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Carotid surgery cognitive function and cerebral blood flow in patients with transient ischemic attacks.

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Carotid Surgery, Cognitive Function,
and Cerebral Blood Flow in Patients
with Transient Ischemic Attacks
Ralf Hemmingsen, MD, Birgitte Mejsholm, Sissel Vorstrup, MD, Jack Lester, MD, Hans Christian Engell, MD,
and Gudrun Boysen, M D
Psychological testing, cerebral blood flow (CBF) measurement, and computed tomographic scan were performed
before and 3 months after operation in 31 patients subjected to endarterectomy of the internal carotid artery (ICA)
because of transient ischemic attacks and in 11 control patients operated on for atherosclerosis of the lower extremities.
In preoperative psychological testing both carotid surgery patients and controls performed somewhat below the
normal level for their age group. Postoperatively, cognitive functions improved in the carotid surgery group but not in
the control group. The improvement was related to the laterality of the operation, being more marked in verbal tests
in patients with left ICA operation and in visuospatial tests in patients with right ICA operation. Postoperatively
regional CBF improved in 2 patients only. Hence the intellectual improvement could not be related to changes in CBF.
Intellectual deterioration in patients with internal carotid atherosclerosis may be delayed or terminated by surgical
abolition of the source of multiple cerebral embolizations.
Hemmingsen R, Mejsholm B, Vorstrup S, Lester J, Engell HC, Boysen G:
Carotid surgery, cognitive function, and cerebral blood flow in patients
with transient ischemic attacks. Ann Neurol 20:13-19, 1986
The possible relationship between atherosclerotic internal carotid artery (ICA) disease and intellectual impairment has been subjected to several investigations,
but inclusion criteria and the clinical condition of the
patients have often been vaguely described o r lacking
(see [3, 111). During the last 10 years, several studies
[ 4 , 11-13, 171 have indicated improvement of intellectual functions after reconstructive operation on the
large neck vessels. Kelly and co-workers [I31 used
patients undergoing peripheral vascular operation as
controls; Jacobs and colleagues 112) compared their
“low flow endangered” patients with a group of patients undergoing carotid endarterectomy for “hemodynamically insignificant stenosis”; and Hemmingsen
and colleagues { 111 used the patients as their own controls by comparing the effects of unilateral operation
on lateralized psychological functions. Taken together,
these studies provide substantial evidence of postoperative intellectual improvement in groups of patients
with ICA disease. These studies, however, did not give
information about structural cerebral lesions or cerebral pathophysiology in the patients and hence the explanations of postoperative intellectual improvement
have been speculative.
The purposes of the present study were (1) to see if
the findings of improvement in intellectual function
after carotid endarterectomy were reproducible; (2) to
compare the results with those in a control group of
patients subjected to vascular operation for atherosclerosis of the lower extremities; and (3) to investigate
whether an increase in cerebral blood flow (CBF) is
related to the postoperative intellectual outcome in
patients with transient ischemic attacks (TIAs) caused
by ICA disease.
From the Departments of Neurology, Psychiatry, Vascular Surgery,
and Neuroradiology, Rigshospitalet, Copenhagen, Denmark.
Address reprint requests to Dr Boysen, Department of Neurology,
Rigshospitalet, 9, Blegdamsvej, DK-2 100 Copenhagen, Denmark.
Received Jan 8, 1985, and in revised form July 8. Accepted for
publication Oct 27, 1985.
Materials and Methods
The demographic data are presented in Table 1. All patients
submitted for ICA endarterectomy had experienced TIAs
including one or more of the following symptoms: amaurosis
fugax, paresis, paresthesias, and aphasia. The neurological
signs had subsided within 24 hours, leaving the patients without neurological deficits. All patients had an angiographically
verified atherosclerotic lesion of the ICA that was judged to
be surgically accessible. The neck vessels were visualized by
aortocervicography; 5 patients had occlusion of the contralateral ICA. The total number of TIAs in each patient varied
between 1 and 50, the majority of patients having experienced 5 to 10 attacks. The interval between the latest TIA
and the preoperative investigation ranged from a few days to
Table 1 . Patient Data
Left ICA Operated
Right ICA Operated
All TIAs Operated
Number and sex
Age (yr); median (range)
17: 10 F, 7 M
59 (37-70)
14: 4 F, 10 M
61 (40-70)
31: 14 F, 17 M
60 (37-70)
1 1 : 2 F, 9 M
62 (41-69)
Occupational level
ICA = internal carotid artery; TIA = transient ischemic attack; F = female: M = male.
several months. At the time of inclusion into the study no
information had been obtained concerning intellectual functions and thus the patient material was selected according to
neurological and angiographic criteria only. The patients
were studied consecutively. If more patients were referred
for operation than the investigational program could accommodate, the youngest patient was included. Informed consent was obtained from all patients.
