Magnetic resonance imaging in experimental subarachnoid haemorrhage.

Abstract

We developed an MRI protocol to measure cerebrovascular diameter and blood flow velocity, and if we could detect cerebrovascular alterations after SAH and their impact on cerebral ischaemia.
SAH was induced in 15 Wistar rats by means of the endovascular filament method; 6 other rats served as control. MRI measurements were performed on a 4.7T NMR spectrometer 1 and 48 hours after SAH and 9 days thereafter. Diffusion-weighted and T2-weighted images were acquired to detect cerebral ischaemia. The arterial spin labelling method was used to measure CBF. MR angiography was used to measure vessel diameter and blood flow velocity, from which the arterial blood flow was calculated.
The ischemic lesion volume increased between 1 and 48 hours after SAH from 0.039 to 0.26 ml (P = 0.003). CBF decreased from 53.6 to 39.1 ml/100 g/min. The vessel diameter had narrowed, the blood flow velocity diminished as did the arterial blood flow in most vessels, but only the vasoconstriction in the right proximal ICA reached significance (0.49 mm to 0.43 mm, P = 0.016). Baseline values were restored at day 9.
We showed that it is feasible to detect alterations of in-vivo vessel diameter and blood flow velocities and their consequences for brain damage after experimental SAH in the rat. The growth of the infarct volume between day 0 and 2 after SAH and the parallel vasoconstriction suggest that delayed cerebral ischaemia after SAH occurs in rats and that this may be caused by vasoconstriction.

Full text

Page 1

Magnetic resonance
imaging in
experimental
subarachnoid
hemorrhagelesion volume after
experimental
subarachnoid
hemorrhage
van den Bergh WM
Schepers J
Veldhuis WB
Nicolay K
Tulleken CAF
Rinkel GJE
Submitted
Chapter 4 Role of magnesium
in the reduction of
ischemic
depolarization and
van den Bergh WM,
Zuur JK,
Kamerling NA,
van Asseldonk JT,
Rinkel GJE,
Tulleken CAF,
Nicolay K
J Neurosurg. 2002 Aug;97(2):416-22
Chapter 5

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Objective
Ischemia-induced tissue depolarizations probably play an important role in
the pathophysiology of cerebral ischemia caused by parent vessel occlusion. Their
role in ischemia caused by subarachnoid hemorrhage (SAH) remains to be inves-
tigated. The authors determined whether ischemic depolarizations (IDs) or corti-
cal spreading depressions (CSDs) occur after SAH, and how these relate to the
extent of tissue injury measured on magnetic resonance (MR) images. In addi-
tion, they assessed whether administration of MgSO4reduces depolarization time
and lesion volume.
Methods
By means of the endovascular suture model, experimental SAH was induced
in 52 rats, of which 37 were appropriate for analysis, including four animals that
underwent sham operations. Before induction of SAH, serum Mg2+levels were
measured and 90 mg/kg intravascular MgSO4or saline was given. Extracellular
direct current potentials were continuously recorded from six Ag/AgCl elec-
trodes, before and up to 90 minutes following SAH, after which serum Mg2+
levels were again measured. Next, animals were transferred to the MR imaging
magnet for diffusion-weighted (DW) MR imaging. Depolarization times per elec-
trode were averaged to determine a mean depolarization time per animal.
Results
No depolarizations occurred in sham-operated animals. Ischemic depolariza-
tions occurred at all electrodes in all animals after SAH. Only two animals dis-
played a single spreading depression-like depolarization. The mean duration of
the ID time was 41 ± 25 minutes in the saline-treated controls and 31 ± 30
minutes in the Mg2+-treated animals (difference 10 minutes; p = 0.31).
Apparent diffusion coefficient (ADC) maps of tissue H2O, obtained using DW
images approximately 2.5 hours after SAH induction, demonstrated hypointensi-
ties in both hemispheres, but predominantly in the ipsilateral cortex. No ADC
abnormalities were found in sham-operated animals. The mean lesion volume, as
defined on the basis of a significant ADC reduction, was 0.32 ± 0.42 ml in
saline-treated controls and 0.11 ± 0.06 ml in Mg2+-treated animals (difference
0.21 ml; p = 0.045). Serum Mg2+ levels were significantly elevated in the Mg2+
-treated group.
Conclusions
On the basis of their data, the authors suggest that CSDs play a minor role, if
any, in the acute pathophysiology of SAH. Administration of Mg2+reduces the
cerebral lesion volume that is present during the acute period after SAH. The
neuroprotective value of Mg2+after SAH may, in part, be explained by a reduc-
tion in the duration of the ID of brain cells.
Chapter 5
56

