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I.
II.
Imaging of the Cervical Carotid Artery for Atherosclerotic Stenosis
Authors:
Alex M. Barrocas, M.D., M.S.
Washington University School of Medicine
St. Louis, MO
Colin P. Derdeyn, M.D.
Associate Professor of Radiology, Neurology, and Neurological Surgery
Washington University School of Medicine
St. Louis, MO
III.
A. Issues
1. What is the imaging modality of choice in symptomatic carotid stenosis?
2. What is the imaging modality of choice in asymptomatic carotid stenosis?
3. What is the role of carotid angioplasty and stenting?
4. What is the role of physiologic imaging in carotid occlusion?
IV.
Key points
A. At present, carotid imaging is performed to identify the presence and measure
the degree of atherosclerotic stenosis, in order to select appropriate candidates
for surgical endarterectomy (Strong evidence). Several different imaging
strategies may be employed in symptomatic patients.
1. Catheter angiography (CA)may be used for this purpose (Strong
evidence).
2. Doppler Ultrasound (DUS), magnetic resonance angiography (MRA), and
computed tomographic angiography (CTA), or some combination, if
adequately validated, may be used to screen patients (those with less than
50% stenosis) prior to catheter angiography (Moderate evidence)
3. DUS, MRA, and CTA, or some combination, if adequately validated, may
be used to identify patients with severe stenosis (greater than 80%) for
surgical endarterectomy (Moderate evidence)
B. Screening of asymptomatic patients with non-invasive methods and highly
specific thresholds may be cost-effective in certain high-risk populations, such
as patients with known atherosclerotic disease in other circulations or the
presence of bruit over the carotid artery on physical examination (Moderate
evidence).
C. More information regarding the safety and efficacy of angioplasty and
stenting relative to surgical endarterectomy is expected in the near future. As
treatment may be incorporated into the diagnostic catheter angiographic
procedure, these recommendations may be revised.
D. Physiological imaging tools identify higher-risk subgroups in patients with
atherosclerotic carotid stenosis and occlusion (Strong evidence).
E. The use of these physiological imaging tools to improve guide therapy and
improve outcome is unproven (Insufficient evidence). A randomized clinical
trial is underway for surgical revascularization of carotid occlusion in patients
selected by PET.
ISSUE 1: What is the Imaging Modality of Choice in Symptomatic Carotid
Stenosis?
Summary:
At present, carotid imaging is performed to identify the presence and measure the
degree of atherosclerotic stenosis, in order to select appropriate candidates for
surgical endarterectomy (Strong evidence). Several different imaging strategies may
be employed in symptomatic patients:
Catheter angiography (CA) can be used for this purpose (Strong evidence).
Doppler Ultrasound (DUS), magnetic resonance angiography (MRA), and
computed tomographic angiography (CTA), or some combination, if
adequately validated at the local institution with quality assurance data, may
be used to screen patients for those with less than 50% stenosis prior to
catheter angiography (Moderate evidence).
DUS, MRA, alone or in combination, if adequately validated locally, may be
used to identify patients for surgical endarterectomy (Limited evidence)
DUS or MRA can be used to both screen for patients with less than 50%
stenosis and reliably identify patients with severe, >80% stenosis. CA is used
to investigate the degree of stenosis for the remaining patients (Moderate
evidence)
ISSUE 2: What is the Imaging Modality of Choice in Asymptomatic Carotid
Stenosis?
Summary:
The benefit of surgery in patients with asymptomatic carotid stenosis is marginal. Two
large randomized trials have found a 1% absolute annual risk reduction for surgery,
compared to best medical therapy. Whether treatment should be pursued will depend on
many factors, including patient age, gender (no definite benefit for women). In one of
these two studies, restricted to highly selected, relatively healthy asymptomatic patients,
20% of the patients were dead at 5 years, many due to vascular disease.
Imaging of asymptomatic patients is necessarily a screening issue. The low risk of stroke
in medically-treated patients and the small risk reduction with surgery remove the harsh
penalties for false-negative or false-positive non-invasive studies that are incurred in
symptomatic patients. Well-validated DUS or MRA laboratories may be used for this
purpose (level 2 – moderate evidence). The critical factors for screening are wellvalidated non-invasive methods and documented low surgical complication rates.
