# Fundamentals of Nerve Conduction Studies

## Principles of Nerve Conduction Studies

### Overview
Nerve conduction studies (NCS) are the objective, quantitative component of the electrodiagnostic examination. Assess the integrity and function of peripheral nerves (motor, sensory, and mixed). Complement the needle EMG and clinical examination to localize and characterize neuromuscular disease. Performed by applying electrical stimuli to peripheral nerves and recording evoked responses.

### Basic Electrophysiology
Peripheral nerves are composed of myelinated and unmyelinated axons. Saltatory conduction occurs along myelinated fibers via nodes of Ranvier. NCS preferentially assess large myelinated fibers (Aalpha and Abeta). Small fiber neuropathies (C fibers, Adelta fibers) are NOT detected by standard NCS. Conduction velocity is proportional to fiber diameter and myelin thickness.

### Components of the NCS Setup
**Stimulator**: delivers supramaximal electrical stimulus to the nerve. **Recording electrodes**: active (E1, G1) placed over the target; reference (E2, G2) placed distally. **Ground electrode**: placed between stimulator and recording electrodes to reduce artifact. **Amplifier**: processes the signal; standard filter settings (20 Hz - 10 kHz for motor; 20 Hz - 2 kHz for sensory). **Display**: oscilloscope or computer screen showing waveform morphology.

## Motor Nerve Conduction Studies

### Technique
Stimulate the motor nerve at two or more points along its course. Record the compound muscle action potential (CMAP) over the target muscle. E1 (active) electrode placed over the motor point (belly-tendon montage). E2 (reference) electrode placed over the tendon distally. Supramaximal stimulation: increase stimulus intensity 20-25% above the level that produces maximum CMAP amplitude.

### Key Parameters

#### Distal Motor Latency (DML)
Time from stimulus at the distal site to the onset of the CMAP. Measured in milliseconds (ms). Reflects conduction through the distal nerve segment plus neuromuscular junction transmission time. Prolongation suggests distal demyelination or nerve compression.

#### CMAP Amplitude
Measured from baseline to negative peak (or peak-to-peak). Reflects the number of muscle fibers activated (and thus the number of functioning motor axons). Reduced amplitude indicates axonal loss or conduction block. Normal values are nerve-specific and laboratory-dependent.

#### Conduction Velocity (CV)
Calculated between two stimulation sites: CV = distance / (proximal latency - distal latency). Measured in meters per second (m/s). Normal motor CV in upper extremities: approximately 50-60 m/s. Normal motor CV in lower extremities: approximately 40-50 m/s. Slowing indicates demyelination; must distinguish from pure axonal loss (mild slowing possible).

#### CMAP Duration
Measured from onset to return to baseline of the negative phase. Prolongation suggests temporal dispersion (desynchronized conduction). Important for identifying acquired demyelinating neuropathies.

<image>Diagram showing motor nerve conduction study setup for the median nerve with stimulator placement at wrist and elbow, recording electrodes on the abductor pollicis brevis (belly-tendon montage), and resulting CMAP waveforms with labeled parameters including distal motor latency, amplitude, duration, and calculation of conduction velocity</image>

## Sensory Nerve Conduction Studies

### Technique
Record the sensory nerve action potential (SNAP) by stimulating a purely sensory nerve or the sensory component of a mixed nerve. **Orthodromic recording**: stimulate distally, record proximally (physiologic direction). **Antidromic recording**: stimulate proximally, record distally (generally produces larger amplitude SNAPs). Ring electrodes commonly used for digital nerves.

### Key Parameters

#### SNAP Onset Latency and Peak Latency
Onset latency: time from stimulus to the initial deflection from baseline. Peak latency: time from stimulus to the negative peak of the SNAP. Peak latency is used more commonly for comparison studies (e.g., median vs. ulnar).

#### SNAP Amplitude
Measured from baseline to negative peak or peak-to-peak. Reflects the number of functioning sensory axons. Reduced amplitude indicates sensory axonal loss. SNAPs are very sensitive to technical factors (temperature, distance, subcutaneous tissue).

#### Sensory Conduction Velocity
Calculated as distance / onset latency (for single-site stimulation). Or distance / (latency difference) for two-site stimulation. Generally slightly faster than motor CV for the same nerve.

### Clinical Significance of SNAP Preservation
In preganglionic lesions (root avulsion, radiculopathy), the dorsal root ganglion (DRG) and distal sensory axon are intact. SNAP is PRESERVED despite clinical sensory loss in radiculopathy. SNAP is LOST in plexopathy and peripheral nerve lesions (postganglionic). This distinction is crucial for localizing lesions to root vs. plexus vs. peripheral nerve.

## Late Responses

### F-Waves
Late motor responses generated by antidromic activation of anterior horn cells. Assess the entire length of the motor nerve (proximal and distal segments). Variable in latency, morphology, and amplitude (different motor neurons activated each time). Minimum F-wave latency is the most commonly used parameter.

Useful for detecting proximal neuropathies (Guillain-Barre syndrome, thoracic outlet syndrome). Non-specific: prolongation can occur with any process affecting nerve conduction.

