Median Motor Response

Needle EMG in the evaluation of PN is helpful for several reasons. First, needle examination of lower extremity muscles can assist in establishing the presence of motor axonal injury by identifying the presence of a gradient of abnormality. In this situation, the distal muscles (e.g., extensor hallucis longs) are most severely affected, with proximal muscles (e.g., tibialis anterior and gastrocnemius) showing more minor changes, and muscles that are even more proximal (e.g., vastus lateralis) appearing entirely normal. Comparing the severity of distal reinnervation between the two legs can also be helpful in some situations. For example, symmetric reinnervation restricted to extensor hallucis longus may be present. Second, needle examination is helpful to evaluate for superimposed processes, most notably lumbosacral radiculopathy that may be contributing to a patient's clinical picture. In fact, in a proportion of elderly individuals referred for symptoms of PN (distal sensory loss), the major pathology identified is polyradiculopathy, often related to lumbar stenosis. Third, in patients with acute demyelinating PNs, reduced recruitment of motor unit potentials is one of the first abnormalities observed, and is often present before nerve conduction abnormalities develop.

Pictures Nerve Conduction Studies
Fig. 1. Slowing of conduction velocity in axon loss lesions. In normal nerve, conduction velocity is measured from the potential onset. When the largest fibers are lost, the fastest conducting fibers can no longer be detected, leading to a slowing of the measured conduction velocity.

4. DIFFERENTIAL DIAGNOSIS OF PN 4.1. Usefulness of Electrophysiological Testing 4.1.1. Axonal vs Demyelinating Injury

One of the most common reasons for performing electrophysiological testing in patients with suspected PN is to differentiate axonal from demyelinating injury. The general rules for separating these two types of disorders are relatively straightforward: axonal PN reduces amplitudes of the recorded motor and sensory responses, whereas demyelinating PN increases the distal latencies, slows conduction velocities, and prolongs F-wave latencies. In addition, conduction block and abnormal temporal dispersion are also present in demyelinating PN.

Unfortunately, however, there is more overlap than this simple description suggests. In fact, some slowing of conduction velocity does occur in axonal lesions, and reductions in amplitude often occur in demyelinating neuropathies. The reason for the reduction in recorded conduction velocity is demonstrated in Fig. 1. A given nerve contains myelinated fibers with a range of conduction velocities, the largest diameter fibers having the fastest velocities and the fibers with the smallest diameters having the slowest velocities. In performing nerve conduction studies, the velocity is typically measured from the onset of the potential, which represents those fibers with the fastest conduction velocities. Now, if an axonal lesion occurs, equally reducing all populations of fibers, the few fastest fibers contributing to the onset of the potential may no longer be present and detectable, and, hence, the measured velocity will be slower.

Likewise, predominantly demyelinating processes can also produce reductions in amplitude in addition to reductions in conduction velocity. These can occur for a number of reasons. First, conduction block may occur. In this situation, the response amplitude is reduced proximally as compared with distally; a reduction in area of greater than 50% is generally considered a conservative estimate. In truth, reductions in area or amplitude of 20% or greater, or increases in duration of 15% or greater, if over short distances, are suggestive of conduction block. If conduction block occurs distal to the most distal stimulation site (e.g., in the hand, if undergoing standard median or ulnar nerve conduction studies), the recorded response amplitude will be low, giving the appearance of axon loss. Second, abnormal

Table 1

Criteria for Acute Inflammatory Demyelinating Polyradiculoneuropathy

Table 1

Criteria for Acute Inflammatory Demyelinating Polyradiculoneuropathy

At least three of the following in motor nerves:

>50% NL CMAP

<50% NL CMAP

CVb

<90% LLN

<80% LLN

DLa

>115% ULN

>125% ULN

F-waveb

>125% ULN

Ratiob

<0.70

Abbreviations: CMAP, compound motor amplitude potential; CV, conduction velocity; DL, distal latency; F-wave, F-wave latency; LLN, lower limit of normal; NL, normal; ratio, proximal/distal CMAP area ratio; ULN, upper limit of normal. aMore than two nerves. bMore than one nerve. Modified from ref. 2.

Abbreviations: CMAP, compound motor amplitude potential; CV, conduction velocity; DL, distal latency; F-wave, F-wave latency; LLN, lower limit of normal; NL, normal; ratio, proximal/distal CMAP area ratio; ULN, upper limit of normal. aMore than two nerves. bMore than one nerve. Modified from ref. 2.

Table 2

Criteria for CIDP

At least three of four in motor nerves:

1. Reduction in nerve conduction velocity in at least two nerves (see chart)

2. Partial conduction block in at least one nerve

3. Prolonged distal latencies in at least two nerves (see chart)

4. Absent F-waves or prolonged F-waves in at least two nerves (see chart)

Abbreviations: CMAP, compound motor amplitude potential; CV, conduction velocity; DL, distal latency; F-wave, F-wave latency; LLN, lower limit of normal; NL, normal; ULN, upper limit of normal.

