Impairments in Cognitive Function and Brain Connectivity
in Severe Asymptomatic Carotid Stenosis
Hsien-Lin Cheng, BS; Chun-Jen Lin, MD; Bing-Wen Soong, MD, PhD; Pei-Ning Wang, MD, PhD;
Feng-Chi Chang, MD; Yu-Te Wu, PhD; Kun-Hsien Chou, PhD; Ching-Po Lin, PhD;
Pei-Chi Tu, MD, PhD; I-Hui Lee, MD, PhD
Downloaded from http://stroke.ahajournals.org/ by guest on July 31, 2017
Background and Purpose—Severe asymptomatic carotid stenosis has been associated with cognitive impairment, but it is
unknown whether this association is attributable to effects on brain connectivity. We present cognitive network
abnormalities in a group of patients at a presymptomatic stage.
Methods—Seventeen patients with ⱖ70% asymptomatic stenosis of unilateral internal carotid artery were compared with 26
healthy controls utilizing a comprehensive neuropsychological battery, the dizziness handicap inventory, and multimodality
neuroimaging including diffusion tensor imaging and resting-state functional connectivity magnetic resonance imaging.
Longitudinally, assessments were completed in a subgroup of 10 patients at 3 months after carotid artery stenting.
Results—Compared with the healthy controls, the patients had worse dizziness scores, poorer memory, complex
visuo-spatial performances, and lower whole-brain mean fractional anisotropy. The Scheltens scores of leukoaraiosis/
infarction were not different between groups. Their seed-based functional connectivity magnetic resonance imaging
showed marked decrements of interhemispheric and intrahemispheric, ipsilaterally to carotid stenosis, functional
connectivity in the frontoparietal network. In the default mode network, the intrahemispheric functional connectivity
was bilaterally impaired. Importantly, the disrupted mean fractional anisotropy in the patients significantly correlated
with the attention and verbal memory functions. After successful carotid artery stenting, small but measurable
increments of the mean fractional anisotropy and little functional connectivity in the default mode network
ipsilateral-to-carotid artery stenting were noted.
Conclusions—We identified for the first time distinct patterns of network disruption that correlate with cognitive fragility
in patients with asymptomatic carotid stenosis. Brain connectivity may provide early and useful biomarkers for brain
ischemia and reperfusion. (Stroke. 2012;43:2567-2573.)
Key Words: asymptomatic carotid stenosis 䡲 cognitive impairment 䡲 diffusion tensor imaging
䡲 endovascular treatment 䡲 functional connectivity magnetic resonance imaging 䡲 resting state 䡲 stent
A
symptomatic stenosis of the internal carotid artery (ICA)
is described as significant atherosclerosis without stroke
or transient ischemic attack in the brain or eyes.1 Retrospective studies comparing asymptomatic subjects with and without ultrasound-assessed carotid stenosis have shown that
subjects with carotid stenosis had significantly poorer performance in tests of attention, psychomotor speed, and memory.2– 4 Moreover, severe ICA stenosis (ⱖ50%) has been
associated with a higher prevalence of silent cerebral infarcts
and white matter hyperintensities,3 indicating that “asymptomatic” carotid stenosis may not be truly asymptomatic. To
understand the pathological changes after chronic cerebral
hypoperfusion, experimental models of vascular cognitive
impairment have been induced by bilateral5 or unilateral6
common carotid artery occlusion. Mice subjected to unilateral
carotid artery occlusion had development of impaired object
recognition and significant white matter damage in the corpus
callosum and frontal-subcortical circuits; however, they
maintained normal spontaneous activity.6 Once their cerebral
ischemia reached a severe degree (⬍10% supply), it caused a
rapid loss of spine and dendrite microstructure within 10
minutes, which could be partially reversible only when
reperfusion occurred within 20 to 60 minutes.7 In the clinical
scenario of carotid revascularization, a few studies showed
that patients with severe asymptomatic ICA stenosis had
cognitive improvements 3 months after carotid artery stenting
Received November 29, 2011; accepted July 6, 2012.
From the Institute of Brain Science (H.-L.C., Y.-T.W., C.-P.L., I.-H.L.); Department of Neurology (B.-W.S., P.-N.W.); Institute of Neuroscience
(K.-H.C., C.-P.L.), School of Medicine, National Yang-Ming University, Taipei, Taiwan; Department of Physical Medicine and Rehabilitation, Taipei
Medical University Hospital, Taipei, Taiwan (H.-L.C.); Department of Neurology (C.-J.L., B.-W.S., P.-N.W., I.-H.L.), Department of Research and
Education (P.C.T.), Department of Psychiatry (P.C.T.), and Department of Radiology (F.C.C.), Taipei Veterans General Hospital, Taipei, Taiwan.