The carotid endarterectomy was performed with the patient under halothane anesthesia at slight hypocapnia. During
operation mean arterial pressure was measured before and
after endarterectomy both proximally and distally to the ICA
lesion, permitting calculation of the pressure gradient across
the stenosis. Postoperatively the pressure gradient was eliminated in all patients. A temporary shunt was inserted during
operation in 7 patients as the pressure in the ICA during
clamping was below 50 mm Hg. Neurological complications
of the operation occurred in 4 patients. Two developed a
transient paresis of the arm. One patient suffered new episodes of TIA postoperatively and occlusion of the operated
carotid artery was revealed by arteriography 3 weeks postoperatively. Another patient developed cerebral infarction
ipsilateral to the operation and the patient suffered a
hemiparesis with aphasia; subsequently the neurological
deficits remitted. The 4 patients suffering postoperative complications are all included in the material. Three of these
patients underwent operation on the left side, one o n the
The control group comprised patients without symptoms
of cerebrovascular or other brain disease. Arteriography of
the neck vessels was not performed in the controls.
All patients studied received anticoagulant treatment with
phenprocoumon (Marcoumar) before operation; this treatment was terminated at the time of discharge from hospital.
This means that with few exceptions anticoagulant treatment
was terminated 1 to 2 weeks postoperatively.
Psychological Investigation-Cognitive Functions
The psychological test battery was designed to assess cognitive functions known to be highly vulnerable to organic impairment, such as learning, memory, and attention. Furthermore, the laterality of cerebral functions was considered
by including some tests reflecting mainly dominant or nondominant hemisphere functions. The test norm values were
established in subjects age-matched to the patients included
in the present study.
To ensure the patient’s motivation and stable cooperation,
tests of relatively short duration were preferred and the
14 Annals of Neurology Vol 20 No 1 July 1986
whole test program was designed to be administered in a.
single session.
In all tests concerning learning and delayed memory (visual gestalts test, word pairs test, and story recall), parallel
sets of tests were used to minimize the retest effect. To
control possible systematic errors caused by minor differences between the sets, the order of the two sets was systematically alternated according to the patient’s number of inclusion in the study. The distribution of the two test sets was
uniform in the two groups of patients and the controls.
In a previous study, tests involving simple perceptual function did not discriminate among patients with mild or moderate impairment [ 111. Hence we included more complex tests
of attention involving concentration, tracking, and scanning
In the visual gestalts
test { 11, the material to be learned consisted of four complex
designs or gestalts, each circumscribed by a well-known
geometric figure (a circle, a square, a triangle, or a semicircle). Each gestalt is easily differentiated into subgestalts. After a 10-second presentation of the design, the patient was
asked to reproduce it on a sheet of paper with only the
circumscribed figure printed. If the drawing was not correct,
the examiner copied the subgestalts already correctly reproduced, using a new sheet with the same circumscribed figure.
After another presentation of the design, the patient was
asked to fill in the rest. This procedure was repeated until the
design was complete. The other three designs were learned
in turn. An hour later, without seeing the design, the patient
was asked to reproduce as much as possible on a sheet with
the outline figure. If there were errors, the examiner added a
subgestalt on the next piece of paper, and sheet by sheet the
patient was provided with an increasing number of clues to
prompt memory for the rest of the pattern. The visualspatial character of the test relates it to right hemisphere
function. To score, the total number of errors was calculated
for the learning and reproduction phases separately. T h e
norm for age level 50 to 59 years was: total number of errors
for learning, 2; total number of errors in reproduction, 6 121.
In the wordpairs test, the patient was presented with a list
of fifteen pairs of nouns. After presentation of the list, learning took place by correcting only the wrong and missing
associations; the correct ones were left out of the sample.
