skull base monitoring
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Neuroelectrophysiological Monitoring
in Skull Base surgeries
What is Intraoperative Monitoring ?•Intraoperative Neurophysiological Monitoring (IONM or IOM)
•utilization of neurophysiological tests in a surgical setting
•tests were originally designed for clinical diagnosis but have been adapted for intraoperative use
•early usage of facial nerve monitoring provided a basis for the usefulness of IOM
• “STANDARD OF PRACTICE”
Why Use Intraoperative Monitoring ?
Why Use Intraoperative Monitoring?• posterior fossa and skull base surgery carries a significant risk of
devastating neurological injury
• detection of evolving iatrogenic neural injury at a stage when it is still reversible
• localization of sensitive neural structures in the presence of pathology
• prognostic value in assessing and anticipating possible postoperative neurological deficits
• allows aggressive resection of tumours that may have been previously considered inoperable
What Tests are Utilized ?•Somatosensory Evoked Potentials (SSEPs)
•Auditory Evoked Potentials (AEPs)
•Cranial Nerve Electromyography (cnEMG)
• skull base and posterior fossa surgery requires the most complex monitoring
• recent advances in computer technology have facilitated significant improvements in MULTIMODALITY MONITORING
• up to 32 channels can be recorded
What Tests are Utilized ?
Cranial Nerve Monitoring• cranial nerves can be grouped as: sensory, motor, mixed
• electromyography has so far proven to be the only practical means of protecting the cranial nerves
• cranial nerve is assessed indirectly by recording from the muscles that it innervates
• provides real-time audio and visual feedback of cranial nerve function
• most effective in cranial nerves that are predominantly motor (facial, spinal accessory, hypoglossal)
• offers limited protection of sensory dominant nerves (trigeminal, glossopharyngeal, vagus)
Cranial Nerve Electromyography Cranial Nerve Monitored Muscle(s)
I olfactory N/A (sensory nerve)
II optic N/A (sensory nerve, use VEP?)
III oculomotor inferior rectus
IV trochlear superior oblique
V trigeminal masseter, temporalis
VI abducens lateral rectus
VII facial orbicularis oculi, orbicularis oris, mentalis
VIII vestibulocochlear N/A (sensory nerve, use BSAEP)
IX glossopharyngeal stylopharyngeus
X vagus cricothyroid
XI spinal accessory trapezius, sternocleidomastoid
XII hypoglossal intrinsic tongue muscles
Cranial Nerve Electromyography• activity can be classified as:
surgically evoked – spontaneouselectrically evoked - triggered
• surgical events – temperature changes, traction, blunt traumaany mechanical manipulation
• sharp dissection is a poor trigger event
• quantification of intraoperative EMG is elusive
• the optimal situation is the lack of activity - neurotonic discharges – “train activity”
amplitude, frequency, duration
• cranial motor nuclei can be mapped within the brainstem
Cranial Nerve Electromyography
Cranial Nerve Electromyography
Cranial Nerve Electromyography• electrically triggered
EMG with the use of a stimulator probe can help the surgeon gauge the proximity to cranial nerves
• stimulation proximally at the brainstem can predict postoperative dysfunction
• functional integrity can be tested and compared to structural integrity
Auditory Evoked Potentials• AEPs are a type of sensory evoked potential (SEP)
• neural responses to acoustic stimuli
• the short latency AEPs are the only useful responses in IOMBrainstem Auditory Evoked Potential (BSAEP)Electrocochleogram (ECochG)VIIIth Cranial Nerve Action Potential (NAP)
• largely unaffected by anaesthesia
• can guard against cochlear, VIIIth nerve and brainstem injury- ischemic or traumatic
Brainstem Auditory Evoked Potential• BSAEP is a short latency
auditory evoked potential
• a multi-peak, far-field response to stimulation of the auditory apparatus, generated in structures from the cochlea to the midbrain
• seven waves (or peaks) are typically recorded in the first ten milliseconds following stimulation
• generator sites are still often in dispute
II
IIII
IIIIII
IVIV
VV
VIVI
VIIVII
Brainstem Auditory Evoked Potential
II
IIII
IIIIII
IVIV
VV
VIVIVIIVII
Peak I - post-synaptic distal cochlear nerve
Peak II - ipsilateral cochlear nucleus in the upper medulla
Peak III - superior olivary complex in the pons (ipsi and contralateral)
Peak IV - ascending lateral lemniscus in the pons (ipsi and contralateral)
Peak V - inferior colliculus in the midbrain (ipsi and contralateral)
