a Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya, Japan.
b Department of Epidemiology, National Institute for Longevity Sciences, Obu, Aichi, Japan.
Reprint requests to: Keiko Mori, Department of Living Science, Chukyo Junior College, 2216 Toki-cho, Mizunami-shi, Gifu, 5096192 Japan.
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Abstract |
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Methods Data were collected from annual health examinations between 1989 and 1997 and reviewed retrospectively. Subjects of the cross-sectional analysis were 70 139 males and females aged 1494 years. Among these subjects, 25 216 males and females who had undergone IOP measurements more than three times were analysed longitudinally. The association between IOP and obesity was examined cross-sectionally and longitudinally.
Results Cross-sectional analysis: The mean IOP at the last visit was 11.6 mmHg. The IOP decreased gradually with age and was significantly higher in males than in females in almost all age groups. Body mass index (BMI) significantly correlated with IOP after controlling for age, gender and blood pressure. Longitudinal analysis: There was a significant association between longitudinal change in IOP and change in weight. This relationship remained significant after controlling for initial BMI, initial blood pressure, change in blood pressure, gender and age.
Conclusion This study showed a significant association between IOP and obesity in both cross-sectional and longitudinal analysis. These findings suggest that obesity is an independent risk factor for increase in IOP.
Keywords Intraocular pressure, obesity, cross-sectional analysis, longitudinal analysis, blood pressure, Japanese population
Accepted 17 December 1999
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Introduction |
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In addition, a previous report demonstrated that relatively high IOP in normal-pressure glaucoma is related to optic nerve damage.5 Therefore it is important to identify factors that influence the level of IOP and prevent increased IOP. A number of studies have attempted to identify risk factors associated with the development of elevated IOP.621 Several cross-sectional studies in western populations have suggested that age and systolic blood pressure (SBP) related positively to IOP.6,9,11,1321 However, there were a few studies that showed a negative association between age and IOP in a Japanese population.22,23
Moreover, some epidemiological studies examined the relationship between obesity and IOP cross-sectionally.15,2225,28 These studies found that obesity was an independent risk factor for increase in IOP, even when considered with age, SBP and diastolic blood pressure (DBP).15,22,23 There were few longitudinal studies that showed a positive relationship between change in IOP and change in SBP.26 In addition, no longitudinal studies have shown an association between IOP and obesity in general populations. However, previous study in our laboratory showed that IOP significantly increased with age in longitudinal analysis, but significantly decreased with age in cross-sectional analysis.29
This study investigated cross-sectional and longitudinal associations between IOP and obesity in a large male and female Japanese population.
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Methods |
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Cross-sectional analysis was based on data obtained at the last visit of each subject. The association of IOP and BMI with age was analysed in a linear regression model. Partial correlation coefficients among IOP, SBP, DBP and BMI controlled for age were examined in males and females. Gender was entered as a dichotomous variable (male = 0, female = 1). The relationship between BMI and IOP controlled for age, gender, SBP and DBP was also studied by analysis of covariance (ANOCOVA).
For longitudinal analysis, the individual-specific linear regression model was used. For each subject, the regression slope from three or more IOP measurements against age was calculated (slope of IOP). Similarly, the slope of weight, slope of SBP, and slope of DBP against age were calculated individually. The dependence of slope of IOP on slope of weight, age, gender, slope of weight, initial SBP, slope of SBP, initial DBP, slope of DBP, initial BMI, and initial IOP was investigated by multiple regression analysis. Moreover, the relationship between slope of weight and slope of IOP were studied by ANOCOVA controlling for age, gender, initial BMI, initial SBP, slope of SBP, initial DBP, slope of DBP, and initial IOP.
All data were processed and analysed by the Statistical Analysis System30 (SAS version 6.12).
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Results |
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Age-specific IOP values estimated by the least square method in the multiple linear regression models controlled for SBP, DBP and BMI are plotted in Figure 1. Age-specific BMI was also plotted. The IOP decreased with age in both genders, and IOP value was generally higher in men than in women. In males, BMI sharply increased with age up to 35 years, but in females increased slowly up to 60 years old and was significantly higher in males than in females up to 60 years. Over 65 years old, BMI gradually decreased in males and there was no gender difference in BMI.
