Nerve Conduction Velocity Flashcards
What 4 pieces of information can nerve conduction velocity provide about peripheral nerve disease?
- Demyelinated nerves
- Axonal degeneration
- Location of injury
- Type of injury
What are the 3 types of nerve injury?
- Neurapraxia
- Axonotomesis
- Neurotemsis
What are the 4 methods of measurement of nerve conduction velocity?
- Latency
- Amplitude
- Duration
- Configuration
What is the difference conduction velocity in a demyelinated motor nerve and a degenerated motor nerve?
A demyelinated nerve has in tact amplitude, but is slowed across the lesion
A degenerated nerve has little or no muscle response.
What is the difference between conduction velocity in a demyelinated sensory nerve and degenerated sensory nerve?
- Slowed velocity in demyelinated nerve
- Reduced amplitude in axonal degeneration
What type of lesion would result in degeneration of sensory fibers?
- Postganglionic (plexopathy/ neuropathy)
Why must a supermaximal stimulation be used for nerve conduction velocity?
Larger fibers are activated before smaller fibers, and if a low magnitude stimulation is used, not all fibers will be generated.
What are the 2 types of action potentials generated in sensory nerve conduction velocity tests?
- Orthodromic: Travels to muscle
- Antidromic: Travels back to spinal cord
How should the patient be prepared prior to performing the test?
- Tell them what to expect
What 2 things should be inspected on the patient before beginning a NCV test?
- Check sensation
- Check skin integrity
How should the patient be positioned, and prepared for the NVC test?
- Clean and mildly abrade skin with alcohol so that less juice needs to be used
- Position patient so that the nerve can be accessed from all positions
Where should the cathode and anode of the recording site be placed? Which is the stimulating electrode, and which is the reference?
- Cathode placed over the muscle belly
- Stimulating
- Anode placed over tendon of insertion distal to the cathode
- Reference
Where is the ground reference placed?
- Over a bony prominence
What are the parameters for stimulation of a motor nerve with NCV tests? Pulse? Frequency? Gain? Sweep?
Pulse: 0.1 ms
Frequency 1Hz; 3 Hz - 10 kHz
Gain: 2mV/ division
Sweep speed: 5 - 10 ms/div
How is conduction velocity calculated?
Distance between stimulating cathodes in mm/ (Latency 2 - Latency 1 in ms)
What type of disease are delayed latencies indicative of?
Demyelinated disease
Where are the stimulation electrodes placed?
Cathode distal
Anode Proximal
Describe the placement of the stimulation and pick-up electrodes for orthodromic sensory NCV tests.
Stimulation: (on fingers)
- Cathode proximal
- Anode distal
Pick-up: (proximally along course of nerve)
- Cathode distal
- Anode proximal
How are the electrodes shaped in sensory orthodromic stimulation?
- rings to be placed around fingers of nerve distribution
Describe the placement of the stimulation and pick-up electrodes for antidromic sensory NCV tests.
Stimulating: (along course of nerve)
- Cathode distal
- Anode proximal
Pick-up (around fingers)
- Cathode proximal
- Anode distal
- Ground over bony prominence
What nerves are usually tested?
- Median
- Ulnar
- Radial
- Peroneal
- Sural
Which type of action potential is preferred for sensory testing? Why?
- Nerves in skin are easier to read
- Antidromic preferred
What are the settings for sensory NVC tests for: EMG? Stimulation?
Pulse: 0.1 ms
Frequency 1Hz; 3 Hz - 10 kHz
Gain: higher than in motor (which is 2mV/ division)
Sweep speed: 5 - 10 ms/div
** Intensity only high enough to produce sensory stimulus **
How are sensory conduction velocities calculated?
- Only latencies are used
- Record from stimulus artifact to peak of 1st phase of AP
What is the calculation for sensory conduction velocity?
distance between stimulation and recording sites/ latency
What can be used to measure nerves more proximally?
H-reflex, and F-waves.
What is an H-reflex?
- A quick delayed reactionary response after a small stimulation.
What does a diminished H-reflex indicate?
The nerve problem is proximal
On which muscle is the H-reflex typically conducted?
- The flexor carpi radialis
What is the F-wave?
- Huge action potential travels back to motor neuron and depolarizes its self, and then returns as a late response
Why must the F-wave be done 20 times? Which latency is used for the measurement?
- The neuron may be in refraction
- Shortest latency is used
What 3 factors may increase or decrease nerve conduction velocity?
- Temperature
- Nerve length (UE vs LE)
- Age
What temperature should the skin be kept at or above?
34 degrees celcius
How much must the conduction velocity be increased by for each degree centigrade the skin falls below 34?
5 %
Alternately, what temperature should the room be kept at during measurements?
21 - 23 degrees C
Why are nerves slower in the LE than in the UE?
Longer nerves.
Is NCV faster in proximal or distal segments?
Proximal
When does NCV velocity reach adult values in children?
3 - 5 yo
When do NCV tend to begin decreasing?
30 - 40 yo
What percentage of adult NCV do infants possess?
About 50 %
At what sites do nerve amplitude and configurations change as a person ages?
At sites of compression
What specifically declines that as people age in NCV tests?
Latency.
What is spread stimulation current?
- Nerves or muscles not being tested may be recruited at higher intensities
What area is specifically susceptible to spread stimulation current?
- Axilla
What anastomosis is present in the forearm that can cause false EMG readings? What nerves are involved? How many people does this occur in?
- Martin-Gruber anastomosis of the median and ulnar nerve
What is present in the leg in 20 - 28 % of people that may complicate NCV?
Accessory deep peroneal nerve branch of superficial peroneal nerve
What effect does the accessory peroneal have on NCV in the leg?
- AP of muscles will be small with stimulation to the deep peroneal nerve at the ankle than with stimulation of the common peroneal nerve at the knee
What is temporal dispersion?
- Different size nerves
- More proximal stimulation means there is a longer duration AP and a smaller amplitude
What physiological process can always be a problem in NCV readings?
Physiological block
What is a motor conduction block? What happens when sensory nerves are blocked?
- If nerve is stimulated proximal to block is reduced in amplitude and disperes
- Sensory nerves are almost unrecordable
What is repetitive nerve stimulation?
- Application of supramaximal stimulation at 2 Hz - 10 Hz.
- Testing to see if there is enough acetylcholine in the synapse
- If not, the amplitude drops
What is needle EMG used for?
- Measures small motor units with a manual contraction
How many times is the same muscle assessed at different points?
3 - 4.