P.C.T. and I.H.L. contributed equally to this work.
The online-only Data Supplement is available with this article at http://stroke.ahajournals.org/lookup/suppl/doi:10.1161/STROKEAHA.111.645614/-/DC1.
Correspondence to I-Hui Lee, MD, PhD, No. 201, Sec. 2, Shih-Pai Road, Taipei, 11217, Taiwan. E-mail [email protected]
© 2012 American Heart Association, Inc.
Stroke is available at http://stroke.ahajournals.org
DOI: 10.1161/STROKEAHA.111.645614
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(CAS), particularly in psychomotor processing speed;8 –11
however, this conclusion is being debated because of lack of
controls. It is unknown whether and how their cognitive
alterations are related to disruptions in brain connectivity.
Diffusion tensor imaging (DTI) and resting-state functional
connectivity magnetic resonance imaging (fcMRI) have been
increasingly used to analyze brain connectivity (ie, structural
and functional connectivity, respectively) in neuropsychiatric
diseases. DTI measures the restricted diffusivity of water
molecules, known as fractional anisotropy (FA), in a spatially
coherent manner and characterizes white matter tracts.12 In
fcMRI, the spatio-temporally organized coherence of spontaneous blood oxygenation level– dependent signals at low
frequencies (⬍0.1 Hz) is thought to reflect the intrinsic
networks of the brain.13 The significant correlations between
brain regions, the so-called functional connectivity (FC),
has been used to identify resting-state networks, including
the sensorimotor network,13 the default mode network
(DMN),14,15 the fronto-parietal network (FPN),16 and the
dorsal attention network.17 Disrupted FC in the DMN, particularly disruption in the posterior cingulate cortex (PCC)
and in the hippocampus, has been implicated in mild cognitive impairment18 and Alzheimer disease.19 Nevertheless, it
has not been studied whether complementary DTI and fcMRI
may detect microstructural and functional alteration of neural
networks in severe asymptomatic carotid stenosis and after
carotid revascularization.20
In this work, we investigated neuropsychological performance, DTI, and fcMRI in 17 consecutive patients with
severe asymptomatic carotid stenosis and compared them
with 26 healthy subjects. To our knowledge, this is the first
study to demonstrate the disconnections of structural and
functional connectivity in correlation with cognitive impairments in patients with severe asymptomatic carotid stenosis.
Subjects and Methods
Subjects and Neuropsychological Tests
We recruited consecutive, testable patients with severe, unilateral
stenosis (ⱖ70% by both ultrasound and magnetic resonance angiography; contralateral carotid stenosis ⬍50% if any) of the extracranial
ICA from our dizziness/cerebrovascular clinic during the period
between February 2010 and January 2012. Many of them had
symptoms of dizziness and amnesia. We also recruited age-eligible,
and education level– eligible healthy controls without significant
(ⱖ50%) stenosis of large cerebral arteries. All of the subjects were
55 to 80 years of age, right-hand-dominant, and free of stroke,
transient ischemic attack, dementia, and depression. Other exclusion
criteria included the presence of any comorbid neurological disease
(such as Parkinson disease), severe medical diseases (such as
advanced malignancy, heart failure, sepsis, cirrhosis, renal failure,
and chronic obstructive pulmonary disease), or functional disability
(modified Rankin Scale ⱖ3). A blinded rater evaluated the participants using the dizziness handicap inventory21 and a battery of
neuropsychological tests,22 including the Mini-Mental State Examination, the Taiwan Geriatric Depression Scale, a working memory
test (backward digit span), a verbal memory test (immediate recall
and delayed recall), an attention test (the symbol digit test, as a part
of the Wechsler Adult Intelligence Scale), executive function tests
(the modified trail-making test A and B, trail-making test A/B, and
the Stroop color-word test), and a complex visuospatial perception
test (the modified complex figure test with copy and recall). The
Ethics Committee of the Taipei Veterans General Hospital
(VGHIRB 98-08-4A, 2011-01-0071C, 2011-12-009GA) approved
this study. Each subject signed the consent form in advance.
