What is a nerve conduction study and how is it used to diagnose and treat nerve damage or dysfunction?

Nerve Conduction Study (NCS)

Definition and Purpose

Nerve conduction studies are electrodiagnostic tests that measure the speed and amplitude of electrical signals traveling through peripheral nerves, providing quantitative assessment of large myelinated nerve fiber function to diagnose and classify peripheral neuropathies. 1, 2

NCS evaluates the peripheral nervous system by measuring two key waveforms: the sensory nerve action potential (SNAP), which assesses sensory nerve axons from distal skin receptors to dorsal root ganglia, and the compound muscle action potential (CMAP), which evaluates motor nerve fibers from anterior horn cells to muscle fibers. 2

Technical Methodology

The procedure involves delivering electrical stimuli to peripheral nerves at accessible body locations and recording the resulting electrophysiological responses. 3 The test measures several critical parameters:

• Conduction velocity (CV): Reflects the speed of action potential propagation along large myelinated axons via saltatory conduction 4
• Amplitude measurements: SNAP amplitude indicates the number of functioning sensory axons, while CMAP amplitude reflects integrated function of motor axons, neuromuscular junction, and muscle 4
• Distal motor latencies: Prolongation suggests demyelination 1
• Temporal dispersion and conduction blocks: Abnormalities indicate demyelinating pathology 1

Clinical Applications and Diagnostic Patterns

Primary Diagnostic Information

NCS provides three essential categories of information: (1) spatial pattern of neuropathy (generalized, focal, or multifocal), (2) distinction between axonal versus demyelinating pathology, and (3) severity of nerve damage. 1

For polyneuropathy, nerve conduction studies show diffuse abnormalities with relatively uniform involvement of tested nerves, differentiating it from multifocal processes. 5

Axonal vs. Demyelinating Patterns

• Axonal neuropathies: Primarily affect SNAP and CMAP amplitudes, with relatively preserved conduction velocities 1
• Demyelinating neuropathies: Cause slowed nerve conduction, increased temporal dispersion, or conduction block 1
• Uniform demyelination: Suggests hereditary neuropathies 1
• Segmental demyelination: Indicates acquired neuropathies 1

Specific Clinical Scenarios

In diabetic sensorimotor polyneuropathy, NCS assesses large myelinated nerve fiber function and has been used as an endpoint in clinical trials based on ease of quantification, reproducibility, and reasonable sensitivity and specificity. 6 However, small fiber neuropathy (affecting 79.6-91.4% of peripheral nerve fibers) may show normal conventional nerve conduction studies and requires skin biopsy to detect decreased epidermal nerve fiber density. 6, 5

For chemotherapy-induced peripheral neuropathy, NCS predominantly reveals low amplitude sensory action potentials consistent with length-dependent, sensory, axonal polyneuropathy with predominant small-fiber involvement. 6 Thalidomide most frequently causes length-dependent axonal neuropathy affecting sensory fibers, while bortezomib causes similar patterns. 6

Diagnostic Utility in Specific Conditions

ICU-Acquired Weakness

In critically ill patients, 84% of studies evaluating ICU-acquired weakness utilized nerve conduction studies. 6 One study showed that the positive predictive value of early ICU EMG for final diagnosis of weakness was 50%, with a negative predictive value of 89%. 6

Timing Considerations

A critical pitfall is performing electrodiagnostic studies too early—studies within the first week of symptom onset may be normal in 30-34% of patients even with active demyelinating disease. 5 Repeat testing 2-3 weeks later is essential when initial studies are normal but clinical suspicion remains high. 5, 7

Pattern Recognition

The "Sural Sparing" pattern (normal sural SNAP with abnormal or absent median and ulnar SNAPs) is highly characteristic of Guillain-Barré syndrome, a prototype acute demyelinating polyneuropathy. 5

When NCS is NOT Indicated

In patients with classic distal symmetric sensory loss in a stocking distribution, reduced ankle reflexes, and known risk factors (such as diabetes), the diagnosis can be made clinically without EMG. 5 The American Diabetes Association confirms that clinical examination alone is adequate when symptoms are symmetric and length-dependent. 5

EMG is not recommended for routine serial monitoring of stable peripheral neuropathy—repeated EMG is only warranted when there is uncertainty about new or worsening neurological processes. 5 Serial neurologic examinations are preferred over repeated EMG for monitoring stable neuropathy. 5

Limitations and Caveats

• Limited to large myelinated fibers: NCS evaluates only large myelinated peripheral nerve fibers and cannot assess small fiber function 8
• Operator-dependent: Results require specialized training and expert interpretation contextualized with clinical findings 9
• Technical factors: Electrode type, age, and muscle-specific variations dramatically affect measurements—monopolar electrodes have 1.5 times the uptake region of concentric electrodes, yielding different values 9
• Normal studies don't exclude neuropathy: Small fiber neuropathy and early disease may show normal NCS 6, 5

Integration with Other Testing

NCS is conventionally performed with needle electromyography (EMG) to provide comprehensive evaluation of suspected neuromuscular impairment. 8 For small fiber neuropathy, skin biopsy with intraepidermal nerve fiber density measurement is valuable when NCS is normal. 5 Autonomic testing should be considered to document autonomic nervous system dysfunction, particularly for suspected autonomic neuropathy. 5