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The multiple linear regression models disclosed the influence of slope of weight and the other variables on slope of IOP (Table 4). Slope of weight, initial BMI, initial SBP, slope of SBP, and initial DBP were positively associated with the slope of IOP (P < 0.0001). Age, gender and initial IOP negatively related to the slope of IOP (P < 0.0001). The slope of DBP was not significantly associated with the slope of IOP. In addition, the relationship between slope of IOP and slope of weight was examined by ANOCOVA after controlling other factors. There was a significant positive trend between slope of IOP and slope of weight after controlling for age, gender, initial BMI, initial SBP, slope of SBP, initial DBP, slope of DBP and initial IOP (trend, P < 0.0001) (Figure 3
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Discussion |
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In this cross-sectional study, the mean IOP using non-contact tonometry was 11.7 mmHg in males and 11.4 mmHg in females. These values were substantially lower than the findings of other surveys using applanation tonometry; the values are between approximately 1417 mmHg.4,11,1315,18,19 Shiose et al., using non-contact tonometers, showed mean IOP values in an apparently normal population of males and females (12.0, 11.5 mmHg, respectively) which were almost equal to our findings.23,24
In our longitudinal study, the initial IOP was highly negatively associated with the slope of IOP. This may be due to regression toward the mean. This phenomenon is widespread in applied science, especially physiological variability. In longitudinal studies, examinations are repeated over time in the same individual. If the initial value is unusually high, it can be expected that the subsequent readings are likely to be closer to the centre of distribution. Because of this phenomenon, the initial value of the examinations tends to correlate negatively with subsequent changes in value.
Many cross-sectional studies in western populations have reported a positive correlation between IOP and age.10,11,13,1721 While there are individual variations, the effects of ageing become more apparent in females than in males over 4045 years old.1719 In cross-sectional studies by Shiose et al.23,24 and Kurokawa27 in Japanese, the IOP decreased with age in both genders. Such a paradoxical result seems to be difficult to explain without considering the effects of other factors. In addition, true changes in IOP with age cannot be determined by cross-sectional statistics alone. A report from the Baltimore Longitudinal Study of Aging showed that there was no consistent relationship between 2-year longitudinal change in IOP and age in a healthy white male, after controlling for other factors.26 However, we found that IOP decreases with age, even when analyses were considered with blood pressure and BMI in cross-sectional study. We also found that longitudinal change in IOP was more strongly influenced by change in weight and change in blood pressure than ageing.
In our cross-sectional data, we confirmed that an increase of SBP and DBP related to IOP, after controlling for age, gender and BMI. For longitudinal study, we recognized that the slope of SBP correlated with an increase in IOP. In addition, slope of SBP was an independent factor influencing increase in IOP, although the standardized coefficient of multiple regression analysis was slightly less than that of the slope of weight. The relationship between IOP and the slope of DBP in longitudinal analyses was smaller than slope of SBP.
A number of papers have reported the positive correlation between IOP and SBP. However, the relation between DBP and IOP was shown in few studies. In addition, data from a longitudinal study by Mcleod et al.26showed that change in IOP was positively correlated with change in SBP over both 1- and 2-year periods. However, change in DBP was negatively correlated with change in IOP over a 2-year period. It appears that our data are generally consistent with previous studies. The results of our cross-sectional and longitudinal study support the hypothesis that increased SBP is closely associated with increased IOP.
The mean BMI in our study was 22.9 for males and 21.6 for females. The prevalence of obesity (BMI 26.4) was 10.9% in males and 6.5% in females. These percentages were slightly lower than those obtained by the National Nutrition Survey conducted in 19901994 in Japan.31 The cross-sectional curve of BMI with age and the prevalence of obesity were similar to reports from other medical centres in Japan.32,33
Obesity is a strong risk factor for hypertension and diabetes mellitus.
In addition to hypertension and diabetes mellitus, the Barbados Eye Study found several other factors associated with an increase in IOP using multiple regression analysis. Larger body size, as measured by BMI, was associated with increasing IOP.22 A high prevalence of obesity has been reported in Barbados. However, the association between larger size and IOP was found to be independent of hypertension and diabetes mellitus, but not directly related with open angle glaucoma. A relation between obesity and IOP was also found in studies by Shiose et al.,23,24 Klein et al.15 and Bulpitt et al.25 (Japanese, American and British populations, respectively). Our cross-sectional data demonstrated that BMI was significantly correlated with IOP after adjusting for age, gender, SBP and DBP. In addition, our longitudinal results provided evidence that, among the factors studied, the strongest relation existed between change in IOP and change in weight and change in weight was an independent risk factor for increase in IOP. In other words, these data suggested a strong positive association between obesity and IOP.
The mechanism has been explained in previous reports22,23,25,34 as follows; IOP may increase due to excess intraorbital fat tissue, an increase in episcleral venous pressure and a consequent decrease in outflow facility. Obesity increases blood viscosity through increasing red cell count, haemoglobin and haematocrit, and consequently increased outflow-resistance of episcleral veins results. Further, obesity is also a risk factor for hypertension. Elevated blood pressure increases IOP by increasing ciliary artery pressure and ultrafiltration of the aqueous humour.
Thus, the combined evidence from several studies now suggests that high levels of BMI and increased BMI are strongly associated with risk of increased IOP. Our outcome reaffirms the importance of weight control in preventing increased IOP.
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Acknowledgments |
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References |
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