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Abbreviations
ADC = apparent diffusion coefficient, CA = carotid artery, CBF = cerebral
blood flow, CSD = cortical spreading depression, DC = direct current, DW = dif-
fusion-weighted, ICP = intracranial pressure, ID = ischemic depolarization, MCA
= middle cerebral artery, MR = magnetic resonance, NMDA = N-methyl-D-
aspartate, SAH = subarachnoid hemorrhage, SD = standard deviation
Much research has been devoted to the treatment of SAH and its complications,
but these have only led to modest improvements in overall outcome.19The lack
of a major improvement is explained by the initial impact of the hemorrhage,
which is responsible for 15% of cases of immediate death from SAH and is also a
major cause of overall morbidity and mortality.4,24,35The initial impact of
aneurysm rupture is probably also associated with the occurrence of secondary
ischemia after SAH, because parameters of the impact of the initial bleeding (that
is, the amount of blood shown on the computerized tomography scan, duration
of unconsciousness, and clinical condition at admission) are the most important
predictors of secondary ischemia.16-18,20The pathophysiology of acute cerebral
damage after SAH remains largely unclear. Increased insight into this pathophysi-
ology may help curb the effects of the initial ischemia and improve prevention
and treatment of secondary ischemia.
57
Figure. 1 Artist’s illustration demonstrating the positions of the six DC-coupled surface electrodes
on the rat skull. The parietal electrodes (1 and 6) were placed 1.5 mm posterior to the bregma and
3 mm under the temporal line, the occipital electrodes (2 and 5) were placed 4.5 mm posterior and
5 mm lateral to the bregma, and the electrodes on the hindlimb area of the cortex (3 and 4) were
placed 1.5 mm posterior and 2 mm lateral to the bregma). Subarachnoid hemorrhage
was induced on the left side (electrodes 4–6). This figure was adapted with permission from
Paxinos GT, Watson C: The Rat Brain in Stereotaxic Coordinates, ed 3. Orlando, FL: Academic
Press, 1996.

