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

2. 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!!

3. History 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])

4. Introduction What is IOM ?
use of neurophysiological recordings for detecting changes 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

5. 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

6. What are the most common types of recording?
Spontaneous activity
EEG
EMG
Evoked responses (through external stimulation of a neural pathway)
Sensory : 1- visual
2-auditory
3-somatosensory
Motor
The type of test to be used and the sites of recording and stimulation are chosen on a case by case

7. Complications during surgery
ischemia
mechanical insult

8. PRACTICAL ASPECTS OF MONITORING SPINAL CORD

9. Spinal Monitoring
Spinal cord, nerve roots, and blood vessels are frequently placed at risk for injury
Electrophysiological modalities for monitoring:
SSEPs
MEPs
free run or spontaneous EMG (sEMG)
triggered EMG (tEMG)

10. 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

11. 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

12. SSEP By electrical stimulation of peripheral nerves
Median nerve at wrist for injury above C8
Posterior tibial nerve at ankle for injury below C8

13. spinal cord through the dorsal roots,
ascending pathways, thalamus
and, finally, to the
primary sensory cortex

14. Recording P9 from brachial plexus P11 Dorsal horn
P Dorsal column nuclei
P Primary sensory cortex(contralat.)
upper limb SSEP
N Primary sensory cortex(contralat.)
lower limb SSEP

16. Location of the stimulating and recording posterior tibial nerve SEPs.

17. 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.

18. 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

19. Normal SSEPs from median nerves and posterior tibial nerves

20. 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

21. Nerve root
SSEP: insensitive to changes
in nerve root function
Why?

22. 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
SSEP
Dermatomal stimulation is better

23. Inhalational anesthetics, cortical responses
Intravenous Agents
- Propofol increases 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, not affect SEPs

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

25. Detection of cord injury due to misplaced instrumentation
just after placement of instrumentation,
both the cortical (peak N45) and
cervical (peak N30) responses disappear

26. SEPs obtained after cross-clamping of the internal carotid ,which resulted 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).

27. Procedures involves the ICA, MCA, PCA, P.Com, or BA?
Median nerve SEPs
procedure involves the
ACA or the A.Com artery?
Posterior tibial nerve

28. MONITORING SPINAL
MOTOR SYSTEM

29. Introduction SSEP for sensory pathway MEP for motor
SSEP + MEP: Small reversible changes in SSEP that occur when motor pathway are injured

30. MEP 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

31. TC-MEPs
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

32. Recording of Muscle Evoked Potentials
Stimulation of cortex, activation of coticospinal, EMG of distal( Hand m., abd hallucis, tibialis anterior)
Muscle relaxant can not be used

33. MEP

34. Interpretation of MEP Recording
4 methods :
1) all-or-nothing criterion: 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

35. TcMEP monitoring contraindicated in
-deep brain stimulators or cochlear implants
Tongue biting is the most common complication

36. Normal MEPs

37. Recording of the response from spinal cord(D, I wave)
Recording from epidural
electrodes
D (direct activation of
corticospinal)
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

38. 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

39. sEMG 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 that interfere with EMG

40. Abnormal sEMG spikes Bursts trains
Trains are continuous, repetitive EMG firing
caused by continuous force applied to the nerve
root.

41. Example of EMG activity indicating irritation of the nerve
Baseline recordings. Note the low amplitude background activity High amplitude spikes are present

42. Artifacts may be mistaken for spikes or trains
a neurostimulator
the surgical table
the surgeon’s head light
bipolar electrocautery device

44. 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

45. 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,

46. 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 lumbar pedicles
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

47. 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

48. Overview of IOM classified by spinal region

49. Conclusions
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

50. 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.