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IONM
for
LUMBOSACRAL SURGERY
ANURAG TEWARI MD
IONM for Lumbosacral Surgery
The adult spinal cord ends opposite the lower border of the L1 vertebra
Introduction
Anurag Tewari MD
IONM for Lumbosacral Surgery
• Below that, the spinal canal contains only individual lumbosacral nerve roots descending
toward their respective neural foramina, collectively known as the cauda equina
Introduction
Anurag Tewari MD
IONM for Lumbosacral Surgery
• There is a 10% risk of new postoperative neurologic deficit from Nerve Root
Injury during spinal fusion for degenerative lumbosacral spine disease
Introduction
Anurag Tewari MD
10-fold greater than the risk of spinal cord injury from scoliosis surgery
IONM for Lumbosacral Surgery
• Correction of spinal deformity during lumbosacral surgery can injure Nerve Roots by
• PROBING
• STRETCHING
• DRILLING
• MISPLACED HARDWARE
Introduction
Anurag Tewari MD
IONM for Lumbosacral Surgery
The most common deficit is a new FOOT DROP from surgical injury to
the L5 nerve root, although findings can include any pattern of
• Dermatomal numbness
• Radicular weakness
• Sphincter dysfunction
Introduction
Anurag Tewari MD
IONM for Lumbosacral Surgery
Nerve Injury
Anurag Tewari MD
IONM for Lumbosacral Surgery
Nerve Injury
Anurag Tewari MD
IONM for Lumbosacral Surgery
• Nerve root injuries are more frequent during revision than primary surgery and
in cases where multiple levels are fused
• Hence, the purpose of IONM with EMG during these lumbosacral spine cases is:
• DETECT early and potentially reversible surgical nerve root irritation
• ALERT the surgeon
• To prevent more significant injury and postoperative deficit(s)
Introduction
Anurag Tewari MD
RESULTS:
The overall incidence of lumbar nerve root palsy was 2.9% with a 1.4%
incidence in primary and 3.8% incidence in revision surgery
https://www.ncbi.nlm.nih.gov/pubmed/16025033
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• Mixed nerve SSEPs and TcMEPs are most often used for spinal cord monitoring
but
THEY ARE NOT SENSITIVE FOR DETECTING NERVE ROOT INJURIES
WHY?
• SSEPs represent the summation of neural signals that enter the spinal cord
through multiple segments, and because of central amplification, they can
remain completely unchanged after an individual nerve root lesion
Background
Anurag Tewari MD
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• TcMEPs recorded from an epidural electrode will obviously not be affected by a
more caudal nerve root injury
• TcMEPs recorded from a single distal limb muscle throughout surgery will not
detect nerve root injuries affecting other myotomes
There have been numerous well-documented cases of patients with acute radiculopathy from spine surgery despite
normal SSEP and TcMEP monitoring
Background
Anurag Tewari MD
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• Dermatomal somatosensory evoked potentials (D-SSEPs) can detect individual
nerve root injuries but are technically difficult to resolve in the operating room
• They need to be averaged many times
• Resulting a slow turnaround time
• They test only dorsal root (sensory) function
Background
Anurag Tewari MD
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• For these reasons, EMG has become the technique most often used to monitor
nerve root function during lumbosacral spine surgery
• EMG is more sensitive than SSEPs and D-SSEPs for detecting nerve root dysfunction, as it
• Provides immediate results without averaging
• Gives instantaneous feedback to the surgeon during critical phases of the procedure
• Can be monitored from multiple channels simultaneously
• (i.e., Multiple myotomes and nerve roots)
Background
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• EMG is recorded from limb muscles using paired intramuscular needle or wire electrodes
Types of electrodes
Recording Electrodes and Parameters
Anurag Tewari MD
Hooked wire electrodes can be used for deeper muscles such as iliacus or rectus abdominus
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
Typical recording parameters for intraoperative EMG are
Recording Electrodes and Parameters
Anurag Tewari MD
Free Running EMG Stimulus-Triggered EMG
Filters 20–10,000 Hz 10–10,000 Hz
Gain 50 μV/div 50 μV/div
Time Base 100 ms/div 10 ms/div
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• Intraoperative CMAP responses are recorded from intramuscular needle electrodes and
submaximal stimulation and are highly complicated polyphasic responses with
• Variable onset Latencies and response Amplitudes
Recording Electrodes and Parameters
Anurag Tewari MD
CMAP responses from intraoperative stimulus-triggered EMG
CMAP response in tibialis anterior from stimulation of an L5 pedicle
screw at 5.3 mA, suggesting pedicular breach
CMAP responses recorded after direct stimulation of exposed cauda
equina (as a positive control) at 4 mA
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• In effect they are all or nothing responses, although the stimulus threshold can
provide some quantitative information regarding nerve function
• A monopolar stimulating electrode is generally preferable during lumbosacral
spine surgeries to avoid current shunting from fluid within the operative field
Stimulating electrodes and parameters
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• The stimulator is a sterile handheld electrode handed to the surgeon during the
procedure
• The surgeon inserts the stimulator into pedicle holes and touches the
stimulator against pedicle screws or any other structure in the operative field
Stimulating electrodes and parameters
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• Although the surgeon holds the stimulating electrode, the IONM technologist
controls the electrical stimulator and will need to be sure that it is active when
the surgeon wants to stimulate
• Stimulation can be with single or recurrent shocks, with a stimulus duration of
0.2 ms and an initial “searching” stimulus intensity of 7 mA
Stimulating electrodes and parameters
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• Recurrent stimuli are delivered at approximately 2 Hz, usually 2.3 Hz to avoid an
exact multiple of 60 and potential artifact, with a stimulus intensity range of 0
to 25 mA
• We notify the surgeon if a CMAP response is seen at the searching stimulus
intensity and then progressively reduce the stimulus intensity and determine
the precise stimulus threshold
Stimulating electrodes and parameters
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
It is important to Discuss The Case With The Surgeon Ahead Of Time to decide
which muscles to monitor, as there are a limited number of available recording
channels and it may not be possible to relocate recording EMG electrodes after the
patent has been prepared and draped for the procedure
Muscle Selection
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• Intramuscular EMG electrodes are usually inserted into the anesthetized
patient without the assistance of voluntary activation to confirm satisfactory
needle placement, and it is important to use muscles that are easily identified
using surface landmarks to assure accurate needle placement
• It is helpful to have an EMG anatomy atlas readily available in the operating
room to check the innervation or location of unfamiliar muscles
Muscle Selection
Anurag Tewari MD
 http://teleemg.sectorlink.org/new/atlas.htm
 EMG Muscle Identification PDF
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• Continuous free-running EMG is monitored from muscles innervated by nerve roots
considered to be at risk for injury during surgery
Technique
Anurag Tewari MD
IONM for Lumbosacral Surgery
Nerve roots at
risk
Appropriate muscle for EMG monitoring
T7–12 External oblique and Rectus abdominus
L1–2 Iliacus
L2–4 Vastus medialis
L4–5 Tibialis anterior
S1–2 Medial gastrocnemius
S3–5 Anal and urethral sphincter
Muscles suitable for Intraoperative Electromyographic Monitoring of Thoraco-lumbosacral
Nerve Root Segments during Lumbar Fusion
Anurag Tewari MD
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• Blunt mechanical trauma to nerve roots causes a depolarization that will be conducted
down the nerve, across the neuromuscular junction, and evoke identifiable motor unit
potentials in the monitored muscle
Technique
Anurag Tewari MD
EMG monitoring should be quiescent
under normal conditions
Blunt mechanical nerve root irritation
activates the motor nerve fibers, is
transmitted down the nerve and across
the neuromuscular junction, and
evokes recordable motor unit
potentials in the monitored muscle
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• Minor nerve root manipulation will result in a short burst of motor unit potentials
Technique
Anurag Tewari MD
A. Minor nerve root manipulation evokes a short burst of motor unit
potentials
B and C. More severe blunt mechanical nerve root injury or retraction
evokes prolonged trains of high-frequency motor unit potentials, known as
neurotonic discharges
D. Rarely, myokymic discharges can be seen, but usually only after very severe
nerve injury
In each case the time base shown is 100 ms/div and the display sensitivity is 50 μV/div.
