Establishing baseline values for brainstem auditory evoked potentials (BAEP)
during microvascular-decompression (MVD) for hemifacial spasm (HFS).
Santhosh Kumar Mohanraj, MD1, Parthasarathy D Thirumala, MD1, MS, Miguel Habeych, MD1, Donald Crammond, PhD1 &
Jeffrey Balzer, PhD1.
1Centre
of Clinical Neurophysiology, Department of Neurological Surgery, University of Pittsburgh Medical Centre.
Introduction
HFS is caused by vascular compression of the
cranial nerve VII (CN VII) at root exit zone. MVD
is a surgical procedure which alleviating HFS by
removing the pressure on CN VII1. During MVD,
BAEP monitoring is done to decrease the
incidence of post operative hearing loss (HLS)2.
In BAEP monitoring changes in the waveforms
are used to alert the surgeon about impending
hearing-loss. These changes are compared to the
baseline BAEP values set at the beginning of the
procedure. Our study evaluates the appropriate
time to establish the baseline values for BAEP
during MVD and its implication on the alarm
criteria
Comparison of HLS in patients with
significant alerts.
Total
Baseline
set at
beginning
of the
procedure
Baseline
set at dura
opening
Patients
Patients with HLS
with
significant
after
number
significant
alerts in
surgery
of
alerts in
AwV at
patients
LwV at
change start
change start
61
61
1
5
•
•
•
We retrospectively identified 61 patients who had
intraoperative monitoring with BAEP during
MVD.
Compared the BAEP values obtained at the
beginning of the procedure and before any major
manipulation (dura opening).
We analyzed the data by comparing the number of
alerts given to the surgeons and the post operative
hearing outcomes between baselines set at the
beginning of the surgery and before major
manipulation.
Latency of wave V (LwV) and amplitude of wave
V (AwV) are the only values considered since it is
the only wave used in the alarm criteria.
•
•
•
61
43
1
4
•
Methods
•
Results
90
80
The latencies of wave I,III and V at dura
opening were significantly increased when
compared to their latency obtained at the
beginning of the procedure. But there were no
significant decrease in amplitudes were seen.
We analyzed the physiological and
pharmacological parameters but they were not
significant.
Higher percentages of alerts were communicated
to the surgeon when baseline values were set
before the beginning of the procedure when
compared to baseline values set before major
manipulation.
There was one patient with HLS when baseline
is set before major manipulation, but on further
analysis of this patient’s data we found this as an
outlier.
Conclusions
70
60
50
40
30
20
10
MAP
Significant variation in BAEP responses exist
before any major manipulations during MVD.
Baselines can be set before any major manipulations
without any loss of sensitivity in value of BAEPs
during MVD.
TEMPERATURE
0
Baseline
Dura
MAC
MAC
Change
start
References
On Skin
TEMPERATURE
MAP
1. Barker FG, 2nd, Jannetta PJ, Bissonette DJ, Shields PT, Larkins MV, Jho HD.
Microvascular decompression for hemifacial spasm. J Neurosurg 1995;82:201210.
2. Radtke RA, Erwin CW, Wilkins RH. Intraoperative brainstem auditory evoked
potentials: significant decrease in postoperative morbidity. Neurology
1989;39:187-191.