### H-Reflexes
Electrically evoked monosynaptic reflex (analogous to the Achilles tendon reflex). Routinely assessed in the tibial nerve (recording from soleus). Assesses the S1 nerve root arc (sensory afferents through dorsal root, synapse in spinal cord, motor efferents). Submaximal stimulation required (H-reflex disappears with supramaximal stimulation).

Side-to-side latency difference >1.5 ms is abnormal. Useful in S1 radiculopathy and early polyneuropathy. Can be obtained from the FCR (C6-C7) in the upper extremity but less commonly used.

## Technical Considerations and Pitfalls

### Temperature Effects
Cold limb temperature is the most common source of error in NCS. Cold temperature SLOWS conduction velocity (approximately 2 m/s per degree Celsius). Cold temperature INCREASES SNAP and CMAP amplitudes (delayed sodium channel inactivation). Cold temperature PROLONGS distal latencies.

Minimum recommended skin temperature: 32 degrees Celsius at the hand, 30 degrees at the ankle. Warm the limb before testing if below threshold.

### Anomalous Innervation

#### Martin-Gruber Anastomosis (MGA)
Most common anomalous innervation pattern (present in 15-30% of individuals). Crossover of motor fibers from the median nerve to the ulnar nerve in the forearm. Typical finding: initial positive deflection of CMAP when stimulating the median nerve at the elbow with recording from APB. Can cause apparent conduction block of the median nerve or spuriously fast ulnar conduction velocity. Three subtypes based on which muscles receive the crossover fibers (FDI, hypothenar, thenar).

#### Riche-Cannieu Anastomosis
Communication between the deep branch of the ulnar nerve and the recurrent branch of the median nerve in the hand. Can result in all thenar muscles being innervated by the ulnar nerve.

#### Accessory Peroneal Nerve
Branch of the superficial peroneal nerve that innervates the lateral portion of EDB. Present in approximately 20-28% of individuals. Suspected when fibular CMAP amplitude at the ankle is lower than at the fibular head. Confirmed by stimulating behind the lateral malleolus.

### Distance Measurement
Measure along the surface of the limb following the nerve course. Errors in measurement directly affect calculated conduction velocity. Minimum distance of 10 cm recommended between stimulation sites to minimize measurement error.

### Stimulus Artifact
Occurs when stimulus current reaches the recording electrodes. Minimize by ensuring proper ground placement, reducing stimulus duration, cleaning skin.

<image>Illustration demonstrating the Martin-Gruber anastomosis showing crossover of motor fibers from the median nerve to the ulnar nerve in the forearm, with corresponding CMAP waveform changes when stimulating at the wrist versus elbow for both median and ulnar nerve recordings</image>

## Distinguishing Axonal vs. Demyelinating Pathology

### Axonal Loss Pattern
Reduced CMAP and/or SNAP amplitude (the hallmark). Conduction velocity mildly slowed (remaining fastest fibers still conduct normally). Distal latency mildly prolonged. No conduction block or temporal dispersion.

### Demyelinating Pattern
Markedly slowed conduction velocity (below 70-80% of normal). Prolonged distal latency (beyond 130% of upper limit of normal). Conduction block: >50% amplitude drop between distal and proximal stimulation sites. Temporal dispersion: prolongation of CMAP duration >30% between stimulation sites. F-wave prolongation or absence.

### Mixed Pattern
Many neuropathies have both axonal and demyelinating features. Secondary axonal degeneration occurs in chronic demyelinating conditions. Primary axonal loss can cause secondary slowing when fastest fibers are lost.

| Parameter | Axonal Loss | Demyelination |
|-----------|------------|---------------|
| CMAP/SNAP amplitude | Reduced (hallmark) | May be normal or reduced |
| Conduction velocity | Mildly slowed | Markedly slowed (<70-80% normal) |
| Distal latency | Mildly prolonged | Prolonged (>130% ULN) |
| Conduction block | Absent | Present (>50% amplitude drop) |
| Temporal dispersion | Absent | Present (>30% duration increase) |
| F-waves | Normal or absent | Prolonged or absent |

## Clinical Pearls

Always check limb temperature before beginning NCS; warming is the single most important quality control step. A normal NCS does not exclude pathology; small fiber neuropathy, early axonal loss, and very proximal lesions may be missed. SNAP preservation in the setting of clinical sensory loss is the key electrodiagnostic finding in radiculopathy. When the CMAP amplitude is low, consider distal stimulation technique (submaximal stimulation is a common error).

Know the anomalous innervation patterns; Martin-Gruber anastomosis can mimic median nerve conduction block. NCS parameters are age-dependent; amplitudes decrease and latencies increase with age. Each laboratory should ideally establish its own normative values using standardized technique and temperature control. The electrodiagnostic study is an extension of the clinical examination and must be interpreted in clinical context.

## References

- Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic-Ultrasound Correlations. 4th Edition. Elsevier. 2021.
- Dumitru D, Amato AA, Zwarts MJ. Electrodiagnostic Medicine. 2nd Edition. Hanley & Belfus. 2002.
- American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM). Practice Guidelines and Position Statements.
- Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. 5th Edition. Oxford University Press. 2013.
- Robinson LR. Traumatic Injury to Peripheral Nerves. Muscle Nerve. 2000;23(6):863-873.