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Intraoperative Monitoring. Behrouz Zamanifekri , MD Neurophysiology Fellow KUMC March 2013. Intraoperative monitoring.

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intraoperative monitoring
Intraoperative Monitoring

BehrouzZamanifekri, MD

Neurophysiology Fellow


March 2013

intraoperative monitoring1
Intraoperative monitoring

The most primitive method of monitoring the patient

50 years ago were continuous palpation of the radial

pulsations throughout the operation or wake up test!!

  • 1921, Dr Penfield, intraoperative neurophysiology research
  • 1950s, Dr Penfield, electrical stimulation to find epileptic foci
  • 1970s, Dr Brown used SSEP for scoliosis operation
  • 1974, among 7,800 operations conducted with Harrington instrumentation, 87 patients had subsequently developed significant spinal cord problems
  • Early 1980, IOM in operations for large skull base tumors
  • 1980, the American Society for Neurophysiological

Monitoring [ASNM])


What is IOM ?

  • use of neurophysiological recordings for detectingchanges caused by surgically induced insults
  • assess the function of specific parts of the nervous system continuously during an operation
  • It is becoming part of standard medical practice
what is the purpose of iom
What is the purpose of IOM?

1. Reduce the risk of postoperative neurological deficits

2. Identify specific neuronal structures and landmarks that cannot be easily recognized

3. Research purposes in basic science, pathophysiology and therapeutic management

what are the most common types of recording
What are the most common types of recording?
  • Spontaneous activity



  • Evoked responses (through external stimulation of a neural pathway)

Sensory: 1- visual




  • The type of test to be used and the sites of recording and stimulation are chosen on a case by case
complications during surgery
Complications during surgery
  • ischemia
  • mechanical insult



spinal monitoring
Spinal Monitoring
  • Spinal cord, nerve roots, and blood vessels are frequently placed at risk for injury
  • Electrophysiological modalities for monitoring:



free run or spontaneous EMG (sEMG)

triggered EMG (tEMG)

spine surgery
Spine Surgery:
  • surgical insults to the ventral parts of the cord, using motor evoked potentials (MEPs)
  • dorsal columns of the spinal cord , SEP
  • the purpose of IOM is to detect response changes due to surgery, not to make a clinical diagnosis
monitoring of somatosensory evoked potential
Monitoring of Somatosensory Evoked Potential
  • Earliest used method in IOM
  • 1970s in operations for scoliosis
  • Stimulation of peripheral nerve and recorded from scalp
  • Only monitor dorsal(sensory) spinal cord
  • patient sensory examination for position and vibration is recommended prior to surgery
  • By electrical stimulation of peripheral nerves
  • Median nerve at wrist for injury above C8
  • Posterior tibial nerve at ankle for injury below C8

spinal cord through the dorsal roots,

ascending pathways, thalamus

and, finally, to the

primary sensory cortex

  • P9 from brachial plexus
  • P11 Dorsal horn
  • P14-16 Dorsal column nuclei
  • P20 Primary sensory cortex(contralat.)

upper limb SSEP

  • N37 Primary sensory cortex(contralat.)

lower limb SSEP


It is important to note

- Earlier peaks tend to be less sensitive to anesthesia

- used to differentiate SSEP monitoring changes resulting from anesthetic effects from surgical manipulation.


Alarm criteria

-50% reduction in amplitude

-10% increase in latency

Factors that affect the SSEP amplitude include halogenated agents, nitrous oxide, hypothermia, hypotension, and electrical interference

ssep in peripheral nerves
SSEP in Peripheral nerves?
  • sciatic nerve injury during pelvic fracture
  • Injuries to brachial plexus in positioning of pt is common
  • Prolong latency of all peaks and decrease amplitude
nerve root
Nerve root
  • SSEP: insensitive to changes

in nerve root function

  • Why?
ssep in nerve root injury
SSEP in nerve root injury?
  • SSEP used during placement of pedicle screws
  • Risk of spinal nerve root injury
  • If one root damaged, no change in


Dermatomal stimulation is better


Inhalational anesthetics, cortical responses

  • Intravenous Agents

- Propofolincreases the latency by approximately10%

- Benzodiazepines reduce the amplitude of cortical SEP

- Etomidate : cortical SEP amplitude augmented 200–600%,

increases SEP latencies

- Opiates, cause a slight increase in SEP latency

- Muscle relaxants, notaffect SEPs


SEP changes due to surgical maneuvers (e.g., spinal distraction) or ischemia (e.g., after placement of an artery clamp) are abrupt and localizedand only one side of the body may be affected

  • whereas changes due to anesthesia or body temperature changes are relatively slower
d etection of cord injury due to misplaced instrumentation
Detection of cord injury due to misplaced instrumentation
  • just after placement of instrumentation,
  • both the cortical (peak N45) and
  • cervical (peak N30) responses disappear

SEPs obtained after cross-clamping of the internal carotid ,whichresulted in ischemia (time 9:45) that later deteriorated (9:55). After placement of a shunt,response amplitude is restored to within normal limits (time 10:01).


