Brainstem Auditory Evoked Potentials
BAEPs are produced in the diagnostic laboratory by delivering
repetitive clicks or tones through headphones. Headphones are not practical for
surgical monitoring of neurosurgical procedures, and click stimuli are delivered
using foam ear inserts attached to stimulus transducers. Stimulus intensity is usually
set at 60 to 70 dB above the patient's click-hearing threshold, although practically,
many intraoperative laboratories establish monitoring after induction of anesthesia
and instead begin with a stimulus intensity of 90 dB nHL. The duration of the click
is approximately 100 µsec, and the stimulus is given usually between 10 and
15 times per second. Clicks are delivered using different polarities; the click
may cause initial movement of the tympanic membrane away from the transducer (i.e.,
rarefaction) or toward the transducer (i.e., condensation). Use of these two different
methods commonly produces very different waveforms, amplitudes, and latencies in
individual patients, and the method is chosen that produces the largest reproducible
response. If stimulus artifact is a serious problem, clicks of alternating polarity
may be used to decrease stimulus artifact, but the waveforms produced are an average
of those produced by either stimulating technique alone and may be more difficult
to monitor. The rate and intensity of stimulus delivery affect the BAEP.[57]
[105]
Unilateral stimulation is used because responses
from the other ear, which may remain normal during surgery, may obscure any abnormal
responses from the monitored ear. Recording electrodes (usually gold cup electrodes)
are placed on the lobe of the stimulated ear and on the top (vertex) of the head.
[105]
White noise is delivered to the contralateral
ear to prevent bone conduction from stimulation of the monitored ear from producing
an evoked response from the contralateral ear. Five hundred to 2000 repetitions
are required on average, because BAEPs recorded from the scalp are far-field potentials
and extremely small (often less than 0.3 µV).[57]
[105]
Peaks in recordings of BAEP are labeled I through VII, and the
purported neural generators for these peaks are shown in Figure
38-11
. As with other SERs, amplitude, absolute latencies, and interpeak
latencies are evaluated to assess integrity of the auditory system, localize the
functional defect when it occurs, and assess peripheral and central conduction times.
Because waves VI and VII are inconsistent and variable, they are not routinely monitored,
[105]
and most papers reporting use of BAEP for
surgical
monitoring in the operating room monitor waves only up to wave V.[106]
[107]
[108]
Surgical Procedures Monitored with Brainstem Auditory
Evoked Potentials
BAEPs have been monitored intraoperatively during procedures involving
or near the auditory pathway and during posterior fossa procedures when global brainstem
function might be compromised. Cases in which BAEPs are commonly monitored include
microvascular decompression of cranial nerves (especially V and VII), resection of
acoustic neuroma, posterior fossa exploration for vascular or neoplastic lesions,
clipping of basilar artery aneurysms, and sectioning of cranial nerve VIII for intractable
tinnitus.[77]
[106]
[107]
Several of these procedures deserve special mention because monitoring
appears to have significantly improved surgical results based on reported series
(not controlled, randomized studies, however). During microvascular decompression
of the facial nerve in patients with hemifacial spasm, hearing loss has been reported
in up to 15% of cases, and this incidence can be greatly decreased with BAEP monitoring.
[109]
[110]
During
this procedure, the cerebellum must be retracted to gain
Figure 38-11
Schematic of auditory neural pathway. The brainstem
auditory evoked potential (BAEP) is initiated by stimulation of the cochlea with
a broadband click stimulus given through an ear insert in the external auditory canal.
Neural generators of the BAEP peaks are shown.
exposure to the nerve root entry zone of the facial nerve. An insulating sponge
is placed between the nerve root and an aberrant artery or, rarely, a vein that compresses
and pulsates against the facial nerve root. Hearing loss may occur because the cerebellar
retraction needed for operative exposure stretches cranial nerve VIII between its
attachments at the brainstem and the internal auditory meatus. Hearing loss after
the more common microvascular decompression of the trigeminal nerve for trigeminal
neuralgia, although less common, has also been reduced with BAEP monitoring.[111]
[112]
[113]
[114]
BAEP monitoring has been used during resection of small acoustic neuromas that have
not yet caused severe, irreversible hearing loss.[113]
[115]
[116]
[117]
[118]
This monitoring has helped surgeons determine
the safety of tumor removal and has helped preserve hearing, particularly in younger
patients in whom preservation of longer-term hearing may be more important. Changes
in the BAEP can alert the surgeon that further resection of tumor will permanently
damage hearing. The surgeon can then decide whether it is better to leave a small
amount of residual tumor or to sacrifice hearing and achieve curative resection.