Cost-Effectiveness Analysis
Screening of asymptomatic patients with non-invasive methods and highly specific
thresholds may be cost-effective in certain high-risk populations, such as patients with
known atherosclerotic disease in other circulations or the presence of bruit over the
carotid artery on physical examination. Different studies addressing the costeffectiveness of screening asymptomatic carotid stenosis resulted in divergent
conclusions. The critical factor in whether intervention is effective is the surgical
complication rates. A one-time screening program of a population with a high prevalence
(20%) of 60% stenosis cost $35 130 per incremental QALY gained. Decreased surgical
benefit (less than 1% annual stroke risk reduction with surgery) or increased annual
discount rate resulted in screening being detrimental, resulting in lost QALYs. Annual
screening cost $457 773 per incremental QALY gained. In a low-prevalence (4%)
population, one-time screening cost $52 588 per QALY gained, while annual screening
was detrimental.
ISSUE 3: What is the Role of Carotid Angioplasty and Stenting?
Summary:
More information regarding the safety and efficacy of angioplasty and stenting relative to
surgical endarterectomy is expected in the near future. As treatment may be incorporated
into the diagnostic catheter angiographic procedure, these recommendations may be
revised.
At present, angioplasty and stenting is accepted as a reasonable therapy for patients with
severe stenosis and recent ischemic symptoms who are not good surgical candidates
(Level 2 – moderate evidence). Patients that are good surgical candidates should be
treated surgically or within clinical trials of stenting versus endarterectomy. Noninvasive screening of symptomatic but surgically-ineligible patients for possible carotid
stenosis prior to angioplasty and stenting (level 2 – moderate evidence). The benefit of
angioplasty and stenting for asymptomatic patients is unproven (level 4 – insufficient
evidence).
ISSUE 4: What is the role of Physiologic Imaging in Carotid Stenosis and
Occlusion?
Summary:
Physiological imaging studies – the identification of compensatory hemodynamic
mechanisms to low perfusion pressure, have been shown to be powerful predictors of
subsequent stroke in patients with symptomatic carotid stenosis or occlusion using some,
but not all physiological imaging methods. The best evidence is for measurements of
oxygen extraction fraction (OEF) with PET and breath-holding transcranial Doppler
studies (Level 1 – strong evidence). There is moderate evidence (level 2) supporting the
use of stable xenon CT and SPECT methods. At present, however, the use of this
information to guide therapy has not been proven to change outcome (level 3 – limited
evidence). The two patient populations in whom these tools are likely to become
important are those with symptomatic complete carotid occlusion and asymptomatic
carotid stenosis.
Cost effectiveness analysis suggest that the use of these physiological tools, even
expensive ones such as PET, would be cost effective for patients with symptomatic
carotid artery occlusion, provided there is a benefit with surgical bypass. The costs of
acute and long term care for stroke victims greatly exceeds the costs of diagnostic work
up and surgery.
In addition to patients with complete carotid occlusion, another promising application for
hemodynamic assessment is in asymptomatic carotid stenosis. The prevalence of
hemodynamic impairment in patients with asymptomatic carotid occlusive disease is very
low. This low prevalence may account in part for the low risk of stroke with medical
treatment, and consequently, the marginal benefit with revascularization. The presence of
hemodynamic impairment may be a powerful predictor of subsequent stroke in this
population. This is one area of research with enormous clinical implications: if a
subgroup of asymptomatic patients at high risk due to hemodynamic factors could be
identified, it would be possible to target surgical or endovascular treatment at those most
likely to benefit.
Only one study has been performed in this population, to date. Silvestrini, et al.,
performed a prospective, blinded longitudinal study of 94 patients with asymptomatic
carotid artery stenosis of at least 70% followed for a mean of 28.5 months. Breathholding TCD was performed on entry, as well as the assessment of other stroke risk
factors. An abnormal TCD study was shown to be a powerful and independent risk
factor for subsequent stroke.
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