Electrodiagnostic Features of Generalized Demyelinating Polyneuropathy
In generalized demyelinating polyneuropathy, look for reduced conduction velocities, prolonged distal motor latencies, abnormal temporal dispersion, partial motor conduction blocks, and prolonged or absent F-waves, with the key distinction being that these abnormalities should be relatively uniform across multiple tested nerves rather than focal. 1
Core Demyelinating Features
The hallmark electrodiagnostic findings that distinguish demyelination from axonal loss include:
- Reduced motor conduction velocities: Slowing to 30-50% below the mean of controls is considered indicative of demyelination, though criteria vary among electromyographers 2
- Prolonged distal motor latencies: Increases of 35-70% above the mean of controls suggest demyelination 2
- Prolonged F-wave latencies:延长至对照上限的120-150%提示脱髓鞘 2
- Partial motor conduction blocks: Defined as 11-50% reduction in CMAP amplitude and/or area between proximal and distal stimulation sites 2
- Abnormal temporal dispersion: This reflects non-uniform slowing of conduction across different nerve fibers 1
Pattern Recognition: Generalized vs. Focal
The critical distinction is that nerve conduction studies show diffuse abnormalities with relatively uniform involvement of tested nerves, differentiating generalized polyneuropathy from multifocal processes 3. In demyelinating polyneuropathy, you should find:
- Multiple nerves affected symmetrically: Both upper and lower extremities show similar patterns of involvement 1
- Length-dependent distribution: Distal nerves are typically more affected than proximal ones 3
- Sensory and motor involvement: Unlike pure motor variants, most demyelinating polyneuropathies affect both sensory and motor fibers 1
Specific Diagnostic Patterns
The "Sural Sparing" Pattern
A highly characteristic finding in Guillain-Barré syndrome (a prototype acute demyelinating polyneuropathy) is sural sparing, where the sural sensory nerve action potential remains normal while median and ulnar sensory nerve action potentials are abnormal or absent 1. This pattern has diagnostic value when present.
Sensory Nerve Involvement
In demyelinating polyneuropathy, 73% of sensory nerves tested typically show abnormalities, which helps distinguish it from pure motor variants like multifocal motor neuropathy 4. The sensory abnormalities include:
- Reduced sensory nerve action potential amplitudes
- Prolonged sensory distal latencies
- Slowed sensory conduction velocities 1
F-Wave and H-Reflex Abnormalities
Absent or prolonged F-waves are among the earliest and most sensitive findings, detected in up to 97% of cases in acute inflammatory demyelinating polyneuropathy 5. The H-reflex is similarly affected early in the disease course 5.
Timing Considerations and Common Pitfalls
A critical pitfall is performing electrodiagnostic studies too early in the disease course. Studies performed within the first week of symptom onset may be normal in 30-34% of patients, even with active demyelinating disease 1, 5. When initial studies are normal or equivocal but clinical suspicion remains high, repeat electrodiagnostic testing 2-3 weeks later is essential 1, 6.
Additional scenarios where studies may be falsely normal include:
- Initially proximal weakness (proximal segments may not be adequately tested)
- Mild disease with slow progression
- Clinical variants affecting primarily nerve roots 1
Amplitude Considerations
While demyelination primarily affects conduction velocity and latency parameters, motor and sensory response amplitudes may also be reduced due to secondary axonal loss or conduction block 7. However, amplitudes are typically less severely reduced in demyelinating polyneuropathy compared to primary axonal polyneuropathies 7.
Conduction Block Specificity
Motor conduction blocks are highly specific for demyelinating pathology when present, occurring in approximately 10% of nerves in acute inflammatory demyelinating polyneuropathy 7. The absence of conduction blocks does not exclude demyelination, but their presence strongly supports it 7.
Electromyography Findings
Needle EMG complements nerve conduction studies by:
- Detecting denervation activity (fibrillation potentials and positive sharp waves) indicating axonal loss
- Assessing chronicity through the extent of reinnervation (large motor unit potentials, reduced recruitment)
- Evaluating topographical distribution of changes 2
In pure demyelinating polyneuropathy without secondary axonal loss, EMG may show reduced recruitment without denervation potentials 2.
Diagnostic Certainty Grading
When interpreting studies, consider a graded approach to diagnostic certainty. The presence of relative sural sparing combined with at least two abnormal upper limb sensory nerve conduction studies and absent or prolonged F-waves provides high specificity (96%) for acute demyelination 7. This combination is particularly useful when distinguishing demyelinating from axonal polyneuropathies 7.