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Shortly after SAH there is a decrease in CBF.14In experimental models of SAH
the decreased CBF has been linked to a rapid and transient increase in ICP in the
initial phase following hemorrhage but after this period CBF is decreased and
thus, probably, the period of raised ICP is accompanied and followed by acute
vasoconstriction, which is independent of changes in ICP and perfusion
pressure.3, 23In patients, this period of diminished CBF is reflected by a period
of unconsciousness, and can lead to brain infarction.39
A recent study in which a fast echoplanar MR imaging diffusion sequence was
used found a sometimes transient decline in the ADC of H2O during the hypera-
cute phase of an endovascularly induced SAH in the rat, indicating that CSDs
occur after SAH.1, 5In the case of focal cerebral ischemia, CSDs are known to
represent undulating changes in extracellular K+ concentration, which occur in
the border zone of evolving brain infarctions and are believed to play a role in
the development of the infarction.6, 21, 26Diffusion-weighted MR imaging is an
important tool in stroke research, because of its ability to visualize ischemic tis-
sue shortly after disease onset. Ischemia is accompanied by a reduction in the
ADC of brain-tissue H2O, which results in regional hypointensities on quantita-
tive H2O ADC maps.
The aim of this work was to measure the DC potential in the rat cortex to deter-
mine whether IDs or CSDs occur after SAH, and to assess lesion volume during
the acute phase following SAH by MR imaging performed shortly after DC
potential measurements. In view of the role of IDs in the pathophysiology of
ischemic stroke, we considered it of interest to examine the possible correlation
between depolarization time and the volume of the lesion as it is depicted on
MR images. Moreover, we studied the effect of MgSO4on CSDs, depolarization
time, and lesion volume following SAH, because we have previously shown that
MgSO4reduces the frequency of CSDs in a rat model of artificially evoked CSD.38
Chapter 5
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Materials and Methods
Animal Preparation
The experiments were performed in 52 male Wistar rats, each of which
weighed between 300 and 380 g. Anesthesia was induced by administering a
subcutaneous injection of a mixture of 0.55 ml/kg fentanyl citrate (0.315
mg/ml), fluanisone (10 mg/ml), and 0.55 ml/ kg midazolam (5 mg/ml). After
transoral intubation had been initiated, anesthesia was maintained by administra-
tion of 0.8% halothane in a 70:30 gas mixture of N2O/O2 and artificial ventila-
tion was regulated at a rate of 30 breaths/minute. The tidal CO2was
continuously monitored and kept within physiological boundaries. Body temper-
ature was maintained at 37 ± 0.5°C by means of a feedback-controlled heating
pad.
The right femoral artery was cannulated with polyethylene (PE-50) tubing for
continuous blood pressure recording and a continuous supply of saline to pre-
vent dehydration, as well as to obtain blood samples for serum Mg2+analysis
before and 90 minutes after SAH induction. Saline or MgSO4was administrated
intravenously as a bolus injection via tail infusion.
A midline incision was made to expose the skull. Six 1.5-mm burr holes were
drilled into the skull, as schematically displayed in Fig. 1, at the parietal,
hindlimb, and occipital cortex areas of the ipsilateral and contralateral hemi-
spheres.30We kept the dura mater intact. After the periosteum had been
removed, the burr holes were covered with an acrylic dental cement mould fixed
with adhesive to anchor the electrodes within the skull. This same mould was
used repeatedly in all rats. One-millimeter-diameter pellet Ag/AgCl electrodes
were enclosed in polyethylene tubes measuring 10 mm long and 1 mm in diam-
eter, which were filled with 0.9% NaCl in agar. Electrical continuity between tis-
sue and electrodes was established by applying electrode cream. The signal from
a bare Ag/AgCl electrode placed in the neck musculature served as a reference.
A ground electrode was connected to the ground of an electric socket.
Induction of SAH
Subarachnoid hemorrhage was induced by advancing a sharpened No. 3.0
Prolene suture through the ligated left external CA and distally through the
internal CA until the suture perforated the intracranial bifurcation of the internal
CA, after which the suture was quickly redrawn. This technique was previously
described by Bederson, et al.,2and Veelken and associates40and is a modification
of the endovascular suture model used for MCA occlusion. A brief decrease in
blood pressure followed by a short spell of raised blood pressure gave us confi-
dence that SAH had been induced. Four rats underwent a similar procedure,
59

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including DC potential recording; however, in these animals, the Prolene thread
was kept in place for less than 2 minutes and the vessel was not perforated
(sham-operated group).
Drug Treatment and Experimental Groups
Animals were randomly chosen for pretreatment with MgSO4or saline,
which was administered 15 minutes before SAH as an intravenous bolus via tail
infusion. Of the 37 surgically treated animals that were used for data analysis, 14
were pretreated with 90 mg/kg MgSO4(treatment group) from a 100 mg/ml
solution, 19 received 0.3 ml of saline (control SAH group), and four animals
underwent sham operation and received saline as well (sham-operated group).
Recording of DC Potentials
Direct current potentials were recorded before and up to 90 minutes follow-
ing SAH by using a homemade six-channel electrometer-amplifier. Amplifier
outputs were transferred to a personal computer and, after analog-to-digital
conversion, were evaluated with the aid of a commercially available technical
graphing software package.
After 90 minutes of recording, the electrodes were removed and the animal was
transferred to the MR imaging magnet.
Chapter 5
60
Figure 2. Typical DC potential recording showing ID on all six electrodes. Electrodes 4 through 6
(E4–E6) are ipsilateral (left sided).

