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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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performed as well as the muscles tested on needle EMG. These findings are
typically presented in a table format. A diagnosis of a radiculopathy based on
findings of fibrillations and positive sharp waves is considered highly
convincing. A diagnosis of a radiculopathy based on findings of decreased
recruitment or increased polyphasicity is considered less strong.
Diagnostic Ultrasonography as an Adjunct to Electrodiagnostic
Testing
The use of neuromuscular ultrasonography has rapidly increased over the
past decade and is a valuable adjunct to electrodiagnostic testing.
Ultrasonography may be considered in situations in which peripheral nerve
entrapment is part of the differential diagnosis, but electrodiagnostic test
findings are negative for evidence of any entrapment neuropathy. Nerve
enlargement in the region of the entrapment site is commonly seen with
ultrasonography; however, the cause of this enlargement is not completely
understood.10 Although the current application of neuromuscular
ultrasonography has focused more on distal peripheral nerve assessment,
recent research has explored the use of ultrasonography in the assessment of
spinal nerve roots.
11
Intraoperative Neurophysiologic Monitoring
James Sanders, MD
Kenneth Foxx, MD
Introduction
Intraoperative neurophysiologic monitoring (IONM) consists of several
modalities combined to provide continuous assessment of neurologic
structures at risk of injury during spine surgery, including the spinal cord,
nerve roots, and peripheral nerves. Iatrogenic injury to these structures can
occur from contusion, mechanical compression, vascular compromise, or a
combination of these factors. IONM can potentially identify evolving insults,
which allows the surgical team to pursue corrective action and prevent
permanent injury.12 The most commonly used modalities in spine surgery
include somatosensory-evoked potentials (SSEPs), motor-evoked potentials
(MEPs), free-running or spontaneous electromyography (sEMG), and
stimulus or triggered electromyography (tEMG). Multimodality IONM is
often used in complex spine procedures, and the selection of the appropriate
modality depends on the anatomic surgical location and associated risks.
The first widely used technique for intraoperative assessment of
neurologic structures was the Stagnara wake-up test.
13-15
Because this
clinically based test involves the patient’s emergence from general anesthesia
and neurologic examination in the operating room, it only provides an
approximate assessment of motor tracts, but no information regarding the
integrity of sensory tracts, specific nerve roots, or specific peripheral nerve
structures. Because feedback is not continuous, it may not allow for timely
corrective action. The Stagnara wake-up test also is associated with risks such
as recalling intraoperative events, extubation, and difficulty performing the
test in certain patient populations such as those with cognitive deficits.
SSEP monitoring was the first type of IONM reported and remains its
cornerstone. MEPs were first described as high-voltage, single-pulse,
transcranial stimulation of the cerebral cortex with induction of contralateral
motor activity, but single-pulse stimulation was highly susceptible to the
effects of anesthesia. The development of multipulse stimulation and
refinement of anesthetic techniques led to the widespread use of MEPs in
spine surgery. tEMG has become popular for assessing pedicle screw
placement, and sEMG is used for the continual evaluation of nerve roots. The
current aggregate of IONM modalities has become a critical tool in spine
surgery.
Neuromonitoring Modalities
Somatosensory-Evoked Potentials
SSEPs are used to monitor the dorsal column-medial lemniscus pathway,
which carries tactile discrimination, vibratory sensation, proprioception, and
stereognosis from the periphery to the postcentral gyrus. The posterior spinal
artery supplies the vascular territory of the dorsal columns. Information from
receptors in skin, tendons, and muscles is relayed via heavily myelinated
fibers from first-order neurons whose soma is in the dorsal root ganglion.
These fibers travel in peripheral nerves, which combine in plexuses of their
extremity, and are distributed to several nerve roots, leading to dermatomal
overlap. Axons from first-order neurons travel in the ipsilateral fasciculi
gracilis and cuneatus and are somatotopically organized with sacral fibers
located medially in the fasciculus gracilis and cervical fibers located laterally
in the fasciculus cuneatus. These axons synapse with second-order neurons in
the nucleus gracilis and nucleus cuneatus, and decussate in the medulla to
travel in the medial lemniscus to the ventral posterolateral nucleus of the
thalamus. Information is then ultimately relayed to the primary
somatosensory cortex in the postcentral gyrus and distributed in the
somatotopically organized sensory cortical homunculus.
13-15
Typical upper extremity SSEP stimulation sites are the median nerve (C6,
C7, C8, and T1 nerve roots) and ulnar nerve (C8 and T1 nerve roots).
