Nerve Conduction Study (NCS): Comprehensive Review

1. Principle

• Depolarization: When an electrical stimulus is applied to the nerve surface via the cathode (negative electrode), negative charges accumulate beneath it, depolarizing the axon membrane and generating an action potential.
• Propagation: This action potential propagates bidirectionally along the nerve fiber.
• Recording: Surface (or needle) electrodes placed at a defined distance downstream detect the traveling electrical wave.

CV = Distance (mm) / [Proximal latency (ms) - Distal latency (ms)]

Fig. 1 - Components of a sensory nerve conduction study: stimulation point (1), recording point (2), distance (d), latency, and amplitude. - Pfenninger & Fowler's Procedures for Primary Care

• Motor CV: 50-70 m/s in upper limbs; 40-60 m/s in lower limbs
• Sensory CV: slightly faster than motor in the same nerve

2. Types of NCS

A. Motor Nerve Conduction Study (MNCS)

Fig. 2 - Compound Motor Action Potential (CMAP) showing stimulus artifact, distal latency, duration, and amplitude - Pfenninger & Fowler's Procedures for Primary Care

Fig. 3 - Motor nerve conduction study of the median nerve, showing distal (1) and proximal (2) stimulations with latencies, amplitudes, durations, and areas. The proximal CMAP has slightly lower amplitude due to physiological temporal dispersion and phase cancellation. CV calculated as: 210 mm / (6.9 ms - 3.5 ms) = 62 m/s - Bradley & Daroff's Neurology in Clinical Practice

• Amplitude (baseline to negative peak, in mV): reflects the number of functional motor axons and muscle fiber integrity
• Duration (negative phase, in ms): reflects synchrony of conduction across all axons
• Area: more accurate than amplitude alone for assessing axonal loss
• Distal latency: time from distal stimulation to CMAP onset; reflects conduction in the fastest fibers of the distal segment plus neuromuscular transmission time

B. Sensory Nerve Conduction Study (SNCS)

• Orthodromic: stimulate distally (e.g., digit), record proximally (e.g., wrist)
• Antidromic: stimulate proximally, record distally (larger amplitude; most commonly used clinically)

C. Mixed Nerve Conduction Study (MNAP)

3. Equipment and Electrode Setup

Stimulators

• Constant voltage stimulator: regulates voltage; current varies inversely with skin-tissue impedance
• Constant current stimulator: adjusts voltage so the current reaching the nerve remains constant despite changes in skin resistance

• Cathode (negative, black): placed closer to the recording electrode - causes depolarization
• Anode (positive, red): placed 2-3 cm proximal to cathode - causes hyperpolarization (can cause anodal block if accidentally placed distally)

Recording Electrodes

• Surface (skin) electrodes: used for routine CMAP and SNAP recording
• Needle electrodes: used when surface recording is inadequate (e.g., deep muscles, severe edema, obesity)
• G1 (active): placed over the motor point (muscle belly) or nerve recording site
• G2 (reference): placed over electrically inactive site (tendon or nearby bony area)

Fig. 4 - Nerve conduction velocity setup showing proximal stimulation (Segment A), site of injury at elbow, distal stimulation (Segment B), and recording electrode at wrist - Campbell's Operative Orthopaedics

4. Special Studies: F Wave and H Reflex

F Wave

• Minimum F latency: the earliest latency among multiple F waves recorded (most sensitive to proximal slowing)
• Chronodispersion: difference between earliest and latest F wave latencies
• Persistence: percentage of stimuli that produce a recordable F wave
• F wave CV: useful when proximal slowing is suspected (e.g., early Guillain-Barré)

Fig. 5 - Multiple CMAP (left) and corresponding F wave (right) responses across four trials, demonstrating the variability in F wave morphology - Pfenninger & Fowler's Procedures for Primary Care

H Reflex

5. Indications

Peripheral Neuropathies

• Symmetric polyneuropathies: diabetic, alcoholic, uremic, nutritional, toxic, hereditary (CMT), inflammatory (CIDP, GBS)
• Focal/multifocal neuropathies: vasculitic, multifocal motor neuropathy

Entrapment Neuropathies (most common indication)

• Carpal tunnel syndrome (median nerve at wrist) - most common
• Cubital tunnel syndrome (ulnar nerve at elbow)
• Tarsal tunnel syndrome
• Peroneal nerve compression at fibular head
• Meralgia paresthetica (lateral femoral cutaneous nerve)

Radiculopathies

• Cervical and lumbar radiculopathies (NCS is often normal in pure root lesions since the DRG is distal to the root; EMG adds diagnostic value)
• F waves and H reflexes are abnormal in severe root lesions

Plexopathies

• Brachial plexopathy (trauma, neuralgic amyotrophy, thoracic outlet syndrome, radiation-induced)
• Lumbosacral plexopathy

Mononeuropathies

• Traumatic nerve injuries (classification as neurapraxia vs. axonotmesis vs. neurotmesis)
• Monitoring recovery after peripheral nerve injury or surgery

Neuromuscular Junction Disorders

• Combined with repetitive nerve stimulation (RNS) for myasthenia gravis, Lambert-Eaton syndrome

Motor Neuron Disease

• To exclude peripheral nerve disease in ALS workup

Systemic Diseases with Nerve Involvement

• Amyloidosis, hypothyroidism, critical illness neuropathy, sarcoidosis, paraneoplastic neuropathy