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Magnetic Resonance Imaging Experiments
During the MR imaging protocol, anesthesia and maintenance of physiological
parameters were achieved in the same fashion as that described earlier. The MR
imaging measurements were obtained approximately 2.5 ± 0.5 hours after SAH
induction by using a 4.7-tesla nuclear MR spectrometer equipped with a gradient
insert of up to 220 mT/m. Each animal's head was fixed in a stereotactic holder.
A Helmholtz volume coil (85 mm diameter) was used for signal excitation,
whereas an inductively coupled 20-mm-diameter surface coil was used for signal
acquisition.
Multislice coronal spin-echo DW images were acquired with a single-shot, trace
ADC sequence (128 ± 64 data matrix, TR 2500 msec, TE 100 msec, and five b
values 1781 seconds/mm).8Nine contiguous 1.5-mm slices located between the
cerebellar sulci and olfactory bulb were imaged. Calculation of quantitative brain
maps of the ADC was performed by monoexponential fitting.
Postmortem Examination
Following the MR imaging study, the animals were killed by administration
of 5% halothane and their brains were exposed and inspected for hemorrhage.
The amount of blood was examined for each hemisphere separately, using scores
ranging from 0 to 3; the total blood score thus could range from 0 (no SAH) to
6 (massive bilateral bleeding).
Statistical Analysis
We measured the duration of depolarizations on a personal computer by
using technical graphing software. As a starting point, the beginning of the
enduring depression of the DC potential baseline was taken. This time point
coincided with the disappearance of the electroencephalographic signal, which
was visible as a high-frequency modulation superimposed on the DC potential
reading. Repolarization was assumed to be complete when the electroencephalo-
graphic signal appeared again and the DC potential had returned as a horizontal
line. Due to a slow electrical drift in the measured signal, this did not necessarily
occur at the same millivolt level as the pre-SAH baseline recording. The dura-
tions of depolarizations of the six electrodes were averaged to obtain the mean
depolarization time per animal. The mean depolarization time was used to relate
the electrophysiological results to the MR imaging data and to test the hypothesis
that Mg2+reduces the severity of the IDs. The amplitude of the DC potential
deflection associated with ID was not taken into account, because this parameter
varied strongly between experiments. The CSDs were defined as depolarizations
with a spreading velocity of approximately 3 mm/minute and a duration of
approximately 1 to 2 minutes.
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We analyzed parametric ADC images by using an image-analysis software pack-
age. The area of the acute ischemic lesion was calculated from the ADC maps by
thresholding, using mean ADC values of sham-operated animals (0.76 ± 0.08
10-3 mm2/second).11, 34An ADC value was considered pathological if it was
two SDs below the mean ADC level of brain-tissue H2O in sham-operated ani-
mals. We determined total lesion volume as well as lesion volume in the ipsilat-
eral (left) and contralateral (right) hemispheres.
We used the independent-samples t-test to compare means for the control SAH
group and the Mg2+-treated group. Data are presented as means SDs. Linear
regression analysis was used to correlate lesion volumes with electrophysiological
data. Data are presented as correlation coefficients with significance levels; proba-
bility values lower than 0.05 were considered significant.
Sources of Supplies and Equipment
Bison Kit Powerglue, purchased from Bison International (Goes, The
Netherlands) was used to anchor the electrodes. The Ag/AgCl electrodes were
manufactured by Harvard Apparatus, Inc. (South Natick, MA). Redux electrode
cream was obtained from Hewlett-Packard Co. (Palo Alto, CA). The technical
graphing software used to evaluate DC potential recordings was KaleidaGraph
version 3.0.9, which was developed by Synergy Software (Reading, PA). The
nuclear MR spectrometer and the ImageBrowser image-analysis software package
were acquired from Varian (Palo Alto, CA).
Results
General Results
Fifty-two rats underwent surgery. In three animals we failed to achieve SAH.
One animal in the control SAH group had an inexplicably high serum Mg2+level
(5.38 mmol/L) before SAH was induced, and was excluded from further analy-
sis. Two animals died of major bleeding from the femoral artery or the CA. Nine
animals (four pretreated with Mg2+) died before we were able to complete the
MR imaging measurements. Data analysis was confined to the 37 animals in
which we completed electrophysiological and MR imaging measurements, among
which there were four sham-operated animals.
Serum Mg2+Levels
The mean pretreatment serum Mg2+level in all animals was 0.91 mmol/L.
There was no difference in the pretreatment serum level of Mg2+between the
two groups. Ninety minutes after SAH induction, the Mg2+level in the control
SAH group was 0.88 mmol/L and the level in the treatment group was 1.29
mmol/L, which was 47% higher (p = 0.001).
Chapter 5
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Direct Current Potentials
Within 10 to 20 seconds after SAH induction, all electrodes simultaneously
demonstrated a small decline in the DC potential, which recovered in 20 to 30
seconds in all cases. In all animals this was followed by depolarization within 1
minute after SAH induction, as indicated by the massive decline in the DC
potential.
Except for one animal, in which the electrodes in the contralateral parietal and
occipital regions did not show a depolarization, all electrodes were involved in
depolarization in all animals. A typical example of a DC potential recording is
shown in Fig. 2. In 19 (58%) of the 33 animals with SAH the decline in the DC
potential was first seen at the ipsilateral parietal electrode, but in 12 animals
(36%) it was observed at several electrodes simultaneously. In 17 animals (52%)
depolarization started ipsilaterally, in nine animals (27%) contralaterally, and in
seven animals (21%) in both hemispheres at the same time. The mean time until
all electrodes were depolarized was 110 seconds, ranging from 0 to 666 seconds,
and was similar for the control SAH group (111 seconds) and the Mg2+-treated
group (120 seconds).
In 26 (79%) of 33 animals repolarization of the DC potential occurred at all
electrodes during the 90-minute observation period. We found a CSD-like depo-
larization, spreading 3 mm/minute from one electrode to the others in only two
animals, both of which belonged to the control SAH group. In both instances,
however, this occurred after repolarization of the ID.
63
Figure 3. Apparent diffusion coefficient maps from DW MR images obtained in three different
animals demonstrating ADC reductions in the cortex of both hemispheres (upper), in the ipsilateral
cortex (center; most common), and in cortical plus hippocampal areas (lower).

