What is the Nerve Conduction Velocity (NCV) test done for?

Nerve Conduction Velocity (NCV) Test: Clinical Applications and Uses

The Nerve Conduction Velocity (NCV) test is primarily used to diagnose peripheral nerve disorders, including neuropathies, entrapment syndromes, and neuromuscular junction disorders, by measuring the speed and strength of electrical signal transmission along nerves. 1

Primary Diagnostic Applications

• NCV testing is essential for detecting and evaluating peripheral neuropathies, particularly diabetic sensorimotor polyneuropathy (DSP), by measuring changes in conduction velocity and amplitude of nerve signals 2
• NCV tests help distinguish between axonal and demyelinating neuropathies by analyzing patterns of conduction slowing and amplitude reduction 1, 3
• The test is valuable for diagnosing nerve entrapment syndromes such as carpal tunnel syndrome by identifying focal slowing across specific anatomical sites 1, 4
• NCV testing can evaluate radiculopathies and plexopathies by assessing patterns of abnormalities in specific nerve distributions 1
• The test assists in diagnosing neuromuscular junction disorders like myasthenia gravis when combined with repetitive nerve stimulation techniques 1

Specific Parameters Measured

• Motor nerve conduction velocity (typically 50-60 m/s in upper limbs and 40-50 m/s in lower limbs) 1
• Sensory nerve action potential (SNAP) amplitude, which reflects the number of functioning sensory axons 3
• Compound muscle action potential (CMAP) amplitude, indicating integrated function of motor axons, neuromuscular junction, and muscle 3
• Distal latency, which helps identify focal compression neuropathies 4

Clinical Significance in Specific Conditions

• In diabetic neuropathy, NCV testing typically includes assessment of sural, peroneal, median sensory, and median motor nerves, with slowing indicating demyelination and reduced amplitude suggesting axonal loss 2, 1
• For suspected entrapment neuropathies, comparison of conduction velocities across and away from potential entrapment sites helps localize the lesion 4
• In peripheral neuropathies, mixed median nerve conduction velocity across the forearm can help differentiate between carpal tunnel syndrome and more generalized neuropathic processes 4

Important Limitations and Considerations

• NCV tests primarily assess large myelinated nerve fibers and may miss small fiber neuropathy, requiring complementary tests like quantitative sensory testing or skin biopsy 2, 1
• Temperature significantly affects results (lower temperature slows conduction) and must be controlled or corrected for accurate interpretation 1
• Early in disease processes, NCV may be normal despite clinical symptoms, making it important to correlate with clinical findings 1, 5
• Age-related changes in NCV must be considered when interpreting results, as studies show NCV declines after age 20 in animal models 6

Complementary Testing

• Electromyography (EMG) is often performed alongside NCV to assess muscle denervation and differentiate between neuropathic and myopathic processes 3
• Quantitative sensory testing can evaluate small fiber function when small fiber neuropathy is suspected 1
• Autonomic testing should be considered when autonomic neuropathy is suspected, as NCV primarily assesses somatic nerve function 2
• Skin biopsy for intraepidermal nerve fiber density measurement may be needed to confirm small fiber neuropathy when NCV is normal despite symptoms 1

Test Procedure

• The patient should lie down or sit comfortably during the test, with skin cleaned at electrode placement sites 1
• Surface electrodes are placed at specific anatomical locations to record and stimulate the nerves 3
• The test typically takes 30-60 minutes to complete, with mild discomfort during electrical stimulation described as brief tingling or shock-like sensations 1
• No special preparation is typically required, but patients should inform their provider about medications they are taking 1

By providing objective measurements of nerve function, NCV testing plays a crucial role in diagnosing and monitoring peripheral nerve disorders, guiding treatment decisions, and assessing treatment efficacy.