Posts Tagged ‘brainstem auditory evoked potential’
IONM: Value of Wave III as an Early Warning Sign in Intraoperative Brainstem Auditory Evoked Potential Monitoring
Yasmine Ashram, Hesham Kozou
Background: The use of Intraoperative brainstem auditory evoked potential monitoring (auditory brainstem response, ABR) has been shown to reduce the risk of hearing impairment during cerebello-pontine angle (CPA) surgeries that place the cochlear nerve at risk. Despite its wide use however, studies defining the changes in intraoperative ABR that correlates to hearing impairment are inconsistent. Loss of ABR wave V although a specific sign of postoeprative hearing impairment is not helpful for hearing preservation because its occurrence is a late indication of compromise of hearing. For increasing the efficacy of hearing preservaton its would be more practical to rely on earlier warning criteria.
Aim: To evaluate the value of wave III as an early warning sign for ABR changes and in predicting postoperative hearing outcome during surgery for microvascular decompression (MVD) procedures and vestibular neurectomies.
Methods: The study was conducted on a total of 30 patients who underwent surgery for MVD procedures and vestibular neurectomies. All patients underwent pure-tone audiometery and speech discrimination immediately before and 10-12 days following surgery. Hearing was classified into four classes based on pure tone average (PTA) and speech discrimination scores. Hearing preservation was attempted in all surgeries using ABR. During the operation ABR was recorded continuously and the latencies and amplitudes of waves I, III and V were compared with those of the baseline recordings. The amplitude of the wave was measured between its peak and the following trough, the ratio of that amplitude value to that of the baseline recording was expressed as a percentage.
Results: Out of 30 patients, 21 (70%) had preserved postoperative hearing and 9 (30%) had reduced hearing postoperatively. ABR changes (latency and/or amplitude) in wave III and V occurred in 25 (83.3%) of cases. Of those 25 cases, wave III changes preceded wave V changes in 14 cases (56%). Delay of latencies of both waves III and V could significantly predict postoperative outcome (cutoff values: 0.6 and 0.62msec respectively). Comparison between areas under the ROC curve (receiver operated characteristic curve) of latency delay of wave III and wave V in relation to postoperative hearing outcome revealed that although delay of latency of wave V had higher value of area under the curve (0.852) than delay of wave III latency (0.733), there was no statistical significant difference between both variables (p=0.240). In addition, percent decrease of amplitude of both waves III and V could significantly predict hearing outcome (cutoff values 40% and 42.8% respectively). Comparison between areas under the ROC curve of percent decrease of amplitude of waves III and wave V in relation to postoperative hearing outcome revealed that the area under the ROC curve of percent decrease of amplitude of wave III (0.929) was significantly higher than that of wave V (0.725) (p=0.028). Further more, the study revealed that 0.4 msec delay of wave III amplitude was an early alert to more significant changes.
Conclusion: Changes in wave III latency and amplitude can predict postoperative hearing outcome as efficient as wave V changes. Furthermore, a more gradual line of ABR changes should be adopted as warning criteria starting with 0.4 msec delay of wave III. A second level of warning at which the surgeon must be alerted includes any of the following single or in combination: a 0.6 msec delay or more of latency of either waves III or V, and more than 40% decrease amplitude of waves III and or V.
Key Words: Intraoperative monitoring, auditory brainstem response, wave III, early warning
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What is surgical neurophysiology?
Surgical neurophysiology, also known as intraoperative neurophysiology monitoring (IONM), is a new and growing allied health field. The surgical neurophysiologist is an integral part of the surgical team, and works closely with the anesthesiologist or anesthetist, the surgeons, and other members of the team. The neurophysiologist performs testing and monitoring of the nervous system during surgery to assist the surgeons in avoiding or reducing complications such as paralysis, hearing loss, or stroke (depending on the type of surgery), by detecting incipient injury in time to prevent or ameliorate it. Surgical neurophysiology also provides information to the surgeon for use in intraoperative decision-making.