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• More severe mechanical nerve root injury or retraction will cause prolonged trains of high-
frequency motor unit potentials, often referred to as neurotonic discharges
Technique
Anurag Tewari MD
NERVE ROOT MONITORING USING FREE-RUNNING EMG
• Repetitive grouped motor unit potentials, MYOKYMIC DISCHARGES, are seen less frequently
and usually indicate very Severe Nerve Injury
• Identification of this abnormal EMG activity can be used to alert the surgeon of inadvertent
trauma to nerve roots
• Allowing evasive action in an effort to prevent more severe or irreversible injury
Technique
Anurag Tewari MD
NERVE ROOT MONITORING USING FREE-RUNNING EMG
The ANAL and/or URETHRAL SPHINCTER muscles should be monitored in cases where the
conus medullaris or lower sacral nerve root are at particular risk for iatrogenic injury
Technique
Anurag Tewari MD
IONM for Lumbosacral Surgery
• EMG monitoring has a high sensitivity for detecting mechanical nerve root manipulation
but
• Has a low specificity and positive predictive value for postoperative nerve root injury
• Significant EMG activity occurs in 80% of monitored lumbosacral spine surgeries
• Although less that 10% of these develop a new persistent postoperative neurologic deficit
Outcome Data
Anurag Tewari MD
https://www.ncbi.nlm.nih.gov/pubmed/15014279
IONM for Lumbosacral Surgery
• SSEP monitoring has a lower sensitivity, but a higher specificity and positive predictive value
• However, SENSITIVITY is preferable to specificity for IONM
• As it is more important to detect a mild degree of nerve root irritation and alert the surgeon in time to take
evasive action, than it is to correctly predict an irreversible postoperative deficit
Outcome Data
Anurag Tewari MD
Sensitivity and Specificity of EMG vs. SEP Monitoring for Detecting 14 New Postoperative
Deficits in 213 Monitored Lumbar Spine Cases
IONM for Lumbosacral Surgery
• There are many published anecdotal case reports touting the benefits of intraoperative EMG
monitoring during lumbar spine surgery, there is a paucity of prospective outcome data
• Some studies have shown reduced incidence of postoperative C5 radiculopathy after
cervical decompression using EMG monitoring from the deltoid muscle, and cases detected
during surgery with EMG monitoring were felt to have been milder, with a more rapid
recovery because the surgeon was alerted immediately
Outcome Data
Anurag Tewari MD
Study EMG Monitoring No EMG Monitoring
Jimenez, 2005 1/116 (0.9%) 4/55 (7.3%)
Fan, 2002 2/68 (2.9%) 6/132 (4.5%)
IONM for Lumbosacral Surgery
• Although EMG monitoring is sensitive for blunt mechanical irritation and injury to motor
nerves, it may remain QUIESCENT DURING SHARP NERVE TRANSECTION
• Even after acute nerve transection the distal nerve stump can still be activated by
mechanical irritation and electrical stimulation
• The presence of these ongoing evoked EMG responses may be mistakenly interpreted as evidence of nerve
root continuity
Precautions
Anurag Tewari MD
IONM for Lumbosacral Surgery
• Mechanical trauma is less likely to evoke neurotonic discharges from abnormal motor nerves
• And EMG monitoring may miss additional intraoperative injury to nerve roots already partially injured by
preexisting lumbosacral radiculopathy
Precautions
Anurag Tewari MD
IONM for Lumbosacral Surgery
• Neurotonic discharges must be differentiated from other EMG findings that can occur during surgery
without nerve injury to avoid false alarms
In patients with preexisting radiculopathy and denervation, Spontaneous Fibrillation Potentials may be
seen throughout surgery
Precautions
Anurag Tewari MD
Spontaneous fibrillation potentials may been seen throughout surgery in patients
with preexisting radiculopathy and denervation
Voluntary motor unit potentials can be seen under conditions
of light anesthesia
Electrocautery artifact seen in EMGNeedle electrode movement artifact
IONM for Lumbosacral Surgery
• Spontaneous fibrillation potentials can be distinguished from neurotonic discharges because
they occur continuously (even before critical phases of surgery) and fire regularly at slow
rates (less than 15 Hz)
• Unlike neurotonic discharges, fibrillations are action potentials of single muscle fibers and
not motor units, so they are simple tri-phasic or biphasic spikes
Precautions
Anurag Tewari MD
IONM for Lumbosacral Surgery
• Voluntary motor unit potentials can be seen under conditions of light anesthesia
• These are semi-rhythmic motor unit potentials that usually occur simultaneously in muscles
from multiple bilateral myotomes
• Electrocautery artifact is easy to recognize but precludes concurrent EMG monitoring
Precautions
Anurag Tewari MD
IONM for Lumbosacral Surgery
• As nerve activation from mechanical irritation is transmitted across the
neuromuscular junction in order to evoke identifiable myogenic responses
• EMG cannot be monitored in the presence of total pharmacologic
neuromuscular blockade
Precautions
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Many spinal instrumentation systems use pedicle screws as a point of fixation
• Holes are drilled blindly through the narrow pedicles into the vertebral body to
facilitate pedicle screw placement
Background
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Clinical studies have detected postoperative symptoms from irritation or injury
to the adjacent nerve roots by misplaced pedicle screws in 5% to 10% cases
• Intraoperative fluoroscopy and radiographs have been used in an attempt to
verify correct pedicle screw placement during surgery but are not always
reliable, perhaps because of limited available projections
Background
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Computed tomography is more definitive, but is not available in real
time in OR
• Direct inspection and palpation of the medial wall of the
instrumented pedicle by the surgeon is the “gold standard” test for
misplaced screws during surgery
• but doing this for every screw may necessitate unnecessary laminectomies
and disproportionate operative times
Background
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Fortunately, stimulus-triggered EMG can quickly verify correct pedicle screw
placement in real time during surgery
Background
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Holes or screws that are correctly positioned within the pedicle wall are
separated from the adjacent nerve roots by a cortical bony layer with high
impedance to the passage of electrical current
Background
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• However, a hole or screw that has perforated the bony pedicle wall will lie
directly against adjacent nerve roots
• Direct electrical stimulation of such misplaced holes and screws activates the
adjacent nerve roots, evoking CMAP responses in muscles from the appropriate
myotomes at lower stimulus intensities than those that lie completely
contained within the pedicle wall
Background
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Stimulus thresholds of less than 4 to 6 mA are suggestive of cortical bony perforation by
pedicular instrumentation when the adjacent nerve root is healthy
Background
Anurag Tewari MD
Structure Stimulus Threshold (mA)
Normal nerve root (0.2–5.7)
Chronically compressed nerve root 6.3–20
Normal hole (16.5–44.3)
Normal screw 24 (12.1–35.9)
Misplaced hole (1–6)
Misplaced screw (1–6)
Stimulus Thresholds for Normal Healthy Nerve Roots, Chronically Compressed Nerve Roots, and Normal and Misplaced Pedicle Holes and Screws
Maguire J, Wallace S, Madiga R, et al. Evaluation of intrapedicular screw position using intraoperative evoked electromyography. Spine 1995;20:1068–1074.
Raynor B, Lenke LG, Bridwell K, et al. Correlation between low triggered EMG thresholds and lumbar pedicle screw malposition: analysis of 4587 screws. Presented at the Scoliosis Research Society, 2005.
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Holes and screws are tested using a monopolar stimulating electrode with a
remote anode needle electrode placed in the surgical wound and stimulus-
triggered EMG recordings made from appropriate limb muscles
• The stimulator is inserted directly into the pedicle hole after drilling
Technique
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• It is preferable to have the stimulator halfway into the hole, so that the
uninsulated tip of the electrode is inside the pedicle close to the adjacent nerve
root and not deep inside the vertebral body
• Screws are tested by touching the stimulator against the exposed shank or port
of the screw, taking care to avoid any intervening soft tissue
Technique
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• It is important to test each pedicle hole and screw individually as each is drilled
and inserted
• A misplaced hole should be redirected and then retested before it is
instrumented
Technique
Anurag Tewari MD
Some particular brands of pedicle screw have an unusually high electrical resistance based on their metallic composition, so just testing the screw (and not the preceding hole) may give rise to a false-negative result
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• To save time, each hole and screw can be initially tested at a searching stimulus intensity of 7
to 10 mA
• If no CMAP response is identified at this intensity level, then the hole or screw is considered
safe
• If a CMAP response is identified at the initial searching intensity, the stimulating current is
progressively reduced, using recurrent stimulation at 2 Hz, in order to determine the precise
stimulus threshold required to evoke the response and hence the probability of pedicular
perforation
Technique
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Pedicle screws or holes with stimulus thresholds of less than 4 mA should be
removed and/or redirected
• Pedicle screws with stimulus thresholds between 4 and 6 mA are considered
borderline and should probably be more closely inspected by the surgeon
• The very same technique of stimulus-triggered EMG using a handheld stimulator in the
operative field can also be used to identify viable nerve roots during dissection of intra-
spinal dumbbell and foraminal tumors and spinal surgery for tethered cord release
Technique
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Based on a stimulus threshold of 6 mA or less, the sensitivity of stimulus-triggered EMG for
identifying misplaced pedicle screws and hole is more than 90%,
• significantly exceeding the sensitivity of intraoperative radiography, which is only 63%
• The probability of a pedicle screw breach increases with decreasing stimulus threshold
Outcome Data
Anurag Tewari MD
Stimulus Threshold and Pedicle Screw Malposition (Confirmed by Palpation and/or Radiographs) for 4,587 Screws From Raynor et al.
