Nerve Conduction Velocity (NCV) Studies

15-Mark MPT-Level Answer (PG Entrance Standard)

1. Definition

2. Principle

• Over the muscle distally (motor fibers) - compound muscle action potential (CMAP)
• Over the nerve more distally (sensory fibers) - sensory nerve action potential (SNAP)
• Over a mixed nerve proximally - mixed nerve action potential (MNAP)

3. Types of NCS

Antidromic SNAP recording is used most often clinically because it has technical advantages over orthodromic techniques.

4. Technique and Parameters Measured

Setup

• Cathode (negative electrode) placed closer to recording site; cathode causes depolarization, anode causes hyperpolarization
• Supramaximal stimulation: intensity increased until CMAP/SNAP no longer grows, then current increased an additional 20-30%

Key Parameters

• CMAP amplitude: in millivolts (mV); reflects the number of functioning motor axons and innervated muscle volume
• SNAP amplitude: in microvolts (uV); reflects number of sensory axons

• Time (ms) from stimulus to onset of CMAP/SNAP at the distal recording site
• Includes neuromuscular junction delay + muscle depolarization time (in motor studies)

• Calculated by stimulating at two sites and measuring the difference in latencies:

CV (m/s) = Distance between two stimulation points (mm) / Difference in latencies (ms)

5. Normal Values

6. Factors Affecting NCV

7. Pathological Patterns - Axonal vs. Demyelinating

Key rule: Conduction velocity slows significantly only in demyelination. In axonal loss, velocity is preserved (or only mildly reduced if >50% axons lost).

8. Late Responses

F-Wave

• Antidromic activation travels up to the anterior horn cell; a proportion of motor neurons "backfire" and send an orthodromic impulse back down to muscle
• Records the total round-trip time through the entire motor pathway (including proximal nerve and spinal cord)
• Clinical use: Detects proximal nerve lesions (Guillain-Barre Syndrome early, thoracic outlet syndrome, radiculopathy)
• Normal F-wave latency: ~25-32 ms for upper limb, ~45-56 ms for lower limb

H-Reflex

• Electrical analog of the ankle jerk reflex (monosynaptic)
• Tibial nerve stimulated at popliteal fossa; response recorded from soleus
• Absent H-reflex: S1 radiculopathy, peripheral neuropathy
• Normal latency: 25-35 ms

9. NCV Findings in Specific Diseases

Guillain-Barre Syndrome (GBS) - Acquired Demyelination

• Markedly slowed CV, prolonged distal latencies, conduction block, temporal dispersion
• Nonuniform slowing across segments (distinguishes from hereditary CMT where slowing is uniform)
• Absent F-waves early (proximal conduction block)
• SNAP amplitudes relatively preserved early

Charcot-Marie-Tooth Disease (CMT Type 1) - Hereditary Demyelination

• Uniform, symmetrical slowing of CV (often < 38 m/s in upper limb)
• No conduction block, no temporal dispersion (hereditary vs. acquired distinction)
• Normal CMAP amplitudes early

Diabetic Neuropathy - Axonal (length-dependent)

• Reduced CMAP and SNAP amplitudes, worse in lower limbs
• Normal or mildly slowed CV
• Absent sural SNAP often the earliest finding
• Later stages: absent responses in lower limb nerves

Carpal Tunnel Syndrome (Median nerve entrapment)

• Prolonged distal motor latency (> 4.5 ms) at the wrist
• Slowed or absent median SNAP across the wrist
• Normal conduction across forearm segment
• Classic: "focal" demyelination at carpal tunnel

CIDP (Chronic Inflammatory Demyelinating Polyneuropathy)

• Similar to GBS but chronic: multifocal demyelination, conduction blocks, temporal dispersion
• Acquired pattern = nonuniform slowing

10. NCV in Nerve Injury (Seddon / Sunderland Classification)

Key point: Immediately after injury, distal conduction is normal even in axonotmesis/neurotmesis. Wallerian degeneration takes 5-10 days to progress distally - so NCV must be repeated at 10-14 days post-injury to differentiate neuropraxia from axonal injury.

11. NCV vs. EMG - Complementary Tests

12. Clinical Applications / Indications

1. Peripheral neuropathy (diabetic, uremic, toxic, hereditary)
2. Entrapment neuropathies (carpal tunnel, cubital tunnel, tarsal tunnel)
3. GBS and CIDP
4. Radiculopathy (with EMG)
5. Plexopathy (brachial, lumbosacral)
6. Mononeuritis multiplex
7. Motor neuron disease
8. NMJ disorders (with repetitive nerve stimulation)
9. Traumatic nerve injuries (prognosis and timing of surgery)

Summary Mnemonics

• Slowed CV + prolonged latency = demyelinating
• Small (reduced) amplitude = axonal loss

NCV Studies - Simple & Exam-Ready 🧠

What is NCV? (The Big Idea)

Normal speed = 50-70 m/s (arms), 40-55 m/s (legs) Legs are slower because the wires are longer and colder.

