Stroke. 1995;26:1817-1819
(Stroke. 1995;26:1817-1819.)
© 1995 American Heart Association, Inc.
Photoreactive Flow Changes in the Posterior Cerebral Artery in Control Subjects and Patients With Occipital Lobe Infarction
P.P. Urban, MD;
A. Allardt, MD;
B. Tettenborn, MD;
H.C. Hopf, MD;
S. Pfennigsdorf, MD
W. Lieb, MD
From the Neurologische Klinik (P.P.U., A.A., B.T., H.C.H.) and the
Augenklinik der Johannes GutenbergUniversität Mainz (S.P., W.L.)
(Germany).
Correspondence to Dr Peter P. Urban, Neurologische Klinik und Poliklinik, Langenbeckstr 1, D 55101 Mainz, Germany.
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Abstract
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Background and Purpose Photoreactive flow changes of the
posterior
cerebral artery (PCA) in control subjects and patients with
unilateral
occipital lobe infarction were investigated to study the
hypothesis
that occipital lobe infarction of varying extent leads to a
reduced
visually activated flow increase in the ipsilateral
PCA.
Methods Maximum mean flow velocity (MFV) of the PCA was
investigated by transcranial Doppler sonography after
photic stimulation of the retina.
Results In 25 control subjects MFV was increased by 30.6±9.7%.
In 13 patients with unilateral occipital lobe infarction the
ipsilateral MFV increase was significantly lower than in control
subjects. Nine patients with homonymous hemianopsia showed an
ipsilateral MFV increase of 3.4±4.1% (P<.001) and four
patients with incomplete occipital lobe infarction and homonymous
quadrantanopsia had an MFV increase of 16.0±12.8%
(P<.05).
Conclusions We conclude that photoreactive flow changes of the
PCA represent a noninvasive and reliable measure of functional
impairment due to occipital infarction.
Key Words: cerebral arteries cerebral infarction ultrasonics hemianopsia quadrantanopsia
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Introduction
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Aaslid
1
first described blood flow velocity changes by TCD
in the PCA under
visual stimulation. In the present study, photoreactive
flow
changes of the PCA and MCAs in control subjects and patients
with
unilateral occipital lobe infarction were investigated
to study the
hypothesis that occipital lobe infarction of varying
extent leads to a
reduced visually activated flow increase in
the ipsilateral
PCA.
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Subjects and Methods
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The MFVs of both PCAs (P2 segment) and MCAs were measured with
a
transcranial 2-MHz pulsed Doppler device (Medasonics
II) in
25 healthy volunteers and 13 consecutive patients with
unilateral
occipital lobe infarction (10 left, 3 right). In 1
additional
healthy subject and 2 additional patients we attained no
reliable
Doppler signal because of an insufficient ultrasonic bone
window.
MFV was calculated by spectral analysis with fast
Fourier transformation.
All measurements were performed in a dark,
silent room while
each subject's eyes were closed. After an adaptation
period
of 10 minutes and immediately before stimulation onset, the
mean
of three consecutive MFV measurements per second was calculated
and
taken as the base. Photic stimulation was performed with
a strobe lamp
placed at a distance of 0.3 m in front of the
subjects. Flash light
stimuli (duration, 10 µs/flash;
power, 0.7 J/flash; Strobotest II) at
a continuously increasing
frequency from 1 to 20 Hz were applied over a
period of 10 seconds.
Immediately after stimulation each subject was
asked to close
his or her eyes. Maximal and minimal MFV values were
documented,
and the percent maximum increase (

1) and percent maximum
decrease
(

2) during and after photic stimulation were calculated.
The
sequence of insonation (PCA or MCA, left or right side) had
no
influence on the results. The structural and functional deficits
were
established in all patients on the basis of cranial CT
or MRI and
static Humphrey perimetry. Student's paired
t test
and the
Mann-Whitney-Wilcoxon
U test were applied for
statistical
analysis.
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Results
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Prestimulus MFV values for the PCA (Fig 1

) in
control subjects
were 35.6±6.0 cm/s and 36.7±5.8 cm/s on the right
and
left side, respectively. The MFV increased (

1) by 30.6±9.7%
during
photic stimulation and in the poststimulus period decreased
(

