Risk Factor for Advanced Glaucoma Damage
Leon W. Herndon, MD;
Jennifer S. Weizer, MD;
Sandra S. Stinnett, DrPH
Arch Ophthalmol. 2004;122:17-21.
ABSTRACT
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Objective To determine if central corneal thickness (CCT) is related to the level of glaucoma severity at the initial examination.
Methods The initial visit to a glaucoma specialist by consecutive patients with primary open-angle glaucoma from 1997 to 2002 was reviewed retrospectively. Each patient's age, sex, race, family history of glaucoma, number of glaucoma medications, visual acuity, spherical equivalent, intraocular pressure, CCT, visual field data, and vertical and horizontal cup-disc ratios were analyzed.
Results Three hundred fifty eyes of 190 patients met the inclusion and exclusion criteria. Central corneal thickness was significantly lower in blacks (mean, 537 µm) than in whites (mean, 556 µm). Lower CCT was significantly associated with worsened Advanced Glaucoma Intervention Study score, worsened mean deviation of visual field, increased vertical and horizontal cup-disc ratios, and increased number of glaucoma medications. In multivariate analysis, lower CCT was significantly associated with worsened Advanced Glaucoma Intervention Study score, worsened mean deviation of visual field, and increased vertical and horizontal cup-disc ratios.
Conclusions Central corneal thickness is a powerful clinical factor in determining glaucoma severity at the initial examination by a specialist. Measuring CCT may aid the ophthalmologist in identification of glaucoma patients at high risk for progression.
INTRODUCTION
Applanation tonometry has been considered to be the gold standard for determining intraocular pressure (IOP) for almost 50 years. It is less likely to be influenced by variables such as scleral rigidity, which can significantly affect measured values obtained by Schiotz tonometry.1 Goldmann and Schmidt2 discussed the influence of variations of central corneal thickness (CCT) on IOP measured by applanation in their landmark article. However, they believed that significant variations in CCT occurred only rarely. After a commercially available optical pachymeter became available, a positive correlation between CCT and IOP measured by applanation was found.3 This relationship has been confirmed in animal and human studies where intraocular cannulation has been performed to experimentally control IOP.4-5 Central corneal thickness has recently been recognized as a significant risk factor for progression of ocular hypertension to primary open-angle glaucoma (POAG) in the Ocular Hypertension Treatment Study.6 This study was the first to prospectively demonstrate that a thinner CCT predicts the development of POAG. Several other studies have shown that eyes with normal-tension glaucoma have a mean CCT less than that of normal eyes, while eyes with ocular hypertension have a mean CCT greater than that of normal eyes.7-14 In the current study, we examined the effect of CCT on the degree of glaucoma damage seen in POAG patients at the initial examination by a glaucoma specialist.
METHODS
All patients were evaluated at the Duke University Eye Center (Durham, NC) and retrospectively selected from our research database. Approval of the study was granted by the Duke University institutional review board. We reviewed the initial visit of each consecutive new POAG patient seen by a glaucoma specialist (L.W.H.) from 1997 to 2002. The diagnosis of POAG was based on patients having an IOP of 22 mm Hg or higher at the initial visit, or historically, characteristic glaucomatous optic neuropathy with diffuse or focal optic rim thinning, hemorrhage, cupping, or nerve fiber layer defects indicative of glaucoma, and corresponding visual field loss. Each of the patients had to have pachymetry performed within 1 month of the initial visit. Stereo disc photographs or a detailed optic nerve drawing, and automated visual field testing (if appropriate) were also required within 1 month of the initial visit. Exclusion criteria were corneal or retinal disease or a secondary cause of glaucoma, including pseudoexfoliation and pigment dispersion syndromes. Patients who had undergone any corneal surgery or retinal laser procedures, including panretinal photocoagulation, were not included in this study. There were 2 patients with normal-tension glaucoma who were included in the study.
