a Department of Public Health, The University of Western Australia, Nedlands, Western Australia.
b Department of Vascular Surgery, Royal Perth Hospital, Perth, Western Australia.
c University Department of Surgery, Fremantle Hospital, Fremantle, Western Australia.
Konrad Jamrozik, Department of Primary Health Care and General Practice, Imperial College School of Medicine, Reynolds Building, St Dunstan's Road, London W6 8RP, UK. E-mail: konrad{at}ic.ac.uk
Abstract
Background We sought to test, in men undergoing ultrasound screening for abdominal aortic aneurysms (AAA) in Western Australia, clinical impressions that the prevalence of AAA is high in Dutch migrants and low in migrants from Mediterranean countries.
Methods In a population-based trial, men undergoing screening for AAA completed a questionnaire covering their place of birth, smoking habits and consumption of alcohol, meat, fish, salt and milk. We examined the variation by place of birth in the mean, median, 95th and 99th centiles of infrarenal aortic diameter and the prevalences of AAA defined by criteria of 30 mm, 50 mm and by the 95th and 99th centiles, in men born in Australia, of aortic diameter adjusted for height.
Findings Overall, 12 203 (70.5%) of the 19 583 men took up the invitation to undergo ultrasound screening. The prevalence of AAA defined by absolute diameter was higher than average in men born in The Netherlands or Scotland (more of whom had ever smoked or smoked currently) and lower in men of Mediterranean origin (more of whom drank alcohol currently). There were no consistent relationships with simple dietary data. Correction of aortic diameter for height eliminated the significant heterogeneity in prevalence of large AAA, although a threefold variation in prevalence of AAA exceeding the 95th centile of height-adjusted diameter in Australian men persisted.
Interpretation In our cohort of men, which is subject to both healthy migrant and survivor effects, if it exists at all, any Mediterranean paradox for AAA is more modest than that for coronary disease.
Keywords Abdominal aortic aneurysm, place of birth, prevalence, normal range
Accepted 19 February 2001
Early in the course of a large, population-based randomized controlled trial of ultrasound screening for abdominal aortic aneurysm (AAA) in men aged 6583 years1 the surgical authors of this paper (MMLB, PEN) suggested that we seek to confirm their strong clinical impression that AAA was differentially common among Dutch migrants to Australia. Once the question of a relationship between place of birth and risk of AAA had been raised, staff conducting the screening clinics soon reported their own observation that it was rare to find an AAA in a man of Italian, Greek or Yugoslav birth. This led us to a hypothesis that the prevalence of AAA might show the same north-south gradient in European men as incidence and mortality from ischaemic heart disease (IHD). This gradient is now widely known as the French paradox,2 although its generalized nature36 suggests that it be more accurately termed the Mediterranean paradox. The paradox to which the phrase refers is that the north-south gradient in intake of fat in Europe is much less steep than that for heart disease. This observation has led to speculation that differences in proportions of poly- and mono-unsaturated fat and in intake of alcohol, and specifically of wine, may modify the atherogenic effects of a given level of fat in the diet.25
We reasoned further that, if the geographic epidemiology of AAA and IHD were similar, we should also see a higher prevalence of AAA in migrants from Scotland, given that the Scots (and the Finns, of whom there are very few in Australia) have the worst profile of coronary disease in Western Europe.6 Fortunately the data collected from men undergoing screening allowed us to test these hypotheses and to examine the relationship between AAA and both self-reported drinking habits and selected aspects of diet. Cross-sectional comparisons based on our data showed that height was independently associated with prevalence of AAA of 30 mm (3% increase in risk per 1 cm increase in height [95% CI : 24%]).1 This prompted us also to examine systematically average aortic diameters in men of different height.
Methods
Source of data
The data in this paper were obtained from the Western Australian AAA Program, the protocol for which is described in full elsewhere.1 Briefly, the core of the Program is a population-based trial of ultrasound screening of men in Perth, Western Australia, who were aged 6579 years and were apparently not resident in nursing homes when originally randomized to screen and no screen groups in April 1996. Men in the screen group were invited to pre-arranged appointments at a dedicated clinic. After providing written consent, they were asked to complete a detailed questionnaire that included aspects of their occupational, demographic and medical histories plus certain aspects of lifestyle including smoking and drinking habits. The questionnaire included items on place of birth and duration of residence in Australia, plus single questions on usual consumption of meat, fish, milk and salt. Following a brief physical examination covering height, weight, girth at the waist and hips and blood pressure and conducted according to a standard protocol, each man underwent an ultrasound scan to establish the maximum diameter, antero-posterior or transverse, of the infrarenal aorta. Scans were performed by qualified ultrasonographers or by specially trained nurses. Quality control systems included video-recording of each man's scan for later checking of measurements, regular measurement of inter- and intra-observer variation, and recall of men whose aorta could not be visualized for a second examination after an overnight fast.
Definitions
We defined a maximum diameter of the infrarenal aorta of 30 mm as indicating that an aneurysm was present, while a diameter of
50 mm indicated a large AAA for which we recommended to the relevant general practitioner that he or she arrange an immediate confirmatory scan and referral to a vascular surgeon. Men born in Australia served as the reference group for comparisons based on place of birth. The three other groups were Mediterranean (Italy, Greece, Spain, Malta, Cyprus, former Yugoslavia), Northern Europe (including the UK and Ireland), and all other countries combined. Additional analyses concerned specifically men born in The Netherlands and Scotland. Height was treated as a continuous variable measured in centimetres. Men were classified into lifelong non-smokers, ex-smokers, and current smokers of 124 or
25 cigarettes daily. The variable for drinking habits separated non-drinkers from those who drank at all currently, and subdivided the latter by increments of 20 g of alcohol (2 standard drinks) daily based on a 7-day diary of usual alcohol consumption. We did not collect data on beer, wine or spirits separately.