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Magnesium treatment led to a reduction in depolarization time for all electrodes.
The mean depolarization time for both hemispheres was 31 minutes (range 1-89
minutes) in the treatment group and 41 minutes (range 5-89 minutes) in the
control SAH group (difference 10 minutes [25%], p = 0.31). In the ipsilateral
hemisphere, the mean depolarization time was 35 minutes (range 1-90 minutes)
in the Mg2+-treated group compared with 45 minutes (range 7-90) minutes in
the SAH control group (difference 10 minutes, p = 0.4). In the contralateral
hemisphere, the mean depolarization time was 27 minutes (range 1-89 minutes)
compared with 37 minutes (range 3-89 minutes) in the SAH control group (dif-
ference 10 minutes, p = 0.2).
Lesion Volumes on DW MR Images
Following the DC potential recordings, DW MR imaging was performed 2.5 ±
0.5 hours after SAH induction to measure the volume of tissue at risk for irre-
versible injury. Typical examples of ADC data are displayed in Fig. 3. The water
ADC maps often showed hypointensities at multiple variable locations, but pre-
dominantly in the ipsilateral cortex. All animals subjected to SAH were found to
harbor lesions in both hemispheres. In 10 animals (30%) lesion volume was
even larger on the contralateral side than on the ipsilateral side. In circumscribed
lesions the mean ADC value (0.54 ± 10-3 mm2/second) was comparable with
that of reported values in infarction areas after MCA occlusion (0.49 ± 10-3
mm2/second).11 As expected, sham-operated animals did not display any ADC
abnormalities.
The mean ( SD) total lesion volume in the saline-treated control animals was
0.32 ± 0.42 ml and the mean volume in Mg2+-treated animals was 0.11 ± 0.06
ml, a reduction of 66% (p = 0.045, 95% confidence interval 0.005-0.4) [Fig. 4].
Chapter 5
64
0.50
0.40
0.30
0.20
0.10
Total lesion volume (cc)
controls
magnesium-
treated
Figure 4. Bar graph demonstrating that lesion volume, as deduced from DW MR images, is reduced
in the Mg -treated group. Error bar represents confidence interval for mean.