This procedure was continued until the correct response was
obtained for all items. Reproduction was tested 1 hour later
by presenting the stimulus words once. The verbal character
of the test relates it to left hemisphere function. To score,
the total number of errors for learning and reproduction
were counted separately. The norm for total number of errors for learning was 18; for total number of errors in reproduction, 5 121
In stovy recall, a short story of 18 items was read aloud to
the patient once. Immediately after the presentation, and
again after 1 hour, the patient was asked to recount the story.
The test is mainly related to left hemisphere function but its
complex organization and the aspect of synthesis involves the
right hemisphere, too. To score, the number of correct units
was counted separately for I, immediate recall, and 11, delayed (1 hour) recall. The norm was: I, 12.7; 11, 10.7 C21).
For digit span, the subtest from the Wechsler
Adult Intelligence Scale (WAIS) was used 125). The verbal
content of the test relates it to left hemisphere function, but
there is evidence that digit span backwards involves visualspatial function and thus reflects right hemisphere function
[7}. The score was based o n the highest number of correctly
repeated digits forward and backward, separately and combined. The combined score was converted into age-corrected
scaled scores (WAIS 1978) (Danish Standardization 1247).
The norm was: forward, 6 digits; backward, 4 digits. Digit
span total score was 10.
For digit symbol, the subtest from the WAIS was used 1257.
It is sensitive to brain damage, but not specifically related to
one hemisphere 114). To score, one point was given for each
square filled in correctly; half credit was given for reversed
symbols. The norm for an age level of 60 was a raw score of
37.5 ( W A S 1978; Danish Standardization 1247).
The trail-making test was given in 2 parts. In part A the
patient drew lines to connect 25 consecutively numbered
circles on a work sheet, and then in part B connected the
same number of consecutively numbered and lettered circles
on another work sheet by alternating between the numbers
and letters. The test is related to both left and right hemisphere function, but it has been suggested that left hemisphere dysfunction could result in relatively greater difficulties on part B than o n part A [18,19]. Scoring was time in
seconds. The norms for an age level of 50 to 59 years were:
A, 38 seconds; B, 98 seconds [87.
At the first psychological test session, hemisphere dominance was evaluated by a hand preference scale [16). All
patients had a marked right hand preference, i.e., left hemisphere dominance. All tests were performed by a clinical
psychologist who was not formally blinded but was, however,
not informed about the laterality of the ICA disease. At the
time of the postoperative psychological investigation the results of the preoperative tests were not available to the investigator.
CBF Measurement and Computed
Tomographic Scanning
Regional CBF was measured using single photon emission
computed tomography (CT), previously described in detail
120, 22). 13jXenon was rebreathed from a closed system for
the first 1.5 minutes, yielding a lung concentration reaching a
maximum of 10 mCiiliter at equilibrium. During this period
and the three subsequent 1-minute periods, four tomographic pictures were recorded for 3 slices of brain tissue
simultaneously. Each slice was 2-cm thick, with an interslice
distance of 2 cm. The resolution was 1.4 to 1.7 cm in the
plane. The midplanes of the 3 slices were routinely placed 1,
5 , and 9 cm above the orbitomeatal plane, respectively.
The arterial input function was estimated from the lung
curve, which was gathered from a narrowly collimated detector placed over the upper part of the right lung. This input
curve, taken together with the sequence of four tomographic
pictures, allows for calculation of CBF by a conventional
back projection filtered algorithm [6}. Although true quantitative CBF data cannot be obtained using an atraumatic
technique, calculation of interpixel or interhemispheric differences remains correct.
Hemispheric flow values as well as mean values for any
region of interest can be calculated by encircling the region
with a cursor, yielding the mean flow for this region and the
symmetrically placed area in the opposite hemisphere. Taking the side-to-side difference as a percentage of the highest
value then gives the degree of asymmetry. In normal controls, the hemispheric CBF as calculated from slice 2 averaged 55
5 mYlOO g d m i n (1 SD), and side-to-side differences for focal areas did not exceed 10%. In patients with
cerebrovascular disease, focal low flow areas are often seen
on the tomographic flow map; calculation of the variance of
CBF changes in such regions in patients in a chronic stable
phase have shown that a change of more than 115% in degree
of side-to-side asymmetry is needed to reach statistical
significance (p < 0.05) when comparing two CBF measurements performed several weeks apart [23].