For each patient, we recorded age, sex, race, family history of glaucoma in a first-degree relative, and presence of diabetes mellitus or systemic hypertension as reported by the patient. For each eye, we recorded Snellen visual acuity, number of glaucoma medications prescribed, spherical equivalent, IOP by Goldmann applanation tonometry, average CCT, visual field data where available, and vertical and horizontal cup-disc ratios. Combination eyedrops, such as timolol/dorzolamide, were counted as 2 glaucoma medications; oral pressure-lowering medications, such as acetazolamide, were counted as 1 glaucoma medication. Goldmann applanation IOP was measured before dilation; when 2 or more predilation measurements were charted, the average of those pressures was recorded. Visual field data included type of visual field analysis performed, Advanced Glaucoma Intervention Study (AGIS) score, mean deviation of visual field, fixation losses, false-negative responses, and false-positive responses. Right and left eyes were analyzed separately. The AGIS score has been described in detail previously.15 In brief, the visual fields are graded on a scale of 0 to 20 based on the degree of damage on the Total Deviation printout. A score of 0 represents a normal visual field; 1 to 5 represents mild disease; 6 to 11, moderate disease; 12 to 17, severe disease; and 18 to 20, end-stage glaucoma.
Each patient's CCT was measured using an ultrasonic pachymeter (Storz Compuscan Ultrasonic Pachymeter System; Storz, St Louis, Mo [used 1997 to 2000]; and DGH 550 Pachette 2; DGH Technology, Exton, Pa [used 2000 to 2002]). The average of 5 CCT readings was recorded. Patients also underwent visual field testing by Humphrey automated 24-2 (Humphrey Systems, Dublin, Calif) or 30-2 full-threshold or Swedish Interactive Thresholding Algorithm (SITA) standard perimetry protocols if their visual function allowed, or by Humphrey automated 10-2 SITA standard perimetry if their glaucomatous damage was deemed very severe. Patients underwent Goldmann manual perimetry if they were not able to complete Humphrey automated visual field testing. Patients whose visual acuity was too poor for automated or manual visual field testing in that eye did not have data for that eye entered into the visual field categories. Only patients who had reliable Humphrey automated 24-2 or 30-2 SITA standard or full-threshold perimetry within 1 month of their initial visit had their visual field data included in the statistical analysis. Reliable Humphrey automated perimetry was defined by having fewer than 2 of the following characteristics: fixation losses less than 20%, false-positive responses less than 33%, or false-negative responses less than 33%. This reliability criteria was adapted from the AGIS reliability ratings.15 Each Humphrey 24-2 or 30-2 visual field was scored by one masked grader (J.S.W.) according to the AGIS scoring system.15 In the case of 30-2 Humphrey visual fields, the outermost circumference of testing points was not included in the scoring so that the remaining testing points fit the AGIS scoring template for 24-2 visual fields. One masked grader (L.W.H.) determined vertical and horizontal cup-disc ratios for each eye. Cup-disc ratios were judged using stereoscopic optic disc photographs from each patient's initial visit when available, or by evaluating detailed chart drawings from the initial visit when photographs were not available.
Initially, descriptive statistics (number and percentage for categorical variables, and number, mean, and SD for continuous variables) were obtained. Subsequently, each variable was assessed individually for its relationship to the degree of damage from glaucoma (AGIS score, mean deviation, and vertical and horizontal cup-disc ratios) and number of medications used for glaucoma. For categorical variables (sex, race, and family history of glaucoma), analysis of variance was used to assess whether differences in the outcome variables exist between categories of these predictor variables. The relationship between continuous predictor variables (corneal thickness, age, spherical equivalent, and IOP) and the outcome variables was assessed using linear regression. Finally, all predictor variables were combined in a single regression model to assess their joint effects on the outcome variables. The model for each outcome variable was reduced using backward elimination until it contained only significant predictors. In all statistical models, the correlation between the 2 eyes was accounted for by using a mixed model with a random subject effect.