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• It is important to recognize that the stimulus thresholds mentioned above refer
to pedicular instrumentation at levels where the bone and adjacent nerve roots
are normal and healthy
• False-positive stimulus-triggered EMGs can be seen in patients with advanced
osteoporosis, presumably because of thin cortical bone with a lower than
expected impedance to the passage of the electrical current
Precautions
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• More importantly, false-negative results may be seen in patients with preexisting
radiculopathy
• The stimulus intensities required to evoke CMAP responses from direct electrical stimulation
of nerve roots are higher in the presence of preexisting axonotmetic radiculopathy,
sometimes as high as 10 to 20 mA
Precautions
Anurag Tewari MD
Stimulus Thresholds for Normal Healthy Nerve Roots, Chronically Compressed Nerve Roots, and Normal and Misplaced Pedicle Holes and Screws
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Thus the failure to evoke a CMAP response from stimulation of a hole or screw
adjacent to a chronically compressed nerve root at the usual searching stimulus
intensity of 7 mA will not necessarily exclude a pedicular wall breach at that
level, with the potential for further iatrogenic nerve root injury
Precautions
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• Clearly, pedicular instrumentation must be tested at higher stimulus intensities
when the adjacent nerve root is known to be axonotmetic
• Axonotmetic nerve root injury should be suspected in the presence of chronic
preoperative radicular motor deficits or abnormal preoperative
electromyographic studies
Precautions
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• When possible it is best to first determine the electrical thresholds required to
evoke CMAP responses in the appropriate muscles from direct stimulation of
the exposed chronically compressed nerve root
• And then test the pedicular instrumentation at that level at an appropriately
higher intensity (perhaps 5 mA higher) in order to exclude a bony cortical
breach
• If the nerve root can not be directly tested, for example if there is no
laminectomy at that level, then it is probably best to empirically test the holes
and screws at a higher searching intensity such as 20 mA
Precautions
Anurag Tewari MD
STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS
• EMG cannot be monitored in the presence of total pharmacologic neuromuscular blockade
• because nerve activation from electrical stimulation must be transmitted across the
neuromuscular junction in order to evoke identifiable CMAP responses
In the presence of partial neuromuscular blockade, a physiologically decremental
response may suggest a falsely high threshold and lead to a false-negative result
• Under these circumstances, it is better to use single shocks at an initial stimulus intensity
where a positive response is likely to be significant (e.g., 4 mA)
Precautions
Anurag Tewari MD
MONITORING NEUROMUSCULAR BLOCKADE
• Nerve activation, whether from mechanical irritation or electrical stimulation,
must be transmitted across the neuromuscular junction in order to evoke
identifiable myogenic responses
• However, muscle relaxation is a critical component of balanced anesthesia
Background
Anurag Tewari MD
MONITORING NEUROMUSCULAR BLOCKADE
• While adequate surgical relaxation may be achieved using inhalation anesthetic
agents alone, patients are more likely to move in the absence of specific
pharmacologic neuromuscular blockade, and this may compromise patient
safety
• Fortunately, EMG can be successfully monitored in the operating room under
conditions of partial (as much as 50% to 75%) neuromuscular blockade using
carefully titrated doses of short-acting agents such as Rocuronium
Background
Anurag Tewari MD
MONITORING NEUROMUSCULAR BLOCKADE
• Neuromuscular blockade has been traditionally monitored by the
anesthesiologist during surgery using the number of visible hand twitches after
a train-of-four peripheral stimulations to the ulnar nerve at the forearm
• However, unfamiliarity with peripheral nerve function and anatomy may result
in inaccurate electrode placement and misinterpretation of direct muscle
activation as twitches by the anesthesiologist, underestimating the degree of
pharmacologic neuromuscular blockade
Background
Anurag Tewari MD
MONITORING NEUROMUSCULAR BLOCKADE
• Muscles from the cranial, cervical, and lumbosacral myotomes have different
susceptibilities to the effects of neuromuscular blocking agents
• In addition, chronically compressed nerves may have enhanced sensitivity to
the effects of neuromuscular blockade
• For these reasons, neuromuscular transmission should be monitored using
repetitive nerve stimulation from a muscle belonging to the appropriate
myotome for the nerves or roots considered at risk from surgery
Background
Anurag Tewari MD
MONITORING NEUROMUSCULAR BLOCKADE
• It is more usual to monitor neuromuscular transmission in anesthetized
patients using repetitive nerve stimulation
• Less than 75% neuromuscular blockade corresponds to a decrement of less
than 100% over four successive supramaximal repetitive nerve stimulations
Technique
Anurag Tewari MD
MONITORING NEUROMUSCULAR BLOCKADE
• It is easy to monitor repetitive nerve stimulation from the tibialis anterior
muscles during lumbar spine surgery because this muscle is routinely used for
EMG monitoring during these cases
• The deep peroneal nerve can be stimulated transcutaneously using needle or
surface electrodes at the fibula head
Technique
Anurag Tewari MD
MONITORING NEUROMUSCULAR BLOCKADE
• Less than four identifiable CMAP responses indicates pharmacologic neuromuscular
blockade exceeding 75%, which might lower the sensitivity of EMG monitoring for the
detection of intraoperative nerve root injury
• As long as four CMAP responses are identifiable from supramaximal repetitive nerve
stimulation at 2 Hz, the degree of neuromuscular blockade is sufficiently low to allow EMG
monitoring
Technique
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• Many patients undergoing lumbosacral spine surgeries will already have