Modified from ref. 1.

aMore than two nerves.

temporal dispersion of recorded compound motor action potentials can also reduce the amplitude of the recorded response. The duration of the response is prolonged, and the amplitude drops, so that the area remains relatively constant. Third, and perhaps most commonly, reductions in amplitude occur because there is secondary axonal loss with many demyelinating processes. Hence, many demyelinating lesions are not purely demyelinating, but, rather, are "mixed" (demyelinating and axonal).

Several attempts have been made to develop criteria to separate demyelinating from axonal processes (Tables 1 and 2). Unfortunately, these criteria have substantial limitations because they were developed mainly for research purposes and for specific disorders, such as acute inflammatory demyelinating polyradiculoneuropathy (AIDP; also known as GBS) or chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). CIDP has stricter criteria than AIDP because the longstanding nature of the disorder leads to more substantial axonal injury.

Needle EMG does not serve a major role in differentiating acute axonal from demyelinat-ing disorders. In acute demyelinating disorders, reduced recruitment will be present often diffusely, whereas, in acute axonal neuropathies, abnormalities may be restricted to nerve

Median Motor Response
Right Median Motor-Nerve Conduction Study

Latency

Amplitude

Area

Distance

CV

Segment

(ms)

(mV)

(ms*mV)

(cm)

(m/s)

APB-wrist

9.8

9.2

32.4

7.8

22.2

Wrist-elbow

20.8

9.2

33.6

24.5

APB, abductor pollicis brevis; CV, conduction velocity.

Fig. 2. Typical motor conduction studies in a patient with Charcot-Marie-Tooth IA. Although the waveform morphology appears normal, there is a marked prolongation of distal latency and severe slowing of conduction velocity.

APB, abductor pollicis brevis; CV, conduction velocity.

Fig. 2. Typical motor conduction studies in a patient with Charcot-Marie-Tooth IA. Although the waveform morphology appears normal, there is a marked prolongation of distal latency and severe slowing of conduction velocity.

segments that are more distal or individual nerves, as occurs in monoeuropathy multiplex. Fibrillation potentials often develop in both types of PN and, hence, are not clearly helpful in establishing the diagnosis. Myokymic discharges often occur early in AIDP, and these potentials, if present, can also be helpful in the diagnosis.

4.1.2. Acquired vs Inherited Demyelinating PNs

Nerve conduction studies are also valuable in differentiating whether a demyelinating process is acquired or inherited. Although the clinical history may make such a distinction inconsequential (e.g., the abrupt onset of numbness and tingling and generalized weakness would make Charcot-Marie-Tooth disease an unlikely consideration), certain features in the electrophysiology may help guide the clinician. In hereditary illnesses, demyelination tends to be distinctly uniform, with the same abnormally slow conduction velocity in all upper extremity nerves. Conduction block is not present and although some abnormal temporal dispersion may occur, the waveforms themselves have a relatively normal morphology. This is demonstrated in Fig. 2, which represents typical motor conduction studies for a patient with Charcot-Marie-Tooth disease IA. Although the motor responses seem normal, the latency is

Demyelinating Nerve Conduction Study
Right Peroneal Motor-Nerve Conduction Study

Latency

Amplitude

Area

Distance

CV

Segment

(ms)

(mV)

(ms*mV)

(cm)

(m/s)

EDB-ankle

7.5

2.67

18.9

8.5

Ankle-fib head

19.6

1.26

6.7

29.8

23.9

Fib head-pop fossa

24.5

1.55

6.4

18.5

21.4

CV, conduction velocity; EDB, extensor digitorum brevis; fib, fibular; pop, popliteal.

Fig. 3. Typical motor conduction studies in a patient with acquired demyelinating neuropathy. Conduction block and temporal dispersion are demonstrated in this study.

CV, conduction velocity; EDB, extensor digitorum brevis; fib, fibular; pop, popliteal.

Fig. 3. Typical motor conduction studies in a patient with acquired demyelinating neuropathy. Conduction block and temporal dispersion are demonstrated in this study.

actually very prolonged (normal <4.4 ms) and the conduction velocity very slow (normal >50 m/s). In patients with acquired demyelinating PNs, one nerve can demonstrate only modest changes, whereas another may be profoundly abnormal, with conduction block and temporal dispersion (Fig. 3). Recently, there have been reports of individuals with hereditary diseases, such as X-linked Charcot-Marie-Tooth disease, with nerve conduction study abnormalities that are more consistent with an acquired picture and, hence, even this simple dichotomy does not seem to hold universally. Caution regarding the interpretation of a given patient's electrophysiological results is warranted.

4.1.3. Evaluation of Small Fiber Neuropathies

Standard electrophysiological testing generally does not involve a detailed analysis of small myelinated and unmyelinated nerve function. Special testing is required to evaluate small fiber PNs. Tests that are useful in the assessment of PNs that specifically or predominantly affect the small sensory fibers include quantitative sensory testing, quantitative sudomotor axon reflex test, thermoregulatory sweat test, sympathetic skin response, and epidermal skin biopsy. An approach to the evaluation of these disorders is discussed in Chapter 24 (Autonomic Nervous System Testing).

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