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Eugen Mattes, Timothy M E Davis, Danian Yang, Dorothy Ridley, Helen Lund and Paul E Norman
Electronically published Monday 28 April 1997. Please submit comments by Monday 26 May 1997.
This article has been accepted for publication in the Medical
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Abstract - Introduction - Methods - Results - Discussion - Acknowledgements - References - Authors' details - Figure 1 - Table 1 - Table 2 - Table 3
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Abstract |
Objective: Many community-based screening studies
suggest that the prevalence of abdominal aortic aneurysm (AAA) in men
over 60 years approaches 5%. Despite being a group subject to regular
screening for vascular complications of their disease, the
prevalence of AAA in patients with diabetes has never been examined.
The aim of this study was to assess the prevalence of AAA in diabetic men
over 60 years. Design and Patients: Abdominal aortic ultrasonography was undertaken in 303 diabetic men aged 60 years and over recruited to the Fremantle Diabetes Study, a large community-based study of diabetes. Main Outcome Measure: An AAA was diagnosed if the aortic diameter was  30 mm or if the subject had undergone previous
surgery for AAA.Results: Of the 303 men scanned, four had already had aortic grafting for AAA and three were found to have previously undiagnosed AAA. The aorta was not visualised in three men. Only one AAA was 50 mm in diameter and hence in need of surgery. This resulted in
an overall prevalence of 2.3% (7/300) which was lower than that
reported previously in the general population. Multivariate
logistic regression analysis of a range of clinical and biochemical
variables revealed statistically significant associations with
fasting triglycerides and a history of intermittent claudication in
our patients.Conclusions: Ultrasound screening was performed easily and was well tolerated by the patients. Although a small number of diabetic men have undiagnosed AAA, the prevalence does not appear to be high enough to warrant targeted ultrasound screening. ©MJA 1997 |
Introduction |
Unlike coronary artery, cerebrovascular and peripheral vascular
disease, abdominal aortic aneurysms (AAA) usually remain
asymptomatic and undetected until they rupture. The mortality from
ruptured AAA is over 75%,1,2 a
statistic which argues strongly for early detection and prompt
elective surgery. Although some AAA are found on careful clinical
examination, the majority are impalpable, particularly in the obese
patient. Because of this, imaging modalities such as ultrasound
scanning are the mainstay of diagnosis.3
The role of ultrasound screening in the early detection of AAA has received considerable attention over the last decade.4-6 Community-based screening, including a study in Western Australia,7 of men (AAA is rare in women) over the age of 60 years has found the prevalence of AAA over 30 mm in diameter to be between 5% and 9%.4,5,8-10 The prevalence of AAA large enough to require elective surgery ( > 50 mm in diameter) is 0.6%-1.4%.4,5,7,11 In an attempt to improve the efficiency of screening, patients with other manifestations of arterial disease have been studied and found to be a high risk group for AAA.12,13 Although some studies have included incidental patients with diabetes7,9,10,12,14 the prevalence of AAA has never been assessed in a well characterised diabetic population. Since diabetic patients are prone to arterial disease15 and may benefit from the early detection and treatment of vascular complications it is important to know the prevalence of a treatable yet lethal condition such as AAA in these patients. We used abdominal ultrasonography to estimate the prevalence of AAA in diabetic men aged 60 years and over recruited to a large community-based study of diabetes in an urban Australian setting. |
Methods | |
Patients | All subjects were participants in the Fremantle Diabetes Study (FDS), which was granted ethical approval by the Fremantle Hospital Ethics Committee. The FDS is a prospective study of the care, metabolic control and complications in diabetic patients living within the primary catchment area of Fremantle Hospital which represents a postcode-defined population of 118 490 people. Based on previously published data,16 approximately 2300 known diabetics were available for recruitment at the time of the study, of which an estimated 25% (575) are men aged 60 years and over. Recruiting strategies include surveillance of hospital inpatient and outpatient clinic lists, notification through local general practitioners and allied health services such as diabetes education, dietetics and podiatry, and advertisements in pharmacies and local media. All subjects have a comprehensive history taken and physical examination performed, and provide blood and urine samples for standard biochemical indices using automated analytical techniques. The aim was to screen all men aged 60 years amongst the first 1000 patients recruited to the study. There were 316 eligible men, of these, 303 (96%) men were asked and gave consent to abdominal ultrasonography in addition to usual study procedures. Another 13 eligible men who attended the study were inadvertently missed. |
Methods | Subjects underwent an abdominal aortic ultrasound examination (Advanced Technology Labs MK 100 series with a 3 MHz probe). Scans were performed without the patients needing to fast. The maximum infra-renal aortic diameter was identified and measured. An AAA was diagnosed if the aortic diameter was 330mm or if the subject had undergone previous surgery for AAA. No other intra-abdominal organs were imaged. The result of the scan was conveyed immediately to the subject and was included in a subsequent letter to the general practitioner. If an aneurysm was detected, a letter to the general practitioner was given to the patient recommending a repeat ultrasound to confirm the diagnosis. |
Data Analysis |
Univariate and multivariate statistical analysis was conducted
using SPSS for Windows.17
Associations between the presence of an AAA and a variety of other
parameters were assessed using Wilcoxon rank-sum test for
continuous variables, and Fisher's exact test for proportions.