RESULTS
Of the 434 medical records reviewed, 350 eyes of 190 patients met the inclusion and exclusion criteria for this study. Descriptive statistics for demographic and background variables are presented in Table 1. The average age of the 190 patients was 65.6 years (range, 22-97 years). Patients were 51% male and were predominantly black (55%) and white (42%). Ninety-nine percent of patients had a diagnosis of POAG, and 39% had a self-reported family history of glaucoma. Nineteen percent had a history of diabetes, and 53% had a history of hypertension. Eighty-one percent of right eyes and 79% of left eyes had Humphrey visual field tests using the SITA standard method.
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Table 1. Descriptive Statistics for Demographic and Background Variables*
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Descriptive statistics for ophthalmic measures are presented separately for right and left eyes in Table 2. Of the 350 eyes, 178 were right eyes and 172 were left eyes. The average visual acuity in the right eyes was 20/38 (0.284 logMAR units) and in the left eyes was 20/42 (0.321 logMAR units). The average spherical equivalent was –0.82 diopters OD and –0.69 diopters OS. Average IOP was about 19 mm Hg OU. The mean CCT was 544 µm OD and 546 µm OS. The average AGIS score was about 6.2 OU. The average mean deviation of visual field was –8.3 dB OD and -8.1 dB OS. The mean vertical cup-disc ratio was about 0.80 OU, and the mean horizontal cup-disc ratio was 0.76 OU. The average number of medications was 1.6 OD and 1.7 OS.
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Table 2. Descriptive Statistics for Ophthalmic Measurements
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In white patients, the mean CCT was 556 ± 41 µm, while in black patients, the mean CCT was 537 ± 38 µm. This was a statistically significant difference (P<.001). There was no statistically significant difference in CCT between men and women, with men having a mean CCT of 544 µm and women having a mean CCT of 547 µm (P = .64). There was also no statistically significant difference in CCT between patients with and without a family history of glaucoma; those with a positive family history had a mean CCT of 547 µm, while those without had a mean CCT of 544 µm (P = .83).
UNIVARIATE MODELING
Results of univariate modeling are presented in Table 3. When considered alone, CCT was a significant predictor of every outcome variable. An increase in CCT was associated with an improved AGIS score (P = .001), an improved mean deviation of visual field (P<.001), a decrease in vertical (P<.001) and horizontal (P = .003) cup-disc ratios, and a decrease in the number of medications used for glaucoma (P = .04).
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Table 3. P Values for Tests of Significance of Variables in Predicting Outcomes of Glaucoma: Univariate Models
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Age was a significant predictor of AGIS score (P = .01) and mean deviation of visual field (P = .002). An increase in age was associated with a worsened AGIS score and a worsened mean deviation of visual field. Intraocular pressure was a significant predictor of mean deviation of visual field (P = .01). An increase in IOP was associated with an improved mean deviation of visual field.
There was a significant difference among races in the number of medications used (P = .01). For both eyes combined, Hispanics used the smallest average number of medications (1.3), followed by whites (1.8), blacks (2.1), Asians (2.7), and those of unknown race (4.5). The number of patients of races other than black or white was very small, however.
There was a significant difference between those with and without a family history of glaucoma in terms of vertical cup-disc ratio (P = .02), horizontal cup-disc ratio (P = .07), and number of medications used (P = .009). Those with a positive family history had a slightly greater vertical cup-disc ratio (0.87) than those without (0.83) and a slightly greater horizontal cup-disc ratio (0.84) than those without (0.80). Those with a family history used more medications (2.3) than those without (1.9). Spherical equivalent was not a significant predictor of any outcome variable. Also, there was no difference between men and women for any outcome variable.
MULTIVARIATE MODELING
Results of multivariate modeling are presented in Table 4. When all variables were considered simultaneously, the only significant predictor of AGIS score was CCT (P = .001). For an increase of 10 µm of corneal thickness, AGIS score improved by 0.31 points.
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Table 4. P Values <.05 for Tests of Significance of Variables in Predicting Outcomes of Glaucoma: Multivariate Models
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Significant predictors of mean deviation of visual field were CCT (P = .006), age (P = .02), and IOP (P = .02). For an increase of 10 µm of CCT, the mean deviation of visual field improved by 0.34 dB; for an increase of 10 years of age, the mean deviation of visual field worsened by 0.88 dB; for an increase of 1 mm Hg of IOP, the mean deviation improved by 0.21 dB.
