In our 5-year prospective study, in clinically healthy, 35–55-year-old, nondiabetic, predominantly non-hypertensive individuals, without known CVD, with low-to moderate calculated fatal risk SCORE, the increase in mean and maximum values of CIMT and FIMT was significant, with almost identical CIMT and FIMT values. The yearly progression rate of IMT was slower in femoral region in comparison with the carotids. IMT>0.9mm (previously identified as hypertension-mediated organ damage) was 5x more frequent in femoral region in comparison with the carotids. On the other hand, the occurrence of FIMT >1.1mm (predictive value of CIMT>0.9mm) [
24] was as low, as CIMT>0.9mm. The presence of age- and sex-adjusted abnormal mean CIMT and FIMT was surprisingly high (mainly carotid) and compared to the beginning of the study, the prevalence was significantly higher by 25.9%. Similarly, a relatively high and similar prevalence of carotid (17.9%) and femoral (17.7%) plaque burden was documented at the end of follow-up, with a more pronounced progression during the follow-up in femoral region.
5.2. CIMT and FIMT Progression
Increased CIMT represents subclinical vascular disease and CVD risk marker [
30,
31], may be related to intimal and/or medial hypertrophy, and may be an adaptive response to changes. Increased CIMT is related to (not clearly synonymous with) subclinical ATS due to similar alterations in the progression of both processes [
30]. The initiation, progression and expression of ATS lesions are mainly artery-related [
32]. Shared common risk factors have different impact in different arterial territories [
33,
34]. Autopsy studies revealed, that in different vascular segments there is no uniform involvement of ATS [
35]. ATS plaques in different segments of the arterial tree have similar cell types, but their relative numbers and amount of connective tissue and lipids can vary considerably [
36]. Twin studies also reported a heritable component on carotid and femoral IMT [
37,
38,
39]. Like carotid, femoral artery wall morphology is correlated with subclinical ATS [
40], is associated with CAC score (CACS) [
10], and is an independent predictor of future CV events [
41,
42,
43,
44,
45]. Some studies have reported that ATS changes are more advanced in the femoral artery than carotid artery [
46,
47], another ones revealed that IMT of femoral artery is a better indicator of extent and severity of coronary artery ATS than in carotid arteries [
48,
49]. Examination of various arterial segments may complement each other in the evaluation of the extent of ATS [
27]. The majority of studies have assessed only common carotid artery IMT, USG of femoral arteries for CV risk modification has not become a part of the routine, moreover, comparative data from the presence and dynamics of vascular target organ damage phenotypes in carotid and femoral arterial segments are scarce [
47].
A systematic review reported the mean CIMT between 0.62-1.07 mm, and CIMTmax between 0.78-1.8 mm in low-to-intermediate risk individuals aged 60±7.6 years [
50]. In the PESA study, with a comparable mean age of study population, similar to our results [
13] the mean CIMT value was 0.59 mm [
11,
29]. The varying progression rate of the mean CCA-IMT was published in different population-based studies, ranged between 0.0038-0.060 mm/year [
51,
52], other studies detected comparable progression rate to ours [
53,
54]. A mildly higher rate of CCA-IMT (0.025 mm/year) was observed in the large Atherosclerosis Risk in Communities (ARIC) Study [
55], lower progression rate of the CCA-IMT was documented in the Carotid Atherosclerosis Progression Study (CAPS) (0.001 mm/year) [
28].
For CVD risk assessment, instead of normative values (i.e. pathological IMT>0.9 mm, reflecting primarily ATS at the carotid bifurcation and hypertension mediated hypertrophy at the level of CCA), carotid USG imaging and measurements should follow protocols with CIMT values in percentiles by age, sex, race/ethnicity and mostly also by side [
22,
23,
56]. In comparison with previous data [
12,
57], the occurrence of CIMT>0.9 mm was rare in our study and not significantly changed after 5-year follow up [
13]. Similarly to us, CIMT>0.9 mm was detected in 1% of participants in the PESA study [
11,
29]. In contrast, there was a 36.7% incidence of CIMT>0.9mm reported by Mitu et al. among apparently healthy individuals, classified mainly in high-risk SCORE [
12] and an incidence of 34% was reported by Novo et al. in an older study group, with a relatively high prevalence of diabetics and hypertensives [
57].
