Neurosurg. Rev.
16 (1993) 71-77

Somatosensory and auditory brainstem evoked potentials
during choroid plexus angioma removal
Alfred Witzmann~, Affred Huber ~, Friedrich Leblhuber 2, and Johannes Fischer~
INeurosurgical Department, 2Department of Gerontopsychiatry, Wagner
Jauregg Hospital, Linz, Austria

Abstract
Intraoperative combined SEP and BAEP monitoring
provides a high level of safety for patients undergoing
high risk intrarcraniat surgical procedures. We performed this type of monitoring in a rare case of
choroid plexus angioma, embedded into the pulvinar
of the right thalamus, which was resected via a transcaliosal approach. The SEP pattern deteriorated during parietal brain retraction and thalamic dissection,
but never completely disappeared. The BAEP pattern
remained unchanged during the entire operation. The
patient suffered incomplete hemiparesis postoperatiw,'ly, but recovered completely within 10 days. After
this time the SEP pattern returned to normal. This
case points out that pathological change of intraoperative SEP pattern without loss of components
suggests an incomplete hemiparesis. There is, however, a good chance of full recovery in the early
postoperative period.

Keywords: Arteriovenous malformations, auditory
brainstem evoked potentials, choroid plexus angiomas, intraoperative monitoring, somatosensory
evoked potentials.
'1 Introduction
Intraventricular vascular disorders such as aneurysms [13] and arteriovenous malformations
[17] occur only rarely. Only four of 500 patients
in YASAROm'S series suffered from ehoroidal
plexus AVM's [17].
The most frequent clinical symptoms of these
detbrmations are sudden onset of severe headache, vomiting, and, sometimes, meningism
and unconsciousness [15] similar to that caused
9 1993 by Walter de Gruyter& Co. Berlin, New York

by subarachnoid hemorrhages [4]. CT shows
intraventricular hemorrhage in most cases, according to the location of the lesion [3, 6, 12,
13, 15, 17].
Supratentorial intraventricular lesions can be
approached either transcortically or transcallosally. Cortical incisions can be made in the
region of the middle temporal gyms [6, 15] or
at the parietal lobe near the midline [3, 4, 5].
Approach via partial temporal lobe resection
[6] or resection of latero-basal parts of the
temporo-occipital region [5] has also been described. YASARGILprefers an approach through
the surface of the precuneus [17]. A further
possibility of approaching the lateral ventricle
can be provided by transcallosal dissection [13].
Regardless of the surgical approach chosen,
surgical treatment of intraventricular vascular
disorders remains a sophisticated and dangerous procedure which involves highly vulnerable
structurs.
A case of small intraventricular AVM in the
trigone is reported. In this case intraoperative
monitoring by means of somatosensory (SEP)
and auditory brainstem (BAEP) evoked potentials greatly reduced the risks of operation.

2 Case report and techniques
In the morning of June 13th, 1988, a 26-yearold pregnant woman suffered a severe attack
of headache followed by vomiting. A few hours
later somnolence developed and the patient was

72

Witzmann et al, Evoked potentials in angioma removal

Figure 1. a: CT presents a small AVM in the right trigone, embedded into the pulvinar of the thalamus.

b: Postoperative CT control: small hypodense area in the former AVM localization.

Figure 2. Preoperative angiography: smaU AVM in the right trigone fed by the medial posterior choroidal
artery. The arrow indicates the AVM. a: ap angiogram, b" lateral angiogram.

admitted to a hospital. The patient was transferred to our d e p a r t m e n t on the evening o f
June 13th, 1988, because o f further clinical
deterioration. U p o n admission she showed
9 signs of severely impaired consciousness, un-

directed reaction to pain stimulus, small pupils
with reduced reaction to light, and a mild tendency to flexion position in the upper extremities. There were no signs o f meningism or
m o t o r neurologic deficit. CT revealed a severe
Neurosurg. Rev. 16 (1993)

Witzmann et al, Evoked potentials in angioma removal

73

Figure 3. Postoperative angiography: the AVM has been completely removed, a: ap angiogram, b: lateral
angiogram.

intraventricular hemorrhage with blood clots
in all four ventricles and dilatation of the supratentorial ventricular system. Immediately
after diagnosis a bilateral frontal burr hole was
cult and ventricular drains were inserted on
both sides. The patient was then transferred to
the intensive care unit where pregnancy was
interrupted on June 27. CT and angiography
pe,rformed after absorption of blood showed a
small intraventricular haemangioma on the
right, which was embedded into the pulvinar
of' the thalamus (Figure la). The angioma was
fed by the medial posterior choroidal artery
(Figure 2). During the following weeks she
recovered completely. On July 26, a right parietooccipital craniotomy was performed and the
angioma was completely removed via a transcallosal approach (Figures lb, 3).

