Intraoperative Monitoring

Question 1

What is the purpose of interoperative monitoring?

Answer 1

To reduce the risk of debilitating permanent neurologic deficits from surgery
To be the voice of the anesthetized patient

Question 2

How do you make IOM your career?

Answer 2

Find someone to train you (hospital or contract company)
Currently no licensing requirements to do this
Obtain CNIM (surgical neurophysiologist) - test is tough (credentials)
Obtain ABNM (remote reader or billable provider)

Question 3

What do you have to do to become a billable provider?

Answer 3

Must have minimum of 5 years experience, with at least 2 years supervising
Possess a minimum of an earned doctoral degree if physical science, life science, or clinical applied health profession (PhD, MD, DO, DC, AuD, D.Pt)
Complete 2 separate graduate level courses, one in neuroanatomy and one in neurophysiology
Statements from 2 surgeons
Case Log with 300 cases, with balance breakdown of minimum requirements
Written exam
Spoken exam

Question 4

Do you do an interview with the patient for IOM?

Answer 4

Yes
Show redundancy that everyone is on the same page
Have the patient sign off on what side will have surgery done
Done by the surgical team

Question 5

Do you set up IOM wherever there is room left?

Answer 5

Yes, might have to squeeze

Question 6

What is the set up of IOM?

Answer 6

Arrive at facility an hour before the case is scheduled
Complete bio-medical check if facility requires
Interview patient to determine deficits and limitations to set-up
Set-up machine, computer, and equipment
Discuss modalities with anesthesia and/or surgeon
Anesthesia will have to adjust their regimen to accommodate your recordings
Patient comes into the room
Patient is moved to OR bed
Anesthesia time out
Patient put under anesthesia and intubated
Place any electrodes that are not on head and/or BAER inserts
Nurse places catheter (folley) if needed
Surgeon adjusts patient; Head leads typically placed at this time
Surgeon takes localizing x-rays to confirm levels
Begin establishing baselines
Troubleshoot
Surgical Time Out
Incision
Set baselines
Discuss baselines with surgeon and pray that everything remains stable throughout the case

Question 7

Do you have to know more of the body mapping for IOM?

Answer 7

Yes
For 10-20 montages

Question 8

What are all of the modalities that you might have to be involved in for IOM?

Answer 8

Somatosensory Evoked Potentials (SSEP)
Transcranial Motor Evoked Potentials (TcMEP)
Spinal Nerve Electromyogram (EMG)
Free Run
Direct Nerve Stimulation
Cranial Nerve Electromyogram (EMG)
Spontaneous Electromyogram (s-EMG)
Triggered Electromyogram (t-EMG)
Brainstem Auditory Evoked Potentials (BAER)
Electroenchephalogram (EEG)
Motor and Sensory Mapping
Phase Reversal
Direct Cortical Motor Evoked Potentials (dcMEP)
Sub-cortical Motor Evoked Potentials

Question 9

What you you stimulate and record for somatosensory evoked potentials?

Answer 9

Stimulate - wrist (median nerve or ulnar nerve, ankle (posterior tibial nerve)
Record evoked response from scalp, cervical or mastoid region, or limb

Question 10

When is SSEP monitoring done?

Answer 10

Anytime spinal cord function is in question
Anytime Somatosensory sulcus is in question
Anytime brainstem is in question
Anytime blood flow in the middle cerebral arteries or anterior cerebral arteries is in question

Question 11

What are the stimulus parameters for SSEP?

Answer 11

Bilateral asynchronous stimulation
Cathodal Stimulation (-) - proximal
Stim Intensity: 25-40 mA
Pulse Rate: 2-8 Hz (avoid 60 Hz) - 2.79, 1.79 & 1.13 (lower rate = stronger signal)
Pulse Width: 0.1-0.3 mS
Averages: > 250, (we use 100)
Bandpass: 30-1000 Hz

Question 12

What are factors that affect SSEP?

Answer 12

Age
Sex
Height
Neuropathy
Anesthesia
Body or limb temperature
Hypotension
Ischemia
Acute traumas
Noise

Question 13

What are common sources of noise?

Answer 13

Recording wires
Poor impedance
Poor Grounding
Fluorescent lights
Infusion Pump
Extension cables
Hospital bed
Electrocautery
Drill (during surgery)
Anesthesia equipment
Ect. ….