Peak VI - medial geniculate in the thalamus
Peak VII - thalamo-cortical auditory radiations or auditory cortex
Brainstem Auditory Evoked Potential• click stimulus is
presented to the auditory apparatus though headphones or silastic tubing and ear inserts
• recorded far-field with electrode combination placed around ear and at vertex
• bilateral recordings are obtained so that a control side is present
Brainstem Auditory Evoked PotentialII
IIIIIIIIII VV
Electrocochleogram• near–field recording of the cochlear response to auditory stimulation
• recorded simultaneously with the BSAEP
• the cochlea is acutely sensitive to ischaemic injury
• a large percentage of postoperative hearing loss can be attributed to the loss of the intracranial blood supply
• ECochG will quickly disappear in situations where the cochlear artery is stressed
• does not test the integrity of the VIIIth nerve or brainstem
Electrocochleogram
•electrode is placed extratympanically or transtympanically to sit in close proximity to the cochlea
Electrocochleogram
IIECochGECochG
BSAEPBSAEP
• a large evoked potential is recorded that mimics peak I of the BSAEP
• up to 10X larger than the BSAEP
• response is minimally averaged and is quickly acquired in almost real-time
• can be recorded even when the BSAEP is almost absent
I V
VIIIth Nerve Action Potential
II
IIII
• electrode is placed on the VIIIth cranial nerve proximally at the brainstem
• best assessment of VIIIth nerve integrity
• presence of a “bulky” electrode in a small surgical site can be interfering
• recording site is not always accessible
VIIIth Nerve Action PotentialNAPNAP
VIIIth Nerve
• large evoked potential is recorded - occurs between peak I and II of the BSAEP
• very short acquisition time – can be recorded live
• near-field response up to 100X larger than the BSAEP
• can be recorded even when the BSAEP is almost absent
Preservation of Hearing• acoustic neuromas above 2.5 – 3.0 cm rarely result in serviceable hearing and is usually not attempted
• even in small tumours (0.5 – 1.0 cm) the success rates vary (20 – 50%) and should not be considered as a standard
• other CP angle tumours usually have better results
Preservation of Hearing
VIIIth NAPVIIIth NAP
ECochGECochG
BSAEPBSAEP III
IV V
I II
• an abnormal or absent BSAEP is often seen even in patients with minor preoperative hearing loss
• widely agreed that the combined use of BSAEP, ECochG and VIIIth NAP results in the greatest chance of hearing preservation
Preservation of Hearing• presence of peak V (A) of the BSAEP is the single best predictor of postoperative hearing even with changes in the other peaks
• absence of peak V (B) rarely results in any serviceable hearing
• presence of peak V does not guarantee hearing but the correlation is strong
• ECochG and VIIIth NAP should remain relatively unchanged
AA
BB
Somatosensory Evoked Potentials• Somatosensory Evoked Potentials (SSEPs) can be obtained
from the upper or lower limbs
• electrical stimulation of the peripheral nervous system recorded centrally
• probably no difference between UL and LL
• ULSSEP is technically easier to record so it is used more often
• like the BSAEP is recorded bilaterally
• when combined with the BSAEP two distinct brainstem pathways are assessed providing more effective monitoring
Somatosensory Evoked Potentials• brainstem generators of
the SSEP remain controversial and elusive
• sensory cortex response is used for interpretation
• changes in amplitude and latency are compared to the baseline and any controls
• capable of detecting conduction blocks through the brainstem -ischaemic or mechanical
cortical
cervicalperipheral
Brainstem
generators?
Somatosensory Evoked Potentials
peripheral control
cervical-medullary junction
cortical response
ULSSEP
Other Modalities ?• Motor Evoked Potentials (MEP)
• limited but increasing use has been reported skull base surgery
• cortical stimulation triggering head and neck muscles directly can be distracting
• more practical in spinal surgery
• Cranial Nerve Sensory Evoked Potentials
• early uses of technique have proven to be inconsistent (trigeminal SEP)
• may eventually evolve into a viable IOM technique
References•Møller AR: Intraoperative Neurophysiologic Monitoring, Luxembourg, Harwood Academic Publishers,1995
•Russell GB: Primer of Intraoperative Neurophysiologic Monitoring, Boston, Butterworth-Heinemann, 1995
•Zouridakis G: A Concise Guide to Intraoperative Monitoring, Boca Raton, CRC Press, 2001
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