2.1 Monitoring procedure and results
Cerebral function monitoring during the surgical procedure was carried out by SEP (median and tibial nerve stimulation) and BAEP
recording. The median nerve was stimulated
simultaneously on both sides. This was folNeurosurg. Rev. 16 (1993)

lowed by alternating stimulation of the right
and left tibia1 nerves and alternating stimulation of the two ears. Electrodes for SEP recording were placed in a modified C3', C4',
and Cz pattern [16]. BAEP recording electrodes
were placed at A1 and A2. Reference electrodes
were placed at FPz (SEP recordings) and Cz
(BAEP recordings). Stimulation and recording
procedures have been described in detail elsewhere [16]. The basic "circular" monitoring was
adapted to the surgical situation and the structure involved. SEP recording in response to
tibial nerve stimulation was performed mainly
during dissection of cortical veins draining to
the sinus sagittalis superior and during parietal
brain retraction. The median nerve was usually
stimulated during thalamic angioma dissection.
A few BAEP recordings were also carried out
during this stage of surgery.
The amplitude of tibial nerve SEP's was depressed to a greater degree in the right hemisphere than in the left, even after induction of
anaesthesia. Parietal brain retraction and vein
dissection led to a considerable increase of amplitude depression as well as prolongation of
the latency of the P40 component (Figure 4).

74

Witzmann et al, Evoked potentials in angioma removal

08:40

LEFT TIBIAL NERVE

RIGHT TIBIAL NERVE

Cz/FP z

Cz/FP z

9

,: ;

ANESTHESIA INDUCED '. "

,l:

PARIETALRETRACTIoNBRAIN ~ ,71

'~

09:30

09:55

A

FURTHER

10:05

10:30

RETRACTION1r~ ~ ~

:ALLOSAL DISSECT ON~i' '~":' :1

ti-,,~.~.,2,2~.~,,..a~,,~e~2~],~,,q,~.:!VENTRICLE OPEN

~'~'~'"~,

i

BRAIN RETRACTION

I
12:50

i:.,,.~:;::~,:..,z/~,,~:,~.~,~<.:,~
l

Figure

END OF THE
OPERATION

.~.~,~

:.;??'~i?'%,,:%..,?;.,. ':'21.1

4. Intraoperative SEP response to tibial nerve stimulation. Parietal brain retraction causes amplitude

depression and prolongation of latency of the P40 component over the right hemisphere. No recovery before
the end of operation.
The thalamic dissection of the angioma was
followed by a severe depression and prolongation of the latency of the cortical response
elicited by median nerve stimulation over the
right hemisphere (Figure 5). These SEP disturbances persisted even after the end of surgery.
BAEP recording was disturbed by external artefacts, but potentials remained interpretable:
they remained unchanged (Figure 6). At the
end of the surgical procedure the tibial nerve
SEP pattern showed remarkable amplitude depression and latency delay. The median nerve
SEP pattern revealed both considerable latency
delay and amplitude depression. The BAEP
pattern remained unchanged and, although
pathological changes in SEP pattern did occur,
the evaluated components (P40, N20) were
never lost throughout the entire procedure.

2.2 Postoperative course

The patient was kept under barbiturates for
two days. After regaining consciousness she
revealed considerable left hemiparesis but recovered completetly within ten days. The patient was then able to support herself without
aid and walk. By the 15th day after the operation the median nerve SEP pattern as well as
the auditory evoked response had completeley
returned to normal (Figures 5, 6). At this point
she began to suffer genuine peripheral facial
nerve palsy. CT did not offer any explanation
for this. She was transferred to a neurological
unit for the medical as well as physical treatment of the VIIth nerve palsy, and recovered
after three weeks of treatment.