13-15
Use
of the ulnar nerve in the upper extremity is preferred by some clinicians
because its lower nerve root entry affords a more complete assessment of the
cervical spinal cord. In the lower extremity, the posterior tibial nerve (L4, L5,
S1, and S2 nerve roots) and peroneal nerve (L4, L5, and S1 nerve roots) are
used. The electrical stimulation applied to a peripheral nerve creates an
afferent signal that is ultimately measured by electrodes on the scalp. On
scalp recordings, a negative potential is seen approximately 20 ms after upper
extremity stimulation and is referred to as the N20 potential. A positive
potential is seen approximately 37 ms after lower extremity stimulation and is
termed the P37 potential.15 Potentials are assessed for both amplitude and
latency (Figure 4). Amplitude is measured in microvolts and defined as the
distance from baseline to peak or from peak to trough. Latency is measured
in milliseconds and refers to the period of time from peripheral stimulation to
the recording of a potential.
A single cortical compound action potential cannot be detected because
of background electroencephalography-related noise captured on scalp
recordings. The signals generated by multiple stimulation sweeps must be
averaged to obtain meaningful results.15 With scalp recordings, the signal-tonoise ratio varies depending on the location of the electrodes relative to the
corresponding cortical region. Recordings with higher signal-to-noise ratios
require fewer sweeps to obtain and produce more reliable results. Scalp
electrodes are generally placed using a standard montage, but fine
adjustments are recommended to optimize the signal-to-noise ratio, which
allows for more rapid and accurate signal change detection.
15,16
Electrodes also are placed to obtain recordings along the dorsal columnmedial lemniscus pathway at the level of the brainstem and select peripheral
Figure 4
nerve structures such as the brachial plexus, popliteal fossa, and lumbar
plexus.14 The signal generated by the medulla and midbrain is referred to as
the subcortical response. A P31/N34 (positive potential 31 ms and negative
potential 34 ms after stimulation) complex subcortical response is seen with
posterior tibial nerve stimulation, and a P14/N18 (positive potential 14 ms
and negative potential 18 ms after stimulation) complex subcortical response
is recorded with median nerve stimulation. Subcortical recordings are less
susceptible to the effects of anesthesia than cortical recordings, and they are
useful in determining whether a change in cortical monitoring is the result of
anesthesia or neurologic insult. However, subcortical recordings are
technically more difficult to obtain and are prone to muscle-related noise.
17
Recordings at peripheral nerve sites are used to determine whether the
peripheral stimulus is adequate, and they are useful in the detection of
peripheral nerve compression and limb ischemia.
14
Photograph of normal somatosensory-evoked potential
tracings for bilateral ulnar and posterior tibial nerve
stimulation. The upper two tracings (A and B) are cortical recordings.
The third tracing from the top (C) is a subcortical recording, and the
bottom tracing (D) is a peripheral recording. The blue tracing
represents the baseline response and the purple tracing represents
the most recent sweep.
A 50% reduction in signal amplitude or a 10% increase in latency are
considered important and require surgeon notification. Even in ideal
situations, SSEPs can take up to 5 minutes to detect a significant
change.
13,15,17
Amplitude changes are more sensitive to the onset of injury
when compared with latency changes.13 In a large multicenter survey, a 92%
sensitivity and 98.9% specificity were reported for the detection of new
postoperative deficits with SSEP monitoring.18 SSEPs can be profoundly
affected by hypotension, hypothermia, halogenated inhalational anesthetics,
and intravenous sedation. The surgical and intraoperative monitoring teams
must work together to recognize and minimize the effects of these
confounders. Anesthetic effects are dose related and can be controlled with
nitrous oxide supplementation.
17
SSEPs are used to assess spinal cord sensory tracts and are good basic
indicators of spinal cord function. However, baseline SSEPs may be
diminished in patients with severe myelopathy, spinal cord tumor, obesity, or
peripheral neuropathy.13 The role of SSEPs in the assessment of the motor
tracts is limited. SSEPs are more sensitive to motor changes secondary to
mechanical injury than changes resulting from vascular insult such as anterior
spinal artery occlusion.17 The corticospinal tracts and the anterior horn cells
are perfused by the anterior spinal artery. During corrective spinal surgery,
small radiomedullary arteries may be stretched or compressed, causing
ischemia or infarction. In addition, SSEPs cannot reliably monitor the
integrity of individual nerve roots.