The end-expiratory COZ tension was measured in conjunction with all CBF studies using a capnograph. These
values varied between 2.7% and 5.95%(vlv), and there were
no systematic differences within or between the groups.
The radiation absorbed dose to the lungs, which are the
organs receiving the highest dose, approximates the dose
obtained during a conventional roentgenographic recording
of the chest [22].
The local ethics committee has approved the CBF technique used in this study.
The C T scans were performed with an EM1 1010 head
scanner. The criteria for cerebral atrophy were as follows:
An Evans ratio (computed as the maximal width of the anterior horns divided by the maximal internal width of the skull)
of less than 0.32 indicated no central atrophy, 0.32 to 0.34
indicated slight atrophy, 0.35 to 0.40 indicated moderate
atrophy, and a ratio larger than 0.40 indicated severe central
atrophy. Patients with a maximum width of sulci of 1 mm
had no cortical atrophy; width up to 1.5 mm indicated mild
atrophy, width of 2 m m indicated moderate, and more than 2
mm indicated severe atrophy. All the figures were obtained
by direct measurement of the C T pictures; the real values
result from multiplication by the diminution factor of 3.3.
Neurological examination, psychological testing, CBF
measurement, and C T scanning were performed 1 to 2
weeks before operation and 3 to 5 months postoperatively.
A neurological examination was also done 1 to 2 weeks after
Wilcoxon’s test for paired data and Mann-Whitney’s test for
unpaired data were used. A p value of less than 5% indicates
Hemmingsen et al: Cognitive Function and Carotid Surgery
statistical significance. Postoperative change in the total ICA
group was tested by paired t test because of the larger number of patients (n = 3 1). Correlations were tested by Spearman's rho test.
Psychological Tests
Results of the preoperative psychometric investigation
in patients with ICA disease and in controls are indicated in Figure 1. There were no systematic differences between the groups. Both the patients with
TIAs and the controls had median test values slightly
inferior to the age-corrected norms, digit span being
the only exception.
Table 2 indicates the psychological test values before and after operation in patients undergoing operation on the left ICA and the right ICA, and in the
controls. Significant improvement occurred in the ICA
patients on several tests, and the improvement was in
tests mainly related to the hemisphere ipsilateral to the
side of endarterectomy. For instance, visual gestalts
test and trail-making B were significantly improved in
the group with right side operation, whereas word
pairs test, story recall I, and trail-making A and B were
significantly improved in the group with left ICA operation. In the control group, significant improvement
occurred only in digit symbol. To summarize, the patients with right ICA operation improved in all 10
psychometric tests, the patients with left ICA opera'
C L i ?
Fig I . Preoperative values (median and quartiles) of psychological tests in patients operated on ldt internal carotid artevy
(ICA)(L) (n = 17). right ICA (R)( n = 141, and in controls
(C) (n = 1I ) . Age-corrected norm (median) are indicated by
dotted lines. There were no significant differences between the
groups in any test.
16 Annals of Neurology Vol 20 No 1 July 1986
tion improved in 9 of 10 tests, whereas the control
group improved in 3, was unchanged in 2 , and worsened in 5 tests. If the postoperative change of intellectual function is tested for the whole ICA group ( n =
3 l), regardless of laterality of operation, significant improvement occurred in all psychometric tests except
digit span. Level of significance was p < 0.05 in visual
gestalts learning and memory, digit symbol, and trailmaking A; in word pairs learning and memory, story
recall I and 11, and trail-making B the p value wa!! less
than 0.01. These results are highly indicative of a difference between ICA patients and controls, the difference being in favor of intellectual improvement in the
ICA groups.
Figure 2 compares quantitatively the changes in the
psychological tests after operation in the ICA groups
and the controls. It shows that in visual gestalts (memory), story recall I, and trail-making A there was a
significantly greater change in the ICA groups compared to controls, and in word pairs test (learning) and
story recall I1 there was a trend in the same direction.
In all tests these significant differences were caused by
improvement in the ICA groups compared to the controls.
CBF Measurements, CT Scanning, and
Psychometric Findings
Table 3 indicates CBF values in the ICA groups according to the degree of cerebral atrophy on CT scans.
CBF values from the control group are also indicated.
There were no significant differences within the
groups before and after operation and no significant
differences between the groups.
