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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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Chapter 7
Electrodiagnostic Testing and
Intraoperative
Neurophysiologic Monitoring
Berdale Colorado, DO, MPH James O. Sanders, MD Kenneth Foxx, MD
Abstract
Electrodiagnostic studies play an important role in the assessment of
patients with neuromuscular disorders. Office-based testing consists
primarily of nerve conduction studies and needle electromyography,
whereas intraoperative testing and monitoring consists of somatosensoryevoked potentials, motor-evoked potentials, and spontaneous and triggered
electromyography. It is helpful to be familiar with all forms of testing,
including the indications and limitations of nerve conduction studies and
needle electromyography for suspected spine disorders, as well as the
clinical practice guidelines for intraoperative neurophysiologic monitoring.
Keywords: intraoperative neurophysiologic monitoring; needle
electromyography; motor-evoked potential; motor unit action
potential; nerve conduction study, somatosensory-evoked
potential
Dr. Sanders or an immediate family member is a member of a speakers’ bureau or
has made paid presentations on behalf of NuVasive; has stock or stock options
held in Abbott, Abbvie, and General Electric; has received nonincome support
(such as equipment or services), commercially derived honoraria, or other non–
research-related funding (such as paid travel) from GreenSun and NuVasive; and
serves as a board member, owner, officer, or committee member of the Pediatric
Orthopaedic Society of North America and the Scoliosis Research Society. Neither
of the following authors nor any immediate family member has received anything
of value from or has stock or stock options held in a commercial company or
institution related directly or indirectly to the subject of this chapter: Dr. Colorado
and Dr. Foxx.
Electrodiagnostic Testing
Berdale Colorado, DO, MPH
Introduction
Electrodiagnostic testing is first and foremost an extension of the clinical
examination. A focused history and physical examination is essential for
guiding electrodiagnostic testing.
Electrodiagnostic studies provide a measure of the electrical activity of
muscles and nerves and can serve as a valuable tool in treating patients with
suspected spine disorders. Although neuroimaging detects structural
abnormalities, only electrodiagnostic testing can assess functional or
physiologic abnormalities. Multiple electrodiagnostic tests exist; however,
nerve conduction studies and needle electromyography (EMG) are the most
commonly performed office-based tests. Information regarding localization,
severity, duration, and prognosis of a nerve disorder often can be obtained
through electrodiagnostic testing. An overview of nerve conduction and
needle EMG studies is presented along with a discussion of the indications
for electrodiagnostic testing for suspected spine disorders, the limitations of
electrodiagnostic testing, and a framework for confirming that appropriate
electrodiagnostic testing has been completed for commonly encountered
diagnoses.
Overview of Testing
Nerve Conduction Studies
Nerve conduction studies measure the ability of peripheral nerves to conduct
electrical impulses. These studies are typically performed before needle EMG
because the results of the nerve conduction studies may influence which
muscles are tested with needle EMG and the interpretation of the needle
EMG findings. Nerve conduction studies can be generally divided into motor
nerve conduction studies, sensory nerve conduction studies, and mixed nerve
conduction studies. The basic setup for nerve conduction studies involves a
Figure 1
ground electrode and two recording electrodes (an active and a reference
electrode). The active electrode is placed over the muscle of interest in motor
nerve conduction studies and over the nerve of interest in sensory nerve
conduction studies. An electrical stimulator is positioned at a specific
distance from the active electrode along the course of the nerve (Figure 1).
When an electrical impulse is applied by the stimulator, an action potential is
recorded at the active electrode and is displayed on the EMG machine as a
waveform (Figure 2). Various waveform parameters, including onset latency,
peak latency, amplitude, and conduction velocity, are measured and
interpreted by an electrodiagnostician (Table 1).
Photograph of the setup for a median motor nerve
conduction study to the abductor pollicis brevis. The ground
electrode is placed on the dorsum of the hand.
Figure 2
Photograph shows an action potential waveform. In this
example, a compound muscle action potential is pictured. a
= onset latency, b = peak latency, c = amplitude (peak to peak), p =
peak, o = onset.
Table 1
The recorded potential in a motor nerve conduction study is known as a
compound muscle action potential (CMAP), which represents the summation
of all underlying individual muscle fiber action potentials. Onset latency (the
time required for an electrical stimulus to initiate an evoked potential) and
amplitude (reflecting the number of muscle fibers activated) of the CMAP are
measured. After a distal and a proximal site have been stimulated, a
conduction velocity can be calculated, which is the speed an impulse travels
along a nerve.
Latency and conduction velocities reflect the fastest conducting fibers and
are primarily dependent on the integrity of the myelin sheath.