What kinds of surgeries can be monitored?
Surgical neurophysiology monitoring employs a wide variety of modalities, each with a very specific application. It is most applicable when there is a specific risk to some part of the nervous system. For some types of surgery, such as cerebellar tumors, there is no suitable monitoring technique. Some of the most commonly monitored surgeries include spinal surgery, certain types of brain surgery, some ENT procedures, peripheral nerve surgery, and vascular surgeries such as carotid endarterectomies and thoracic-abdominal aortic aneurysms (TAAA).
What testing modalities are performed in intraoperative monitoring?
Many different modalities can be used in the OR. Frequently several modalities, such as SSEP, EMG and MEP (see below) are used together in the same surgery. Some of the most widely used modalities include:
- SSEP (Somatosensory Evoked Potentials)—the response recorded from the brain, nerve, or spinal cord to electrical stimulation of peripheral nerve. Used most often to monitor the integrity of the dorsal columns of the spinal cord during spine surgery; also used in some brain surgeries and peripheral nerve surgeries.
- TCeMEP ( Transcranial Electrical Motor Evoked Potentials): an electrical stimulus is applied to the motor cortex of the brain, and a response recorded from the spinal cord or from limb muscles. Works like SSEP (see above), but in the opposite direction, to monitor function of the motor tracts of the spinal cord.
- BSEP (Brainstem Auditory Evoked Potentials)—an electrical response, originating in the brainstem, to an auditory stimulus, usually a click delivered through small in-the-ear earphones. Used to monitor brainstem function and to help preserve hearing in acoustic neuroma and brainstem tumor cases.
- EMG (electromyography)—spontaneous EMG is used to detect incipient nerve damage in spine surgery (spinal nerve roots) and in skull base surgery (facial nerve and other cranial nerves). Evoked EMG, using an electrical stimulus delivered through a hand-held probe used by the surgeon, is also used to identify and test nervous structures.
- Pedicle Screw Stimulation: evoked EMG obtained by stimulating a screw placed in part of a vertebra called the pedicle. Since a nerve root lies immediately beneath each pedicle, a response obtained at too low a stimulus intensity level indicates a breach in the pedicle. Used to avoid nerve root damage caused by such a breach.
- EEG (Electroencephalogram)—spontaneous brain activity is recorded to monitor functional integrity of the cerebral cortex, specifically to avoid injuries caused by ischemia (reduced blood flow) during carotid endarterectomies and aneurysm clippings.
- ECOG (Electrocorticography)—EEG recorded directly from the exposed surface of the brain to help define the borders of resection (tissue removal) in epilepsy surgeries and craniotomies for brain tumors.
- Direct Cortical Stimulation: Also used in epilepsy and tumor surgeries, to identify and map eloquent areas of the brain (speech and motor areas)
- TCD (Transcranial Doppler)—blood flow velocity in the internal arteries of the brain is measured using an ultrasound beam, analogous to clocking a baseball pitch with a radar gun. Used to monitor cerebral blood flow in carotid endarterectomies.
Who are surgical neurophysiologists?
Surgical neurophysiology, though rapidly evolving into an established profession, began as an interdisciplinary field. Neurophysiologists come from a variety of backgrounds, including medicine (especially neurology and physiotry); audiology; neuroscience; and neurodiagnostic technology.
How did surgical neurophysiology develop?
The earliest intraoperative neurophysiology was probably the famous work of Wilder Penfield and others in the 1920’s. Penfield mapped exposed motor and speech cortex by electrical stimulation. In the 1960’s and 1970’s, EEG recordings were made from exposed cerebral cortex in epilepsy and tumor surgeries.