Figure 6. Intraoperative brainstem auditory evoked potentials: traces are contaminated by artefacts from

thalamic dissection until the end of operation. Components are, however, recognizeable. No change throughout the entire operation.
Neurosurg. Rev. 16 (1993)

W]tzmann et al, Evoked potentials in angioma removal

75

N13
LEFT HEMISPHERE

" - ....

-

RIGHT HEMISPHERE

C3'/FPz,

C4'/FP z
I

[
N 20

I

08:30 i

--

:~:--~

.....

-1 "

.::=-=':INDUCED

__..- ~",

11:20:

[

THALAMIC I

--,,,,

1125

/.

;

N 20

ANESTHESIA ':: . . . . . . . .

.....

k

-.-4"-1":

_

I

k......

...... l

TH,,.-c I

-

.--

I

DISSECllORI

/ .......

....

.....

I

_~

-

11:40,
,,
12:251

5

12:45 . . . . .

/"~-~

....

L

',-"

]
- |

....

IWMOVIIID

~

. .~OF~I

'

~

t

--~

!

15 D A Y S

.....

Ii

AFTER THE] ~: _ _ . = _ _ ~ . . . .

=~I

O~RAnON l

-

__l': ....

I

"~
.......":-'-:=-~::':::

" .......... ....
-

Figure 5. Intraoperative SEP in response to median nerve stimulation. Thalamic dissection is followed by
considerable depression and latency delay of the N20 component over the affected hemisphere. No recovery
before the end of operation. Full recovery at the 15th day after operation.

LEFT EAR STIMULATION
A1/Cz
IH

RIGHT EAR STIMULATION
A2/Cz
III
v

v

08:50 :!,-~:.:~l-:"~ ~

ANESTHESIA INDUCED

09:45

DURA OPEN

11:30

THALAMIC DISSECTION

13:00

END

15 DAYS

t , ~I/,\4~,/,~
Neurosurg. Rev. 16 (1993)

AFTER THE OPERATION

__[_;,3.v

76

Witzmann et al, Evoked potentials in angioma removal

3 Discussion

We chose the transcallosal approach because
of the relatively short distance to the intraventricular space and the small risk of approachrelated neurologic deficits [13].
SEP monitoring is now well established in aneurysm surgery [ 9 - 1 1 , 14] and has been used
increasingly often during intracranial tumor removal [1, 16]. BAEP monitoring has mainly
been used during surgery for infratentorial lesions [8, 16]. There are only a small number of
reports concerning the use of evoked potential
monitoring in spinal [7] or intracranial [2] AVM
resection in the literature.
As demonstrated previously [16], intraoperative
loss of the SEP component N20 and BAEP
components III and V before the end of operation indicates poor recovery in the postoperative period and a poor final outcome. The

attending surgeon should, therefore, promptly
change his strategy or even interrupt surgery if
loss of SEP and/or BAEP components occurs.
The case discussed in this report demonstrates
the significance of a combined SEP and BAEP
monitoring during high risk operations. The
SEP responses deteriorated during the operation, but were never lost. This bioelectrical phenomenon was reflected in postoperative incomplete hemiparesis which recovered within 10
days. This hemiparesis was probably partly due
to the mechanical disturbance caused by thalamic AVM dissection, but mainly due to temporary change of the regional cerebral blood
flow pattern. After clinical recovery the SEP
pattern also reverted to normal.
In conclusion, this case stresses the usefulness
and significance of multimodal intraoperative
evoked potential monitoring for the prognosis
of outcome of high risk surgical procedures.

References

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(1982) 6 4 - 6 7
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J Neurosurg 45 (1976) 218-222
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[t3] UNGERSBOCKK, A PERNECZKY:Intraventricular
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[14] VALENCAK E, A WITZMANN, F REISECKER, W
LOFFLER: Intraoperative somatosensorisch evozierte Potentiale (SSEP) in der Chirurgie cerebraler Aneurysmen: Topische Aussagen. Schw
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77
[16] WITZMANN A, F REISECKER: Auditory and somatosensory evoked potentials monitoring in
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[17] YASARGIL MG: Microneurosurgery iII A.
Thieme, S t u t t g a r t - N e w York 1987
Submitted January 7, 1991. Accepted May 2,
1991.
Univ. Doz. Dr. Alfred Witzmann
Neurochirnrgische Abteilung
Landeskrankenhaus Feldkirch
Carinagasse 47
A-6807 Feldkirch
Austria

Neurosurg. Rev. 16 (1993)