MRI Acquisition
The images were acquired with a 3.0 GE Discovery 750 MRI
scanner. A standard head coil with foam padding was used to restrict
head motion. All of the imaging sections were acquired along the
anterior–posterior commissural plane, as identified by multiplanar
T1-weighted BRAVO anatomic images (repetition time, 12.2 ms;
echo time, 5.2 ms; flip angle, 12 degrees; voxel size, 1⫻1⫻1 mm; field
of view, field of vision⫽256 mm). Standard fluid attenuation
inversion recovery images were also acquired to assess structural
lesions. For the fcMRI, we recorded blood oxygenation level–
dependent signals from 1 task-free run (124 time points/372 s) with
a gradient-echo echo-planar imaging sequence (repetition time/echo
time, 3000/30 ms; flip angle, 90 degrees; field of vision, 222 mm;
thickness, 3 mm). The subjects were asked to open their eyes without
thinking or moving. For the DTI, we used a single-shot diffusion
spin-echo echo-planar imaging sequence (repetition time/echo time,
9500/85.6 ms; thickness, 2 mm; matrix, 128⫻128; field of vision,
256 mm; 30 directions). The MRI was longitudinally acquired twice
in patients receiving CAS (before CAS and 3 months after the CAS).
MRI Analysis
A blinded neurologist assessed all of the images. The presence of
lacunes (diameter ⬍15 mm) and leukoaraiosis (hypointensity on the
T1-weighted images and hyperintensity on the fluid attenuation
inversion recovery images) were all depicted on a Montreal Neurological Institute template with the MRIcron software. The semiquantitative Scheltens scale23 for leukoaraiosis/infarcts is evaluated
in details in the Supplementary Methods.
For the DTI analysis, the Diffusion Toolbox package and tractbased spatial statistics from the FMRIB Software Library (FSL 3.2,
http://www.fmrib.ox.ac.uk/fsl) were used to perform preprocessing
and voxel-wise FA analysis, respectively.24,25 The details are described in the Supplementary Methods. For the FA maps and FC
correlation maps, the hemisphere ipsilateral to the carotid stenosis
was set to the left side by flipping along the midsagittal axis.
For the fcMRI analysis, a preprocessing procedure was performed
using our previously described method26 (details provided in the
Supplementary Methods). Regions of interest (ROI) representing
the seed regions for 3 resting-state networks, including the DMN, the
FPN, and the dorsal attention network, were predefined according to
the literature17 (Supplementary Table I). All of the ROI were 4 mm
in radius. The Pearson correlation coefficient (r) for the temporal
correlation between the blood oxygenation level– dependent signals
from each ROI-to-ROI pair, ie, the FC value, was calculated with a
Fisher r-to-z transformation to yield an approximately normally
distributed measurement.13,17 The voxel-wise correlation z maps
from a single ROI were computed with 1-sample t test by using The
Statistical Parametric Mapping (SPM8; Wellcome Department of
Cognitive Neurology in London) for within-group analysis. We then
compared the mean (homologous) interhemispheric and intrahemispheric FC values of the ROI-to-ROI pairs between the 2 groups (a
total of 17 ROI pairs compared: 9 in DMN; 4 in FPN; 4 in dorsal
attention network) and between the baseline and the 3-month
follow-up periods in the patients receiving CAS.
Statistical Analysis
We used the SPSS software (version 18.0) for data analyses. The ␹2
or Fisher exact test (when the expected number ⱕ5) was used to
compare the categorical demographic variables. The nonparametric
Mann-Whitney U test was used to compare the dizziness scores, the
neuropsychological scores (Bonferroni correction n⫽10 items), the
Scheltens scores, the FC, and the mean FA values between groups.
To investigate these longitudinal changes after CAS, we adopted the
nonparametric Wilcoxon signed-rank test for differences between the
baseline and the 3-month follow-up. Significance was defined as a
corrected P⬍0.05.
Cheng et al
Table 1.
Basic Characteristic of Study Subjects
Characteristics
Age, y
Male:female (male %)
Education, y
TGDS
Patients
(n⫽17)
Connectivity in Asymptomatic Carotid Stenosis
Table 2.