preoperative weakness
• Their preoperative EMG reports may show evidence of chronic motor axonal
loss from lumbar radiculopathy in the form of low amplitude CMAP responses
or denervation on needle EMG examination
• These patients have preexisting axonotmetic lumbar nerve root injuries,
necessitating special precautions to exclude pedicular breach with stimulus
triggered EMG at those levels
Preoperative Studies
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• Neuromuscular blockade must be carefully monitored and documented
throughout surgery
• The expected and preferred result is quiescent EMG monitoring, but quiescent
EMG is only meaningful when the IONMtechnologist is able to demonstrate
less than 75% pharmacologic blockade at the same time
Neuromuscular Blockade
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• Otherwise the false-negative absence of EMG activity may give the surgeon a
false sense of security and give rise to a surgical outcome that is worse than it
would have been with no monitoring at all
• Hence, the surgeon should be notified right away whenever excessive
pharmacologic neuromuscular blockade is detected; he or she will then likely
order reversal if approaching a critical phase of the procedure
Neuromuscular Blockade
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• Only one ground electrode should be attached to the patient, preferably a large adhesive
surface electrode, which has a higher contact area than a needle and therefore receives a
reduced current density in the event of current leakage,
• Thus minimizing the risk of a skin burn
• All electrical devices attached to the patient should share the same ground circuit as the
EMG machine
• The EMG machine should be turned on before any electrodes are attached to the patient
and turned off only after all electrodes have been disconnected so as to avoid power surges
that could be transmitted to the patient
Patient Safety
Anurag Tewari MD
TECHNICAL CONSIDERATIONS for Lumbosacral Surgery
• There have been skin burns at the site of EMG needle electrode insertion because of current
shunting from faulty electrocautery units during surgery; and the following steps should be
taken to avoid this:
• Ground pads from all electrocautery units must be properly positioned to ensure a large
area of contact with the patient’s skin and placed as close to the surgical site as possible
• Each electrocautery unit should contain a built-in ground monitoring system to warn the
surgeon in case of circuit discontinuity with a potential for current shunting through other
electrodes attached to the patient
• Needle electrodes should not be placed in close proximity to electrocautery ground pads
and, if possible, should not be placed within the predicted pathway between an active
electrocautery electrode and its ground pad
• Finally, electrocautery units, EMG monitoring devices, and all operating room electrical
circuits should be regularly checked for current leakage
Patient Safety
Anurag Tewari MD
Anurag Tewari MD

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IONM for Lumbosacral Surgery

  • 2. IONM for Lumbosacral Surgery The adult spinal cord ends opposite the lower border of the L1 vertebra Introduction Anurag Tewari MD
  • 3. IONM for Lumbosacral Surgery • Below that, the spinal canal contains only individual lumbosacral nerve roots descending toward their respective neural foramina, collectively known as the cauda equina Introduction Anurag Tewari MD
  • 4. IONM for Lumbosacral Surgery • There is a 10% risk of new postoperative neurologic deficit from Nerve Root Injury during spinal fusion for degenerative lumbosacral spine disease Introduction Anurag Tewari MD 10-fold greater than the risk of spinal cord injury from scoliosis surgery
  • 5. IONM for Lumbosacral Surgery • Correction of spinal deformity during lumbosacral surgery can injure Nerve Roots by • PROBING • STRETCHING • DRILLING • MISPLACED HARDWARE Introduction Anurag Tewari MD
  • 6. IONM for Lumbosacral Surgery The most common deficit is a new FOOT DROP from surgical injury to the L5 nerve root, although findings can include any pattern of • Dermatomal numbness • Radicular weakness • Sphincter dysfunction Introduction Anurag Tewari MD
  • 7. IONM for Lumbosacral Surgery Nerve Injury Anurag Tewari MD
  • 8. IONM for Lumbosacral Surgery Nerve Injury Anurag Tewari MD
  • 9. IONM for Lumbosacral Surgery • Nerve root injuries are more frequent during revision than primary surgery and in cases where multiple levels are fused • Hence, the purpose of IONM with EMG during these lumbosacral spine cases is: • DETECT early and potentially reversible surgical nerve root irritation • ALERT the surgeon • To prevent more significant injury and postoperative deficit(s) Introduction Anurag Tewari MD RESULTS: The overall incidence of lumbar nerve root palsy was 2.9% with a 1.4% incidence in primary and 3.8% incidence in revision surgery https://www.ncbi.nlm.nih.gov/pubmed/16025033
  • 10. NERVE ROOT MONITORING USING FREE-RUNNING EMG • Mixed nerve SSEPs and TcMEPs are most often used for spinal cord monitoring but THEY ARE NOT SENSITIVE FOR DETECTING NERVE ROOT INJURIES WHY? • SSEPs represent the summation of neural signals that enter the spinal cord through multiple segments, and because of central amplification, they can remain completely unchanged after an individual nerve root lesion Background Anurag Tewari MD
  • 11. NERVE ROOT MONITORING USING FREE-RUNNING EMG • TcMEPs recorded from an epidural electrode will obviously not be affected by a more caudal nerve root injury • TcMEPs recorded from a single distal limb muscle throughout surgery will not detect nerve root injuries affecting other myotomes There have been numerous well-documented cases of patients with acute radiculopathy from spine surgery despite normal SSEP and TcMEP monitoring Background Anurag Tewari MD
  • 12. NERVE ROOT MONITORING USING FREE-RUNNING EMG • Dermatomal somatosensory evoked potentials (D-SSEPs) can detect individual nerve root injuries but are technically difficult to resolve in the operating room • They need to be averaged many times • Resulting a slow turnaround time • They test only dorsal root (sensory) function Background Anurag Tewari MD
