Abstract
Neurophysiologic studies are useful for the evaluation of patients with pain. This chapter describes electrical tests such as electromyography, nerve conduction testing, somatosensory evoked potential as well as quantitative sensory testing, laser evoked potential, and contact heat evoked potential. The tests for sympathetic nervous system include sympathetic skin response and quantitative sweat test. Nociceptive reflex test is briefly touched. Indications, methodology, and interpretation as well as limitation of each test are discussed in detail.
Keywords
contact heat evoked potentials, electromyography, laser evoked potentials, nerve conduction study, nociceptive reflex, quantitative sensory testing, somatosensory evoked potentials
Electrophysiologic testing, when properly applied, is a useful tool for the evaluation of patients with pain. Understanding the indications and limitations of each test is absolutely essential for appropriate diagnosis and subsequent treatment.
Electrophysiologic studies are a very sensitive indicator of central and peripheral nervous system involvement but do not indicate underlying disease. For example, testing can diagnose radiculopathy but cannot determine if it is caused by osteophytes, a herniated disc, or diabetes. This chapter describes conventional electrophysiologic tests such as electromyography (EMG), and short latency somatosensory evoked potentials (SSEPs), as well as newer techniques including quantitative sensory testing (QST), laser evoked potentials (LEPs), and contact heat evoked potentials (CHEPs). Invasive testing such as microneurography will not be discussed here.
The role of sympathetic nervous system in the production of pain is complex and controversial; nonetheless, testing of the autonomic function is also important for the evaluation of pain complaints because it gives an objective measure of small nerve fiber involvement, as well as evidence of the therapeutic interventions such as sympathetic nerve blocks. The most frequent referrals to the autonomic laboratory are patients with painful peripheral neuropathy such as diabetic polyneuropathy, and so-called complex regional pain syndrome/reflex sympathetic dystrophy (CRPS/RSD). Based on accuracy, reproducibility, and ease of implementation, sudomotor function tests such as sympathetic skin response (SSR) and quantitative sweat test are discussed here. Other quantitative autonomic measures for adrenergic function (Valsalva maneuver, head up tilt) and for cardiovagal function (heart rate variability to deep breathing) are beyond the scope of this discussion.
Finally, although controversial, the value of nociceptive reflexes such as blink reflex, masseter inhibitory reflex (MIR), and flexor reflex for the evaluation of neuropathic pain will be discussed briefly.
Electromyography and Nerve Conduction Studies
When strictly defined, EMG indicates only a needle examination of muscles. However, EMG is often used to include both needle studies and nerve conduction studies (NCS). EMG/NCS is extremely useful in the evaluation of the peripheral nervous system. Indeed, the three most common diagnoses in EMG laboratories—peripheral neuropathy, carpal tunnel syndrome, and lumbosacral radiculopathy—all cause pain. EMG/NCS 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. This data is clinically relevant for pain management, as it can confirm pathophysiology, assist with prognosis, and guide treatment (optimal site/level for injection).
Nerve Conduction Studies
By stimulating peripheral nerve with supramaximal intensity, compound muscle action potential (CMAP) for motor nerve and sensory nerve action potential (SNAP) for sensory nerve are recorded. Amplitude of action potentials as well as the time from stimulation to response is recorded. Latency is the interval between the onset of a stimulus and the onset of a response, expressed in milliseconds. Conduction velocity is 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 seconds (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 Aδ and C fibers.
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. Most laboratories have their own normal values for major motor and sensory nerves, with minor differences occurring among laboratories. A lower temperature will prolong distal latencies, reduce conduction velocities, and increase the amplitude of CMAP and SNAP. Age also affects NCSs. Adult values are not attained until 4 years of age, and they decline after age 60 years at a rate of 1–2 m/s per decade. Waveform analyses of CMAP and SNAP help determine normal versus abnormal nerve function ( Fig. 8.1 ). The amplitude of a response should be similar when the same nerve is stimulated proximally and distally. A greater than 20%–50% reduction between distal and proximal stimulation of a motor nerve suggests an abnormal block in conduction between two stimulation points. Many laboratories are now computerized, and the area under an action potential curve can be calculated. Greater than 20%–40% reduction in area also suggests conduction block. 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 that is due to different rates of conduction of each nerve fiber. This also suggests nerve pathology between the proximal and distal stimulation sites.