Controls
(n⫽26)
P Value
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Neuropsychological Measures of Study Subjects
Patients
(n⫽17)
Controls
(n⫽26)
P Value
Corrected
73 (11)
70 (7)
0.101
Dizziness Handicap Inventory
24 (29)
2 (4)
⬍0.001*
12:5 (70.6)
12:14 (46.2)
0.206
Mini-Mental State Examination
28 (3)
29 (1)
0.07
12 (8)
12 (8)
0.778
Working memory test
6 (3)
4 (5.5)
0.281
Stenotic degree, % (n)
Backward digit span
4 (2)
5 (2.25)
0.03*
Verbal memory test (12 items)
Left
80 (7)
NA
Total immediate recall
Right
81.4 (10)
NA
Delayed recall
Hypertension
64.7 (11)
42.3 (11)
0.215
Symbol digit test
Diabetes mellitus
41.2 (7)
15.4 (4)
0.080
Executive function test
Ischemic heart disease
11.8 (2)
3.8 (1)
0.552
PAOD
11.8 (2)
0
Hypercholesterolemia
35.3 (6)
26.9 (7)
Smoking
35.3 (6)
11.5 (3)
0.122
47 (15)
56.5 (7)
0.06
9 (4)
12 (9.75)
0.01*
34 (42)
56 (61.75)
0.13
Modified trail-making test A (sec)
15 (10)
9.5 (5)
0.62
0.151
Modified trail-making test B (sec)
45 (32)
28 (19)
0.45
0.736
Stroop color-word card
38 (23)
41.5 (11.5)
1.23
Modified complex figure test, copy
14 (3)
17 (5)
0.03*
Modified complex figure test, recall
8 (8)
13 (4)
0.03*
Risk factors, presence % (n)
Attention test
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NA indicates not applicable; PAOD, peripheral arterial occlusive disease;
TGDS, Taiwan Geriatric Depression Scale.
Values expressed as median (interquartile range).
For FC statistical adjustment, we adopted the false discovery rate
correction27 for multiple comparisons (n⫽17 ROI pairs) and significance was defined as q⬍0.05. For voxel-wise FA analyses, we used
the nonparametric permutation-based cross-subject statistics28
implemented in the FSL. This nonparametric test was used to
compare the FA values between groups and the changes from
baseline after CAS in the patient group. The threshold-free cluster
enhancement29 was used to avoid choosing an arbitrary cluster
threshold for voxel-wise inference. Significance was defined as a
corrected P⬍0.05 after controlling the family-wise error rate
(n⫽5000 random permutations).30
To evaluate the correlations between the mean FA values with the
dizziness severity and neuropsychological performance,13,31,32 we
calculated the nonparametric Spearman correlation coefficients (rs)
separately for the patients (n⫽17) and the controls (n⫽26) with
Bonferroni correction (n⫽5 tests, including correlations with the
scores of dizziness, the backward digit span of working memory, the
immediate recall of verbal memory, the symbol digit test of attention,
and the modified trail-making test B of executive functions). These
items were found to be significantly correlated with the mean FA
values (all corrected P⬍0.05) in the combined population (n⫽43)
after Bonferroni correction (n⫽11 dizziness and neuropsychological
tests).
Results
Patient Characteristics and
Neuropsychological Evaluation
We enrolled 17 consecutive patients with unilateral, severe,
asymptomatic carotid stenosis and 26 healthy controls (Table
1). There were no significant differences in age, gender ratio,
educational years, depression scores, and vascular risk factors
between groups. Compared with the controls, the patients had
significantly worse dizziness scores and poorer cognitive
performance on the memory (working and verbal) and complex visuospatial perception tests. The Mini-Mental State
Examination scores and the attention and the executive scores
were not different after corrections for multiple comparisons
(Table 2).
Complex visuospatial perception
Values expressed as the median (interquartile range).
*Significance defined as a corrected P⬍0.05.
White Matter Hyperintensities and Impaired
Structural Connectivity
The semi-quantitative Scheltens scores were not different
between groups (6.6⫾3.6 vs 5.6⫾2.4; P⫽0.44; Figure 1A).
Nevertheless, the patient group had lower whole-brain mean
FA (0.45⫾0.03 vs 0.6⫾0.02; P⬍0.001) and diffuse decrements of FA compared with the control group (Figure 1B, C)
in bilateral white matter tracts, particularly in frontoparietal
regions ipsilateral to the stenosis and in the PCC. These
findings are indicative of poorer diffusivity and microstructural disruption of white matter integrity.
Brain Connectivity in Correlation With
Neuropsychological Measurements
Compared with the healthy controls, the patients had a
markedly decreased blood oxygenation level– dependent correlation in the opposite hemisphere when the ROI were
predefined on the stenotic side (flipped to the left) (Figure
2A), suggesting a disruption of interhemispheric connectivity. The within-group analysis performed with 1-sample t test
also showed that the patients had a more asymmetrical
network distribution in the cortical surface areas (Figure 2B).
