Electrodiagnostic Findings in Neuromuscular Disorders

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Chapter 4 Electrodiagnostic Findings in Neuromuscular Disorders

Details of the electrodiagnostic findings in various neuromuscular disorders are outlined within the case studies in this book. The following is a brief summary of these findings in the most common neuromuscular disorders.

FOCAL MONONEUROPATHIES

Compression, traction, laceration, thermal, or chemical injury may damage one or more components of the peripheral nerves, including the myelin, axons, or supporting nerve structures (endoneurium, epineurium, and perineurium). The pathophysiologic responses to peripheral nerve injuries have a limited repertoire; that is demyelination, axon loss, or a combination of both.

Demyelinative Mononeuropathy

With focal injury to myelin, conduction along the affected nerve fiber is altered. This may result in slowing of conduction or conduction block along the nerve fibers or a combination of both.

1. Focal slowing. This is usually the result of widening of the nodes of Ranvier (paranodal demyelination). Focal slowing may be synchronized when demyelination affects all the large myelinated fibers equally. When the focal lesion is distal, there is prolongation of distal and proximal latencies while the proximal conduction velocity remains normal. If the focal lesion is between the distal and proximal stimulation sites, there is prolongation of proximal latency only resulting in slowing in proximal conduction velocity while the distal latency remains normal (Figure 4-1). With lesions manifesting as focal synchronized slowing, the CMAP amplitudes, durations, and areas remain normal and do not change significantly following proximal and distal stimulation. Desynchronized (differential) slowing occurs when conduction time is reduced at the lesion site along a variable number of the medium or small nerve fibers (average or slower conducting axons). Here, the CMAP is dispersed with prolonged duration on stimulations proximal to the lesion (Figure 4-2). The latency and conduction velocity along the injury site remain normal, since at least some of the fastest conducting axons are spared. When the largest axons are also affected, the dispersed CMAP with prolonged duration is also accompanied by slowing of distal latency (in distal lesions) or conduction velocity (in proximal lesions).

2. Conduction block. This is usually the result of focal loss of one or more myelin segment (segmental or internodal demyelination) which leads to interruption of action potential transmission. A nerve lesion manifesting with conduction block is best localized when it can be bracketed by two stimulation points, one distal to the site of injury and one proximal. In conduction block, stimulation distal to the lesion elicits a normal CMAP, whereas proximal stimulation elicits a response with reduced amplitude (partial conduction block) or absent response (complete conduction block) (Figure 4-3). The percentage drop in amplitude and area (amplitude or area decay) are calculated as follows:

Figure 4-1 Nerve conduction studies showing focal slowing in distal segment (a) resulting in slowing of distal latency only, and in proximal segment (b) resulting in slowing of conduction velocity only.

(Reprinted from Wilbourn AJ. Nerve conduction studies. Types, components, abnormalities and value in localization. Neurol Clin 2002;20:305–338, with permission.)

Figure 4-2 Nerve conduction studies showing desynchronized slowing. The response with proximal stimulation is dispersed consistent with differential slowing of nerve fibers in the proximal nerve segment.

(Reprinted from Wilbourn AJ. Nerve conduction studies. Types, components, abnormalities and value in localization. Neurol Clin 2002;20:305–338, with permission.)

Figure 4-3 Nerve conduction studies showing partial or complete conduction blocks in the proximal segment of the nerve.

(Reprinted from Wilbourn AJ. Nerve conduction studies. Types, components, abnormalities and value in localization. Neurol Clin 2002;20:305–338, with permission.)

There are several limitations to the definitive diagnosis of demyelinative conduction block:

(A) A reduced CMAP size may result from phase cancellation between action potentials peaks of opposite polarity because of abnormally increased temporal dispersion. Such excessive desynchronization often develops in acquired demyelinative neuropathies. If the distal and proximal responses have dissimilar waveforms, the discrepancy in amplitude between the two may represent in part a phase cancellation rather than true conduction block. Hence, in true partial conduction block, the significant drop of CMAP amplitude, with stimulation proximal to the lesion site compared with the CMAP distal to it, should not be accompanied by significant (>15%) prolongation of CMAP duration and should be supported by significant drop in CMAP area. In general, more than 50% decrease of the CMAP amplitude across the lesion along with more than 50% drop in CMAP area, are definitive signs of conduction block. A 20–50% drop CMAP amplitude and area may be consistent with conduction block in situations when the two stimulation sites are close (usually 10 cm or less) such as across the fibular neck or elbow (Table 4-1).