Preoperatively, 7 patients showed a focal low flow
area, i.e., a side-to-side asymmetry of more than 1096,
on the side of endarterectomy. In 2 patients, this corresponded to a somewhat smaller hypodense lesion on
the CT scan, whereas 5 patients showed no such focal
abnormality. Postoperatively, only 2 of these 7 patients
showed a significant CBF increase in the focal lowflow area. Both these patients had severely stenosing
(threadlike) arteriosclerotic lesions of the ipsilateral
ICA; 1 patient also showed an occlusion of the contralateral ICA. The remaining 5 patients all showed a
persistent side-to-side asymmetry.
Taking all ICA patients together, the mean CBF
value in the hemisphere ipsilateral to the side of endarterectomy averaged 54 my100 g d m i n both before
and after reconstructive operation.
A total of 5 patients in the ICA group had focal
lesions on the CT scan. Except for 1 patient, these
lesions were all definitely minor. This fact may explain
why the CBF measurement did not reveal a low-flow
area in 3 patients having a lesion seen on CT. In 1
patient suffering a postoperative infarction, a focal lowflow area was seen at the follow-up CBF measurement.
Table 2. Median Psychological Test Values (Quartiles in Parentheses) before (1) and after (2) Operation"
Left ICA Operated
(n = 17)
Visual gestalts
Word pairs
Story recall
Digit symbol
Digit span
Right ICA Operated
(n = 14)
(n = 11)
4 (NS)
10 (NS)
(6-2 1)
8 (NS)
13 (NS)
23 (NS)
4 (NS)
(9-1 3)
40 (NS)
10 (NS)
8 (NS)
(2-2 1)
(8- 13)
( 10- 15)
( 10- 13)
(8- 13)
6 (NS)
4 (NS)
10 (NS)
6 (NS)
5 (NS)
12 (NS)
6 (NS)
10 (NS)
(73- 138)
40 (NS)
(70- 112)
(72- 160)
47 (NS)
111 (NS)
(75-1 18)
"Statistical test indicates results of comparing preoperative and postoperative values by means of Wilcoxon's test for paired data.
bp < 0.10; ' p < 0.05; ' p < 0.02; ' p < 0.01.
internal carotid artery; NS = not significant.
Concerning the psychological measures, there was a
trend that patients with definite or severe atrophy performed worse preoperatively than did patients with no
or only slight atrophy, but this reached significance
only in visual gestalts ( p < 0.05). Postoperatively the
ICA patients without atrophy improved less than did
the other groups in 7 of 10 tests, this difference reaching statistical significance in story recall I and I1 ( p <
The relationship between postoperative change in
psychological tests and change in mean hemisphere
CBF ipsilateral to the ICA operation was investigated
and no significant correlations were revealed ( p >
0.10; Spearman).
Two of the 11 patients in the control group showed
hypodense lesions on the CT scan, even though they
had no history of neurological deficits, transient or permanent. These 2 patients both showed corresponding
regional CBF changes. Seven of the control patients
had some degree of cerebral atrophy.
Psychological Findings
The findings at the preoperative test sessions indicate
that both ICA patients and age-matched control patients operated on for atherosclerosis of the lower extremities had a slightly subnormal level of intellectual
functions when compared to the age-corrected norms.
As far as the ICA group is concerned, this is in agreement with previous findings [ l l , 151. The lack of a
preoperative difference between ICA patients and
atherosclerotic controls is in accordance with the report of Kelly and co-workers 1131. The subnormal
functions in both groups probably reflect the common
underlying disease, generalized atherosclerosis. The
control group is suspected of having suffered subclini-
Hemmingsen et al: Cognitive Function and Carotid Surgery 17
had improved in almost all tests. When studied according to laterality of the operation, this improvement
reached significance mainly in tests related to the
hemisphere ipsilateral to the operation. This is in
agreement with our previous findings in a group of 25
ICA-diseased patients { 111. In the control group the
postoperative test results were unchanged or even
worsened and it may be concluded that (1) in ICA
patients suffering TIAs, test results improved after
carotid endarterectomy; (2) the improvement was expressed most markedly in the tests relating to intellertual abilities lateralized to the side of the operation;
and (3) the control group differed from the ICA
groups at the postoperative test session; i.e., they were
essentially unchanged after operation.
In any study involving repeated psychometric mea.sures the question of the practice effect (retest effect.)
is pertinent [3, 151. However, the relationship between improvement and laterality of operation cannot
be caused by retest effects; also, the lack of improvement in the controls justifies the conclusion that the
postoperative improvement in the ICA patients cannot
be explained mainly by a retest effect.