Especially in the FPN, the long-distance interhemispheric FC
between the bilateral dorsal lateral prefrontal cortices
(q⫽0.033) and between the bilateral anterior inferior parietal
lobules (q⫽0.007) were significantly reduced in the patients
compared with the controls (Figure 2C). The short-distance
interhemispheric FC between the bilateral hippocampi in the
DMN was preserved. Also, the intrahemispheric FC was
specifically decreased between the dorsal lateral prefrontal
cortices and the anterior inferior parietal lobules in the FPN
ipsilateral to the stenosis (q⫽0.021), and so were the FC
between the PCC and the medial prefrontal cortex (q⫽0.022),
and between the PCC and the hippocampus bilaterally (ipsi-
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Figure 1. A, White matter hyperintensities from the
17 patients (PA, red) and the 26 healthy controls
(HC, green) are overlaid on a standard Montreal
Neurological Institute template. B, The fractional
anisotropy (FA) maps. The carotid stenotic side
was set to the left. The patient group had a diffuse
and significant FA decrease (red–yellow; white
matter skeleton shown in green) compared with
the healthy controls, particularly in the frontoparietal regions ipsilateral to the stenosis and in the
PCC (arrowheads). C, The bar chart of the wholebrain mean FA values of the 2 groups. The error
bars represent the standard error of the mean.
*P⬍0.05, family-wise error rate corrected.
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lateral q⫽0.009 and contralateral q⫽0.014, respectively) in
the DMN compared to those of the healthy controls (Figure
2D). Taken together, the interhemispheric FC in the FPN and
the intrahemispheric FC in the FPN (ipsilateral to the steno-
sis) and in the DMN were specifically susceptible in the
patients with severe asymptomatic carotid stenosis.
The whole-brain mean FA significantly correlated with the
symbol digit test scores of attention function (P⫽0.005) and
Figure 2. Disrupted default mode network (DMN) and the frontoparietal network (FPN) in the patients with severe asymptomatic carotid
stenosis. A, The functional connectivity (FC) correlation maps from the predefined regions of interest (ROIs) showed more asymmetry
and decreased correlation in the contralateral hemisphere in the patients (PA) than in the healthy controls (HC). The ROIs were located
at the posterior cingulate cortex (PCC), the dorsolateral prefrontal cortex (DLPFC), and the frontal eye field (FEF; blue circles) of the
DMN, the FPN, and the dorsal attention network (DAN), respectively. The carotid stenotic side was set to the left. B, The within-group
analysis of 3 resting-state networks from the same ROIs (blue circles) viewed from lateral, midsaggital, and top cortical surface. Compared with the healthy controls, the patients had significantly disrupted (C) interhemispheric and (D) intrahemispheric FC in the FPN
and the DMN. AH, affected hemisphere; UH, unaffected hemisphere. The error bars represent the standard error of the mean. *q⬍0.05,
false discovery rate corrected.
Cheng et al
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Figure 3. Scatter plots of the correlation analyses between the whole-brain mean fractional anisotropy (FA) and (A) the symbol digit
test scores (attention function), (B) the immediate recall scores (verbal memory function), and (C) the dizziness scale scores. Regression
lines are calculated separately in the control (HC, dashed lines) and the patient (PA, solid lines) groups. All probability values are
adjusted by Bonferroni correction.
the immediate recall scores of verbal memory function
(P⫽0.03) in the patients, but not in the controls (Figure 3).
There were no case-control differences in the symbol digit
test or the immediate recall scores (Table 2). In contrast, we
did not find correlations between the Scheltens scores and
the neuropsychological measures (data not shown). There
were no correlations of the mean FA with the dizziness
scores, the working memory (backward digit span), or the
executive (trail-making test B) scores. Taken together, decreased whole-brain mean FA values in the patients are
indicative of attention and verbal memory impairments.
