Atherosclerosis and Lipoproteins |
From the Department of Neurology, Innsbruck University Hospital, Innsbruck, Austria.
Correspondence to Dr J. Willeit, Department of Neurology, Innsbruck University Hospital, Anichstr. 35, A-6020 Innsbruck, Austria.
| Abstract |
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Key Words: carotid arteries atherosclerosis atherothrombosis population study
| Introduction |
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| Methods |
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Scanning Protocol
Sonographic assessment of the extracranial carotid arteries was
performed using a duplex ultrasound system (ATL8, Advanced Technology
Laboratories) with 10-MHz scanning frequency in B-mode and 5-MHz
scanning frequency in pulsedDoppler mode. All subjects were
examined in a supine position. The scanning protocol included imaging
of the right and left common carotid arteries (CCA) and internal
carotid arteries at the following locations: proximal CCA (15 to
30 mm proximal to the carotid bulb), distal CCA (<15 mm
proximal to the carotid bulb), and proximal internal carotid artery
(ICA) (carotid bulb, identified by loss of the parallel wall
present in the CCA and the initial 10 mm of the vessel above
the flow divider between external and internal carotid arteries). For
each segment, the sonographer imaged the vessel in multiple
longitudinal and transversal planes to identify the largest axial
diameter of focal plaques and to adequately visualize the interface
required to measure intima-media thickness (IMT). Pulsed Doppler
was used to provide information on blood flow velocity and to identify
the various arteries.
Atherosclerotic lesions were defined by 2 ultrasound criteria: (1) wall surface (protrusion into the lumen or roughness of the arterial boundary) and (2) wall texture (echogenicity). We did not use an IMT cut-off to discriminate plaques from wall thickening. The maximum axial diameter of each plaque was measured as the distance from the leading edge of the lumen-intima interface to the leading edge of the media-adventitia interface. For the assessment of stenosis, Doppler criteria or, when no hemodynamic disturbances were detectable, the percentage of maximum diameter reduction in the B-mode images was applied.17 Peak systolic velocities exceeding 180cm/s and 250cm/s were considered indicative of stenosis >60% and 80%, respectively. Scanning was performed twice, namely in 1990 and 1995, by the same experienced sonographer, who was unaware of the subjects' clinical and laboratory characteristics. For documentation purposes, short segments of real time ultrasonography and frozen longitudinal and transversal images were recorded for each vessel segment.
Reproducibility, Validity, and Data Quality
To assess the reproducibility of the ultrasound technique
applied in the current evaluation, rescanning was performed in a
representative subsample by the same sonographer
(n=100). To avoid memory effects, we left a waiting period of 6 to 8
weeks between both assessments. In all, 800 vessel segments and 180
plaques served as the basis for computation of intra-observer
variability. Main focus was on the quantification of plaque diameters.
Relative measurement errors that describe the intra-observer error as a
percentage of the pooled mean were low at 10% (CCA) and 15% (ICA),
which enabled us to monitor changes in atherosclerosis
in individuals over time. In an attempt to subdivide the overall error
into various components
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At the lowest end of the plaque size distribution the above relative errors may not fit the data well. As neither surface nor texture criteria used in the definition of plaques are so-called "hard criteria", counting of small lesions may ultimately depend on the investigator's judgment. Diagnostic shifts from wall thickening to atherosclerotic plaques and vice versa represent the main source of error in the quantification of such small lesions. Concordance of classifications markedly increased with lesion size and was near perfect (>95%) for plaques exceeding 0.7 mm (CCA) and 1.1 mm (ICA). These limits were introduced in the definition of incident atherosclerosis as a minimum diameter requirement.
Statistical Analysis
Age-specific incidence/progression rates were assessed for age
strata of 5 years each and expressed as "incidence/progression of
atherosclerosis per 1000 person-years." Rates were
calculated under the assumption of a consistent probability of
incident/progressive atherosclerosis across the 5-year
age intervals. Equal risks were allocated from the first to
fifth year of follow-up; cases contributed, on average, 2.5 years of
follow-up to the denominator of the incidence formula (for details see
Reference 1818 ). Cumulative
rates were converted into cumulative
risks by means of the formula P=1-exp (-
).18 Standardization of rates was performed
according to the guidelines given by Breslow and Day.18
Strength and type of association between cigarette smoking and various
categories of atherosclerosis progression was assessed
by logistic regression analysis (SPSS-X statistical
software).19
| Results |
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4 incident plaques: 57%, 27%, 12%, and 4%).
Figure 2
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For comparison purposes, rates were adjusted to the age/sex structure of the standard European population,18 thereby obtaining 1 universal estimate of atherosclerosis incidence in the middle-aged and elderly (aged 40 to 84 years), which amounted to 69.9 per 1000 person-years.
Next, we focused on spontaneous progression of atherosclerotic lesions.
Plots of plaque growth against the lesion size ultimately achieved
identified 2 distinct types of growth kinetics.
