Introduction

An arterial aneurysm is defined as a permanent localized dilatation of an artery, having at least a 50% increase in diameter when compared to its expected normal diameter¹. The normal diameter of the infrarenal aorta is traditionally based on the findings of Horejs et al.² who retrospectively reviewed 260 computed tomography (CT) scans. Although ethnicity was not mentioned in that publication, it could be assumed that most patients were Americans, as the study was performed in Chicago. Since then, the American³ and European⁴ guidelines define an abdominal aortic aneurysm when the aortic diameter is greater than 30 mm.

Ultrasound (US), magnetic resonance imaging (MRI), and CT play a central role in the evaluation of aortic diseases, with CT being the most accurate modality to determine outer vessel wall diameter². There is frequently a poor agreement between US and CT diameters, particularly when close to the treatment threshold. Guidelines recommend to use the former for surveillance of small AAA and the latter for pre-operative imaging⁵.

An AAA is a common, potentially fatal, but treatable disease, particularly if detected before rupture with a clear indication threshold based on its diameters. Therefore, it is important to accurately determine the normal size of the abdominal aorta, so clinicians can determine when an aorta becomes aneurysmal, for guiding therapeutic decisions⁶. This is also of relevance regarding secondary risk prevention as the guidelines advice to start with platelet inhibitors and statins in all patients diagnoses with an AAA to reduce cardiovascular morbidity and mortality.

There are no studies that describe the normal size of the aorta in Mexican people, which led us to implement this study. Given the specific ethnic variation in Mexico, with many Native Americans, the normal diameters could defer from those of Horejs et al.² The purpose of the present study is to assess the paired differences in aortic diameter measurements, obtained by CT scans, in patients without aortic pathology from Mexico and to compared them with patients from other countries.

Methods

A transversal study was conducted among five countries by six experienced vascular specialists. Data were obtained anonymously and retrospectively reviewed. The study was approved by ethics committee, and Institutional Review Board from all the centers involved and has, therefore, been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.

The inclusion criteria were contrast-enhanced CT scans obtained for non-vascular disorders, slide thickness of ≤ 0.5 mm, and age ≥ 18 years. Patients with aortic aneurysm were excluded from the analysis.

The collected data included country, gender, age, length, weight, and body mass index (BMI). The patients were divided into two groups: Mexicans and control group, of non-Mexican patients. Furthermore, a division by ethnicities was made into three groups, Hispanic (Latin America), European, and North America.

The aortic diameter was measured including the wall (outer-to-outer diameter) and perpendicular to the centerline of the vessel in multiplanar reconstructions using dedicated software packages. OsiriX (Pixmeo, Geneva, Swiss), Horos (Nimble Co LLC Purview, Annapolis, MD, USA), or Carestream Vue (Onex Corporation, Rochester, USA) software were used, based on preferences or availability of each center. Two other researchers, one vascular surgeon (MAF) and one radiologist (MAC), randomly audited the 30% of the measurements obtained and reported by local investigators.

The aortic diameter was measured at the level of celiac axis (CA), superior mesenteric artery (SMA), right renal artery (RRA), inferior mesenteric artery (IMA), aortic bifurcation, right common iliac artery (RIA), and left common iliac artery (LIA).

Results

The study included 166 patients, 106 (63.8%) were enrolled in México and 60 (36.1%) in the control group, enrolled in four countries including 21 (12.6%) patients from Argentina, 12 (7.2%) patients from Costa Rica, 16 (9.6%) patients from Spain, and 11 (6.6%) patients from the United States. Seventy-seven (46.3%) patients were male and the mean age was 51.3 ± 17 years. There was no difference in age, sex, and BMI between the two groups, although the BMI was not reported in patients from Argentina and the USA (Table 1).

Discussion

Ever since the “suggested standards for reporting on arterial aneurysms” were published in 1991, an aneurysm was defined in size as 50% increase in diameter compared to the expected normal diameter of the artery¹. Consequently, it became mandatory to determine which is the normal diameter of the aorta to define an aortic aneurysm. This normal aortic diameter was mostly based on studies that were performed on an occidental population, and their results were assumed as a the ground truth². In France, Brivady et al.⁷ assessed the diameter of infrarenal abdominal aorta using ultrasonography in 1413 patients with atherosclerosis of the lower limbs. They reported a mean diameter of 15.9 mm in women and 20.4 mm in men. In a similar study performed by Ouriel et al.⁸ on American individuals, the diameter was 19 mm in females and 23 mm in males.

