What is involved in a Nerve Conduction Velocity (NCV) limbs test?

Nerve Conduction Velocity (NCV) Limbs Test

A Nerve Conduction Velocity (NCV) test is a diagnostic procedure that measures how quickly electrical signals travel through peripheral nerves to assess nerve function, identify damage, and diagnose conditions such as peripheral neuropathy.

What is an NCV Test?

• NCV testing is part of electrodiagnostic studies that evaluate the function of peripheral nerves by measuring the speed and amplitude of electrical signals as they travel through nerves 1
• It is often performed alongside electromyography (EMG) to provide a comprehensive assessment of the peripheral nervous system and skeletal muscle 2, 1

How the Test is Performed

Equipment and Setup

• The test requires specialized equipment including:
  • Electrical stimulator
  • Recording electrodes
  • Amplifiers
  • Computer system for data analysis 1
• Temperature monitoring and control are essential as nerve conduction velocity changes approximately 2.0 meters per second per degree Celsius of temperature change 3

Testing Procedure

1. Patient Preparation:

   • The patient lies down or sits comfortably
   • The skin is cleaned at electrode placement sites
   • Surface electrodes are placed on the skin over the nerves being tested 1

2. Stimulation and Recording:

   • A small electrical stimulus is applied to the nerve at specific points
   • Recording electrodes detect the electrical response at different points along the nerve
   • The time it takes for the impulse to travel between points is measured 1, 4

3. Measurements Obtained:

   • Motor nerve conduction velocity (motor NCV)
   • Sensory nerve conduction velocity (sensory NCV)
   • Compound motor action potential (cMAP) amplitude
   • Sensory nerve action potential (SNAP) amplitude
   • Distal motor latency (DML) 1, 5

Types of NCV Tests

Motor Nerve Conduction Studies

• Measures how quickly electrical signals travel along motor nerves that control muscle movement
• Commonly tested motor nerves include median, ulnar, peroneal, and tibial nerves 1
• Evaluates the compound motor action potential (cMAP) amplitude and conduction velocity 1

Sensory Nerve Conduction Studies

• Assesses the function of sensory nerves that carry signals related to sensation
• Common sensory nerves tested include median, ulnar, radial, and sural nerves 1
• Measures sensory nerve action potential (SNAP) amplitude and conduction velocity 1, 5

Repetitive Nerve Stimulation (RNS)

• Involves stimulating a nerve repeatedly at specific frequencies (2-3 Hz or 30-50 Hz)
• Used to identify neuromuscular junction disorders like myasthenia gravis 2, 1

Clinical Applications

Diagnostic Uses

• Detecting peripheral neuropathies, including diabetic neuropathy 2
• Identifying nerve entrapment syndromes (e.g., carpal tunnel syndrome) 5
• Distinguishing between axonal and demyelinating neuropathies 1
• Evaluating radiculopathies and plexopathies 2
• Assessing neuromuscular junction disorders 2

Specific Testing Protocols

• Diabetic Neuropathy Assessment: Typically includes testing of sural, peroneal, median sensory, and median motor nerves 2
• Carpal Tunnel Syndrome: Focuses on median nerve conduction across the wrist, comparing to ulnar nerve 5
• Lower Back Pain Evaluation: May include NCV testing when radiculopathy is suspected 2

What to Expect During the Test

Patient Experience

• The test typically takes 30-60 minutes to complete
• Mild discomfort is common during electrical stimulation, often described as brief tingling or shock-like sensations 2
• The procedure is generally well-tolerated with minimal risks 1

Potential Discomfort

• Electrical stimulation may cause momentary discomfort
• Higher intensity stimulation for motor studies may be more uncomfortable than sensory studies 2
• Repetitive nerve stimulation at high frequencies (30-50 Hz) can be particularly uncomfortable 2

Interpretation of Results

Normal Values

• Normal conduction velocities vary by:
  • Specific nerve being tested
  • Patient's age
  • Limb temperature 3
• Generally, normal motor NCV ranges from 50-60 m/s in upper limbs and 40-50 m/s in lower limbs 2

Abnormal Findings

• Slowed conduction velocity: Indicates demyelination of nerves 1
• Reduced amplitude: Suggests axonal damage or loss 1, 5
• Conduction block: Indicates focal demyelination or compression 1
• Temporal dispersion: Seen in acquired demyelinating neuropathies 6

Limitations and Considerations

• NCV tests primarily assess large myelinated nerve fibers and may miss small fiber neuropathy 2
• Temperature significantly affects results and must be controlled or corrected for accurate interpretation 3
• Early in disease processes, NCV may be normal despite clinical symptoms 2
• The test is operator-dependent and requires specialized training for both performance and interpretation 2
• Results should always be interpreted in conjunction with clinical findings 2, 1

Complementary Tests

• Electromyography (EMG): Often performed with NCV to assess muscle electrical activity 1
• Quantitative sensory testing: Evaluates small fiber function 2
• Skin biopsy: Measures intraepidermal nerve fiber density 2
• Autonomic testing: Assesses autonomic nerve function 2

Preparation for the Test

• No special preparation is typically required
• Patients should inform their provider about medications they are taking
• Lotions and oils should not be applied to the skin on the day of the test
• Normal body temperature should be maintained before and during the test 3