Significant predictors of vertical cup-disc ratio were CCT (P<.001) and family history of glaucoma (P = .01). For an increase of 10 µm of CCT, the vertical cup-disc ratio decreased by 0.008. At an average value of CCT (ie, 545 µm), patients with a family history of glaucoma had a vertical cup-disc ratio that was 0.043 greater than those without a family history of glaucoma.
Significant predictors of horizontal cup-disc ratio were CCT (P = .003) and family history of glaucoma (0.042). For an increase of 10 µm of CCT, horizontal cup-disc ratio decreased by 0.007. At an average value of corneal thickness (ie, 545 µm), patients with a family history of glaucoma had a horizontal cup-disc ratio that was 0.038 greater than those without a family history.
Significant predictors of number of medications used for glaucoma were age (P<.001), race (P<.001), spherical equivalent (P = .04), and family history of glaucoma (P = .03). For an increase of 10 years of age, the number of medications increased by 0.23; for an increase of 1 diopter of spherical equivalent, the number of medications decreased by 0.06. Adjusting for all other variables, patients with a family history of glaucoma used 0.31 more medications that those without a family history. The pattern among races was similar to that in the univariate model.
We mentioned previously that black patients had significantly thinner central corneas compared with white patients. The measured IOP in black patients (19.4 ± 5.9 mm Hg) was not different compared with that in white patients (18.4 ± 6.0 mm Hg). Ehlers and associates4 created an additive correction table for CCT measurements greater or less than the mean CCT in their study. They extrapolated that applanation tonometry is overestimated or underestimated by approximately 5 mm Hg for every 70-µm difference in measured CCT from normal CCT. If we adjust the measured IOPs by this correction factor to determine the true IOPs, using 545 µm as a normal CCT, there is a significant difference (P<.001) between blacks and whites (20.0 ± 6.4 mm Hg vs 17.6 ± 6.3 mm Hg, respectively). This difference between measured and true IOPs is shown in Figure 1.
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True intraocular pressure (IOP) vs measured IOP. For every 1–mm Hg increase in measured IOP, the true IOP increases by 0.931 mm Hg in white subjects and by 0.994 mm Hg in black subjects. The correction for true IOP was made based on the additive correction table by Ehlers et al.4
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COMMENT
To our knowledge, we are the first investigators to report the association of CCT with POAG severity. The effect of CCT on ocular hypertension and normal-tension glaucoma has already been well documented.7-14 In our study, patients with POAG who had a thinner CCT tended to have more severe glaucomatous damage on initial examination by a glaucoma specialist. Central corneal thickness was the most consistent predictor of degree of glaucomatous damage as measured by the outcome variables.
It has been suggested that a thicker CCT may be protective against glaucomatous damage, since CCT in ocular hypertensive patients tends to be thicker than in POAG patients.7, 9-10,14 It is well known that IOP measured by applanation should be adjusted to correct for CCT measurements that are higher or lower than the mean CCT of approximately 545 µm in the general population.12 In our univariate and multivariate analyses, increased IOP was associated with an improved mean deviation of visual field and was not significantly associated with any other outcome variable. Thinner CCT was associated with worsening in all outcome variables except number of glaucoma medications. It is unclear why having a higher IOP should be associated with an improved mean deviation of visual field in our study. Perhaps the association of higher IOP with increased CCT explains this phenomenon, although after correcting for the true IOP, the difference still persisted.