Carotid Revascularization on Cognitive Functions
and Brain Connectivity
We investigated the changes in cognitive function and brain
connectivity after CAS at 3 months with an uncontrolled
design. Eleven of the 17 patients accepted CAS rather than
carotid endarterectomy (CAS is one of the hospital’s bestknown procedures), and the other 6 preferred aggressive
medical treatments only. Successful carotid revascularization
(defined as being free of periprocedural complications within
1 month and having residual stenosis ⬍50%) was achieved in
all 11 patients (100%), and 10 of them completed the
longitudinal follow-up (Supplementary Table II). After CAS,
these patients had insignificant improvements in their dizziness and the neuropsychological scores after corrections for
multiple comparisons (Figure 4D). Interestingly, small but
measurable increases in the mean FA (P⫽0.017; Figure 4A)
and in the FC between the posterior inferior parietal lobules
and the hippocampus ipsilateral to CAS (q⫽0.035) and in that
between the PCC and the medial prefrontal cortex (q⫽0.027)
in the DMN were observed (Figure 4B, C), implying a causal
relationship between reperfusion and connectivity increments. In addition, we found that 2 of the 10 patients had 1 to
3 asymptomatic new MRI embolic infarctions after CAS
(data not shown), which might have had an impact on
cognitive functions, although it was not evident in our
examinations at 3-month follow-up, consistent with a previous randomized comparison between the CAS and carotid
endarterectomy.33
Discussion
Here, we show that patients with severe asymptomatic ICA
stenosis had more dizziness, poorer cognitive performance
(including working and verbal memory and complex visuospatial perception). Importantly, the patients had further
diffuse impairments of white matter structural connectivity
(FA) and regionally specific disruptions of FC in the FPN and
the DMN. The disrupted mean FA in the patient group
correlated significantly with the attention and the verbal
memory impairments. Their cognitive impairments are likely
attributed to both structural and functional disconnections.
The DTI and fcMRI in these patients provide sensitive and
objective information to detect network abnormalities early,
even at a presymptomatic stage.
Our major finding is that patients with severe asymptomatic unilateral carotid stenosis had significantly reduced
whole-brain mean FA and FC, particularly in the interhemispheric FPN (between bilateral dorsal lateral prefrontal cortices and bilateral anterior inferior parietal lobules) and the
intrahemispheric FPN (dorsal lateral prefrontal cortices–
anterior inferior parietal lobules ipsilateral to the stenosis)
and the DMN (PCC– hippocampus and PCC–medial prefrontal cortex). The PCC, supplied by the precuneal artery from
its origins at the ICA, has been shown to be particularly
vulnerable and is involved in degenerative cognitive decline.18,19,34 The hippocampus is mainly supplied by the posterior cerebral artery and was less affected in these patients.
Moreover, the medial prefrontal cortex and the frontoparietal
region receive blood from the ICA, and therefore they are
directly affected. Consistent with other findings, the disrupted
connectivity between these hubs were correlated with cognitive
impairments.18,19 Importantly, significant impairment of FA
supports the aforementioned functional discontinuity and pro-
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Figure 4. The changes in brain connectivity and neuropsychological measurements at 3 months after carotid artery stenting
(CAS). A, A modest increase in fractional anisotropy (FA) (red to
yellow: P⬍0.05) and the whole-brain mean FA value (bar chart
of baseline [pre] and 3 months [3 m] after CAS). B and C, Note
few increments of intrahemispheric FC in the default mode network (DMN) at the medial prefrontal cortex (MPFC) and the hippocampus (HP) ipsilateral to the CAS (arrowheads). *q⬍0.05,
false discovery rate corrected. D, The dizziness scores and the
neuropsychological scores were insignificantly changed after the
CAS. DHI, dizziness handicap inventory; BDS, backward digit
span; SDT, symbol digit test; TMT-A/B: trail-making test A/B.
The error bars represent the standard errors of the mean.
vides a hemodynamic independent index of white matter integrity12 in relation to cognitive performance and cortical network
efficiency.10 Moreover, the whole-brain mean FA can be a
useful biomarker for the attention and the verbal memory
impairments in these patients.
Consistent with previous studies of patients with asymptomatic carotid stenosis,2,4,35 our patients showed poorer
cognitive performance than the healthy controls, but not in
Mini-Mental State Examination. All of the patients’ MiniMental State Examination scores were ⬎24, which is a
cut-off value for mild dementia suggested by the large
community-based domestic (Taiwan) studies of healthy elderly subjects (26.52⫾2.93).22,36 Hence, cognitive alterations
in asymptomatic patients with severe carotid stenosis may not
meet the criteria of vascular cognitive impairment, which has
been vaguely constructed to encompass any vascular disorder
that is sufficient to produce abrupt cognitive impairment
beyond a standard deviation from the mean.37 Given the
common cardiovascular risk factors in the elderly with either
vascular or nonvascular cognitive impairment38 and the high
prevalence of mixed pathologies of microvascular infarct and
Alzheimer amyloid plaque,39 early identification of treatable
vascular condition is warranted, even in those with neurodegenerative diseases.