In the 1970’s, following the development of commercial evoked potential equipment, SSEP was used to prevent paralysis in scoliosis surgeries; BSEP and facial nerve EMG began to be used in skull base surgeries at about this time to prevent facial paralysis and hearing loss, and EEG monitoring began to be used in carotid endarterectomies to prevent ischemic strokes during surgery. The use of SSEP monitoring has become generalized to a wide variety of spinal and other surgeries, and some form of intraoperative neurophysiology monitoring has become the standard of care in many types of surgeries.
With the widespread popularity of several modalities of IONM, the specialty began to emerge as neurophysiologists, audiologists, technologists and others began to develop the skills to perform multiple types of monitoring. The technology has steadily improved, the knowledge base has greatly expanded with research and clinical experience, and new applications have been developed. The most recent major advance in the field has been the development of transcranial electric motor evoked potential (TCeMEP) monitoring.
Where is surgical neurophysiology headed in the future?
Surgical neurophysiology continues to advance, with the development of new applications such as brainstem mapping, spinal cord mapping, monitoring for position-related nerve injuries, and many others. The surgical neurophysiologist requires increasing knowledge, versatility and sophistication. The greatest challenge faced by this evolving field is the need for standardized education, training and credentialling. Many neurophysiologists envision a structure like that of audiology, with graduate degrees in the field and state licensure.
© Copyright, 2004. Jerry Larson, CNIM, D. ABNM, email@example.com.
Cranial Nerve Compression Syndromes and Neurophysiological Monitoring: Brainstem Auditory Evoked Potentials (BAEP) Monitoring During Microvascular Decompression (MVD) for Trigeminal Neuralgia, Hemifacial Spasm and Vago-Glosso-Pharyngeal Neuralgia
Marc Sindou, MD*
Gustavo Polo, MD*, Catherine Fischer, MD**
*Department of Neurosurgery and ** Department of Clinical Neurophysiology
Hôpital Neurologique, University of Lyon, France
•Among the classically-named hyperactive Cranial Nerve Compression Syndromes (CNCS), Trigeminal Neuralgia
(TN), Hemifacial Spasm (HFS) and Vago-Glosso-Pharyngeal Neuralgia (VGPN) are the most commonly CNCS
treated by neurosurgery (4). Symptoms are caused by neurovascular conflicts at (or in the vicinity of) the Root Entry
Zone (REZ) (for the trigeminal or the vagal/glosso-pharyngeal nerve) or the Root Exit Zone (for the facial nerve).
More exactly the vascular cross-compression is situated on the central portion of the root, that is the segment
between brainstem and the Obersteiner-Redlich transitional zone of the root, the later being the borderline in
between the oligodendrocytic (= central) and the schwannian (= peripheral) portions of the root. The central
portion is on average 2.6 mm in length for the Trigeminal nerve, 1 mm for the Facial nerve and 1.2 mm for the
Vagal-Glossopharingal nerves (VGP)– whereas 8.3mm for the Vestibulo-Cochlear nerve complex (1).
MicroVascular Decompression (MVD) has a long-term success rate of 70% to 80% for TN, 83% to 97% for HFS
and 87% to 95% for VGPN, according to main series’publications. Because MVD is functional neurosurgery,
complications and/or side-effects have to be minimal. For CNCS located in the cerebello-pontine angle, function of
the cochlear nerve is especially at risk during surgery (5). Dangerous steps are: first, cerebellum retraction at the very
beginning of the intradural approach; second, manipulations of CPA arteries; third, prosthesis insertion to maintain
apart the offending vessel(s) from the nerve(8,9).
Correlation studies between post-operative hearing loss or intraoperative brainstem auditory evoked potential
(BAEP) electrophysiological changes and surgical manoeuvres shows that auditory function is at risk when
cerebellar retraction is performed from laterally to medially (5,6,8).
Intraoperative neurophysiologic monitoring (IOM) and testing are medical procedures that have been in standard practice for almost 30 years. The procedures allow monitoring of neurophysiologic signals during a surgical procedure whenever the neuroaxis is at risk as a consequence of either the surgical manipulation or the surgical environment.