  • 13. NERVE ROOT MONITORING USING FREE-RUNNING EMG • For these reasons, EMG has become the technique most often used to monitor nerve root function during lumbosacral spine surgery • EMG is more sensitive than SSEPs and D-SSEPs for detecting nerve root dysfunction, as it • Provides immediate results without averaging • Gives instantaneous feedback to the surgeon during critical phases of the procedure • Can be monitored from multiple channels simultaneously • (i.e., Multiple myotomes and nerve roots) Background Anurag Tewari MD
  • 14. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • EMG is recorded from limb muscles using paired intramuscular needle or wire electrodes Types of electrodes Recording Electrodes and Parameters Anurag Tewari MD Hooked wire electrodes can be used for deeper muscles such as iliacus or rectus abdominus
  • 15. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery Typical recording parameters for intraoperative EMG are Recording Electrodes and Parameters Anurag Tewari MD Free Running EMG Stimulus-Triggered EMG Filters 20–10,000 Hz 10–10,000 Hz Gain 50 μV/div 50 μV/div Time Base 100 ms/div 10 ms/div
  • 16. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • Intraoperative CMAP responses are recorded from intramuscular needle electrodes and submaximal stimulation and are highly complicated polyphasic responses with • Variable onset Latencies and response Amplitudes Recording Electrodes and Parameters Anurag Tewari MD CMAP responses from intraoperative stimulus-triggered EMG CMAP response in tibialis anterior from stimulation of an L5 pedicle screw at 5.3 mA, suggesting pedicular breach CMAP responses recorded after direct stimulation of exposed cauda equina (as a positive control) at 4 mA
  • 17. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • In effect they are all or nothing responses, although the stimulus threshold can provide some quantitative information regarding nerve function • A monopolar stimulating electrode is generally preferable during lumbosacral spine surgeries to avoid current shunting from fluid within the operative field Stimulating electrodes and parameters Anurag Tewari MD
  • 18. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • The stimulator is a sterile handheld electrode handed to the surgeon during the procedure • The surgeon inserts the stimulator into pedicle holes and touches the stimulator against pedicle screws or any other structure in the operative field Stimulating electrodes and parameters Anurag Tewari MD
  • 19. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • Although the surgeon holds the stimulating electrode, the IONM technologist controls the electrical stimulator and will need to be sure that it is active when the surgeon wants to stimulate • Stimulation can be with single or recurrent shocks, with a stimulus duration of 0.2 ms and an initial “searching” stimulus intensity of 7 mA Stimulating electrodes and parameters Anurag Tewari MD
  • 20. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • Recurrent stimuli are delivered at approximately 2 Hz, usually 2.3 Hz to avoid an exact multiple of 60 and potential artifact, with a stimulus intensity range of 0 to 25 mA • We notify the surgeon if a CMAP response is seen at the searching stimulus intensity and then progressively reduce the stimulus intensity and determine the precise stimulus threshold Stimulating electrodes and parameters Anurag Tewari MD
  • 21. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery It is important to Discuss The Case With The Surgeon Ahead Of Time to decide which muscles to monitor, as there are a limited number of available recording channels and it may not be possible to relocate recording EMG electrodes after the patent has been prepared and draped for the procedure Muscle Selection Anurag Tewari MD
  • 22. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • Intramuscular EMG electrodes are usually inserted into the anesthetized patient without the assistance of voluntary activation to confirm satisfactory needle placement, and it is important to use muscles that are easily identified using surface landmarks to assure accurate needle placement • It is helpful to have an EMG anatomy atlas readily available in the operating room to check the innervation or location of unfamiliar muscles Muscle Selection Anurag Tewari MD  http://teleemg.sectorlink.org/new/atlas.htm  EMG Muscle Identification PDF
  • 23. NERVE ROOT MONITORING USING FREE-RUNNING EMG • Continuous free-running EMG is monitored from muscles innervated by nerve roots considered to be at risk for injury during surgery Technique Anurag Tewari MD
  • 24. IONM for Lumbosacral Surgery Nerve roots at risk Appropriate muscle for EMG monitoring T7–12 External oblique and Rectus abdominus L1–2 Iliacus L2–4 Vastus medialis L4–5 Tibialis anterior S1–2 Medial gastrocnemius S3–5 Anal and urethral sphincter Muscles suitable for Intraoperative Electromyographic Monitoring of Thoraco-lumbosacral Nerve Root Segments during Lumbar Fusion Anurag Tewari MD
  • 25. NERVE ROOT MONITORING USING FREE-RUNNING EMG • Blunt mechanical trauma to nerve roots causes a depolarization that will be conducted down the nerve, across the neuromuscular junction, and evoke identifiable motor unit potentials in the monitored muscle Technique Anurag Tewari MD EMG monitoring should be quiescent under normal conditions Blunt mechanical nerve root irritation activates the motor nerve fibers, is transmitted down the nerve and across the neuromuscular junction, and evokes recordable motor unit potentials in the monitored muscle
  • 26. NERVE ROOT MONITORING USING FREE-RUNNING EMG • Minor nerve root manipulation will result in a short burst of motor unit potentials Technique Anurag Tewari MD A. Minor nerve root manipulation evokes a short burst of motor unit potentials B and C. More severe blunt mechanical nerve root injury or retraction evokes prolonged trains of high-frequency motor unit potentials, known as neurotonic discharges D. Rarely, myokymic discharges can be seen, but usually only after very severe nerve injury In each case the time base shown is 100 ms/div and the display sensitivity is 50 μV/div.