Question 14

What are some ways to reduce noise?

Answer 14

Braid wires
Keep impedances below 5kOhms
Plug equipment into hospital grade outlets (Red)
Don’t use extension cords
Unplug bed
Increase distance from anesthesia equipment or other noise sources
Pause recording during electrocautery or drill use
Change rep rate
Increase number of averages

Question 15

What are transcranial motor evoked potentials?

Answer 15

TcMEPs
Travel efferently through the ventral columns of the spinal cord
Stimulating the head; recording from muscle groups
Use of quad-polar to reduce stimulation levels

Question 16

What are the three types of electromyographic activity that are often seen during vestibular schwannoma surgery?

Answer 16

Dense tonic (sustained) activity, often associated with nerve stretch and demonstrating a sinusoidal pattern
Lower tonic activity, called popcorn activity
Phasic (transient) burst activity typically associated with direct contact with the nerve (such events are not of major significance unless they involve large-amplitude trains and occur during critical stages of dissection)
Burst activity superimposed on ongoing small-amplitude train; it is important not to overlook such events overlapping on background activity, because they may pass unnoticed despite their significance

Question 17

Can EMG be measured on any of the cranial nerves?

Answer 17

Yes
Changes site of electrode placement

Question 18

What are the recording parameters for EMG?

Answer 18

Want Filters to be wide open
Free run EMG
Filters: 10-30 Hz to 16,000 Hz
Direct Nerve Stimulation
Filters: 1-250 Hz to 1500 Hz
Stimulation intensity: 0.1-5 mA
Bipolar best for specificity
Monopolar best for sensitivity

Question 19

What factors affect CN EMG?

Answer 19

Muscle relaxant - false negative
Patient alertness - false positive; activity seen on free run EMG not due to nerve stimulation
Previous injury to facial nerve - false negative; may require higher stimulation thresholds

Question 20

Do we typically only monitor the motor component of facial nerve EMG?

Answer 20

Yes
Monitor for irritation or injury

Question 21

What muscles are monitored for facial nerve EMG?

Answer 21

Intracranial - orbicularis Oculi and Orbicularis Oris
Extracranial - Frontalis, Orbicularis Oculi, Orbicularis Oris, Mentalis

Question 22

What do BAEPs measure?

Answer 22

Cranial Nerve VIII
Sensory only
Monitor auditory pathway and brainstem integrity

Question 23

What factors affect BAEP?

Answer 23

Post Auricular Muscle Reflex - large wave that occurs around 12-15 mS after stimulus
Anesthesia - or the most part…CNS depressants cause minimal effects
Body Temperature
Temperature dropping: Bilateral Increased IPL latency, (affects wave V slightly more)
Severe hypothermia (20-25 C): absent Bilateral BAER
Hypotension (MAP below 40) - bilateral decreased amplitude, increased absolute latency
Local Nerve cooling - increase ipsilateral I-III IPL or transient loss of III-V
Drilling of the cranium - absent Bilateral BAER due to masking affect
Cerebellar Retraction - increased III-V IPL
Nerve Manipulation - increased I-III IPL
Ischemia (labyrinthine artery, branch of AICA) - absent BAER

Question 24

When is CN VII monitored?

Answer 24

Acoustic Neuroma
Brainstem Tumor
Cerebellopontine Angle (CPA) Tumor
Microvascular Decompression
Parotidectomy
Tympanomastoidectomy
Tympanoplasty
Fourth Ventricle Mapping

Question 25

When is VIII monitored?

Answer 25

Acoustic Neuroma Aneurysm (cranial) Brainstem Tumor Cerebellopontine Angle (CPA) Tumor Chiari Malformation Microvascular Decompression Fourth Ventricle Mapping

Question 26

What are the surgical approaches for acoustic neuromas?

Answer 26

Retrosigmoid / Suboccipital - possible to preserve hearing, risk of facial nerve injury (not visible until tumor is out) Translabyrinthine - hearing is sacrificed, facial nerve easily preserved (drill through mastoid) Transcochlear - hearing sacrificed, facial nerve likely sacrificed (just like translabyrinthine, but larger exposure)