Chapter 8 Neurophysiologic Testing for Pain Flashcards
KEY POINTS 1. Electrophysiologic studies are very sensitive indicators of central and peripheral nervous system involvement but do not indicate underlying disease. 2. EMG/NCV studies can identify the anatomic site of injury, the type of neurons or fibers involved, the nature of the pathologic alteration, and severity of injury. 3. In QST, cold threshold measures Ad fiber function, whereas warmth, heat pain, and cold pain thresholds reflect the function of C-fibers. 4. SSEPs provide a quantit
Electrophysiologic studies indication
are a very sensitive indicator of central and peripheral nervous system involvement but do not indicate underlying disease
Conventional Electrophysiologic Tests
Electromyography (EMG), Short-Latency Somatosensory-Evoked Potentials (SSEPs), quantitative sensory testing (QST),
Laser-Evoked Potentials (LEPs), and
Contact Heat–Evoked Potentials (CHEPs)
EMG
EMG indicates only a needle examination of muscles. However, EMG is often used to include both needle studies and nerve conduction studies
Nerve conduction studies are often referred to by
the letters NCV, with “V” standing for velocity, although nerve conduction
studies measure more than velocity. For clarity, EMG/NCV to indicate the combination of needle electromyography
and nerve conduction studies
three most common diagnoses in EMG laboratories
peripheral neuropathy, carpal tunnel syndrome, and lumbosacral radiculopathy
EMG/NCV can identify
the anatomic site of injury (anterior horn cell, spinal
root, plexus, nerve, neuromuscular junction, or muscle), the type of neurons or fibers involved (motor, sensory, or autonomic), the nature of pathologic alteration (demyelination,
or axonal degeneration), time course (acute, subacute, or chronic), and severity of injury
What is recorded when stimulating a peripheral nerve with supramaximal intensity?
Compound muscle action potential (CMAP) for motor nerve and sensory nerve action potential (SNAP)
for sensory nerve are recorded. The amplitude of action potentials as well as the time from stimulation to response is recorded
Latency
the interval between the onset of a stimulus and the onset of a response, expressed in milliseconds.
Conduction velocity
obtained by dividing the
distance between two stimulation points (mm) of the same nerve by the difference between proximal and distal latencies (ms). This calculated velocity, expressed in meters per second (m/s) represents the conduction velocity of the fastest nerve fibers between two points of stimulation. It is important to note that studies may be normal if a disorder is limited to small nerve fibers such as Ad and C-fibers.
Amplitude of CMAP
Amplitude of SNAP
The amplitude of CMAP is measured from baseline to
negative peak in millivolts, and the amplitude of SNAP is measured from the first positive peak to negative peak in microvolts.
Effect of lower temperature on distal latencies, conduction velocities, amplitude of CMAP and SNAP
will prolong distal latencies, reduce conduction velocities, and increase the amplitude of CMAP and SNAP
Amplitude of a response when the same nerve is stimulated proximally and distally
The amplitude of a response should be similar when the same nerve is stimulated proximally and distally. A 20% to 50% reduction between distal and proximal stimulation of a motor nerve suggests an abnormal
block in conduction between two stimulation points
Conduction block on EMG
Greater than 20% to 40% reduction in area also suggests conduction
block.
Temporal Dispersion on EMG
A significant reduction in amplitude from proximal to distal stimulation sites without a reduction in area under the response curve, and a significant increase in
duration (>15%) suggest temporal dispersion resulting
from a relative desynchronization of the components of an action potential, which is due to different rates of conduction
of each nerve fiber. This also suggests nerve pathology
between the proximal and distal stimulation sites.
H-reflex
The H-reflex is the electrophysiologic equivalent of a muscle stretch reflex. A sensory nerve is stimulated with submaximal intensity, and a late motor response is recorded owing to reflex activation of motor neurons
In adults, H-reflexes are easily obtained from
Soleus muscle and less easily from flexor carpi radialis muscle following the stimulation of tibial and median nerves, respectively. The tibial H-reflex is useful in identifying S1 radiculopathy.
F-waves
F-waves are late response recorded from muscle after
supramaximal stimulation of a motor nerve. F-waves represent a response to a stimulus that travels first to and then from the cord via motor pathways; thus, F-waves are useful in studying the proximal portion of motor nerves
Repetitive nerve stimulation (RNS) studies
are used primarily for evaluation of neuromuscular junction disorders like myasthenia gravis.
Needle examination electrical activity
The electrical activity is evaluated by sight and sound, as specific activities have specific wave forms and characteristic sounds
Insertion activity
Insertion activity, also referred to as injury potential, is caused by movement of the needle electrode, resulting in mechanical damage to the muscle fibers. Increased insertion activity consists of unsustained fibrillation potentials and positive sharp waves. A muscle at rest should be electrically silent. Spontaneous activity in a resting muscle usually suggests a pathologic condition
What happens as a muscle contracts?
motor unit action potentials
(MUAPs) are observed. MUAP represents the summation of muscle fiber action potentials of a given motor unit. With increasing voluntary muscle contraction, individual
motor units fire more frequently, and more motor units are recruited to fire. The term “onset frequency” is used to describe the firing rate of a single MUAP maintained at the lowest voluntary muscle contraction (normally ,10 Hz).
Recruitment frequency
defined as the frequency of first MUAP when the second MUAP is recruited (normally ,15 Hz). A reduced number of MUAPs (high recruitment frequency) can be seen in neuropathic processes. An increased
number of MUAPs (low recruitment frequency),
however, can be seen in myopathic disorders or defects of the neuromuscular junction
MUAPs are analyzed in terms of
amplitude, duration, number of phases, and stability.
The morphology of the MUAPs is affected by
the type of needle electrode used, location of the needle within the motor unit territory, age, temperature, and specific muscle being examined.
In general, changes in conduction, either a prolonged distal latency or a low velocity, suggest a
pathologic lesion between the site of stimulation and the recording site.
PATHOPHYSIOLOGY: IS THE LESION AXONAL
OR DEMYELINATING?
If an injury occurs at the cell body or axon, axonal degeneration results. If an injury is directed against the myelin, demyelination ensues
Demyelinating neuropathies
Demyelinating neuropathies
can be further divided into segmental (acquired) and
uniform (hereditary) types. In the former, nonuniform
slowing in individual myelinated nerve fibers results in conduction block and temporal dispersion. In the latter, prolonged latency and slowing of conduction predominate as a result of uniform involvement of all myelinated fibers
FIBER TYPE SPECIFICITY: IS THE LESION MOTOR, SENSORY, OR AUTONOMIC?
In a case of distal sensory or motor neuropathies,
amplitudes as well as velocities are abnormal.
With a dorsal root ganglia lesion or anterior horn cell disease, NCV studies show small amplitude SNAP or CMAP, respectively, and as a rule normal velocity. Routine EMG/ NCV studies do not test the integrity of the autonomic nervous
system
DISTRIBUTION: IS THE LESION FOCAL, MULTIFOCAL, OR DIFFUSE?
neuropathy, for example, can be further divided into mononeuropathy, multifocal neuropathy, and polyneuropathy