The most common BAEP change observed during monitoring of these
operations is an increase in I-V interpeak latency and loss of amplitude of wave
V. Other intraoperative BAEP changes include permanent or transient obliteration
or severe distortion of waveforms distal to the operative site and, rarely, obliteration
of BAEP contralateral to the operative side.[79]
[80]
[106]
[107]
The most common BAEP change is usually caused by intraoperative stretch of cranial
nerve VIII because of cerebellar retraction. Such changes may be reversed with repositioning
or removal of the retractor. More significant changes may be caused by deliberate
or unintentional sectioning of cranial nerve VIII, loss of blood supply to the nerve,
brainstem compression or compromise to its blood supply, operative manipulation of
cranial nerve VIII, severe cerebellar edema, and positioning of the head for retromastoid
craniotomy[79]
[106]
[107]
[119]
( Table
38-8
). Technical and physiologic factors that may significantly alter
the BAEP include drilling around the internal auditory canal (i.e., interferes with
stimulus delivery), irrigation of cranial nerve VIII with cold saline, and hypotension
and hypocarbia.
Patients with transient or persistent mild increases in latency
or decreases in amplitude can be expected to have unchanged or only slightly worsened
hearing postoperatively. Patients with complete but reversible loss of BAEP also
have unchanged or mild worsening of hearing postoperatively. Patients with complete
irreversible loss of BAEP have complete or near-complete loss of hearing in the ipsilateral
ear postoperatively unless the change was caused by a technical problem or a neuropraxic
type injury.[77]
[79]
[80]
[107]
TABLE 38-8 -- Correlation between brainstem auditory evoked potential changes and associated
clinical events
|
Brainstem Auditory Evoked Potential Change |
Associated Events |
|
Transient latency increase, amplitude decrease |
Drilling, irrigation, retraction, surgical irritation, hypocarbia
and hypotension, positioning |
|
Persistent latency increase, amplitude decrease |
Retraction and stretching of the auditory nerve |
|
Transient loss of evoked potential |
Retraction, pressure, surgical dissection |
|
Permanent loss of ipsilateral evoked potential |
Surgical interruption of auditory pathway or interruption of
cochlear blood supply |
|
Loss of contralateral evoked potential |
Cerebellar edema, damage to the brainstem or its blood supply |
One series had a single false-positive result with a complete loss of BAEP with intact
hearing postoperatively.[79]
False-negative results
are rare. In one series, BAEPs were monitored in a patient undergoing resection
of a meningioma in the lateral ventricle and were unchanged intraoperatively and
postoperatively; however, this patient did not regain consciousness postoperatively.
[80]
This case illustrates the fact that BAEP monitoring
assesses only the health of the auditory pathway and involved structures. Function
of the ascending reticular system and cortical functions are not monitored directly
by the BAEP.
BAEPs are considered the easiest of the SERs to monitor intraoperatively
and are least sensitive to changes in perioperative variables. Ability to record
technically adequate BAEP has been reported in 90% to 100% of cases in which monitoring
was attempted.[77]
[79]
[106]
[107]
Unlike
SSEPs, BAEPs are resistant to the effects of inhalation and intravenous anesthetics.
Although latency and amplitude are altered, especially by inhalation anesthetics,
the degree of change is small enough that changes in latency and amplitude associated
with anesthetic agents used in clinically relevant doses would not be confused with
significant surgically induced changes. Preservation of BAEP intraoperatively indicates
preserved hearing postoperatively, and persistent changes indicate significant risk
of injury.