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In the ipsilateral hemisphere the reduction was 67% (from 0.18 ± 0.22 ml in the
SAH control group to 0.06 ± 0.04 ml in the treatment group; p = 0.037, 95%
confidence interval 0.008-0.2), and in the contralateral hemisphere the reduction
was 64% (0.14-0.05 ml, p = 0.068).
Correlation Between Depolarization Time and Lesion Volume on DW MR
Images
The Pearson correlation coefficient between depolarization time and lesion vol-
ume was 0.39 (p = 0.025) [Fig. 5]. When one considers only the ipsilateral side,
the correlation coefficient increased to 0.46 (p = 0.007).
Postmortem Findings
There was no evidence of SAH in any sham-operated animal. In all other ani-
mals, however, extensive SAH was identified, with blood distributed around the
circle of Willis and a thin layer overlying the cortex and around the brainstem
[Fig. 6]. The amount of subarachnoid blood was equal in the two groups in
which SAH was induced.
65
Saline
Magnesium
Lesion volume (cc)
-5 102540-557010085
-0.1
0.7
0.9
0.1
0.3
0.5
1.3
1.5
1.1
Depolarization time (min)
Figure 4. Graph demonstrating correlation between lesion volume and depolarization time.

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Discussion
Our data demonstrate that prolonged depolarizations occur immediately after
SAH and that the duration of these depolarizations is related to the extent of
ischemic lesions observed on MR images. Moreover, we found that pretreatment
with Mg2+reduces the extent of the ischemic lesions.
Direct current potential deflections can be divided into two types: CSDs, which
are inherently transient in nature and have a very characteristic migration pat-
tern, and IDs, which, depending on regional perfusion status, may be transient or
permanent.29The spreading velocity, duration, and pattern of depolarizations in
our study are typical of IDs. Cortical spreading depressions were rarely detected
in our study and, therefore, seem to be of minor importance for the develop-
ment of ischemic lesions during the acute phase following SAH. Two other stud-
ies in which DC potential recording was used after acute SAH detected
depolarization and transient depression in electrocortical activity, but the method
used to imitate an SAH was direct application of blood or artificial cerebrospinal
fluid containing the hemolysis products K+and hemoglobin on the cerebral cor-
tex.12, 22Using the same endovascular filament method to induce SAH as we did,
Beaulieu and associates1 detected CSD in an indirect manner, namely by the
occurrence of transient ADC reductions in the ultraacute phase following SAH.
Nevertheless, ADC changes can occur before membrane depolarization and thus it
is not certain that these transient reductions are actually CSDs.23
The absence of CSDs after SAH may be explained by the extent of hypoxia. The
ischemic lesions that occur after SAH are the result of a global hypoxia, in con-
trast with the regional area of hypoperfusion with penumbra seen in models of
MCA occlusion, from which these CSDs are considered to originate. The global
ischemia and absence of a zone of marginally perfused tissue probably preclude
the occurrence of CSDs in the acute phase following SAH. The IDs that we
observed demonstrate that the intensity of the ischemia is sufficient to induce
CSDs. The appearance of depolarizations in both hemispheres indicates that the
impact of SAH from a ruptured artery affects the whole brain and not only the
region of the ruptured artery. This is also supported by our finding that the
lesion volume on the contralateral side was almost identical to that on the side
of the ruptured artery.
Cerebral arteries have been shown to respond to SAH with a biphasic constric-
tion pattern. An acute constriction begins within minutes after the bleeding,
whereas delayed vasospasm develops 48 hours later.9Although the significance of
delayed vasospasm for ischemic brain damage after SAH is recognized, the con-
tribution of acute vasoconstriction is less clear.36, 37The CBF decreases rapidly
and ischemic injury occurs after SAH in both experimental and clinical studies.
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Bederson and colleagues3described a pattern in which CBF reduced to 16.5% of
baseline within 5 minutes and remained reduced to 44% 60 minutes after exper-
imental SAH. This reduction in CBF is initially due to raised ICP, but later is