The impact of carotid revascularization on cognition and
brain connectivity needs further investigation given the limitation of the small subpopulation subjected to CAS. Because
of the class II recommendations (benefit greater than risk) for
carotid revascularization in selected asymptomatic patients
who have ⱖ70% stenosis of the ICA and low perioperative
risk,40 there has been hesitation for randomized controlled
studies for the effects of CAS and the uncontrolled observations therefore have been challenged.8,9,11 We found small but
measurable increments of the whole-brain mean FA and focal
FC in the DMN ipsilateral to CAS. Nevertheless, the neuropsychological measures and most disrupted FC did not
change obviously after statistical adjustments. The strengthened FC neighbor to some disrupted areas at 3 months after
CAS may be attributable to restored perfusion and vasomotor
reactivity,20 enhanced axonal transport, and/or functional
plasticity of already existing pathways. A study with a larger
population and longer observation is needed to determine
whether cognitive impairment can be enhanced after CAS
and whether the increased brain connectivity may correlate
with cognitive improvement.
There are several limitations to this study. First, the sample
size and the number of multiple comparisons made between
patients and controls were small. Second, we used a rapid
fcMRI acquisition (⬇6 minutes) to estimate the FC strength.
A single 4-minute fcMRI run is sufficient to estimate connectivity,26,41 but increasing the acquisition time can improve
the signal-to-noise ratio. Third, blood oxygenation level–
dependent signals are sensitive to impaired perfusion and
vasomotor reactivity in cerebrovascular diseases in which the
coupling between neuronal activity and hemodynamic activity has been affected.42 Hence, we adopted complementary
DTI and had been cautious in interpretation of these changes.
Further investigation using electroencephalography or magnetoencephalography can provide direct evidence for neural
activity. Fourth, we compared FC of homologous ROI pairs
between groups and might have underestimated differences
outside the ROI we selected a priori or between nonhomologous ROI pairs. A brain-wise comparison between groups is
currently undergoing investigation. Given the small sample
size and the uncontrolled results of the comparisons before
and after CAS, larger studies to investigate the alternations
found in brain connectivity are needed in the future.
Conclusions
Patients with severe asymptomatic carotid stenosis have an
increased risk of cognitive fragility and white matter tract
disruption. The FPN and DMN have high susceptibility to
chronic cerebral hypoperfusion. Complementary DTI and fcMRI in these patients provide sensitive and objective information on the functional status of the network connectivity.
Cheng et al
Acknowledgments
The authors thank Chia-Feng Lu for technical assistance in the image
analysis.
Sources of Funding
The National Science Council (NSC-100-2314-B-075-021, NSC-993111-B-010-004), the National Yang-Ming University (101AC-B16),
and the Taipei Veterans General Hospital (V100C-045, V100E1-009,
V101C-062, V101E1-008) in Taiwan supported this work.
Disclosures
None.
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Impairments in Cognitive Function and Brain Connectivity in Severe Asymptomatic
Carotid Stenosis
Hsien-Lin Cheng, Chun-Jen Lin, Bing-Wen Soong, Pei-Ning Wang, Feng-Chi Chang, Yu-Te
Wu, Kun-Hsien Chou, Ching-Po Lin, Pei-Chi Tu and I-Hui Lee
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Stroke. 2012;43:2567-2573; originally published online August 30, 2012;
doi: 10.1161/STROKEAHA.111.645614
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Copyright © 2012 American Heart Association, Inc. All rights reserved.
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SUPPLEMENTAL MATERIAL
Impairments in cognitive function and brain connectivity in severe
asymptomatic carotid stenosis
Supplemental Methods
MRI Scheltens scale
The whole brain was divided into periventricular, deep white matter and basal ganglia. The
periventricular region was further divided into the frontal caps, occipital caps and the bands
beside lateral ventricles. Each region was separately scored as follows: absent as 0, white
matter lesions up to 5 mm as 1 and larger than 5 mm as 2. The deep white matter was divided
into frontal, parietal, occipital and temporal regions. The basal ganglia were divided into
caudate nucleus, putamen, globus pallidus, thalamus and internal capsule. Each of these
regions was scored as follows: absence of lesions as 0, <6 lesions with a diameter smaller
than 3 mm as 1, >5 lesions with a diameter smaller than 3 mm as 2, <6 lesions with a
diameter of 4–10 mm as 3, >5 lesions with a diameter of 4–10 mm as 4, at least one lesion 10
mm diameter as 5, and confluent lesions as 6. All of the scores of the different regions were
summed to obtain a total scale score of an individual.