  • 27. NERVE ROOT MONITORING USING FREE-RUNNING EMG • More severe mechanical nerve root injury or retraction will cause prolonged trains of high- frequency motor unit potentials, often referred to as neurotonic discharges Technique Anurag Tewari MD
  • 28. NERVE ROOT MONITORING USING FREE-RUNNING EMG • Repetitive grouped motor unit potentials, MYOKYMIC DISCHARGES, are seen less frequently and usually indicate very Severe Nerve Injury • Identification of this abnormal EMG activity can be used to alert the surgeon of inadvertent trauma to nerve roots • Allowing evasive action in an effort to prevent more severe or irreversible injury Technique Anurag Tewari MD
  • 29. NERVE ROOT MONITORING USING FREE-RUNNING EMG The ANAL and/or URETHRAL SPHINCTER muscles should be monitored in cases where the conus medullaris or lower sacral nerve root are at particular risk for iatrogenic injury Technique Anurag Tewari MD
  • 30. IONM for Lumbosacral Surgery • EMG monitoring has a high sensitivity for detecting mechanical nerve root manipulation but • Has a low specificity and positive predictive value for postoperative nerve root injury • Significant EMG activity occurs in 80% of monitored lumbosacral spine surgeries • Although less that 10% of these develop a new persistent postoperative neurologic deficit Outcome Data Anurag Tewari MD https://www.ncbi.nlm.nih.gov/pubmed/15014279
  • 31. IONM for Lumbosacral Surgery • SSEP monitoring has a lower sensitivity, but a higher specificity and positive predictive value • However, SENSITIVITY is preferable to specificity for IONM • As it is more important to detect a mild degree of nerve root irritation and alert the surgeon in time to take evasive action, than it is to correctly predict an irreversible postoperative deficit Outcome Data Anurag Tewari MD Sensitivity and Specificity of EMG vs. SEP Monitoring for Detecting 14 New Postoperative Deficits in 213 Monitored Lumbar Spine Cases
  • 32. IONM for Lumbosacral Surgery • There are many published anecdotal case reports touting the benefits of intraoperative EMG monitoring during lumbar spine surgery, there is a paucity of prospective outcome data • Some studies have shown reduced incidence of postoperative C5 radiculopathy after cervical decompression using EMG monitoring from the deltoid muscle, and cases detected during surgery with EMG monitoring were felt to have been milder, with a more rapid recovery because the surgeon was alerted immediately Outcome Data Anurag Tewari MD Study EMG Monitoring No EMG Monitoring Jimenez, 2005 1/116 (0.9%) 4/55 (7.3%) Fan, 2002 2/68 (2.9%) 6/132 (4.5%)
  • 33. IONM for Lumbosacral Surgery • Although EMG monitoring is sensitive for blunt mechanical irritation and injury to motor nerves, it may remain QUIESCENT DURING SHARP NERVE TRANSECTION • Even after acute nerve transection the distal nerve stump can still be activated by mechanical irritation and electrical stimulation • The presence of these ongoing evoked EMG responses may be mistakenly interpreted as evidence of nerve root continuity Precautions Anurag Tewari MD
  • 34. IONM for Lumbosacral Surgery • Mechanical trauma is less likely to evoke neurotonic discharges from abnormal motor nerves • And EMG monitoring may miss additional intraoperative injury to nerve roots already partially injured by preexisting lumbosacral radiculopathy Precautions Anurag Tewari MD
  • 35. IONM for Lumbosacral Surgery • Neurotonic discharges must be differentiated from other EMG findings that can occur during surgery without nerve injury to avoid false alarms In patients with preexisting radiculopathy and denervation, Spontaneous Fibrillation Potentials may be seen throughout surgery Precautions Anurag Tewari MD Spontaneous fibrillation potentials may been seen throughout surgery in patients with preexisting radiculopathy and denervation Voluntary motor unit potentials can be seen under conditions of light anesthesia Electrocautery artifact seen in EMGNeedle electrode movement artifact
  • 36. IONM for Lumbosacral Surgery • Spontaneous fibrillation potentials can be distinguished from neurotonic discharges because they occur continuously (even before critical phases of surgery) and fire regularly at slow rates (less than 15 Hz) • Unlike neurotonic discharges, fibrillations are action potentials of single muscle fibers and not motor units, so they are simple tri-phasic or biphasic spikes Precautions Anurag Tewari MD
  • 37. IONM for Lumbosacral Surgery • Voluntary motor unit potentials can be seen under conditions of light anesthesia • These are semi-rhythmic motor unit potentials that usually occur simultaneously in muscles from multiple bilateral myotomes • Electrocautery artifact is easy to recognize but precludes concurrent EMG monitoring Precautions Anurag Tewari MD
  • 38. IONM for Lumbosacral Surgery • As nerve activation from mechanical irritation is transmitted across the neuromuscular junction in order to evoke identifiable myogenic responses • EMG cannot be monitored in the presence of total pharmacologic neuromuscular blockade Precautions Anurag Tewari MD
  • 39. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Many spinal instrumentation systems use pedicle screws as a point of fixation • Holes are drilled blindly through the narrow pedicles into the vertebral body to facilitate pedicle screw placement Background Anurag Tewari MD