accompanied and followed by acute vasoconstriction of large cerebral arteries.
The duration of the ID we found is in line with the aforementioned pattern of
CBF changes, which recovered to values above the ischemic threshold within 60
minutes.27
Magnesium treatment led to a reduction in lesion volume, although its effect on
the duration of the depolarizations was not statistically significant. This might be
caused by the fact that measurements of DC potentials were restricted to 90
minutes after SAH induction. At this time point, persistent depolarization was
found on some electrodes in six animals in the control group and in only one in
the Mg2+-treated group. This difference in the distribution of the two groups can
partly be obviated by the use of a nonparametric (Mann-Whitney) test. When
this test was used, Mg2+therapy led to a reduction in the duration of depolariza-
tion time that was nearly significant (p = 0.053).
Depolarization time, as detected by DC potential measurements, was found to
correlate with lesion volumes on DW MR images. Similar observations have been
made in rat models of ischemic stroke, indicating that IDs may play a role in the
pathophysiology of SAH.10
Our data support the concept that acute ischemia-induced reductions in the ADC
primarily reflect cellular swelling (that is, cytotoxic edema), because the depolar-
ization-induced ion shifts are accompanied by intracellular H2O accumulation.15,
21, 28, 41
Magnesium is readily available and inexpensive, and has a well-established clini-
cal profile in obstetric and cardiovascular practice. It is a promising agent for
suppressing cell membrane depolarization during ischemia, and it passes the
blood-brain barrier.13Neuroprotective mechanisms of Mg2+include inhibition
of the release of excitatory amino acids and blockade of the NMDA-glutamate
receptor.33 The excitatory amino acid glutamate is released in excess during
brain ischemia and is a reliable predictor of outcome in experimental SAH.3The
NMDA receptors are activated by glutamate and other excitatory amino acids,
after the voltage-dependent removal of channel-blocking Mg2+. The influx of Ca
ions via the NMDA receptor is an important mechanism in the pathogenesis of
ischemic cerebral injury.7Magnesium delays anoxic depolarization, in contrast to
the potent NMDA-receptor antagonist MK-801.38This implies that Mg2+does
not exert its anoxic depolarization-postponing effect through NMDA receptor
blocking alone. Magnesium is also a noncompetitive antagonist of voltage-
dependent Ca++channels, displays cerebrovascular dilatory activity,31can reverse
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delayed cerebral vasospasm after experimental SAH in rats,32and is an important
cofactor of cellular adenosine triphosphatases, including the Na+/K+- adenosine
triphosphatase.
Our results on the neuroprotective value of Mg2+are in agreement with those of
others who reported reduction in infarction volume in rats that were pretreated
with 90 mmol/L MgSO4before being subjected to 1.5 hours of MCA occlusion.25
In patients with SAH, not only initial ischemia but also secondary ischemia plays
an important role. The pathogenesis of secondary ischemia remains to be eluci-
dated. In our model, which closely resembles aneurysm rupture in humans, the
mortality rate from the initial bleeding was 20%, which is lower than expected.
Because of the relatively low case fatality rate, this model probably can also be
used for long-term survival experiments after SAH to study delayed ischemia
associated with SAH. The aim of the present experiments was to investigate
whether this model is suitable to test the effect of promising neuroprotective
agents by means of both electrophysiological and MR imaging data. The possibil-
ity of detecting an effect is maximum when this medication is given before hem-
orrhage occurs; however, this design is not in accordance with the clinical
situation. The results of this study show that our method of inducing SAH is
valid. To approach the clinical situation, additional studies are needed. These
should include extended observation periods after the SAH and administration of
Mg2+after induction of SAH.
Acknowledgments:
Gerard van Vliet is gratefully acknowledged for his expert technical assistance.
Chapter 5
68
Figure 6. Basal view of rat cerebrum after a small SAH.

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