6. Methodology - Step by Step

Step 1: Patient Preparation

• Patient is positioned comfortably; the limb to be tested is relaxed and supported
• Skin is cleaned with alcohol to reduce impedance
• Limb temperature is measured: nerve conduction is temperature-dependent (CV decreases ~2 m/s per °C drop); if limb temperature is below 32°C, warm the extremity before testing or apply a correction factor
• Skin-to-electrode impedance is checked (ideally <5 kΩ)

Step 2: Electrode Placement

• Recording electrodes (G1 and G2) placed per standard anatomical landmarks for each nerve
• Ground electrode placed between stimulating and recording electrodes to reduce stimulus artifact
• Stimulating electrode placed over the nerve, with cathode facing the recording site

Step 3: Stimulation

• Start with submaximal stimulation, gradually increase intensity
• Identify the threshold response, then increase to supramaximal (20-30% above plateau)
• Stimulate at distal site first, then proximal site(s)

Step 4: Recording and Measurement

• For CMAP: measure distal latency, amplitude (baseline to negative peak), duration, and area from distal stimulation; then measure proximal latency to calculate CV
• For SNAP: measure onset latency (or peak latency), amplitude (baseline to negative peak or peak-to-peak), and duration
• Digital averaging used for low-amplitude SNAPs (signal-to-noise ratio improves with √n trials)

Step 5: F Wave Recording

• Nerve stimulated supramaximally at a distal site; multiple stimulations (minimum 10-20) given
• Late responses recorded and minimum latency identified

Step 6: Data Interpretation

• Prolonged distal latency
• Slowed conduction velocity (<75% lower limit of normal)
• Prolonged F wave latency
• Increased CMAP duration (temporal dispersion)
• Conduction block (>50% drop in CMAP amplitude proximal vs. distal)
• Relatively preserved amplitude early on

• Reduced CMAP and/or SNAP amplitude
• Normal or minimally slowed CV (fast fibers preserved)
• Normal or mildly prolonged distal latency
• Wallerian degeneration distal to lesion after 5-10 days

7. Interpretation of Injury Patterns

• CMAP amplitude falls to its nadir 5-6 days after axonal injury
• SNAP amplitude falls to its nadir 10-11 days after axonal injury (NMJ failure accounts for the earlier CMAP drop)

8. Limitations and Disadvantages

Technical Limitations

1. Temperature dependence: Cold limbs slow conduction and increase amplitude artifactually, mimicking or masking pathology. Strict temperature control is necessary.
2. Operator dependence: Accurate electrode placement, distance measurement, and supramaximal stimulation require training and experience. Errors in distance measurement directly cause errors in CV calculation.
3. Stimulus artifact: Large electrical artifacts from the stimulator can obscure early responses, especially short-distance SNAPs.
4. Volume conduction: Potentials from distant muscles (via anomalous innervation or stimulus spread) can generate an initial positive deflection, indicating incorrect G1 placement.
5. Anatomical variations: Anomalous innervation (e.g., Martin-Gruber anastomosis of median-ulnar in the forearm, occurring in ~15-30% of people) can produce misleading results if unrecognized.

Diagnostic Limitations

1. Cannot assess small fibers: NCS evaluates only large myelinated A-alpha (motor) and A-beta (sensory) fibers. Small unmyelinated C fibers (pain, temperature) and thinly myelinated A-delta fibers cannot be assessed - normal NCS does not exclude small fiber neuropathy.
2. Normal in pure radiculopathy: Because the dorsal root ganglion (DRG) lies distal to the nerve root, sensory axons remain intact in root lesions, making SNAPs normal. NCS cannot directly assess nerve roots.
3. Cannot assess proximal nerve segments directly: Plexus and root segments are not directly accessible to percutaneous stimulation; F waves and H reflexes provide only indirect information.
4. Limited in early disease: In the first 5-10 days after axonal injury, distal Wallerian degeneration is incomplete and the distal nerve remains excitable, masking the true extent of axonal loss.
5. Does not assess neuromuscular junction directly: RNS and SFEMG are needed for NMJ disorders; routine NCS is insufficient.
6. Low sensitivity for mild polyneuropathy: Subclinical or length-dependent polyneuropathies may show NCS changes only when nerve fiber loss is substantial.

Patient-Related Limitations

1. Pain and discomfort: Electrical stimulation causes discomfort, particularly with supramaximal stimulation. It is poorly tolerated in children and anxious patients.
2. Obesity and edema: Increased subcutaneous tissue distances reduce amplitude and may prevent adequate stimulation, making results unreliable.
3. Lymphedema: Alters electrode-skin contact and tissue impedance.
4. Pacemakers/implanted devices: Electrical stimulation near pacemakers requires caution (relative contraindication), though NCS is generally safe at standard intensities with proper precautions.

Interpretive Limitations

1. Only evaluates fastest conducting fibers: CV reflects only the fastest-conducting (largest-diameter, most myelinated) fibers in the nerve. Selective loss of slow fibers may be missed.
2. Temporal dispersion: With proximal stimulation, phase cancellation between faster and slower fibers causes a physiological reduction in SNAP/CMAP amplitude and prolongation of duration, which can be misinterpreted as pathological.
3. Age, height, and sex affect normal values: Normal ranges must be adjusted; elderly patients and tall individuals normally have slower conduction in the legs.

9. Summary Comparison: Demyelination vs. Axonal Loss

• Bradley and Daroff's Neurology in Clinical Practice, 8th Ed., Ch. 36 - Clinical Electromyography
• Pfenninger and Fowler's Procedures for Primary Care, 3rd Ed., Ch. 224 - Nerve Conduction Studies
• Campbell's Operative Orthopaedics, 15th Ed. 2026, Ch. 67 - Nerve Conduction Velocity
• Adams and Victor's Principles of Neurology, 12th Ed.