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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_6032_Библиотеки_им_академика_М_И_Перельмана
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Figure 6
Photograph of spontaneous electromyography tracings
recorded from the following bilateral muscles: vastus
medialis (A), tibialis anterior (B), and gastrocnemius (C). Left-sided
recordings are on the left and right-sided recordings are on the right.
Activity is indicated in the left tibialis anterior muscle.
A normal resting nerve root in the anesthetized patient does not
significantly activate the muscles that it innervates, which translates to a flat
line and silence on sEMG recordings.23 Abnormal recordings can be
described as spikes, bursts, or trains. Spikes and bursts are the result of nerve
root irritation from traction, compression, or thermal injury and can inform
the surgeon of proximity to a nerve root14 (Figure 6). With an increasing
degree of trauma to the nerve, the amplitude and frequency of these
discharges increases and trains of activity are observed. Trains of activity
represent extensive nerve fiber recruitment and indicate impending nerve
injury with sustained manipulation.
14
sEMG provides continuous feedback throughout the surgical procedure
and has a high sensitivity for the detection of nerve root injury. A review of
Figure 7
213 patients who underwent thoracolumbar spine surgery with sEMG
recordings found a 100% sensitivity but only a 23.7% specificity for the
detection of new postoperative neurologic deficits.24 sEMG recordings are
sensitive to temperature changes such as irrigation with cold water or the use
of cautery devices. Electrocardiography leads and use of a high-speed drill
can produce artifact. The presence of preoperative neurologic deficits or
underlying neuromuscular disorders may make sEMG recordings
unreliable.
14
Photograph of triggered electromyographic tracings
recorded from the following bilateral muscles: vastus
medialis (A), tibialis anterior (B), and gastrocnemius (C). Left-sided
recordings are on the left and right-sided recordings are on the right. A
left L5 pedicle has been inserted and is being stimulated at 9 mA.
There are responses in the left tibialis anterior and gastrocnemius.
Triggered Electromyography
A major risk of pedicle screw placement is a medial or an inferior breach
resulting in injury to the dura, nerve roots, or spinal cord.
14,15
The tEMG
technique relies on the principle that the electrical resistance of bone is higher
than that of surrounding tissues. With accurate placement of the screw, the
surrounding cortical bone acts as an insulator to electrical conduction. If a
medial breach occurs, a low resistance pathway is formed between the screw
and the adjacent tissues, allowing for nerve root stimulation and generation of
a CMAP in the muscles innervated by that nerve root.
23
tEMG is performed by directly stimulating a pedicle screw with
monopolar, cathodal, and constant current via an insulated ball-tip probe,
with an anodal reference needle electrode placed in the paraspinal muscle.
The current is gradually increased until CMAPs are elicited. It is important to
place the stimulation probe directly on top of the screw, not on the tulip, to
optimize the current flow from the screw to the reference electrode.14 A close
correlation exists between the intensity of stimulation required to elicit
CMAPs and the risk of neurologic injury.23 Stimulation thresholds differ
among cervical, thoracic, and lumbar pedicles because of the variations in
thickness and shape of the bone.14 A stimulation threshold more than 15 mA
indicates safe placement of the pedicle screw regardless of the level. Lumbar
and thoracic pedicle screws thresholds of less than 7 mA and 6 mA,
respectively, are suggestive of a medial wall breach.
14,25-27
Although tEMG is
rarely used in posterior cervical fusion surgery, it has been reported that a
stimulation threshold of less than 10 mA warrants further investigation.
28
Although stimulation threshold values can be predictive of screw
misplacement, their false-positive rate is quite high, with more than 75% of
the threshold values below 6 mA representing appropriately placed
screws.
23,26
A 6 mA threshold value in conjunction with a 60% to 65%
decrease from the average of the threshold values obtained from all other
pedicle screws may be a more reliable warning sign of a medial pedicle
breach.26 An example of tEMG tracing is shown in Figure 7.
tEMG also is prone to false-negative findings resulting from anesthesia,
technical difficulties, or preexisting nerve injury. Paralytic agents should be
avoided, and four of four twitches should be present on train-of-four
testing.14 Current shunting can occur because of the presence of soft tissues,
blood, or other fluid around the screw head.
14,15
The use of pedicle screws
coated with hydroxyapatite, which reduces the conductive capacity of the
screws, may produce false-negative results.14 Chronically irritated nerve
roots typically have higher direct triggering thresholds ranging from 6 mA to
more than 10 mA, compared with 2 mA in a normal nerve root.15 Performing
a direct stimulation test is recommended to establish a baseline stimulation
threshold when a nerve root is known to be chronically irritated.
14
Multimodal Techniques and Guidelines
IONM should be tailored to the location and type of surgical procedure
because each modality has limitations, and none can sufficiently monitor all
neural structures at risk of injury.14 Several groups have published position
statements on IONM use.