Similarly to our results [
13] the 75th percentile of the CCA-IMT distribution was established at 0.58 and 0.59 mm in healthy females and males without CV RFs, over 40 years of age [
58,
59]. In a recent study of an apparently healthy population aged 57.7±10.4 years, without exclusion of diabetics, the distribution of pathological CIMT>0.74 mm (75th percentile) was 25.96% (lower than in our study), but it followed a higher cut-off level in comparison with us [
60].The prevalence of CIMT>75th percentile for the patient’s age, sex and race/ethnicity was approximately 12% across the Framingham Heart Study, but at intermediate Framingham risk score (FRS), 22–58% of patients had increased CIMT [
61]. However, no data are available on the progression rate of pathological age- and sex-adjusted CIMT in the literature.
Very similar to our mean FIMT values were found by Deparion et al. [
62] in healthy subjects aged 20-60 years, without CV RFs: 0.543±0.063 mm and 0.562±0.074 mm for women and men resp. The estimated increase per year was less than in our study, (0.0031 mmm for men and 0.0012 for women), probably because the fact, that they screened subjects without CV RFs, in our study the presence of RFs was not an exclusion criterion. In some studies the mean CFA IMT was higher than in our: in size and risk profile similar study to our, regardless of sex, the FIMT was 0.80±0.2mm [
47]; in another one the FIMT was 0.72/0.73mm (left/ right), probably due to the effect of older age and presence of DM [
63]. A bit higher value of mean FIMT (0.64mm females /0.75mm males) was measured in healthy participants of the Bogalusa Heart Study (71% white, aged 24–43 years), but only single measurement of the left common femoral artery was provided [
26]. The population based French (low-risk country) AXA Study (Sex and Topographic Differences in Associations Between Large-Artery Wall Thickness and Coronary Risk Profile in a French Working Cohort) in subjects (employees of an insurance company AXA, Paris La Défense, France) aged 17-65 years, with no exclusion of CVD and CV RFs, documented mean FIMT 0.43±0.06 mm for women and 0,50±0.11mm for men (thinner than FIMT in our study) with progression rate 0.003 and 0.005mm/year for women and men, resp. [
64]. The Asclepios Study in apparently healthy population aged 35-55 years without exclusion of DM documented thinner FIMT than in our cohort in females (0.49 mm) but not in males [
25] (due to thickened femoral IMT measurement site, incorrectly classified as plaque, however, we did not evaluate separately IMT for females and males).
FIMT>0.9mm, age and sex adjusted pathological FIMT occurrence have received less attention to date in the literature. Langlois et al. found the maximal FIMT 0.59 (0.51–0.70) mm in females and 0.71 (0.60–0.87) mm in males [
25], which is similar to our results, even though we did not determine FIMT separately for men and women, but the presence of age- and sex-adjusted FIMT. In the same cohort, [
25] with no exclusion of DM, 26,3% of subjects had FIMT>0.9mm, more than in our study.
Rietzschel et al. in a population of 156 apparently healthy normotensive Caucasian volunteers between 18 and 65 years revealed, similarly to us, identical right common femoral and carotid mean IMT (0.52 mm) [
65]. In above mentioned studies [
47,
63] the mean and maximal femoral IMT were greater than the mean and maximal carotid IMT. In accordance with us, the CIMT was greater in other studies [
25,
26], also the progression rate was higher for CIMT than for FIMT in the AXA study and in the study conducted by Markus [
64,
66].