2)
by 33.6±11.2% to values slightly below the base. Differences
between

1 and

2 and between the left and right PCA were not
statistically
significant.
In the 13 patients with unilateral occipital lobe infarction, the PCA
showed normal Doppler spectra, and the prestimulus MFV values of
the affected PCA (31.1±12.6 cm/s) and the unaffected side (30.8±10.1
cm/s) were not significantly different.
1 and
2 of the unaffected
PCA were not significantly different from the control values.
In the affected PCA of 9 patients with homonymous hemianopsia due to
occipital lobe infarction, visual stimulation induced only a small flow
increase (
1, 3.4±4.1%) (Table
). This increase was
significantly lower (P<.001) compared with the unaffected
side and control subjects. In 4 patients with partial occipital lobe
infarction and homonymous quadrantanopsia (2 each of the upper and
lower visual hemifields), the evoked flow increase (
1, 16.0±12.8%)
was significantly higher (P<.05) than that in the
hemianoptic patients but less than that in control subjects
(Table
).
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Table 1. Prestimulus Values and Increase and Decrease in MFV During and
After Photic Stimulation in the PCAs of Control Subjects and Patients
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In the MCA territory (Fig 2
), the MFV increase during
flash light stimulation (
1) of control subjects and patients (n=13)
was 7.1±8.9% and 3.0±5.2%, respectively, and the poststimulus
decrease (
2) was 6.3±9.0% and 4.1±6.0%, respectively. In 1
patient with hemianopsia, the macular region of the visual field defect
was spared but
1 (5%) and
2 (8%) of the ipsilateral MCA were in
the same range as in the MCA of the 8 patients with hemianopsia and
macular involvement (ipsilateral MCA
1, 5.5±3.9%;
2,
5.5±2.4%). The differences between control subjects and patients and
between the left and right MCAs of either group were not statistically
significant.
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Discussion
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Cerebral blood flow is coupled with brain metabolism
and brain
function.
2 Functional mapping studies with
positron emission
tomography
3 and MRI
techniques
4 showed a localized increase
in blood volume in
the primary visual cortex during photic stimulation.
Both methods have
a low temporal resolution and limitations
in quantifying the degree of
functional activation.
5 TCD allows
on-line monitoring
of blood flow changes. The relationship between
blood flow velocity and
blood volume within the large basal
intracranial arteries is linear if
alterations of the cerebrovascular
bed are restricted to the small
cortical resistance vessels.
6 TCD studies have shown that
changes in the diameter of the
large basal arteries are negligible and
intraindividual changes
in blood flow velocity reflect changes in
volume flow.
6 7 8
Photic stimulation induces a marked MFV increase in the PCA of healthy
subjects. Optimum differentiation between control subjects and patients
is achieved by administering intermittent light stimuli with a strobe
lamp. Stimuli frequencies ranging from 10 to 20 Hz have been shown to
be most effective.9 The maximum flow increase
(30.6±9.7%) was similar to that reported by Conrad and
Klingelhöfer,10 who used 10 Hzcheckerboard
stimulation (30.9±7.2%) and by Sitzer et al,11 who
showed a color video film (30.4±6.4%). Aaslid1 obtained
lower values by use of a whole-screen white-dark change causing
an on-off stimulus (16.4±1.5%). In the present study, the MFV
during photic stimulation rose steeply, reached a maximum flow increase
that was followed by a slight decrease, and continued as a plateaulike
velocity response. Thus, the maximum flow increase comprises an initial
"overshooting" due to autoregulative mechanisms.10
The subsequent MFV decrease to the "working level" is explained
by adaptation mechanisms of the retina and the visual
cortex.1
PCA lesions causing visual field defects significantly reduce the MFV
increase on the affected side during visual stimulation. The residual
MFV reactivity was considerably less in patients with complete
homonymous hemianopsia than in patients with quadrantanopsia. The
differences between the three groups (control subjects and
patients with quadrantanopsia and hemianopsia) were statistically
significant. We feel that impairment of photoreactive flow changes in
the PCA is a semiquantitative measure of the visual field defect. With
flashlight stimulation, the flow response is independent of the
patient's cooperation.
In 8 of 9 patients with complete unilateral occipital lobe infarction,
perimetry demonstrated that the homonymous hemianopsia also involved
the macular region. Therefore, the small MCA flow increase found
ipsilateral to the affected side cannot be explained by sparing
of the occipital pole, the blood supply of which is derived from
MCA branches in about 10% of subjects.12 As do Droste et
al,13 we suggest that the small bilateral MCA flow
increase is due to unspecific effects such as increased attention and
arousal evoked by photic stimulation.
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Selected Abbreviations and Acronyms
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| MCA |
= |
middle cerebral artery |
| MFV |
= |
mean flow velocity |
| PCA |
= |
posterior cerebral artery |
| TCD |
= |
transcranial Doppler sonography |
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Received May 11, 1995;
revision received June 30, 1995;
accepted July 3, 1995.
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