4. Discussion
The atherosclerotic plaque is a dynamic lesion that undergoes continuous
remodelling of the extracellular matrix on which its structural
stability depends. Previous studies demonstrate that acute changes
within the carotid plaque, such as IPH, fibrous cap rupture or
ulceration can lead to the onset of ischemic cardiovascular events
[6]. Recent reports suggest that MMP-9 may have a key role in all
stages of the atherosclerotic process during which are being secreted by
inflammatory cells and VSMC, although there is still lack of evidence
whether MMP-9 can affect the incidence of adverse events [7, 8, 34].
The present pilot trial has demonstrated that the prevalence of unstable
plaque was higher in symptomatic than asymptomatic patients, although
this was not statistically significant (63% vs. 37%, p =0.077).
Our main findings were that the expression of MMP-9 in CD68 cells within
the carotid plaque was found to be significantly higher in symptomatic
as compared to asymptomatic patients (86% vs. 25% with the highest
expression, p =0.014). Importantly, the average MVD by
immunohistochemical staining of CD34 was found to be higher and lipid
core area larger among both symptomatic patients and unstable carotid
plaque specimens, although this did not reach statistical significance
(p =0.064 and p =0.132, p =0.360 and p =0.569,
respectively). Our study demonstrates the ability of MRI to identify
patients with high-risk carotid plaque and to differentiate better which
patients could benefit from early surgical revascularization. These
findings are in line with the results of other studies and help in
understanding that MRI can be the most accurate advanced imaging
modality in distinguishing vulnerable from stable carotid plaques
preoperatively, although further large prospective comparative studies
are still required [7, 19, 20, 28, 35, 36]. Importantly, the most
recent ESVS guidelines and recommendations of multicentric randomised
controlled trials highlight clinical/imaging features in asymptomatic
patients associated with an increased risk of stroke despite the best
medical treatment that indicate necessity for carotid revascularisation
[1, 4, 25-27]. A recent report comparing carotid MRI, ultrasound and
histological findings of carotid plaque specimens has demonstrated that
the findings of carotid MRI and ultrasound are closely associated with
specimen morphology, but ultrasound alone is insufficient to detect the
type of lesion accurately.[36] In addition, we have found that the
most common type of carotid plaque as per modified AHA classification by
MRI was type IV-V (10 patients, 65%), while type VI was the most common
(89 out of 252 plaque specimens) in the report by Cai et al.[28]. Similarly, Million et al. have reported the ability of
MRI to identify high-risk carotid plaque and to differentiate
symptomatic from asymptomatic patients, although their study was
performed using a 3-Tesla system [19].
Furthermore, our results showed that MMP-9 may play a key role in
remodelling and destabilization of the carotid plaque. As we
hypothesized, the expression of MMP-9 in CD68 cells was significantly
higher among symptomatic patients. Similarly, it was also higher in SMA
cells in symptomatic patients, but this did not reach statistical
difference (p =0.143). Interestingly, symptomatic patients had
significantly higher preoperative hsCRP which is in accordance with the
inflammatory response and infiltration related to the destabilization of
the plaque and potential increased release of MMP-9. Similarly, recent
reports showed higher expression levels of MMP-9 obtained from
symptomatic when compared to asymptomatic patients [8, 37, 38].
These findings highlighted MMP-9 as one of the most important enzymes
related to the plaque instability. On the other hand, Baroncini et
al. reported higher MMP-9 concentration in normal tissues and in
asymptomatic patients suggesting that MMP-9 is a part of the
atherosclerotic process although not associated to acute plaque
disruption [15, 16]. Still, this finding remains part of debate asHeo et al. have reported that MMP-9 is significantly associated
with plaque rupture [38]. However, recent evidence suggests that an
increase in proteolytic activity may lead to an overall increase in
matrix degradation which could explain our results which demonstrate
that the average MVD could be higher and lipid core area larger among
both symptomatic patients and unstable carotid plaque specimens.
There are several limitations to our study. Our results showed a higher
prevalence of unstable plaque in symptomatic patients, and a higher
average MVD and larger lipid core area among both symptomatic patients
and unstable carotid plaque specimens. However, the differences did not
reach statistical significance probably because of the lack of power of
the study and smaller sample size which are limitations of the present
pilot study. Cappendijk et al. also failed to demonstrate a
significant difference between symptomatic and asymptomatic patients
[39]. However, in times of constraints on resources, both additional
MRI analysis and detailed histologic work-up of plaques is costly and
time-consuming, thus explaining the necessity of conducting a pilot
trial. Although we obtained the best possible resolution for carotid
MRI, another limitation of the study could be related to the use of only
non-contrast MRI and motion artefacts. Further improvement in coil
design, use of 3-Tesla system and modification of imaging parameters
could be required, while the use of contrast-enhanced MRI would be less
justified in terms of routine preoperative diagnostic work-up. Another
limitation is that we have analysed our results semiquantitatively by
immunohistochemistry, and the results do not allow accurate
quantification. Nevertheless, our estimates were performed independently
by two investigators. In addition, it is also possible that we have
excluded some regions of plaques where inflammation was more advanced.