- - H - c I ) - - - c - u -
Fig 2. Postoperative change in psychological tests in patients
operated on left internal carotid artery (ICA) (L) (n = 171,
right ICA (R) (n = 141, and in controls (C) (n = 11). Zero
indicates unchanged test result. Medians and guartiles are given
for the postoperative changes, which are calculated by subtracting
the postoperative test value from the preoperative test value in
each patient. ICA groups are compared with controls. (")p <
0.10; *p < 0.05; **p < 0.01.
Bra in Stractzl re, Pathophysiology, and
Psychometric Findings
From the comparison between intellectual functions,
CT scans, and CBF tomograms, it is clear that the
present study material does not comprise patients with
fully developed multiinfarct dementia but rather those
with mildly stigmatized atherosclerosis. The intellectual impairment was more severe in the group with
unequivocal cerebral atrophy, but the differences were
not large. This is in accordance with the CBF median
values being essentially the same among the groups
investigated. It is to be remembered that the patients
were referred because of TIAs, not because of any
signs of intellectual impairment.
cal cerebral (embolic) ischemic episodes even though
no neurological symptoms were reported. This suggestion is supported by the finding of 2 patients with
completed cerebral infarcts and 7 with cerebral atrophy among the 11 atherosclerotic controls.
At the postoperative test session the ICA patients
Table 3. Median Preoperative and Postoperative Cerebral Blood Flwu Hemisphere Values (Quartiles in Parentheses)
in Patients According to Degree of Cerebral Atrophy and Controls"
CBF after Operation
(mV100 g d m i n )
CBF before Operation
(mV100 g d m i n )
Carotid patients
No atrophy (n = 12)
Slight atrophy (n = 11)
Definite or severe atrophy (n = 8)
Controls (n = 1 1 )
Left Hemisphere
h g h t Hemisphere
Left Hemisphere
Right Hemisphere
(49- 5 7)
(50-6 1)
(50- 5 6 )
(49-5 7 )
(47-5 5 )
"There were no statistically significant differences within or between the groups.
cerebral blood flow.
18 Annals of Neurology
Vol 20
N o 1 July 1986
How is the improved intellectual function after endarterectomy to be explained? Although there was a
trend that patients with more pronounced cerebral atrophy improved more than others, the improvement
was by no means confined exclusively to patients with
structural brain damage. Hence at least in patients with
a relatively mild deficit, the structural findings are not
discriminative as far as postoperative intellectual improvement is concerned. This conclusion is not very
surprising and thus we may turn to the more controversial question of whether hemodynamic variables
may predict or relate to the psychometric findings.
From the findings in this study, the answer is no. In
only 2 patients did the perfusion of a focal low-flow
area increase after endarterectomy, and the changes in
intellectual functions in these 2 patients were nor at
the extremes of the groups investigated. A frequency
of such “hemodynamic cases” of about 10% is in
agreement with the original peroperative findings of
Boysen { S } . Although neither structural nor hemodynamic factors revealed on CBF tomograms explain the
intellectual improvement, such improvemenr did occur
in the ICA patients and not in the controls.
One explanation for intellectual improvement might
be that it reflects the natural course after termination
of a series of TIAs. It is thus possible that cessation
of microembolization may normalize cerebrovascular
reactivity, leading to improved brain function. An
evaluation of this possibility would require randomization of patients to either operation or antithrombotic
drug treatment, a project that has not been feasible.
Recent investigation by positron emission tomography has shown that the main physiological change
induced by operation for occlusive carotid artery disease is a reduction in cerebral blood volume as a result
of an improved perfusion pressure [9}. Whether a
reduction of cerebral blood volume correlates with
postoperative intellectual improvement awaits further
Whatever the correct explanation might be, the
present findings indicate that reconstructive vascular
operation may stop or delay intellectual deterioration
caused by atherosclerotic neck vessel disease. The
main indication for carotid operation still is prevention
of major stroke. However, the possibility of interfering with the more subtle process of progressive multiinfarct lesions [lo] by means of operation has gained
further support.
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Hernrningsen et al: Cognitive Function and C a i x i d Surgery
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flow, carotid, patients, transiente, ischemia, attack, surgery, cognitive, function, cerebral, blood
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