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
Demyelinative Mononeuropathy
There are several limitations to the definitive diagnosis of demyelinative conduction block:
Definite in Any Nerve* |
* 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.
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.
Figure 4-5 Effect of wallerian degeneration on distal CMAP and SNAP amplitudes following acute nerve lesion.
On motor NCS, a conduction block is present soon after axonal injury. However, as the distal axons undergo wallerian degeneration, this is replaced by unelicitable or low CMAP amplitudes with both distal and proximal stimulations corresponding to complete or partial motor axonal loss lesions respectively (see Figure 4-4