Our study confirms the observation that mean CCT in black patients is lower than mean CCT in white patients. La Rosa et al16 found that the mean difference in CCT between all black and white subjects was 27.3 µm, while the mean difference between black and white glaucoma patients was 32.2 µm. Nemesure et al17 measured pachymetry on 1142 participants in the Barbados Eye Study and found that black participants tended to have thinner corneas (mean thickness, 529.8 µm) than mixed (black and white) (537.8 µm) and white (545.2 µm) participants, respectively. In our study, the mean difference in CCT between black and white glaucoma patients was 19 µm. This may help to explain why blacks are 4 to 5 times more likely than whites to be diagnosed as having glaucoma.18 In the Ocular Hypertension Treatment Study, when race is entered into a multivariate analysis, it is no longer a statistically significant predictor of progression to glaucoma; CCT and vertical cup-disc ratio remain significant predictors, however.6 In our univariate and multivariate analyses, black race significantly predicts an increase in the number of glaucoma medications, although race is not significantly associated with any other outcome variables.
The Baltimore Eye Survey (BES) showed that the average IOP among black and white subjects in the general population was similar.19 In both races, the screening IOP was higher in glaucomatous eyes than in eyes in the general population. In white subjects, eyes that received glaucoma treatment had an IOP 2 to 3 mm Hg higher than eyes without glaucoma, on average. Glaucomatous eyes that were not yet receiving treatment had an IOP 6 to 7 mm Hg higher than normal eyes. In black subjects, treatment status made little difference; the IOP in glaucomatous eyes was 4 to 5 mm Hg higher than in the rest of the population whether they received treatment or not. Untreated black subjects with POAG had lower IOP than untreated white subjects in the BES, which would suggest that the optic nerve in black subjects may be more sensitive to IOP than that in white subjects. It is difficult to draw comparisons between our study and the BES, as most of our patients were receiving treatment; however, perhaps the true IOP of the untreated black subjects in the BES (as corrected for variations in CCT) would be higher than that of the untreated white subjects, which would account for the seemingly increased susceptibility to glaucomatous optic nerve damage among blacks.
We found that having a family history of glaucoma was associated with having worse disease, as defined by increased horizontal and vertical cup-disc ratios. Also, patients with a family history of glaucoma used more glaucoma medications. Having a family history of glaucoma, however, had no effect on the degree of visual field loss as defined by AGIS scores and mean deviation indices. These findings are in agreement with a recent article by Landers et al,20 who found that a family history of POAG had no influence on the severity of visual field at diagnosis. They did find, however, that having a family history of glaucoma was associated with a better visual field at diagnosis in patients younger than 50 years but not in patients 50 years or older.
There are some minor limitations in our study. We did not intend to follow patients' progression of glaucomatous damage. Rather, our data reflect the findings at the initial examination by a glaucoma specialist. Also, we are aware that there may be some referral bias in our patient population. Patients referred to a glaucoma specialist at a tertiary care institution may have more advanced, intractable glaucoma than those in the general population and therefore may not represent the majority of POAG patients. Because of a lack of standardized grading methods, we were not able to grade the few Goldmann manual perimetry studies and 10-2 Humphrey visual fields that were performed, so patients with the most severe glaucomatous visual field damage could not be included in the visual field analysis because they were unable to perform SITA standard or full-threshold Humphrey visual field testing. Finally, the AGIS grading system was designed for full-threshold strategies, but most of our patients had SITA analysis. It is not clear if one can directly apply AGIS criteria to SITA in a seamless manner; however, we were consistent with all of our interpretation, which should quantify our results in a uniform fashion.
In conclusion, CCT is a significant predictor of glaucomatous damage as measured by AGIS score, mean deviation of visual field, and vertical and horizontal cup-disc ratios in patients at the initial examination by a glaucoma specialist. Measuring CCT in glaucoma patients may help identify those patients who are at higher risk for developing severe glaucomatous sequelae, thus enabling the ophthalmologist to treat their disease more aggressively.
AUTHOR INFORMATION
Corresponding author and reprints: Leon W. Herndon, MD, Duke University Eye Center, Box 3802, Duke University Medical Center, Durham, NC 27710-3802.
Submitted for publication April 28, 2003; final revision received August 17, 2003; accepted August 25, 2003.
This study was supported in part by Clinical Vision Research Development Award EY11725 from the National Eye Institute, National Institutes of Health, Bethesda, Md (Dr Stinnett).
From Duke University Eye Center, Durham, NC. The authors have no relevant financial interest in this article.
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