The H reflex is the electrophysiological 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 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 ( Fig. 8.2 ). Unfortunately there is no consensus as to methodology for obtaining responses, and to the patterns of abnormality to be identified.
Electromyography
The electrical activity in a muscle can be measured using disposable needle electrodes. Needle examination is performed in proper steps. An examiner observes activity on insertion of a needle (insertion activity), activity when the needle is maintained in a relaxed muscle (spontaneous activity), and activity during varying degrees of voluntary muscle contraction. The electrical activity is evaluated by sight and sound, as specific activities have specific waveforms and characteristic sounds. Observations are made by the electromyographer during the study; therefore the results of a needle examination are dependent on the experience of the examiner.
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. The type and significance of various spontaneous activities are summarized in Table 8.1 , and some examples are shown in Fig. 8.3 .
| Spontaneous Activity | Firing Pattern | Frequency | Waveform | Amplitude | Duration | Significance |
|---|---|---|---|---|---|---|
| Complex repetitive discharge | Regular, abrupt onset and cessation, “motor cycle idling” | 5–100 Hz | Polyphasic or serrated, MFAP | 100 μV–1 mV | — | Neurogenic (chronic), myopathic (dystrophy) |
| Cramp discharge | Increase and subside gradually | (1) <150 Hz (2) 4–15 Hz | MUAP | — | — | (1) Ischemic, ↑ Na, (2) ↓ Ca, ↓ Mg, ↑ K |
| End plate noise | Dense and steady, “sea shell” “hissing” | >150 Hz | Monophasic (negative), MEPP | 10–20 μV | 0.5–1 ms | Normal |
| End plate spike | Irregular short burst, “sputtering fat in a frying pan” | 50–100 Hz | Biphasic (negative-positive) MFAP | 100–300 μV | 2–4 ms | Decrease in denervated muscle, increase in reinnervated muscle |
| Fasciculation potential | Spontaneous, sporadic, “typing on card board” | 0.1–10 Hz | MUAP | >1 mv | >5 ms | Normal, neurogenic (motor neuronopathy), myopathic |
| Fibrillation potential | Regular, “rain on tin roof” “ticking of clock” | 1–50 Hz | Biphasic (positive-negative) MFAP | <1 mV | <5 ms | Neurogenic, NMJ defect, myopathic |
| Myokymic discharge | Semiregular, “marching soldiers” | (1) 2–60 Hz brief (2) 1–5 Hz continuous | MUAP | — | — | Limb (entrapment, radiation), face (MS, brainstem tumor, Bell palsy) |
| Myotonic discharge | Wax and wane, “dive bomber” | 20–80 Hz | (1) Biphasic (positive-negative) (2) Positive MFAP | <1 mV <1 mV | (1) <5 ms (2) 5–20 ms | Myopathic (myotonic syndromes), ↑K, Schwartz-Jampel |
| Neuromyotonic discharge | Start and stop abruptly, wane, “pinging” | 150–300 Hz | MUAP | — | — | Isaac syndrome, stiff-man syndrome, tetany |
| Positive sharp wave | Regular | 1–50 Hz | Biphasic (positive-negative) MFAP | <1 mV | 10–100 ms | Same as fibrillation |
| Neurotonic discharge | Irregular | 30–100 Hz | MUAP | — | <200 ms | — |
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 less than 10 Hz). Recruitment frequency is defined as the frequency of first MUAP when second MUAP is recruited (normally less than 15 Hz). Reduced number of MUAP (high recruitment frequency) can be seen in neuropathic processes. An increased number of MUAP (low recruitment frequency), however, can be seen in myopathic disorder or defect of neuromuscular junction. During maximum contraction, a full interference pattern consisting of overlapping motor units is seen. 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. Large, long duration polyphasic units suggest denervation and re-innervation. Short-duration, small polyphasic units can be seen in myopathic processes. EMG findings in neuropathic and myopathic disorder are summarized in Table 8.2 .
| EMG | Normal | Neurogenic (Axonal) | NMJ Defect | Myopathic |
|---|---|---|---|---|
| Insertional activity | N | ↑ | ↑ | ↑ |
| Spontaneous activity | — | + | + | + |
| MUAP amplitude | 0.1–5 mV | ↑ | ↓ | ↓ |
| Duration | 3–15 ms | ↑ | ↓ | ↓ |
| Phase | <5 | ↑ | ↑ | ↑ |
| Stability | N | N | Variable | N |
| Recruitment | N | ↓ | N | ↑ |
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