Procedures involves the ICA, MCA, PCA, P.Com, or BA?

Median nerve SEPs

  • procedure involves the

ACA or the A.Com artery?

Posterior tibialnerve




  • SSEP for sensory pathway
  • MEP for motor
  • SSEP + MEP: Small reversible changes in SSEP that occur when motor pathway are injured
  • 1990s, TC-MEP as a method to monitor the corticospinal tracts
  • Prior to MEP monitoring,

the only way to assess corticospinal

tract during surgery was wake-up test



  • stimulation through the skull with signal recording at the level of

muscle (CMAP)

nerve (neurogenic MEP)

spinal cord ( D-wave )

-the newer technologies is continuous free-running EMG throughout the surgery

recording of muscle evoked potentials
Recording of Muscle EvokedPotentials
  • Stimulation of cortex, activation of coticospinal, EMG of distal( Hand m., abdhallucis, tibialis anterior)
  • Muscle relaxant can not be used
interpretation of mep recording
Interpretation of MEP Recording
  • 4 methods :

1) all-or-nothingcriterion: the most used method,

complete loss of the MEP signal from a baseline recording is indicative of

a significant event

2) amplitude criterion: 80% amplitude decrement in at least 1

out of 6 recording sites

3) threshold criterion: increases in the threshold of 100 V or more

required for eliciting CMAP responses that are persistent for 1 h or more

4) morphology criterion: changes in the pattern and duration of MEP

waveform morphology

tcmep monitoring

contraindicated in

-deep brain stimulators or cochlear implants

Tongue biting is the most common complication

recording of the response from spinal cord d i wave
Recording of the response from spinal cord(D, I wave)
  • Recording from epidural


  • D (direct activation of


  • I ( indirect, through transsynaptic)
  • Not affected by muscle relaxants, but latencies increase with cooling
  • Subdural electrodes can be substituted for epidural electrodes
  • Needle electrodes can be place in interspinous ligaments both sides of surgery area
  • major benefits reported during intramedullary spinal cord tumor resection
  • a complete loss of MEPs with at least 50% preservation of the D-wave amplitude generally results in a transient paraplegia
spontaneous emg
Spontaneous EMG
  • monitor nerve roots
  • recording electrodes placed in the muscles
  • no stimulation is performed
  • monitoring of 2 muscles is recommended
  • C5 nerve root injury, The deltoid and biceps brachii
  • MEPs be obtained intermittently
  • no paralytic agents
  • train-of-4 testing should indicate that at least 3 out of 4
  • Myasthenia gravis, Botox treatments, and muscular dystrophy are classic conditions thatinterfere with EMG
abnormal semg
Abnormal sEMG
  • spikes
  • Bursts
  • trains
  • Trains are continuous, repetitive EMG firing

caused by continuous force applied to the nerve


example of emg activity indicating irritation of the nerve
Example of EMG activityindicating irritation of the nerve

Baseline recordings. Note the low

amplitude background activity

High amplitude spikes are present

artifacts may be mistaken for spikes or trains
Artifacts may be mistaken for spikes or trains
  • a neurostimulator
  • the surgical table
  • the surgeon’s head light
  • bipolar electrocautery device
triggered emg pedicle screw stimulation
Triggered EMG (Pedicle Screw Stimulation)
  • used to determine whether screws have breached the medial or inferior pedicle wall and thus pose a risk to the exiting nerve root at that level
  • When a pedicle screw is accurately placed, the surrounding bone acts as an insulator to electrical conduction, and a higher amount of electrical current is thus required to stimulate the surrounding nerve root.
  • When a medial pedicle wall breach occurs, the stimulation threshold is significantly reduced
false negative response
False negative response
  • muscles relaxants
  • fluid, blood, or soft tissue around the head of the screw , shunt current away from the screw
  • it is important that the stimulation probe be placed directly on the top of the screw and not the tulip, as these 2 structures are not structurally fused
  • Presence of preexisting nerve root injury. Injured nerve roots will have higher triggering thresholds,

Due to the variation in thickness and shape between thoracic and lumbar pedicles, different stimulation thresholds exist for these regions

  • A threshold < 10 mA for screw stimulation, suggest a medial wall breach in the lumbarpedicles
  • A thresholds > 15 mA indicate a 98% likelihood of accurate screw positioning
  • For thoracic pedicle screw placement, stimulation threshold < 6 mA suggest a medial pedicle breach

In cervical and thoracic procedures, the spinal cord are of greater importance

  • Conversely, in lumbar or sacral procedures the nerve roots are at greater risk of injury
  • Multimodality neurophysiological monitoring is extremely valuable in the prevention of neurological injury
  • Knowledge of the benefits and limitations of each modality helps maximize the diagnostic value of IOM during spinal procedures

Neurosurg Focus / Volume 27 / October 2009

  • A concise guide to intraoperative monitoring / George Zouridakis, Andrew C. Papanicolaou.2001
  • Intraoperative neurophysiological monitoring / Aage R. Moller. -- 2nd ed.