Causes of Decreased Conduction Velocity in Nerve Conduction Studies
Demyelinating disorders are the primary cause of decreased conduction velocity in nerve conduction studies, with other causes including axonal neuropathies with severe fiber loss, temperature effects, and anatomical factors. 1
Primary Causes of Decreased Conduction Velocity
1. Demyelinating Disorders
- Pathophysiology: Loss of myelin sheath disrupts saltatory conduction, forcing action potentials to travel continuously along the axon membrane
- Characteristic findings:
- Marked slowing of conduction velocity in all segments of the nerve
- Temporal dispersion of impulses
- Conduction block in severe cases 2
- Common conditions:
2. Axonal Neuropathies with Severe Fiber Loss
- Pathophysiology: Selective loss of large, fast-conducting fibers
- Characteristic findings:
- Preferential slowing when recording from distal muscles
- Relatively normal conduction when recording from proximal sites
- Reduced CMAP amplitudes 6
- Common conditions:
3. Temperature Effects
- Pathophysiology: Decreased muscle temperature slows action potential propagation
- Characteristic findings:
- Decreased conduction velocity with lower temperature
- Increased latency
- Can be corrected by warming the limb 1
4. Anatomical Factors
- Pathophysiology: Nerve compression or entrapment
- Characteristic findings:
- Focal slowing at sites of compression
- Prolonged distal motor latencies
- Normal conduction in other segments 4
- Common sites:
- Carpal tunnel (median nerve)
- Cubital tunnel (ulnar nerve)
- Fibular head (peroneal nerve)
Distinguishing Features Between Demyelinating and Axonal Neuropathies
Demyelinating Neuropathies
- Marked slowing of conduction velocity (typically <70-80% of lower limit of normal)
- Prolonged distal motor latencies
- Temporal dispersion and conduction block
- Slowing present in both proximal and distal segments 6
- Often have normal or near-normal CMAP amplitudes early in the disease
Axonal Neuropathies
- Mild to moderate slowing of conduction velocity
- Reduced CMAP amplitudes
- Preferential slowing when recording from distal muscles
- Relatively preserved conduction velocity in proximal segments 6
Technical Considerations Affecting Conduction Velocity
- Electrode placement: Electrodes positioned near innervation zones can produce complex interference patterns 1
- Cross-talk: Contamination from adjacent muscles can affect readings 1
- Changes in muscle length/chest wall configuration: Can affect amplitude and frequency content 1
- Temperature: Muscle cooling decreases conduction velocity 1
Clinical Pearls
- In patients with markedly reduced CMAP amplitudes, comparing conduction velocities through the same nerve segment when recording from both proximal and distal muscles can help differentiate between axonal and demyelinating processes 6
- Anti-MAG neuropathy shows a distinctive pattern of distal accentuation of conduction slowing with prolonged distal motor latencies disproportionate to proximal conduction velocities 5
- Hereditary neuropathy with liability to pressure palsies (HNPP) has a characteristic pattern of diffuse sensory nerve conduction velocity slowing with disproportionate distal motor latency prolongation 4
- Always consider temperature effects when interpreting nerve conduction studies, as cooling can significantly slow conduction velocity 1
Diagnostic Approach
- Measure conduction velocity in multiple nerves and compare with normal values
- Assess for uniform vs. focal slowing patterns
- Compare proximal vs. distal conduction velocities
- Evaluate for temporal dispersion and conduction block
- Correlate with clinical presentation and other electrodiagnostic findings
- Consider specialized techniques like single-fiber EMG for early detection of neuromuscular junction disorders 1