  • 40. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Clinical studies have detected postoperative symptoms from irritation or injury to the adjacent nerve roots by misplaced pedicle screws in 5% to 10% cases • Intraoperative fluoroscopy and radiographs have been used in an attempt to verify correct pedicle screw placement during surgery but are not always reliable, perhaps because of limited available projections Background Anurag Tewari MD
  • 41. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Computed tomography is more definitive, but is not available in real time in OR • Direct inspection and palpation of the medial wall of the instrumented pedicle by the surgeon is the “gold standard” test for misplaced screws during surgery • but doing this for every screw may necessitate unnecessary laminectomies and disproportionate operative times Background Anurag Tewari MD
  • 42. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Fortunately, stimulus-triggered EMG can quickly verify correct pedicle screw placement in real time during surgery Background Anurag Tewari MD
  • 43. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Holes or screws that are correctly positioned within the pedicle wall are separated from the adjacent nerve roots by a cortical bony layer with high impedance to the passage of electrical current Background Anurag Tewari MD
  • 44. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • However, a hole or screw that has perforated the bony pedicle wall will lie directly against adjacent nerve roots • Direct electrical stimulation of such misplaced holes and screws activates the adjacent nerve roots, evoking CMAP responses in muscles from the appropriate myotomes at lower stimulus intensities than those that lie completely contained within the pedicle wall Background Anurag Tewari MD
  • 45. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Stimulus thresholds of less than 4 to 6 mA are suggestive of cortical bony perforation by pedicular instrumentation when the adjacent nerve root is healthy Background Anurag Tewari MD Structure Stimulus Threshold (mA) Normal nerve root (0.2–5.7) Chronically compressed nerve root 6.3–20 Normal hole (16.5–44.3) Normal screw 24 (12.1–35.9) Misplaced hole (1–6) Misplaced screw (1–6) Stimulus Thresholds for Normal Healthy Nerve Roots, Chronically Compressed Nerve Roots, and Normal and Misplaced Pedicle Holes and Screws Maguire J, Wallace S, Madiga R, et al. Evaluation of intrapedicular screw position using intraoperative evoked electromyography. Spine 1995;20:1068–1074. Raynor B, Lenke LG, Bridwell K, et al. Correlation between low triggered EMG thresholds and lumbar pedicle screw malposition: analysis of 4587 screws. Presented at the Scoliosis Research Society, 2005.
  • 46. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Holes and screws are tested using a monopolar stimulating electrode with a remote anode needle electrode placed in the surgical wound and stimulus- triggered EMG recordings made from appropriate limb muscles • The stimulator is inserted directly into the pedicle hole after drilling Technique Anurag Tewari MD
  • 47. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • It is preferable to have the stimulator halfway into the hole, so that the uninsulated tip of the electrode is inside the pedicle close to the adjacent nerve root and not deep inside the vertebral body • Screws are tested by touching the stimulator against the exposed shank or port of the screw, taking care to avoid any intervening soft tissue Technique Anurag Tewari MD
  • 48. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • It is important to test each pedicle hole and screw individually as each is drilled and inserted • A misplaced hole should be redirected and then retested before it is instrumented Technique Anurag Tewari MD Some particular brands of pedicle screw have an unusually high electrical resistance based on their metallic composition, so just testing the screw (and not the preceding hole) may give rise to a false-negative result
  • 49. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • To save time, each hole and screw can be initially tested at a searching stimulus intensity of 7 to 10 mA • If no CMAP response is identified at this intensity level, then the hole or screw is considered safe • If a CMAP response is identified at the initial searching intensity, the stimulating current is progressively reduced, using recurrent stimulation at 2 Hz, in order to determine the precise stimulus threshold required to evoke the response and hence the probability of pedicular perforation Technique Anurag Tewari MD
  • 50. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Pedicle screws or holes with stimulus thresholds of less than 4 mA should be removed and/or redirected • Pedicle screws with stimulus thresholds between 4 and 6 mA are considered borderline and should probably be more closely inspected by the surgeon • The very same technique of stimulus-triggered EMG using a handheld stimulator in the operative field can also be used to identify viable nerve roots during dissection of intra- spinal dumbbell and foraminal tumors and spinal surgery for tethered cord release Technique Anurag Tewari MD
  • 51. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Based on a stimulus threshold of 6 mA or less, the sensitivity of stimulus-triggered EMG for identifying misplaced pedicle screws and hole is more than 90%, • significantly exceeding the sensitivity of intraoperative radiography, which is only 63% • The probability of a pedicle screw breach increases with decreasing stimulus threshold Outcome Data Anurag Tewari MD Stimulus Threshold and Pedicle Screw Malposition (Confirmed by Palpation and/or Radiographs) for 4,587 Screws From Raynor et al.