14
Motor-Evoked Potentials
MEPs monitor the corticospinal tract, which originates in the neurons of the
primary motor cortex of the precentral gyrus. The upper motor neuron axons
converge in the internal capsule and continue to the medulla before reaching
the spinal canal. Seventy-five percent to 90% of the axons cross in the
pyramidal decussation and continue in the contralateral lateral corticospinal
tract. The ventral corticospinal tract contains the remaining ipsilateral axons,
which cross over at their dedicated spinal level. A small number of upper
motor neurons synapse directly with alpha motor neurons in the anterior gray
matter, and the remainder act through intermediate neurons. Each alpha
motor neuron innervates a single motor unit.
15
MEPs are produced by transcranial anodal stimulation with subdermal
scalp electrodes placed over the primary motor cortex.
14,15
A low-output
impedance electrical stimulator generates a high-volume, short-duration pulse
train via the subdermal electrodes.13 With application of a short train of 5 to 7
electrical pulses, the alpha motor neurons receive enough stimulation to reach
their firing thresholds. MEPs can be recorded either as D-waves over the
spinal cord with epidural electrodes or as CMAPs via recording needles
placed in the muscles of interest.
19
D-waves do not require activation of lower motor neurons, which renders
the technique more robust and less variable, with proven utility in
intramedullary spinal cord tumor resection. Complete loss of MEPs with at
least 50% preservation of D-wave amplitude is usually associated with
transient motor deficit, whereas a complete loss of D-wave amplitude is
predictive of permanent paralysis.14 However, the D-wave recording has
been shown to have a high rate of false-positive results during scoliosis
surgery, and it is unable to detect a unilateral spinal cord injury. The
recording electrode must be placed in the epidural space, either
percutaneously or through a laminotomy distal to the surgical level. D-wave
recordings can be made only at spinal levels where the corticospinal tract is
large enough to generate a recordable signal, which restricts its use to above
the T11 level.
15
CMAP recordings are used much more frequently than D-wave
recordings because of their relative ease of use, ability for use distal to T11,
and ability to detect unilateral injury. CMAPs are more easily monitored from
more distal muscles because of their richer corticospinal tract innervation
compared with proximal muscles. Commonly used muscles include the
abductor pollicis brevis, abductor hallucis brevis, gastrocnemius, and tibialis
anterior (Figure 5), although the biceps, deltoid, and quadriceps also can be
monitored.
19
Figure 5
Photograph of transcranial motor-evoked potential tracings
recorded from the following bilateral muscles: abductor
pollicis brevis (A), tibialis anterior (B), gastrocnemius (C), and
abductor hallucis brevis (D). The blue tracing represents the baseline
recording and the purple tracing represents the most recent response.
A possible change in the left abductor hallucis brevis is indicated.
Prior to initiation of a surgical procedure, baseline MEP threshold
voltages and CMAP baseline amplitudes are obtained for each side of the
body. The starting stimulus voltage is typically 100V and is increased in 50V
increments until a CMAP response can be recorded in each of the monitored
muscles. The stimulation voltage required to produce a CMAP response in
each of the monitored muscles is termed the threshold voltage. CMAPs are
polyphasic waveforms that are variable in morphology.15 CMAP amplitude is
typically calculated as the area under the curve of the recorded potential.
Latency time is usually 20 ms in the hand and 45 ms in the foot, although
these values are dependent on a variety of factors, including body
temperature, patient height, and preexisting neuromuscular pathology.
13,14
As a result of the variability of MEP responses, four methods of
interpretation have been developed. (1) The all-or-nothing criterion requires a
complete loss of the baseline MEP signal. It is indicative of a clinically
important event and warrants an immediate corrective action per
recommendations of the American Society of Neuromonitoring (ASNM).
15,20
This method of interpretation may not be appropriate for degenerative and
spine deformity surgery in which a temporary postoperative motor deficit is
unacceptable. It may be more suitable for intradural surgery. (2) The
amplitude criterion requires an 80% decrease in signal amplitude in at least
one of the six recording sites for a change to be considered clinically
important.15 This technique has shown 100% sensitivity and 91% specificity
for the detection of a postoperative motor deficit.14 (3) The threshold
criterion involves analysis of increases in the stimulation threshold required
to maintain CMAP responses.14 If an increase of greater than 100V is
required for at least 1 hour to maintain CMAP responses, a neurologic insult
is presumed to have occurred.14 The amplitude and threshold criteria are
considered moderate criteria by the ASNM because of their tendency to
generate false-positive results.