29-34
A joint position statement by the American
Association of Neurological Surgeons and the Congress of Neurological
Surgeons Joint Section on Disorders of the Spine and Peripheral Nerves
states that IONM may assist in diagnosing neurologic injury, but notes that
“there currently exists no evidence such monitoring either reduces the
incidence of neurological injury or mitigates the severity of it.”33 A
recommendation is made that IONM “should be performed in procedures
when the operating surgeon feels that the diagnostic information is of value,
such as deformity correction, spinal instability, spinal cord compression,
intradural spinal cord lesions, and when in proximity to peripheral nerves or
roots.”33 The statement also makes specific recommendations regarding the
use of sEMG and tEMG.
Cervical and Thoracic Spine Surgery
Preservation of spinal cord integrity is of prime importance during cervical
and thoracic spine surgery. Combined SSEP and MEP monitoring provides
an excellent assessment of the entire spinal cord, and their combined use
increases the sensitivity and specificity of detecting an injury. Some authors
recommend the use of sEMG during surgical procedures performed in
proximity to the C5 nerve root, which is at particular risk of injury. The
reliability and use of electrophysiologic monitoring during surgery for
cervical myelopathy or radiculopathy is not clear.30 Because the thoracic cord
is at risk of ischemia during anterior procedures involving segmental artery
ligation, some authors recommend temporary occlusion of segmental arteries
under MEPs before ligation to identify critical vessels and avoid spinal cord
infarction.
13
Lumbosacral Spine Surgery
In the lumbosacral spine, the focus of IONM shifts from the spinal cord to the
nerve roots. The use of SSEPs in combination with sEMG, as the optimal
multimodal technique for continuously monitoring nerve root integrity, has
been recommended by some authors.14 tEMG can be used during pedicle
screw placement as a method for detecting medial pedicle screw breach,
although this technique has a high false-positive rate.
23,26
Minimally Invasive Spine Surgery
The limited exposure and confined work space afforded by minimally
invasive surgery can place neurologic structures at risk. These conditions
make IONM particularly attractive, and the modalities used depend on the
location of the procedure, with sEMG and tEMG potentially helpful for
monitoring at-risk nerve roots or peripheral nerves.
Spine Deformity Surgery
Neurologic impairment is a rare but potentially devastating complication of
modern surgery for spine deformity. The probability of hardware
misdirection is probably higher in surgery to correct a deformity because of
the abnormal curvature and rotation of the vertebrae, and the risk of spinal
cord injury is likely higher because the spinal cord tends to be positioned
along the medial wall of the concave pedicles. However, the greatest risk of
spinal cord injury occurs during distraction of the spine. In an information
statement, the Scoliosis Research Society (SRS) states that IONM “can assist
in the early detection of complications and possibly prevent postoperative
morbidity.”34 The SRS also notes that the use of IONM in deformity surgery
is not investigational and is used routinely by most SRS members. The SRS
considers IONM the “preferred method for the early detection of an evolving
or impending spinal cord deficit during surgical manipulation.” The SRS also
notes that the wake-up test is a useful adjunct to IONM.
34
Figure 8
Example of a checklist for use in spinal deformity surgery
for patients with a stable spine. This checklist would not be
applicable for vertebral column resections or a patient with an
unstable spine. (Reproduced with permission from Vitale MG, Skaggs
DL, Pace GI, et al: Best practices in intraoperative neuromonitoring in
spine deformity surgery: Development of an intraoperative checklist to
optimize response. Spine Deform 2014;2[5]:333-339.)
The value of using surgical checklists, particularly during high-stress
situations such as IONM alerts, has been demonstrated. Some institutions
also incorporate discussion of the specific IONM modalities to be used,
anesthetic issues, and a review of alarm criteria in the initial surgical
timeout.23 A consensus clinical practice guideline established an
intraoperative checklist to optimize responses to IONM changes in deformity
surgery29 (Figure 8). A systematic process for the team to follow is divided
into the following five main categories: (1) gain control of the room, (2)
anesthetic/systemic, (3) technical/neurophysiologic, (4) surgical, and (5)
ongoing considerations (Table 3).
Summary
Electrodiagnostic testing is valuable in identifying or excluding peripheral
nerve disorders that may mimic radicular symptoms. In addition to localizing
a nerve disorder, electrodiagnostic testing can provide information regarding
the severity, duration, and prognosis for the spinal disorder.