What Does NCV Measure? → CADS

Motor vs Sensory NCS

Memory trick: CMAP = Car goes to Muscle. SNAP = Sensory Nerve Action Potential

The #1 Most Important Concept

⚡ "SLOW = Myelin problem. SMALL = Axon problem."

DEMYELINATION          AXONAL LOSS
(myelin sheath gone)   (wire itself lost)

CV is SLOW ✓           CV is NORMAL
Latency is LONG ✓      Latency is NORMAL
Amplitude = normal     Amplitude is SMALL ✓
Conduction BLOCK ✓     No conduction block
Temporal DISPERSION ✓  No dispersion
No fibrillations       FIBRILLATIONS on EMG ✓

One-line Memory Hook:

"Demyelination = Delay. Axonal = Absent (amplitude)."

Factors That Slow NCV (Exam Traps)

Cold trap in exams: A cold limb gives falsely prolonged latency and falsely high amplitude - looks like demyelination but isn't! Always warm limb to >33°C before testing.

Normal Values to Remember

Upper limb nerves:   50 - 70 m/s
Lower limb nerves:   40 - 55 m/s
Median distal latency:  < 4.5 ms  (carpal tunnel if prolonged)

Late Responses - Simple Version

F-Wave = "The Bouncer"

• Impulse goes UP to spinal cord, bounces back DOWN
• Tests proximal nerve (you can't stimulate there directly)
• Absent/prolonged in: GBS (early sign), radiculopathy

H-Reflex = "Electric Ankle Jerk"

• Electrical version of tapping the Achilles
• Tests S1 nerve root
• Absent in: S1 radiculopathy, peripheral neuropathy

Memory: H = Heel reflex (S1). F = Far away (proximal conduction).

Disease Patterns - The Big 5

1. Carpal Tunnel Syndrome

• Prolonged distal latency of median nerve at wrist (> 4.5 ms)
• Slow SNAP across wrist
• Everything normal above the wrist
• Think: "Traffic jam at the toll booth (wrist)"

2. Diabetic Neuropathy

• Reduced SNAP amplitude (axonal, length-dependent)
• Legs worse than arms
• Sural SNAP - earliest to disappear
• CV normal or mildly slow
• Think: "Wires dying from the tips upward"

3. GBS (Guillain-Barre Syndrome)

• Acquired demyelination - patchy and non-uniform
• Slowed CV + conduction block + temporal dispersion
• F-waves absent early
• Think: "Random potholes on the highway (non-uniform)"

4. CMT Type 1 (Hereditary)

• Uniform demyelination - symmetric slowing
• CV often < 38 m/s
• NO conduction block (this is the key difference from GBS)
• Think: "Evenly bad road (uniform)"

5. Nerve Injury - Neuropraxia vs Axonotmesis

Both injuries:
  - Immediately after: Distal conduction = NORMAL
  - Wait 10 days!

After 10 days:
  Neuropraxia:    Distal conduction still NORMAL (just a block)
  Axonotmesis:    Distal conduction GONE (Wallerian degeneration)

Exam pearl: Never judge axonal injury in the first week. Repeat NCV at 10-14 days.

GBS vs CMT - The Classic Comparison

One-liner: "GBS = Patchy road with roadblocks. CMT = Uniformly bad road, no roadblocks."

NCV vs EMG - Two-Second Version

Rule: Do BOTH. NCV and EMG are partners, not alternatives.

Master Flowchart for Any Neuropathy

Neuropathy on NCV?
        |
   _____|_____
  |           |
SLOW CV     NORMAL CV
  |           |
Demyelinating  Axonal
  |           |
Uniform?      Amplitude reduced?
  |           |
YES = CMT     YES = diabetes, toxins,
NO = GBS/CIDP      uremia, B12 deficiency

Quick Revision Table - Exam Day

The 5 Things an Examiner Always Asks

1. NCV formula? → Distance ÷ Difference in latencies
2. What does NCV NOT test? → Small fibers (pain & temperature - C fibers, A-delta)
3. Why repeat NCV after 10 days in trauma? → Wallerian degeneration takes 5-10 days
4. Cold limb effect? → Slows NCV, mimics demyelination
5. GBS vs CMT distinction? → Conduction block + non-uniform slowing = GBS/acquired

Golden Rule for the exam: "Slow and Blocked = Demyelination (GBS/CIDP/CTS) Small and Gone = Axonal (DM/toxin/trauma)"