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