The recorded potential in a sensory nerve conduction study is known as a
sensory nerve action potential (SNAP), which represents the summation of all
the individual sensory fiber action potentials. Because nerve conduction
occurs in both directions following nerve depolarization, sensory nerve
conduction studies can be performed using either orthodromic (in the
direction of physiologic conduction) or antidromic (opposite of physiologic
conduction) techniques. Although the antidromic technique is often preferred,
particularly because of the higher amplitude compared with the orthodromic
technique, both techniques have advantages and disadvantages. Peak latency
(reflecting the latency along most of the sensory fibers) and amplitude
(reflecting the number of sensory fibers activated) of the SNAP are measured.
Late response studies such as the F-wave and H-reflex studies are
sometimes used to assess more proximal nerve segments. These studies can
provide additional information in the evaluation of radiculopathies,
plexopathies, polyneuropathies, and proximal peripheral neuropathies.
Taken altogether, the numeric values of the latency, amplitude, and
conduction velocity of the various nerve conduction studies can point to a
focal nerve lesion such as an entrapment or a diffuse neurogenic process such
as a peripheral polyneuropathy.
Needle Electromyography
Needle EMG is typically performed after completion of the nerve conduction
studies. The basic setup for needle EMG involves a needle electrode (either
monopolar or concentric), a reference electrode (if using a monopolar
needle), and a ground electrode (Figure 3). Monopolar and concentric
needles each have advantages and disadvantages, but both are widely used.
The needle is inserted into each muscle of interest, and several additional
Figure 3
small, brief insertions are made to adequately sample each muscle. Insertional
activity and spontaneous activity are assessed while the muscle is at rest. In a
normal muscle, there should be electrical silence shortly after needle
movement. Increased insertional activity is defined as any electrical activity
that lasts longer than 300 ms after brief needle movement, with the exception
of end plate potentials, which can be seen if a needle is placed near the motor
end plate. Increased insertional activity is seen with denervation.
Spontaneous activity is defined as any electrical activity at rest that lasts
longer than 3 seconds. Two types of spontaneous activity commonly seen
with denervation are fibrillations and positive sharp waves. These represent
abnormal electrical activity generated from denervated single muscle fibers.
Fibrillations and positive sharp waves are graded based on their distribution
and intensity, ranging from 1+ (persistent single runs in two areas) up to 4+
(continuous discharges in all areas of the muscle).
Photograph shows the basic setup for needle
electromyography.
After assessment of insertional and spontaneous activity, analysis of
motor unit action potentials (MUAPs) is performed. This assessment involves
voluntary contraction of the muscles of interest. MUAPs are analyzed for
amplitude, duration, number of phases, and recruitment (Table 2).
Amplitude is measured from the most positive to the most negative peak,
and the normal value is typically 1-2 mV. Amplitude can be increased in the
setting of reinnervation, often indicating a chronic neurogenic disorder.
Duration is measured from the initial baseline departure to the final return to
baseline, and the normal value is typically 5-15 ms. Duration can be
increased in the setting of reinnervation, often indicating a subacute
neurogenic disorder.
Phases of the MUAP refer to the number of times the MUAP crosses the
baseline; the normal value is typically two to four phases. Increased phases,
or polyphasicity, can be seen in the setting of reinnervation, often indicating a
subacute neurogenic disorder. With increasing muscle contraction force,
additional MUAPs can be activated, and the MUAP recruitment pattern can
be analyzed. In normal recruitment, when the first MUAP fires at a rate of
approximately 10 Hz, a second MUAP is recruited and begins to fire at a rate
of approximately 5 Hz. As the firing frequency increases by approximately 5
Hz, an additional MUAP is recruited. With maximal contraction, a complete
interference pattern may be seen in which the screen of the EMG machine is
filled with four or more overlapping MUAPs and no individual MUAP can
be distinguished. With denervation, recruitment is often reduced and only one
or a few MUAPs are seen, even with maximal contraction. In this case, the
interference pattern would be reduced.
Table 2
Electrodiagnostic Testing for Spine Disorders
The timing of performance of electrodiagnostic testing is dependent on the
clinical differential diagnosis. Electrodiagnostic testing may be indicated in
the assessment of any suspected nerve disorder. Patients with symptoms such
as arm or leg pain, weakness, or numbness or tingling are commonly referred
for electrodiagnostic testing. Testing can confirm the presence of a
radiculopathy; rule out peripheral nerve disorders, which can affect similar
anatomic distributions; and identify the presence of multiple coexistent nerve
disorders. For example, a patient may simultaneously have a cervical
radiculopathy, carpal tunnel syndrome, and a peripheral polyneuropathy.
Identifying these disorders may have a substantial effect on the treatment
plan. Electrodiagnostic testing also can be helpful in patients with a physical
examination that does not clearly localize the etiology of their symptoms or
with imaging findings that do not correlate with their symptoms. The needle
EMG and nerve conduction studies may be abnormal when all other test
findings, including neuroimaging, are unremarkable.