(B) A distal demyelinating lesion, causing conduction block of the nerve segment between the most distal possible stimulating point and the recording site, manifest as unelicitable or low CMAP amplitude at both distal and proximal stimulation sites. This finding mimics the findings encountered with axonal degeneration (see axon-loss mononeuropathy). Such a demyelinative lesion could only be distinguished from axon loss lesion by repeating the NCS after several weeks. Often in demyelinative lesions, the distal CMAP improves rapidly, a finding that is not consistent with axon loss.

(C) With proximal demyelinative conduction block lesions, it may not be technically possible to bracket the lesion with two stimulation sites since the lesion is extremely proximal. Examples include demyelinating lesions at the root level, femoral nerve, facial nerve, sciatic nerve, or lumbosacral plexus. In these situations, stimulation distal to the lesion can only be achieved. The diagnosis of conduction block is made only by inference, when a normal or nearly normal CMAP is coupled with significant reduction of recruitment on needle EMG of the recorded muscle.

(D) The prominent temporal dispersion due to phase cancellation seen in evaluating normal SNAPs precludes the use of these potentials in the diagnosis of conduction block (see Chapter 2, Figure 2-15).

(E) Conduction block pattern is often seen with acute axonal loss nerve lesions before the completion of wallerian degeneration (see axon-loss mononeuropathy).

3. Focal slowing with conduction block. Demyelinating lesions presenting with both focal slowing and partial conduction block are not common. They are usually seen at chronic entrapment sites such ulnar nerve lesions across the elbow. In such situations, a significant number of fibers have internodal demyelination resulting in conduction block (drop in amplitude and area across the abnormal segment) while other fibers have paranodal demyelination manifesting as focal slowing of the same nerve segment.

Table 4-1 Electrodiagnosis of Conduction Block

Definite in Any Nerve^*

1. ≥50% drop in CMAP amplitude with a 15% prolongation of CMAP duration and with ≥50% drop in CMAP area

2. ≥30% drop in amplitude and area over a short nerve segment (e.g., radial across the spiral groove, ulnar across the elbow, peroneal across the fibular head)

Possible in Median, Ulnar, and Peroneal Nerves Only

1. 20–50% drop in CMAP amplitude with ≤15% prolongation of CMAP duration and with 20–50% drop in CMAP area

* Caution should be taken in evaluating the tibial nerve, where stimulation at the knee can be submaximal, resulting in 50% or at times greater than 50% drop in amplitude and area, especially in overweight and very tall patients.

Axon-Loss Mononeuropathy

Following acute focal axonal damage, the distal nerve segment undergoes wallerian degeneration. However, early after axonal transaction, the distal axon remains excitable. Hence, stimulation distal to the lesion elicits a normal CMAP, whereas proximal stimulation elicits an absent response (complete conduction block) when the lesion is total and reduced CMAP amplitude (partial conduction block) when the lesion is incomplete (Figure 4-4). In an attempt to distinguish this pattern from a demyelinative conduction block, some refer to this pattern as an axonal noncontinuity, early axon loss, or axon discontinuity conduction block.

Figure 4-4 Nerve conduction studies in partial axon loss lesion. Initially, there is an axon discontinuity conduction block. Following wallerian degeneration, the distal response drop in amplitude and the subsequent study showed uniformly low amplitude CMAP irrespective of the site of stimulation.

(Reprinted from Wilbourn AJ. Nerve conduction studies. Types, components, abnormalities and value in localization. Neurol Clin 2002;20:305–338, with permission.)

Wallerian degeneration of the axons distal to the nerve lesions is completed in 7–11 days. In the first 1–2 days, the distal CMAP and SNAP are normal. The distal CMAP amplitude then decreases and reaches its nadir in 5–6 days, while the distal SNAP amplitude lags slightly behind. It starts declining in amplitude after 4–5 days and reaches its nadir in 10–11 days (Figure 4-5). The earlier decline of the CMAP amplitude comparing to the SNAP amplitude following axon-loss nerve lesion is likely related to the early neuromuscular transmission failure that affects the recording of the CMAP amplitudes only. This is supported by the fact that MNAPs, recorded directly from nerve trunks, follow the time course of SNAPs.