  • 52. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • It is important to recognize that the stimulus thresholds mentioned above refer to pedicular instrumentation at levels where the bone and adjacent nerve roots are normal and healthy • False-positive stimulus-triggered EMGs can be seen in patients with advanced osteoporosis, presumably because of thin cortical bone with a lower than expected impedance to the passage of the electrical current Precautions Anurag Tewari MD
  • 53. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • More importantly, false-negative results may be seen in patients with preexisting radiculopathy • The stimulus intensities required to evoke CMAP responses from direct electrical stimulation of nerve roots are higher in the presence of preexisting axonotmetic radiculopathy, sometimes as high as 10 to 20 mA Precautions Anurag Tewari MD Stimulus Thresholds for Normal Healthy Nerve Roots, Chronically Compressed Nerve Roots, and Normal and Misplaced Pedicle Holes and Screws
  • 54. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Thus the failure to evoke a CMAP response from stimulation of a hole or screw adjacent to a chronically compressed nerve root at the usual searching stimulus intensity of 7 mA will not necessarily exclude a pedicular wall breach at that level, with the potential for further iatrogenic nerve root injury Precautions Anurag Tewari MD
  • 55. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • Clearly, pedicular instrumentation must be tested at higher stimulus intensities when the adjacent nerve root is known to be axonotmetic • Axonotmetic nerve root injury should be suspected in the presence of chronic preoperative radicular motor deficits or abnormal preoperative electromyographic studies Precautions Anurag Tewari MD
  • 56. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • When possible it is best to first determine the electrical thresholds required to evoke CMAP responses in the appropriate muscles from direct stimulation of the exposed chronically compressed nerve root • And then test the pedicular instrumentation at that level at an appropriately higher intensity (perhaps 5 mA higher) in order to exclude a bony cortical breach • If the nerve root can not be directly tested, for example if there is no laminectomy at that level, then it is probably best to empirically test the holes and screws at a higher searching intensity such as 20 mA Precautions Anurag Tewari MD
  • 57. STIMULUS-TRIGGERED EMG MONITORING TO VERIFY CORRECT PLACEMENT OF PEDICLE SCREWS • EMG cannot be monitored in the presence of total pharmacologic neuromuscular blockade • because nerve activation from electrical stimulation must be transmitted across the neuromuscular junction in order to evoke identifiable CMAP responses In the presence of partial neuromuscular blockade, a physiologically decremental response may suggest a falsely high threshold and lead to a false-negative result • Under these circumstances, it is better to use single shocks at an initial stimulus intensity where a positive response is likely to be significant (e.g., 4 mA) Precautions Anurag Tewari MD
  • 58. MONITORING NEUROMUSCULAR BLOCKADE • Nerve activation, whether from mechanical irritation or electrical stimulation, must be transmitted across the neuromuscular junction in order to evoke identifiable myogenic responses • However, muscle relaxation is a critical component of balanced anesthesia Background Anurag Tewari MD
  • 59. MONITORING NEUROMUSCULAR BLOCKADE • While adequate surgical relaxation may be achieved using inhalation anesthetic agents alone, patients are more likely to move in the absence of specific pharmacologic neuromuscular blockade, and this may compromise patient safety • Fortunately, EMG can be successfully monitored in the operating room under conditions of partial (as much as 50% to 75%) neuromuscular blockade using carefully titrated doses of short-acting agents such as Rocuronium Background Anurag Tewari MD
  • 60. MONITORING NEUROMUSCULAR BLOCKADE • Neuromuscular blockade has been traditionally monitored by the anesthesiologist during surgery using the number of visible hand twitches after a train-of-four peripheral stimulations to the ulnar nerve at the forearm • However, unfamiliarity with peripheral nerve function and anatomy may result in inaccurate electrode placement and misinterpretation of direct muscle activation as twitches by the anesthesiologist, underestimating the degree of pharmacologic neuromuscular blockade Background Anurag Tewari MD
  • 61. MONITORING NEUROMUSCULAR BLOCKADE • Muscles from the cranial, cervical, and lumbosacral myotomes have different susceptibilities to the effects of neuromuscular blocking agents • In addition, chronically compressed nerves may have enhanced sensitivity to the effects of neuromuscular blockade • For these reasons, neuromuscular transmission should be monitored using repetitive nerve stimulation from a muscle belonging to the appropriate myotome for the nerves or roots considered at risk from surgery Background Anurag Tewari MD
  • 62. MONITORING NEUROMUSCULAR BLOCKADE • It is more usual to monitor neuromuscular transmission in anesthetized patients using repetitive nerve stimulation • Less than 75% neuromuscular blockade corresponds to a decrement of less than 100% over four successive supramaximal repetitive nerve stimulations Technique Anurag Tewari MD
  • 63. MONITORING NEUROMUSCULAR BLOCKADE • It is easy to monitor repetitive nerve stimulation from the tibialis anterior muscles during lumbar spine surgery because this muscle is routinely used for EMG monitoring during these cases • The deep peroneal nerve can be stimulated transcutaneously using needle or surface electrodes at the fibula head Technique Anurag Tewari MD
  • 64. MONITORING NEUROMUSCULAR BLOCKADE • Less than four identifiable CMAP responses indicates pharmacologic neuromuscular blockade exceeding 75%, which might lower the sensitivity of EMG monitoring for the detection of intraoperative nerve root injury • As long as four CMAP responses are identifiable from supramaximal repetitive nerve stimulation at 2 Hz, the degree of neuromuscular blockade is sufficiently low to allow EMG monitoring Technique Anurag Tewari MD
  • 65. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • Many patients undergoing lumbosacral spine surgeries will already have preoperative weakness • Their preoperative EMG reports may show evidence of chronic motor axonal loss from lumbar radiculopathy in the form of low amplitude CMAP responses or denervation on needle EMG examination • These patients have preexisting axonotmetic lumbar nerve root injuries, necessitating special precautions to exclude pedicular breach with stimulus triggered EMG at those levels Preoperative Studies Anurag Tewari MD
  • 66. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • Neuromuscular blockade must be carefully monitored and documented throughout surgery • The expected and preferred result is quiescent EMG monitoring, but quiescent EMG is only meaningful when the IONMtechnologist is able to demonstrate less than 75% pharmacologic blockade at the same time Neuromuscular Blockade Anurag Tewari MD
  • 67. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • Otherwise the false-negative absence of EMG activity may give the surgeon a false sense of security and give rise to a surgical outcome that is worse than it would have been with no monitoring at all • Hence, the surgeon should be notified right away whenever excessive pharmacologic neuromuscular blockade is detected; he or she will then likely order reversal if approaching a critical phase of the procedure Neuromuscular Blockade Anurag Tewari MD
  • 68. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • Only one ground electrode should be attached to the patient, preferably a large adhesive surface electrode, which has a higher contact area than a needle and therefore receives a reduced current density in the event of current leakage, • Thus minimizing the risk of a skin burn • All electrical devices attached to the patient should share the same ground circuit as the EMG machine • The EMG machine should be turned on before any electrodes are attached to the patient and turned off only after all electrodes have been disconnected so as to avoid power surges that could be transmitted to the patient Patient Safety Anurag Tewari MD
  • 69. TECHNICAL CONSIDERATIONS for Lumbosacral Surgery • There have been skin burns at the site of EMG needle electrode insertion because of current shunting from faulty electrocautery units during surgery; and the following steps should be taken to avoid this: • Ground pads from all electrocautery units must be properly positioned to ensure a large area of contact with the patient’s skin and placed as close to the surgical site as possible • Each electrocautery unit should contain a built-in ground monitoring system to warn the surgeon in case of circuit discontinuity with a potential for current shunting through other electrodes attached to the patient • Needle electrodes should not be placed in close proximity to electrocautery ground pads and, if possible, should not be placed within the predicted pathway between an active electrocautery electrode and its ground pad • Finally, electrocautery units, EMG monitoring devices, and all operating room electrical circuits should be regularly checked for current leakage Patient Safety Anurag Tewari MD