DTI analysis
The Diffusion Toolbox package and tract-based spatial statistics (TBSS) from the FMRIB
Software Library (FSL 3.2, http://www.fmrib.ox.ac.uk/fsl)1 were used to perform
preprocessing and voxel-wise FA analysis, respectively. The raw DTI images were corrected
for eddy currents and head motion. To create a brain mask from the b=0 images, the
non-brain structures were removed using the brain extraction tool2 in FSL. The diffusion
tensor was then fitted to a diffusion tensor model at each voxel of the preprocessed data using
the DTIfit program in FSL, and the FA was calculated. The hemisphere ipsilateral to the
carotid stenosis was set to the left side by flipping along the mid-sagittal axis. The resulting
FA images were fed into TBSS to perform the voxel-wise statistical analysis. All of the FA
maps were spatially aligned in a 1×1×1 mm standard MNI to generate a mean FA image. The
threshold of the mean FA map was defined as a mean FA value of 0.2 to generate a white
matter skeleton mask.
fcMRI analytic preprocessing
The preprocessing procedure included steps to reduce scanner artifacts, correct head motion
and transform the data into a standard space. Atlas registration was achieved by joining the
fMRI run with the T1-weighted anatomical images. The correlation maps were generated as
previously described: the data were band-pass filtered for frequencies between 0.009 and 0.08
Hz and then spatially smoothed using a 6-mm, full-width, half-maximum Gaussian kernel.
The sources of spurious or regionally nonspecific variance were removed, including 6
parameters obtained from the rigid body head motion correction, the signal averaged over the
whole brain, the signal averaged over the lateral ventricles and the signal averaged over the
deep white matter.
Supplemental Tables
S1. Regions of interest
Resting-state networks
Default mode network (DMN)
Posterior cingulate cortex (PCC)
Medial prefrontal cortex
Left hippocampus
Right hippocampus
Left posterior inferior parietal lobule (pIPL)
Right posterior inferior parietal lobule
Frontoparietal network (FPN)
Left dorsal lateral prefrontal cortex (DLPFC)
Right dorsal lateral prefrontal cortex
Left anterior inferior parietal lobule (aIPL)
Right anterior inferior parietal lobule
Dorsal attention network
Left frontal eye field (FEF)
Right frontal eye field
Left intraparietal sulcus (IPS)
Right intraparietal sulcus
Atlas coordinates represent the MNI coordinate system
x
y
z
0
0
-22
24
-50
62
-22
-20
22
26
-18
-18
-47
50
-71
-64
29
27
-50
46
-52
52
20
14
-49
-46
34
43
47
46
-25
-8
50
27
-27
24
-8
-52
-56
50
57
55
S2. Patient characteristics
Age
†
69*
62*†
79
70*†
76
72*†
Gender
Side, Stenosis%
Symptoms
Risk factors
MRI interval
F
M
M
M
M
M
R,99
R,95
R,90
R,80
R,70
R,80
D,
no
D
D
D
no
HTN,DM,IHD
HTN,S
HTN,PAOD
S
no
C
108
91
65*
75
80*†
80*†
76
M
F
M
F
M
R,80
R,80
R,70
R,70
L,100
D
A
no
D
D
C
DM
HTN,DM,IHD
HTN,C
HTN,DM,C,S
90
55*†
79*†
70
73*†
F
M
M
M
L,95
L,80
L,70
L,70
no
D,A
A
D,A
HTN
HTN,DM,C,S
HTN,DM,S,PAOD
HTN,S
95
95
†
96
89
97
92
103
60*
M
L,75
no
HTN
73
F
L,70
D,A
DM,C
* Subjects undergoing CAS; † MRI before and 3 months after CAS with indicated MRI
interval in days. The median time between first and 3-month MRI is 95 (interquartile range: 5)
days. F: female; M: male; R: right side; L: left side; D: dizziness with DHI>0; A: amnesia;
HTN: hypertension; DM: diabetes mellitus; IHD: ischemic heart disease; PAOD: peripheral
arterial occlusive disease; C: hypercholesterolemia; S: smoking.
Supplemental References
1.
2.
Woolrich MW, Jbabdi S, Patenaude B, Chappell M, Makni S, Behrens T, et al.
Bayesian analysis of neuroimaging data in fsl. Neuroimage. 2009;45:S173-186
Smith SM. Fast robust automated brain extraction. Hum Brain Mapp.
2002;17:143-155