15,20
(4) The morphology criterion focuses on
changes in the pattern and duration of MEP waveforms. A transition from a
polyphasic to a biphasic waveform is indicative of a motor deficit. A
complete loss of waveform is associated with a higher risk of permanent
injury. Morphologic changes also can occur because of anesthetic agents,
hypothermia, hypotension, patient positioning, or technical issues.14 The
ASNM has not made recommendations for using this criterion in spine
surgery.
15,20
Although MEPs can effectively monitor the corticospinal tracts, they are
subject to several limitations and confounding factors. As with SSEPs, it may
be impossible to obtain MEP signals in patients with preoperative neurologic
deficits. CMAPs are dependent on transsynaptic activation of spinal motor
neurons, and paralytic agents need to be avoided entirely or maintained at a
subparalytic level.19 Because halogenated agents can depress synaptic
transmission and spinal motor neuron function, total intravenous anesthesia is
strongly preferred.19 CMAP interpretation also may be confounded by
substantial variability in waveform amplitude and morphology secondary to
activation of a small number of different low-threshold spinal motor neurons
by each descending corticospinal volley.19 MEP monitoring may be difficult
in some children younger than 6 years because of incomplete
electrophysiologic maturation of the corticospinal tract, which finalizes by
age 13 years. However, it often can be used effectively in young children.
Because MEPs only monitor 4% to 5% of the corticospinal tract neuron pool,
a complex motor deficit may rarely occur without any monitoring
abnormalities.21 In addition, as a result of radicular overlap and limited
sampling, a single nerve root injury will frequently be missed with MEP
monitoring.
17
Although MEP monitoring is generally considered quite safe, the
possibility of injury exists.17 Thermal injuries, which are estimated to occur
in 0.01% of cases, can result from improper technique. Bite injuries, which
occur in approximately 0.2% of cases, are the result of contraction of the
mastication muscles, which is likely mediated by corticobulbar pathway
activation, trigeminal nerve stimulation, or direct muscle stimulation. A soft
bite-block is a standard precaution. Seizures from transcranial stimulation are
rare, with an estimated incidence of 0.03%, and are usually self-limited.
Cardiac arrhythmia also is possible, but its occurrence has been rarely
reported. The ASNM guidelines do not include any absolute
contraindications to MEP monitoring, but epilepsy, cortical lesions, skull
defects, intracranial vascular clips, shunts, cranial electrodes, pacemakers,
and other implanted bioelectric devices are listed as relative
contraindications. However, the ASNM guidelines also note that no proof
exists that any of these conditions increase complication rates, and many
patients with one or more of these conditions have undergone uneventful
MEP monitoring.
20
Electromyography
Neither SSEP nor MEP monitoring provide reliable monitoring of single
nerve root function during spine surgery. Electromyographic techniques can
detect excessive nerve root retraction, mechanical injury, thermal injury, or
medial cortical breach of a pedicle screw.
Segmental nerve root monitoring can be used to assess the function of
motor unit axons via sEMG and tEMG. CMAPs are recorded from needle
electrodes placed in muscles innervated by the cervical, thoracic, lumbar, and
sacral nerve roots. The typically monitored muscles innervated by cervical
nerve roots include the following: (1) trapezius and sternocleidomastoid (C2,
C3, C4), (2) biceps and deltoid (C5, C6), (3) flexor carpi radialis (C6, C7),
and (4) abductor pollicis brevis and abductor digiti minimi (C8, T1).
Commonly used muscles innervated by thoracic nerve roots include the
following: (1) upper rectus abdominis (T5, T6), (2) middle rectus abdominis
(T7, T8), (3) lower rectus abdominis (T9, T10, T11), and (4) interior rectus
abdominis (T12). Typically monitored muscles innervated by lumbar and
sacral nerve roots include the following: (1) vastus medialis (L2, L3, L4), (2)
tibialis anterior (L4, L5, S1), (3) peroneus longus (L5, S1), (4) gastrocnemius
(S1, S2), and (5) the external anal sphincter (S2, S3, S4).
22
Spontaneous Electromyography
Surgical manipulation such as traction or compression of a nerve root
produces neurotonic discharges, which results in the production of CMAPs
that can be recorded from the muscles innervated by the nerve root. Because
there is a certain degree of redundancy in muscle innervation, monitoring one
muscle group per nerve root is usually considered adequate.
14,23
The only
exception is the C5 nerve root, which is at high risk of injury during cervical
procedures, and concurrent monitoring of the deltoid and biceps is
recommended. sEMG is continuously recorded and may be sent to a speaker
to provide auditory feedback. Paralytic agents should not be used, and trainof-four testing should indicate that at least three of four twitches are
present.
14
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