5.3. Carotid and Femoral Plaque Progression
Carotid IMT and plaque are markers for measuring ATS burden and strongly associated with vascular RFs and the incidence of CV events [
31]. ATS progression predicts CV events [
67]. The occurrence of carotid plaques is variable in the general population and might be explained by age, CV RFs and geographical influence [
12]. According to a systematic review [
50], the occurrence of plaque in asymptomatic, low-to-intermediate risk cohorts, with different age and risk profile was an average of 35%. Some studies [
12,
57,
60,
68] in comparison to our results, reported a higher prevalence of carotid plaque (40%, 25%, 34%, 78%, resp.) probably due to the enrollment of older subjects. Studies with asymptomatic, middle-aged individuals documented higher occurrence of carotid plaques (29.3% in subjects with risk SCORE <5% [
12], 31% in the PESA Study [
29]). In the Refine study among 50–69-year-old participants, after a 4.2-year follow-up, in those patients without plaque at the first visit, the rate of plaque burden was 29.7%, which is a higher progression than in our study, but in a population with worse risk profile, with no exclusion of CVD [
54]. Similar to our data, 20.5% of subjects developed new carotid artery plaques during a 5-year follow-up in a community in Taiwan (older subjects, no exclusion of DM) [
53].
There is a slight difference in the genesis of ATS plaques in CCA and CFA, supported by pathology [
36], biochemical studies [
25], different distribution of plaques in carotid and femoral sites [
36], as well as by significant side-difference in IMT of CFA but not of CCA , underlining a possible role of local geometry in the development of ATS [
69]. However, this side difference was not observed by Lucatelli at al. [
63] by us not even in carotid area.
Although ATS is considered a generalized disease process, the extent of ATS and its underlying risk factors differ among arterial sites [
70], confirmed by autopsy studies [
35]. It has been shown that ATS lesions are more frequent and advanced in femoral arteries than in carotid arteries independent of increasing number of risk factors [
71,
72,
73]. ATS in femoral arteries occurred earlier than carotid arteries [
72] and femoral artery is more susceptible to the atherogenic influence of risk factors [
73].
In our study the occurrence of carotid plaque was slightly higher than femoral plaque, mainly at baseline, the difference practically disappeared at the end of follow-up due to higher progression rate in femoral region. Generally, high plaque frequency was documented among participants aged 45–64 years, with 17.5% diabetics, in study conducted by Yerly at al. [
74]: 73.4 % of participants had ≥1 plaque (defined as IMT ≥1.2 mm) at carotid level and 67.5 % at femoral level. In contrast, among healthy adults (subpopulation of international twin study), aged 20-78 years, with 4.1% presence of DM, with higher prevalence of smokers, the plaque prevalence was significantly higher in the CFA compared to the CCA (40.7% vs 30.4%), the progression rate was not followed [
63]. Among PESA participants, plaques were most common in the iliofemorals (44%), followed by the carotids (31%) aorta (25%) and coronary arteries (18%). Interestingly, among participants with low Framingham Heart Study (FHS) 10-year risk, subclinical disease at all was detected in 58% (higher than in our study, but in mul-titerritorial location). Nearly 60% of those, with CACS=0, had plaques at other vascular sites implying, that in low-risk sample, the absence of CAC does not necessarily indicate that a participant is disease free [
11]. In the large Asklepios Study cohort of asymp-tomatic subjects aged 35 to 55 years without exclusion of DM/impaired fasting glucose (21.4%) the occurrence of carotid and femoral plaque was generally high, 43.6% in ca-rotid and 54.9% in femoral region [
25]. In the Cafes-Cave 10-year, prospective study with 10 000 healthy, low risk individuals without AH, DM and DLP, aged 35-65 years, 10.8% of study population had ATS plaque either at the femoral, or carotid level (less than in our study, but their study population was free of 3 main modifiable RFs). Moreover, the authors documented a difference in morphology between carotid and femoral arteries: in 51% of subjects the carotid was the most advanced artery and in 52.4% the right (carotid or femoral) arteries were more advanced than the left [
9]. We observed almost the same prevalence on both arterial sites but did not evaluate side difference.