MONITORING OPERATIONS IN THE SKULL BASE

CHAPTER VI

Monitoring of cranial nerves in skull base operations:

what to monitor?1. Extraocular muscles 2. Facial muscles3. Masseter muscle4. Tongue5. CNIX pharynx6. CNX larynx7. ABR8. CAP from CNVIII and cochlear nucleus

Monitoring other cranial motor nerves

Extraocular muscles• CN III• CN IV• CN VILower cranial nerves• CN IX• CN X• CN XI• CN XII

Monitoring other cranial motor nerves

• Monitoring the facial nerve is a model for monitoring other cranial motor nerves

How to activate the motor system?

• Electrical stimulation of motor nerves• Magnetic stimulation of motor nerves• Electrical stimulation of the motor cortex• Magnetic stimulation of the motor cortex

How to record the response?

• Recording of electromyographic (EMG) potentials

• Mechanical recordings of muscle contractions

• Recording of motor nerve CAP

Recording of EMG potentials

Recording muscle responses

• Muscle relaxants cannot be used

MONITORING OTHER CRANIAL MOTOR NERVES

EXTRAOCULAR MUSCLES• CN III• CNVI• CNIVLOWER CRANIAL NERVES• CNIX• CNX• CNXI• CNXII

Monitoring nerves that innervate extraocular muscles

CN III, CN IV, CN VI

Recording EMG potentials from extraocular muscles

• Using needle electrodes

Recording from extraocular muscles:Place needle electrodes percutaneously so they come close to respective musclesMedial rectus for CNIIILateral rectus for CNVISuperior oblique for CNIV

EMG potentials recorded from extraocular muscles in response to electrical stimulation of respective cranial nerves intracranially

Recording EMG potentials from extraocular muscles

• Using non-invasive electrodes

Electrical stimulation of the oculomotor nerve (CN III)

Middle rectus

Lateral rectus

Masseter

Monitoring multiple systems simultaneously

• Extraocular muscles• Facial muscles• Masseter muscle• Tongue• Neck muscles• BAEP• VEP

Typical placements of recording electrodes used in skull base operationsRecording from extraocular muscles: Place needle electrodes percutaneously so they come close to respective muscles Medial rectus for CNIII Lateral rectus for CNVI Superior oblique for CNIV

Monitoring other cranial motor nerves

Lower cranial motor nerves

CN IX, CN X, CN XI, CN XII

FROM YINGLING

Monitoring CN X can be done by placing recording

electrodes in the vocal folds

FROM YINGLING

Monitoring CNX

FROM YINGLING

Auditory neuromonitoring

Recording of auditory evoked potentials in operations in the

posterior fossa

Monitoring of ABR can detect manipulations of the

brainstem before cardiovascular signs change

Waveform analysis of the ABR provides information about the

anatomical location of an injury

Different conventions for display of BAEP

NEURAL GENERATORS OF THE ABR:

• Peak I: distal auditory nerve• Peak II: central auditory nerve• Peak III: mainly cochlear nucleus• Peak IV: unknown• Peak V: termination of the lateral lemniscus in the contralateral inferior colliculus

Waveform analysis of the BAEP provides information about the anatomical location of an injury

PEAK III

PEAK VIpsilateralstimulation

Contralateral stimulation

Waveform analysis of the BAEP provides information about the anatomical location of an injury

PEAK III

PEAK V