Table 3
The most commonly used IONM modalities are SSEPs, MEPs, sEMG,
and tEMG. The dorsal column-medial lemniscus pathway, which carries
tactile discrimination, vibratory sensation, proprioception, and stereognosis
from the periphery to the postcentral gyrus are monitored with SSEPs. MEPs
are used to monitor the corticospinal tract, which originates in the upper
motor neurons of the primary cortex and extends to their connections with
alpha motor neurons. sEMG continuously monitors the integrity of individual
nerve roots. tEMG is used to detect a medial breach of pedicle screws.
Multimodality IONM is often used in complex spinal procedures, and
appropriate selection depends on the anatomic location and associated risks.
Guidelines and consensus statements have been published regarding use of
IONM.
Key Study Points
Electrodiagnostic testing is valuable in identifying or excluding peripheral
nerve disorders that may mimic radicular symptoms.
Electrodiagnostic testing has moderate sensitivity but high specificity for
radiculopathy.
SSEPs and MEPs primarily monitor the integrity of the spinal cord.
sEMG is used to continuously monitor the integrity of individual nerve
roots.
tEMG is used to detect medial breach of pedicle screws.
Acknowledgment
Dr. Sanders and Dr. Foxx would like to acknowledge Sarah Gannon, R.
EEG/EP T./CNIM for her help obtaining and preparing some of the figures
used in this chapter.
Annotated References
1. Dillingham TR: Evaluating the patient with suspected radiculopathy. PM R
2013;5(5suppl):S41-S49.
An overview of the electrodiagnostic evaluation of patients with suspected
radiculopathy is presented.
2. Dillingham TR, Pezzin LE, Lauder TD: Cervical paraspinal muscle abnormalities and
symptom duration: A multivariate analysis. Muscle Nerve 1998;21(5):640-642.
3. Pezzin LE, Dillingham TR, Lauder TD, et al: Cervical radiculopathies: Relationship
between symptom duration and spontaneous EMG activity. Muscle Nerve
1999;22(10):1412-1418.
4. Dillingham TR, Pezzin LE, Lauder TD, et al: Symptom duration and spontaneous
activity in lumbosacral radiculopathy. Am J Phys Med Rehabil 2000;79(2):124-132.
5. Buschbacher RM, Prahlow ND: Manual of Nerve Conduction Studies, ed 2. New York,
NY, Demos Medical, 2006.
6. Tong HC: Specificity of needle electromyography for lumbar radiculopathy in 55- to
79-yr-old subjects with low back pain and sciatica without stenosis. Am J Phys Med
Rehabil 2011;90(3):233-238, quiz 239-242.
An assessment of the specificity of needle EMG for lumbar radiculopathy in patients
with low back pain and sciatica without stenosis is presented.
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AAEM minimonograph 32: The electrodiagnostic examination in patients with
radiculopathies. Muscle Nerve 1998;21(12):1612-1631.
8. Dillingham TR, Lauder TD, Andary M, et al: Identifying lumbosacral radiculopathies:
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9. Dillingham TR, Lauder TD, Andary M, et al: Identification of cervical radiculopathies:
Optimizing the electromyographic screen. Am J Phys Med Rehabil 2001;80(2):84-91.
neuropathies. Muscle Nerve 2013;48(5):696-704.
This study reviews common ultrasound findings in entrapment neuropathies and
focuses on the use of ultrasonography in four common entrapment neuropathies: carpal
tunnel syndrome, ulnar neuropathy at the elbow and wrist, and fibular neuropathy at the
knee.
in cervical radiculopathy: A pilot study. Am J Phys Med Rehabil 2015;94(2):159-164.
The use of high-resolution ultrasonography in patients with cervical radiculopathy is
explored. The cross-sectional areas of the nerve roots between the affected and
unaffected sides are compared.
surgery in patients with cerebral palsy: Assessment of risk factors. Eur Spine J
2016;25(3):795-800.
This prospective case-control study of patients undergoing spine surgery for idiopathic
scoliosis reported that identification and correction of offending factors after an
intraoperative alert resulted in normal neurologic function equivalent to that of patients
who had no alerts during surgery. These results stress the importance of acting properly
when IONM alerts occur. Level of evidence: II.
surgery. J Am Acad Orthop Surg 2007;15(9):549-560.
neurophysiological monitoring during spine surgery: A review. Neurosurg Focus
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This is a comprehensive review article of intraoperative neurophysiologic monitoring.
Level of evidence: V.
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popliteal fossa, optimized P37, standard P37, and P31 potentials. Clin Neurophysiol
2005;116(8):1858-1869.
surgery. Spine (Phila Pa 1976) 2010;35(25):2167-2179.
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The authors present a comprehensive review of MEP monitoring technique and a
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evidence: V.
using motor-evoked potentials. Curr Opin Anaesthesiol 2008;21(5):560-564.
free-run and electrically-triggered electromyography and spinal cord function with
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This study documents the inclusion of IONM in the surgical pause and its ability to
make the surgical team aware of issues that may need to be addressed.
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