Statistical analysis
We began by comparing the mean age and height, smoking and drinking habits and responses to the dietary questions of the four groups of men classified by place of birth, and, separately, men born in Scotland or in The Netherlands. Next, we computed the crude mean, standard deviation, median, inter-quartile range, 95th and 99th centiles of aortic diameter for each subset of men defined by place of birth. This was supplemented by a calculation of the prevalence of any and of large AAA for each subset, standardizing these proportions using the direct method, 5-year increments of age, and weights from Segi's world population as the reference distribution.7 We then repeated this calculation after adjusting each man's aortic diameter for his height using the formula:
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and redefining any AAA and a large AAA as, respectively, scores exceeding the 95th and 99th centiles of this index among men born in Australia.
Ethical considerations
The protocol for the trial of screening was approved by the Committee for Human Rights of the University of Western Australia.
Results
Overall, 12 203 (70.5%) of the 19 583 men who were invited to screening and were not deemed ineligible by reason of absence from Perth, being housebound or having a known or treated AAA, underwent an ultrasound examination.
Table 1 shows that men born in the Mediterranean region were obviously shorter, while those born in The Netherlands were distinctly taller, than any other group. The latter surprised us given that, having been born in the 1920s, most of these men had their adolescence during World War II and the Dutch famine of 1943. Dutch men were also distinguished by having the highest current (18.6%) and lifetime (87.2%) prevalence of smoking, although only fractionally fewer men born in Scotland were current smokers (18.3%). The highest proportion of non-drinkers (25.6%) occurred in men born outside Europe and Australia and the lowest among Mediterranean men (13.8%). Among men who reported drinking at all at the time of screening, the proportion consuming an average of less than two drinks (20 g) of alcohol daily varied from 83.2% of men born in Australia to 93.4% of men born in The Netherlands. The differences in smoking and drinking habits across groups defined by place of birth were highly statistically significant.
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We have confirmed the original clinical observation that AAA is significantly more common among migrants to Western Australia of Dutch origin than among the Australian-born population, which is largely of British stock. The excess risk in Dutch men is not explained by differences in age, but the relatively narrow period of migration to Australia from northern continental Europe after World War II8 means that we cannot exclude the possibility of a cohort effect within the Dutch community itself, perhaps related to the mid-war famine. Data from an ongoing trial of ultrasound screening in Rotterdam should shed light on that hypothesis.9 Most of the Dutch men in our study were in their teens during the War but they still emerged as the tallest group in adulthood. This suggests that any effect of the famine involved nutrients that were not related to skeletal growth.
Our further analyses showed that, using a criterion of 30 mm, men born in Scotland also had a very high age-adjusted prevalence of AAA while those of Mediterranean origin, principally from Italy, Greece and the former Yugoslavia, had distinctly lower prevalence relative to Australian-born men. This follows the well-described gradient in incidence and mortality from coronary disease6 that Richard originally dubbed the French paradox.2 Of the potential explanatory variables that we examined, high lifetime and current prevalences of smoking were seen in men of Dutch and Scottish origin, while Mediterranean men showed the lowest proportion who reported not drinking alcohol at all. Answers to simple questions relating to consumption of meat (a major source of saturated fat in the diet), fish, milk and salt did not consistently fit the pattern of aneurysms. None of these emerged as an independent risk factor for AAA in a multivariate logistic model based on data from all 12 203 men.1 That analysis confirmed that current and former smoking are associated with significantly increased risks,1012 as are symptomatic coronary and peripheral arterial disease,10,11 abdominal obesity, and treatment for hypertension or hypercholesterolaemia.10 In common with others, we found no association between AAA and diabetes13 or a history of stroke.11
In the present analyses, once we adjusted aortic diameter for height, significant variation in the prevalence of large aneurysms was no longer evident, although there was still greater than threefold variation in the prevalence of AAA defined by the 95th centile of height-adjusted aortic diameter in men born in Australia. Thus, this simple adjustment, which reproduces prevalences similar to those based on absolute measures of aortic diameter, appears to have eliminated at least part of the north-south gradient in risk of AAA evident in men born in Europe.
We recognise that our cohort is subject to both healthy migrant and survivor effects, that the different groups defined by place of birth will have had different periods of acculturation in Australia,8 and that cross-sectional studies of dietary factors are a poor substitute for prospective investigations of the aetiology of conditions with very long natural histories. Nevertheless we conclude that, if it exists at all, any Mediterranean paradox for AAA is more modest than that for coronary disease, emphasising that risk factors for aneurysms and for occlusive vascular disease are probably not identical.
KEY MESSAGES
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The Western Australian Abdominal Aortic Aneurysm Program is supported by a grants-in-aid from the National Health and Medical Research Council and the National Heart Foundation of Australia. The authors are also grateful for assistance received from the State Electoral Commission, the Australian Bureau of Statistics, the Registrar General of Births, Deaths and Marriages, and the Health Department of Western Australia, and to hospitals in Perth for providing space in which to conduct screening. Other staff associated with the Program are: Professor Jim Dickinson, Yvonne Allen, Ann Blakemore, Michelle England, Lorili Jacobs, Gill Kaye, Max Le, Janet Mitchell, Richard Parsons, Carol Pearce, Lisa Rich, Lyn Schofield, Jan Sleith, Raywin Tuohy, Teresa Warner and Raylene Williamson.
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