The first published study that evaluated ethnical differences in the normal aortic diameters was performed on a Saudi population using ultrasonography. No significant differences with occidental population were observed, with a mean aortic diameter of 18.7 mm in females and 19.3 mm in males⁹. More recently, there were a few studies that observed differences between the aortic diameters in their populations compared to literature. In a Korean study that included 1218 people, the mean diameter of the infrarenal aorta was 19.0 mm in males and 17.9 mm in females¹⁰. Furthermore, Sariosmanoglu et al.¹¹ reported a mean aortic diameter of 16 mm in males and 15 mm in females in the Turkish population. Both studies used ultrasound as imaging modality. A similar study performed on 142 Indian subjects, using contrast-enhanced CT showed a median aortic diameter of 12.0 mm and 13.8 mm in women and men, respectively¹². Finally in a Thai population, studied with CT scan, a mean diameter of 13.3 mm in women and 14.9 mm in men was reported¹³. In our study, we find mean aortic diameter of Mexican patients at the level of the IMA of 16.63 mm and 15.14 mm in males and females, respectively.

As to our knowledge, there is only one other study evaluating aortic diameter differences between races. In this multi-ethnic study of atherosclerosis, aortic diameter in people of Chinese, African, and Hispanic descendant was smaller than the aortic diameter of Caucasians, even after adjusting for differences in body size and other factors. Measurement was done with CT but all 6814 patients were recruited from six American field centers¹⁴. Our results show that the normal aortic diameter is smaller in Mexican population compared with three other countries from America and Spain, but only at the level where aortic abdominal aneurysms mostly occur.

Our study also collected information on the family origin with various options, such as European, Hispanic, or North Americans. Most patients included were classified as being Hispanic. This generic division, however, made is impossible to determine the true ethnicity of our study groups. Moreover, it is very complex to define ethnicity in the American population, since actual population is the result of a complex admixture between Native Americans and immigrants from diverse origin. Furthermore, their distribution is heterogeneous even between Hispanic countries. Genomic composition of argentine population is composed by 67% of European, 27% of Native American, 0.3% of West African, and 0.1% of East Asian¹⁵. In Costa Rica, genomic ancestry proportions are composed by 54% European, 32% Native American, and 13% West African¹⁶. In Mexico, the Native American, European, and West African account for 56%, 37%, and 5% of the population, respectively¹⁷.

The results from the present study are hypothesis generating. We clearly observed a smaller diameter in the overall Mexican population and the question remain whether is influences by the relatively high number of Native Americans in this group. Moreover, it is unclear whether this observation would have repercussions for the threshold for preventive treatment of an AAA. Commonly elective treatment is indicated when the aortic diameter is > 55 mm, assuming 20 mm normal aortic diameter. In the Mexican study population, a similar relative increase would reflect an aortic diameter of 43.5 mm³,⁴. These observations encouraged us to design a new study in larger subset of healthy volunteers from different ethnicities based on genomic data and determine the anatomical differences between them. This could further support the need of redefined the size of abdominal aortic aneurysm in the specific subgroups of the population.

The present study has limitations, such as vascular specialists based on their own experience and availability chose the measurement software tool. Although, there is some evidence on difference of aortic diameter related to its measurement during systole or diastole, it is not yet accurately assessed, and ECG-gated scans are not widespread; therefore, no direction was done on this topic¹⁸. Another weakness of our study is the sample bias. The sample was gathered by a retrospective fashion rather than a calculated one. Furthermore, there was a lack on the report of length and weight in patients from Argentina and the USA, making not possible to compare these variables with the aortic diameter. In the same way, the relation between male-female genders is not the same between the two groups, and it could be a cause of bias. Furthermore, Mexican patients were collected in a single center, so it may not be representative of the varying ethnic and racial groups in our country. A larger population-based multicenter study with an adequate representative sample, ideally in normal subjects would be beneficial in obtaining more representative values.

Conclusion

We have shown that the normal aortic diameters in the Mexican population, both males and females, are smaller, compared to other countries, particularly at the infrarenal level. A Hispanic background and female gender are also related to smaller diameters. However, these conclusions required subsequent validation by a study with a larger population and a representative sample. The clinical consequences of these observations remain to be shown.

Disclaimer

This collaborative study is part of HRG initiative.