Atherosclerosis that did not cause lumen obstruction
>40% usually grew slowly and evoked (over)compensatory local dilation
of vessel segments (Figure 3
). In
contrast, stenosis >40% usually originated from occasional
marked increases in plaque size and insufficient or even lacking
vascular remodelling (Figure 3
). Both types of lesion
progression started from plaques of similar diameters (Figure 3
;
1.3 versus 1.6 mm), with the distribution of changes in plaque
size showing only minor overlap (mean [95%CI]: 1.1 mm [0.1 to
2.2 mm] versus 2.4 mm [1.3 to 3.6 mm];
P=0.0001). Progression rates of nonstenotic
atherosclerosis showed age- and sex-trends similar to
those observed for incident atherosclerosis (Tables 1
and 2
) and an amplification of disease activity with an
increasing number of preexisting plaques (Figure 2
). In
contrast, incidence rates of stenosis in middle-aged and
elderly subjects emerged as independent of sex and age (Table 2
). In most instances, nonstenotic atherogenesis
manifested in several plaques simultaneously, whereas
incident stenosis usually occurred only once in the 5-year
follow-up period (Figure 4
). Further
differences in the epidemiology of both
processes are opposed in Table 3
.
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The risk profile of nonstenotic atherosclerosis consisted of traditional risk factors (high LDL and low HDL cholesterol, hypertension, and smoking) supplemented by less well-established risk conditions such as prominent body iron stores,20 21 severe alcohol consumption, or chronic infections (unpublished data from the Bruneck Study). Stenotic atherosclerosis emerged as a domain of a procoagulant state involving high fibrinogen and Lp(a), low antithrombin III and APC ratio (factor V mutation), and clinical conditions known to shift hemostasis toward coagulation. As to cigarette smoking, the risk of nonstenotic atherosclerosis was best described by measures of cumulative exposure (pack-years) (OR [95%CI], 1.26 [1.06 to 1.50] for a 1-standard deviation unit change) and did not normalize within a 5- to 10-year period after cessation (1.24 [1.03 to 1.49]). In contrast, peak levels of exposure (number of cigarettes currently smoked) were superior to cumulative measures in predicting the risk of stenotic atherosclerosis (2.57 [1.74 to 3.80]). After secession of smoking, the risk of focal stenotic disease normalized (OR for ex-smokers 1 to 5 years after cessation, 1.29 [0.33 to 5.04]).
| Discussion |
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As detailed previously, differences in prevalence and incidence of
carotid atherosclerosis evident between premenopausal
women and men disappeared when sex variations in body iron stores were
taken into account.20 21 Iron accumulation in
(post)menopause corresponded excellently with the breakdown of female
protection against atherogenesis (Figure 1b
). Growing
epidemiological and experimental evidence suggests a crucial role of
prominent iron stores in lipid-induced atherogenesis even though a
general consensus in this field has not yet been
reached.22
As expected, the proximal ICA was the site of predilection for
manifestation of atherosclerosis. In 1 out of 2
subjects with incident atherosclerosis, plaques
occurred simultaneously in 2 or more carotid segments. The
risk of incident atherosclerosis amplified with size
and number of preexisting lesions (Figure 2
), which may well
reflect a perpetuation of adverse risk profiles but could,
speculatively, also point to some kind of auto-catalytic propagation of
atherosclerosis. A previous investigation in this
cohort yielded evidence of an (auto)-immune component in human
atherogenesis and identified heat-shock protein 65 as a potential
target antigen.23 Occasionally, plaques evolved as
focal lesions within a normal vasculature. In most instances, however,
atherosclerosis developed at sites with an IMT beyond
the 50th percentile (ICA, 88.8%; CCA, 97.8%). Thus, our survey
supported the view that wall thickening commonly precedes definite
atherosclerosis, possibly in the sense of a precursor
lesion of one and the same disease process.
Progression of Atherosclerosis
From a pathoanatomical perspective, 2 distinct types of
atherosclerosis progression may be distinguished: 1) In
small and medium-sized lesions slow and continuous plaque growth
predominates, which is mediated by a variety of complex biological
step-by-step phenomena such as lipid-induced atherogenesis or smooth
muscle cell proliferation. 2) This type of plaque growth may be
occasionally accelerated by plaque fissuring, thrombosis and fibrous
organization of mural thrombi.24 25 The latter mechanism,
further on referred to as plaque thrombosis, gains increasing weight
with advancing lesion size and may even be the key event in the
development of stenotic lesions. A comprehensive discussion of
the dualism of conventional atherosclerosis and
atherothrombosis in human vessel pathology is given by Fuster et
al26 and Badimon et al.27 The current
prospective survey provides strong in vivo support for this concept and
suggests that a shift in the relevance of both pathomechanisms occurs
when a plaque causes >40% diameter reduction (Figure 3
and
Table 3
).
