What is the typical Nerve Conduction Velocity (NCV) picture in Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)?

Typical NCV Picture in CIDP

CIDP demonstrates a characteristic demyelinating pattern on nerve conduction studies, featuring marked slowing of conduction velocities, prolonged distal latencies, temporal dispersion, and conduction block—with focal and asymmetric abnormalities being more specific than uniform slowing.

Key Electrophysiological Features

Primary Demyelinating Parameters

Motor nerve conduction velocities are significantly reduced, typically showing:

• Median motor nerve CV often reduced to approximately 30-50 m/s (normal 50-60 m/s in upper limbs) 1
• Peroneal motor nerve CV typically 38-41 m/s (normal 40-50 m/s in lower limbs) 2
• The degree of slowing reflects active demyelination but may overlap with hereditary neuropathies 1

Distal motor latencies are markedly prolonged, particularly:

• Distal compound muscle action potential (dCMAP) duration >9 ms is highly specific for CIDP and helps differentiate it from hereditary neuropathies like CMT1a 3
• Median nerve distal latency prolongation is frequently observed 4

Distinguishing Features from Hereditary Neuropathies

The pattern of abnormalities is more important than absolute values:

• Focal conduction blocks or focal conduction delays are characteristic of CIDP rather than the longitudinal uniformity seen in hereditary neuropathies 1
• Temporal dispersion with irregular multiphasic waves indicates acquired demyelination 1
• Asymmetric involvement between corresponding nerves (>10 m/s difference in CV between two corresponding nerves) strongly suggests CIDP over CMT1a 3

Sensory Nerve Involvement

Sensory nerve conduction abnormalities provide high specificity:

• Sensory conduction velocity slowing is a highly specific marker for CIDP, though it has lower sensitivity 5
• Abnormal median/normal sural SNAP (AMNS) pattern is useful in distinguishing CIDP from CMT1a 3
• Sensory nerve action potentials may be absent in more severe cases (approximately 67% of patients) 1
• Sural nerve CV typically ranges 39-42 m/s in affected patients 2

Conduction Block Characteristics

Conduction block is a hallmark finding:

• Present in approximately 65-78% of CIDP patients 4, 1
• Incomplete conduction block in the peroneal nerve is frequently observed 4
• Conduction blocks reflect focal demyelination and are rare in hereditary neuropathies 1

F-Wave Abnormalities

F-wave latencies are prolonged in approximately 90% of cases, reflecting proximal demyelination 4

Common Diagnostic Pitfalls

Beware of diabetic neuropathy mimicking CIDP electrophysiologically:

• Approximately 3.7% of diabetic patients with moderate-to-severe neuropathy can fulfill electrophysiologic CIDP criteria without having CIDP 4
• Focal compression neuropathies (carpal tunnel, peroneal nerve at fibular head) can create false-positive findings 4
• Clinical and CSF criteria must be used to exclude diabetic neuropathy in patients meeting electrophysiologic criteria 4

Timing of studies matters:

• Early studies (within 1 week) may be normal even in active disease 6
• Repeat studies 2-3 weeks later may be necessary for accurate diagnosis 6

Atypical Variants

Recognize that CIDP has several atypical presentations including multifocal acquired demyelinating sensory and motor neuropathy (Lewis-Sumner syndrome) and distal acquired demyelinating symmetric neuropathy, which may show different electrophysiological patterns 7

Complementary Testing

When NCV studies are equivocal:

• Nerve biopsy may be useful in atypical CIDP forms 8
• Neuroimaging can demonstrate nerve hypertrophy 7
• Testing for antibodies against nodal proteins (neurofascin, contactin-1) may help identify specific subgroups 7