Interrelationship among blood pressure, intraocular pressure, and life-style in middle-aged and older Japanese residents ()
1. INTRODUCTION
Glaucoma is the second leading cause of blindness worldwide. It is estimated that the number of people with glaucoma will be 80 million in 2020, and more than 70% 0f those will have open-angle glaucoma (OAG) [1]. Ocular hypertension, which is usually defined to be in traocular pressure (IOP) higher than normal, IOP >21 mmHg, in the absence of optic nerve damage or visual field loss [2], is one of the major risk factors for the development of OAG [3]. Several studies have reported that relatively high IOP can cause optic-nerve damage and visual-field abnormalities, even if it is within the normal range [4,5]. High IOP can also affect the progression of visual-field defects in patients with OAG, particularly those with normal tension glaucoma (NTG) [6,7].
Many cross-sectional epidemiological studies have investigated the risk factors associated with elevated IOP. Although positive associations have been found between IOP and blood pressure (BP), and several life-style related factors such as body mass index (BMI), drinking, smoking, and exercise habits [8-18], most of these reports were conducted in Western countries. Few epidemiological studies have examined this relationship in a Japanese population [16,18]. Up to the present, there have been no studies investigating the interrelationship among BP, IOP, and life-style related factors. Moreover, the role of life-style related factors as a cause for “high BP accompanied by high IOP” remains obscure. Therefore, in the present study, we attempted to clarify whether high BP was associated with elevated IOP, and whether there were some particular life-style related factors specific to “high BP accompanied by high IOP”. We analyzed health examination data from middle-aged and older Japanese residents in a prefectural capital close to Tokyo. The results of this study might contribute to preventing the progression of IOP-related optic-nerve damage and visual-field defects.
2. METHODS
2.1. Subjects
A total of 1,320 residents of the Ibaraki prefecture, Japan, underwent an annual health examination between April 2001 and March 2004 at a general hospital in Mito, which is the capital city of the prefecture. From this initial group, 1113 individuals (829 men and 284 women), ranging in age from 28 to 79 years, who had not undergone any ocular surgery or medical treatment for hypertension, ocular hypertension, and glaucoma, were selected for cross-sectional analysis.
This study was conducted in accordance with the Declaration of Helsinki of the World Medical Association and was approved by the human ethics review committees of Mito Red Cross Hospital and Kyorin University School of Medicine in Japan.
2.2. Health Examination
The health examination consisted of a questionnaire that assessed demographic and lifestyle-related factors, along with the following measurements and tests: height, weight, BP, IOP, hematological and serum biochemistry, chest X-rays, electrocardiography, and fundus photography. All subjects were requested not to consume any food or alcohol after 21:00 h on the day before the examination.
The topics covered in the questionnaire included age, marital status, occupation, residence, current status and past history of medication, family medical history, drinking history, smoking history and number of cigarette smoking per day, and exercise habits. Alcohol consumption was classified into the following four categories: never or seldom, several times per month, several times per week, and everyday. BP was measured with a sphygmomanometer using the right arm; two consecutive measurements were taken after subjects had rested in a sitting position for at least 5 min. IOP was measured using a non-contact tonometer (NT-3000, Nidek, Japan) and three consecutive measurements were taken per eye.
2.3. Statistical Analyses
SBP and DBP values for each subject were calculated as the mean of two measurements. For IOP, the mean of three measurements was calculated for each eye; however, as there was a strong correlation (r = 0.84) between the mean IOPs of the right and left eyes in each individual, the mean of these two values was used as a single measure of IOP. The associations of age with IOP, SBP, DBP, and BMI—calculated as weight (kg)/height squared (m2)—were determined by comparing the mean values of these four parameters among four age groups (<40 years, 40 - 49 years, 50 - 59 years, and ³60 years) by gender, using the Bonferroni multiple-comparison method [19]. The Student’s t-test was used to analyze differences in the mean IOP, SBP, DBP, and BMI between the two sexes by age group.
To analyze the associations of BP and life-style related factors with IOP, first, the partial regression coefficients and the standardized partial regression coefficients for IOP of the following independent variables were determined by multiple-regression analysis: SBP (model 1), DBP (model 2), and BMI, alcohol consumption (never or seldom = 0; several times per month = 1; several times per week = 2; everyday = 3), number of cigarettes smoking per day, and regular exercise (no = 0; yes =1). Second, to analyze the interrelationship among BP, IOP, and life-style related factors, the adjusted mean of the eligible etiologic factors, calculated by analysis of covariance, were compared between the four groups classified by BP and IOP levels; Group A: hypertensives (high-normal BP or hypertension) with high IOP (highnormal IOP or ocular hypertension), Group B: hypertensives without high IOP, Group C: normotensives with high IOP, Group D: normotensives without high IOP. The definition of hypertensives, normotensives, and high IOP in the present study was as follows: hypertensives (SBP > 130 mmHg and/or DBP > 85 mmHg), normotensives (SBP < 130 mmHg and DBP < 85 mmHg), and high IOP (IOP > 15 mmHg). All analyses were conducted using the SAS statistical software package, version 8.2. [20].
3. RESULTS
The relationships between age and IOP, SBP, DBP, and BMI are shown in Table 1. The mean IOP was highest in the <40 age group for both sexes. In both sexes, the highest mean SBP, DBP, and BMI values were found in the age group of ≥60, 50 - 59, and 50 - 59, respectively. All parameters measured were significantly higher in men than in women (IOP, P < 0.05; SBP, DBP, and BMI, P < 0.01). The Bonferroni multiple-comparison analysis revealed that the mean IOP was significantly lower in the ³60 age group than in the <40 age group in both sexes (P < 0.05).
Table 2 shows the partial regression coefficients and the standardized partial regression coefficients of SBP,