Multivariate analysis was by logistic regression on EGRET18 with the binary outcome or
dependent variable being the presence or absence of an AAA while the
independent variables included the following: History: age, duration of diabetes, heart disease, stroke, claudication, age commenced smoking, packyears, diabetic therapy. Examination: body mass index, presence of at least one carotid bruit, ankle:brachial doppler ratio, supine blood pressure. Biochemistry: Fasting serum: creatinine, uric acid, total cholesterol, HDL:total cholesterol ratio and triglycerides, and glycosylated haemoglobin level. |
Results |
The 303 males who participated in the present study formed just under
one-third of the FDS patient database at the time ultrasound
screening was stopped in October 1994. Since then, a further 126 males
over 60 years have been recruited leading to a total of 442 (31%) older
men of all 1430 FDS patients. An additional 230 older diabetic men have
been identified in the study catchment area but not recruited.
Therefore, our screened sample covers 45% of the 672 older diabetic
males identified in an urban Australian population of almost 120 000
people. Ultrasonography was well tolerated by all patients,
screening was completed in under 5 minutes in all but three obese
patients in whom the aorta was not adequately visualised. The median
diameter of abdominal aorta was 16 mm. The frequency distribution of
the diameter of the abdominal aorta is shown in Figure 1. Three AAA were detected, measuring
30 mm, 46 mm and 60 mm in diameter respectively. The patient with the 60
mm AAA has since undergone successful elective surgery. Four
patients reported a previous operation for AAA resulting in an
overall prevalence of 2.3% (7/300) with 95% CI 0.6%-4.0%. The median
age of patients without AAA was 68.8 (range, 60-86) years and 68.7
(range, 61-84) years for those with AAA.
The variables included in the statistical analysis were risk factors for cardiovascular disease and diabetes-specific variables which are known to determine macrovascular status. No important known risk factors for AAA were excluded from the analysis. Univariate analysis demonstrated that patients with AAA had a significant higher mean fasting triglyceride level and body mass index, and started smoking at a younger age than those without (Table 1, below).
In the logistic regression analysis, clinical and biochemical variables were tested with forward and backward stepwise regression. The main effect variables left in the model are shown in Table 2 (below). No significant interactions were found between risk factors for cardiovascular disease and diabetes-specific variables. In addition, the squares of all continuous main effect variables, such as age and duration of diabetes, were found not to be significant. This analysis revealed that increased fasting serum triglycerides and a history of intermittent claudication were associated with AAA. |
Discussion | |
Low prevalence |
Ultrasound screening of a population of diabetic patients for AAA has
not been undertaken previously. This study found a surprisingly low
prevalence of 2.3% AAA amongst older diabetic men. Other
community-based studies did not include AAA which had undergone
previous surgical repair in their estimates of AAA prevalence (see Table 3). Hence, for comparability our
prevalence figure would be 1.0% (3/296) with 95% CI 0%-2.1% . Thus,
using either definition of AAA prevalence, the prevalence of AAA
amongst older men with diabetes appears to be well below that in the
general male population (between 4% and 9%).
A limitation of the present study is that ultrasonographic data from a sample of non-diabetic subjects in the same postcode-defined population were not available for comparison. In a recent community-based study of 654 self-referred urban-dwelling Western Australian men aged 60 years and over screened used identical techniques to those of the present study, an AAA prevalence of 4.7% (95% CI, 3.1%-6.3%) was found. 7 Given that the asymptomatic nature of AAA would tend to limit selection bias in such a sample and that the age range and geographical location were similar to those of our subjects, it can be inferred that the prevalence of AAA in diabetic males is no greater than in the non-diabetic population and could even be lower (1.0% v. 4.7%; P = 0.004). This result was unexpected given that diabetes is a major risk factor for macrovascular disease 15,19,20 and subjects with macrovascular disease have a reported prevalence of AAA as high as 11%-15%. 12,13 We raise two possible explanations for this low prevalence of AAA amongst older diabetic men. First, there was potential for selection bias, with greater (or lesser) severity of diabetes or the presence of complications influencing participation in the FDS. Almost half our patients had a glycosylated haemoglobin within the poor control range ( > 8.0%), suggesting a bias towards more severe diabetes. If diabetes were a risk factor for AAA, this bias would be expected to increase and not decrease the prevalence of AAA. Second, patients with diabetes and at risk of developing AAA die of other macrovascular complications such as coronary artery or cerebrovascular disease before developing AAA. Investigation of this hypothesis would be difficult because it would depend largely on post mortem information on aortic disease in diabetic patients dying at a relatively young age from other causes. In any case, our results are consistent with preliminary data from previous community-based studies involving small numbers of incidental diabetic subjects which indicate that diabetes is not a risk factor for AAA. 7,9,10,14 |
The association | Logistic regression analysis found a significant association between the detection of an AAA and a history of intermittent claudication in our patients. This is consistent with a number of studies showing the prevalence of AAA in subjects with occlusive peripheral vascular disease to be as high as 11%-15%. 12,13 The association with elevated fasting triglycerides confirms the findings in a study of familial AAA 21 although this was not found in a population-based study. 22 So far there has been little evidence to implicate serum lipid abnormalities in the pathogenesis of AAA in the general community. However, the association of AAA with hypertriglyceridaemia in our patients is consistent with elevated triglycerides being a possible risk factor for macrovascular disease in the diabetic population. 23 |
Screening for | Our study found that out of 300 men in whom the aorta was imaged, only one previously undiagnosed aneurysm was of a size ( > 50 mm in diameter) indicating the need for surgery. Thus, although all men over the age of 60 years are at increased risk of AAA, the results of the present study do not support selective screening of diabetic men using abdominal ultrasound. |
Acknowledgements | We are grateful to the Biochemistry Department, Fremantle Hospital for performing all chemical assays, and to Giovanna Stuccio, Pat Mumford and Debbie Mulcahy for logistic and secretarial support. The FDS is supported by the Raine Foundation, University of Western Australia. At the time of the study Eugen Mattes was the recipient of an NHMRC (PHRDC) Research Scholarship. |
References |
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