The timing or performance of electrodiagnostic testing also depends to
some extent on the type of information desired. A common misconception is
that it is necessary to wait 2 to 3 weeks after a nerve injury before obtaining
needle EMG or nerve conduction studies. This misconception is based on the
idea that wallerian degeneration, which may take several days to weeks, must
first occur before meaningful information can be obtained. However, a
substantial axonal injury can be seen immediately, and this information may
aid in early localization of the injury. The primary limitation to early
performance of electrodiagnostic testing is the difficulty in differentiating a
conduction block from an axonal injury immediately after a nerve injury. In a
patient with a cervical or lumbosacral radiculopathy, it is a commonly held
belief that changes of denervation on needle EMG are seen first in the
paraspinal muscles at approximately 2 weeks. This is followed by changes of
denervation in the peripheral muscles (proximal to distal) at approximately 3
weeks. This belief is not supported in the literature; several studies have
concluded that radiculopathies do not follow a predictable time course of
denervation.
1-4
Pitfalls and Limitations of Electrodiagnostic Testing
Nerve Conduction Studies
Nerve conduction studies can be affected by numerous factors that must be
taken into account when interpreting data. These can be subdivided into
physiologic and nonphysiologic factors. Temperature is the most important
physiologic factor affecting nerve conduction studies. A decrease in
temperature can cause prolonged latency, increased amplitude, increased
duration, and decreased conduction velocity. Conduction velocity decreases
approximately 2.4 m/s per 1°C decrease, and distal latency can be prolonged
by 0.2 ms per 1°C decrease. Normal temperature is approximately 32°C for
the upper extremity and 30°C for the lower extremity. Other physiologic
factors that can influence nerve conduction studies include age, height, sex,
edema, obesity, and anomalous innervation.5 Therefore, these factors must be
taken into consideration when determining what is “normal.”
Nonphysiologic factors affecting nerve conduction studies refer to
technical considerations that can influence the recorded data. These include
improper electrode placement, suboptimal stimulation, inaccurate
measurements, and failure to recognize artifacts. These factors can create
inaccurate latency, amplitude, and conduction velocity measurements, which
can result in incorrect conclusions. Recognition of potential technical factors
is imperative when conducting nerve conduction studies.
Needle Electromyography
In the electrodiagnostic evaluation of a cervical or a lumbosacral
radiculopathy, nerve conduction studies are typically normal, and the
diagnosis is made primarily on the results of needle EMG. Needle EMG is
the best electrodiagnostic procedure for detecting radiculopathies. However,
the primary limitation of needle EMG is that it does not detect all
compressive radiculopathies. A “negative needle EMG” cannot be used to
exclude a radiculopathy. Sensitivities of needle EMG for cervical
radiculopathy range from 50% to 71%, and sensitivities for lumbosacral
radiculopathy range from 49% to 86%.1 Specificity of needle EMG for
radiculopathy is high. A 2011 study reported that when only positive sharp
waves or fibrillations were considered abnormal, specificity ranged from 92%
to 97% in lumbosacral radiculopathy.6 Low sensitivity can result from an
inadequate number of affected motor nerve root fibers, inadequate sampling
of fibers during needle EMG, or because the needle EMG was performed
after reinnervation had occurred. A pure sensory radiculopathy would result
in a normal needle EMG because of sparing of the dorsal root ganglion.
These are important considerations when electrodiagnostic testing is negative
for needle EMG abnormalities.
Needle EMG has limited effectiveness for identifying the cause or
etiology of a nerve lesion. Also, the specific nerve root involved may not be
accurately identified by the needle EMG because of myotome overlap or
anomalous root innervation. These are important considerations when
electrodiagnostic testing is positive for needle EMG abnormalities.
Interpretation of Electrodiagnostic Testing
Although detailed interpretation of electrodiagnostic data can be challenging
to a clinician who does not perform such testing, a general understanding can
help clinicians identify a high-quality EMG report.
The electrodiagnostic testing for radiculopathy should include both nerve
conduction studies and needle EMG. Although the nerve conduction studies
are typically normal, they are necessary to exclude a peripheral neuropathy.
Low CMAP amplitudes may occur, however, in the setting of a radiculopathy
with substantial axonal degeneration. At least one motor nerve conduction
study and one sensory nerve conduction study should be performed in the
involved limb. The sensory nerve conduction study ideally should be in the
distribution of the suspected radiculopathy. The screening for a suspected
radiculopathy should examine at least five to seven muscles, including the
paraspinal muscles.7 The screening should include muscles that represent all
relevant myotomes of the involved limb, with emphasis on the myotome in
question.
No consensus exists regarding which muscles should be included in the
initial screening. If one muscle is abnormal, the screening is expanded. The
optimal number of muscles tested for both cervical and lumbosacral
radiculopathy screenings has been proposed to be six muscles and should
include the paraspinal muscles.
8,9
A needle EMG study is considered
“positive” for a radiculopathy if EMG abnormalities are present in two or
more muscles that receive innervation from the same root (preferably by
different peripheral nerves), but muscles innervated by adjacent nerve roots
are normal. The EMG report should include the nerve conduction studies
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