Nonstenotic atherosclerosis expanded slowly
(Figure 3
). Such processes usually paralleled in several
atherosclerotic lesions independently of plaque location (Figure 4
), ie, they were a continuous and ubiquitous process. In
analogy to the initiation of atherosclerosis, the risk
of disease progression amplified with advancing age and number of
atherosclerotic lesions (Figure 2
). Once more than 3 plaques
preexisted in a single subject, further disease extension was an almost
obligatory phenomenon (78%), which reinforces the possibility of an
auto-catalytic component in this type of atherogenesis. Compensatory
enlargement of the vessel at the site of active
atherosclerosis effectively preserved a normal lumen or
was even over-compensatory in the early course of disease (see Part II:
Vascular Remodeling). Diffuse dilative atherosclerosis
may be assumed as a final stage of this type of disease
progression.
In contrast, stenosis >40% usually developed focally at sites
of high hemodynamic stress (ICA) based on occasional
marked increases in plaque size (mean, 2.4 mm). Subjects with
preexisting stenotic disease were at a clearly elevated risk of
developing a further stenosis in a different segment of the
carotid arteries (23 of 55 [41.8%]). On the other hand, such events
appeared to be so rare that manifestation of more than 1
stenosis during the 5-year follow-up period was definitely the
exception and not the rule (Figure 4
). Compensatory enlargement
of the vessel as typical for nonstenotic
atherosclerosis did not occur at all or only
insufficiently, which acted synergistically with the rapid growth
pattern in producing significant lumen compromise (Figure 3
).
Actually, 95% of stenosis >40% arose from this synergism.
Carotid stenosis did not develop before age 45 in men and 55 in
women. Thereafter, segment-based incidence rates were constant across
the whole age range and in sexes (Table 2
). The higher
prevalence of stenosis in men is thus not a consequence of
enhanced disease activity but simply reflects the higher prevalence of
nonstenotic atherosclerotic lesions at risk of stenotic
transformation. Lack of a significant age trend, which at first glance
may surprise, is explained by peculiarities in the etiology of
stenosis. Consistent with the concept of underlying
plaque thrombosis, this process primarily relied on procoagulant risk
factors, most of which did not show prominent age-dependency.
Both types of disease progression preferentially started from small- to
medium-sized plaques of similar diameter. Stenotic
atherosclerosis should not be viewed as a simple
perpetuation of disease mechanisms relevant to early atherogenesis nor
did it in most instances superimpose on advanced nonstenotic
disease. Actually, both types of atherogenesis develop and proceed
independently of each other (Table 3
).
Presentation of detailed risk profiles goes far beyond the scope of this article. Instead, we attempted to clarify main differences in the way risk factors relate to either type of disease progression using the example of cigarette smoking. The risk of nonstenotic atherosclerosis primarily relied on measures of cumulative smoking exposure and did not normalize within a 5- to 10-year period after cessation whereas peak levels of exposure appeared to be of significance for stenotic atherosclerosis. Consistent with a reversible pro-coagulant state, the risk of focal stenotic disease normalized after smoking cessation.
Briefly, the risk profile of nonstenotic atherosclerosis consisted of traditional risk factors and that of stenotic atherosclerosis of markers of enhanced thrombotic activity, attenuated fibrinolysis, and clinical conditions known to interfere with coagulation.
Regression
A variety of angiographic and ultrasound follow-up evaluations
outlined the possibility of spontaneous or drug-induced regression of
increased IMT and atherosclerotic lesions.28 29 30 31 In our
survey, long-lasting regression was quite rare (5.2% of all lesions,
n=45) and confined to lesions with signs of plaque hemorrhage
(19 of 45 [42%]), healing of ulcerated
atherosclerosis (2 of 45 [4%]), and disappearance of
small plaques (18 of 45 [40%]).
Limitations and Merits of the Study Design
1) Accuracy of descriptive epidemiologic data mainly depends on
the choice of an appropriate study population and the sample size. The
current cohort was a random sample of the general population with a
near complete follow-up (96.5%) and an observation period of more than
33 000 segment-years (4000 person-years). 2) The progression model
developed and applied in the current study permitted us to monitor and
characterize changes in vascular status in individuals (person-based
approach), which is a clear advantage over the more commonly used
approach of assessing changes in IMT over time. 3) Ultrasound
evaluations were performed twice, namely in 1990 and 1995. Systematic
interim scanning was not available. Thus, we cannot construct precise
growth curves of atherosclerotic lesions. In particular, it remains to
be elucidated whether slow plaque growth stands for a continuous or
stepwise process or for successive plaque propagation and repair
(regression) with progression overweight.
Conclusions
The current study may well be the first to provide
population-based incidence rates of carotid
atherosclerosis and detailed insights into the course
of early disease progression. These findings contribute to a better
understanding of the nature of atherogenesis and possibly assist in
clinical decision making. Atherogenesis was found to be a
heterogeneous disease that subsumes epidemiologically and
etiologically distinct disease entities. Fighting the same risk
factors in all individuals, as is common clinical practice, ignores the
actual complexity of the disease.
| Appendix 1 |
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Received June 12, 1998; accepted November 30, 1998.
| References |
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