Technical Factors EMG/NCS

Question 1

High frequency filters do what?

Answer 1

• Exclude signals above a set frequency
• Allow lower frequency signals to pass through (low pass)

Question 2

Low frequency filters do what?

Answer 2

• Excludes signals below a set frequency from passing through
• Allows higher frequency signals to pass through (high pass)

Question 3

Reducing a low frequency filter allows more low frequecy signals to pass through. What does this do to the waveforms?

Answer 3

It increases the duration of the waveforms because the duration is primarily a lower frequency response

Question 4

As a high frequency filter is lowered more high frequency signals are excluded. What does this do to the waveform?

Answer 4

It decreases the amplitude of the waveform because amplitude is primarily a higher frequency response

Question 5

Does depolarization take place beneath the cathode or the anode?

Answer 5

Cathode

Question 6

Waveform measurement should be made between the _______ and __

Answer 6

Cathode and G1

Question 7

What physiologic effects do cold temperatures have on Na channels and what does this do to depolarization?

Answer 7

It delays inactivation of Na channels and prolongs the time of depolarization

Question 8

The time of depolarization increases with decreasing temperature. What does prolonged depolarization do to conduction velocity?

Answer 8

It slows conduction velocity down. The colder the temperature, the slower the conduction velocity.

Question 9

For every 1⁰ C drop in temperature, what happens to motor and sensory conduction velocity and distal latency?

Answer 9

• Conduction velocity decreases by 1.5 to 2.5 m/s
• Distal latency prolongs by 0.2 ms

Question 10

Why do CMAPs and SNAPs increase in amplitude and duration with cooler temperatures?

Answer 10

Cooler temperatures result in longer channel opening which results in a larger influx of Na which lengthens the depolarization

Question 11

What effect does cooler temperature have on EMG MUAPs?

Answer 11

• Increased amplitude
• Increased duration
• Number of phases may also increase

Question 12

What is the ideal distal limb temperature for EMG/NCS?

Answer 12

32º to 34º C

Question 13

Why does conduction velocity increase from birth to adult?

Answer 13

Myelination increases

Question 14

What happens to conduction velocity with increasing age after adulthood? At what age is this most prominent?

Answer 14

• It decreases because of demyelination
• Most prominent after age 60

Question 15

At what age is peripheral myelination complete?

Answer 15

Ages 3 to 5

Question 16

After age 60 how much does conduction velocity decrease per decade?

Answer 16

0.5 to 4.0 m/s/decade

Question 17

What affect does increasing age have on SNAP amplitudes?

Answer 17

It decreases amplitudes up to 50% by age 70

Question 18

What effect does increasing age have on EMG MUAP?

Answer 18

It increases MUAP duration due to the slow dropout of motor units with some subsequent reinnervation

Question 19

What affect does height have on nerve conduction velocity and why?

Answer 19

• Increasing height = slower conduction velocity
• The taller the individual, the longer the limb and the more tapered the distal nerve is
  
  • Conduction velocity is directly proportional to nerve diameter (larger = faster)
  • Limbs are cooler distally than proximally and since legs are longer than arms, conduction velocity in the legs is slower than in the arms

Question 20

What is common mode rejection?

Answer 20

This occurs when the same electrical noise is present at both the active and reference electrodes, they cancel each other out and only the signal of interest is amplified

Question 21

In recording motor conduction studies, what is the belly-tendon method?

Answer 21

Active electrode (G1) is placed on the muscle belly and the reference electrode (G2) is placed on the muscle’s distal tendon

Question 22

Antidromic stimulation vs orthodromic stimulation

Answer 22

Antidromic stimulation

• Stimulate proximal nerve and record distal nerve
• Example: stimulating median nerve at wrist and recording at digit 2

Orthodromic stimulation

• Stimulate distal nerve and record at proximal nerve
• Example: stimulating at digit 2 and recording median nerve at wrist

Question 23

Why does antidromic stimulation of a sensory nerve result in a higher amplitude

Answer 23

• SNAP amplitude is directly proportional to the distance between the recording electrodes and the nerve
  
  • For most antidromic potentials, the active recording electrodes are closer to the nerve
• Example: stimulating median sensory nerve at the wrist and recording on digit 2

Question 24

Disadvantage of antidromic sensory study

Answer 24

• Motor and sensory fibers are stimulated
• Usually the SNAP shows up before the volume conducted motor potential
• If the SNAP is absent the motor potential can be confused for the SNAP

Question 25

What is the effect on onset latency, peak latency and amplitude of a sensory nerve if the recording electrode is lateral or medial to the nerve instead of directly on top?

Answer 25

• Onset latency is shorter (seems like it would be the opposite)
• Peak latency is the same (again, seems weird)
• Amplitude is lower (this makes sense)

All of the above occurs because of volume conduction through tissue

Question 26

What effects do sweep speed and sensitivity have on the recorded latency of sensory and motor potentials?

Answer 26

• As the sweep speed is decreased (2 ms to 1 ms for example), onset latency measurement increases
• As sensitivity is increased (1 mV to 100 µV), onset latency measurement decreases

Question 27

What is the transmembrane potential of skeletal muscle?

Answer 27

-90 mV

Question 28

What does the endoneurium do?

Answer 28

Surrounds individual axons

Question 29

What does the perineurium do?

Answer 29

It binds each fascicle and may act as a blood-nerve barrier

Question 30

What does the epineurium do?

Answer 30

It binds fascicles together and merges with the dura mater of the spinal roots

Question 31

Do changes in sweep speed and sensitivity have an effect on onset and peak latency?

Answer 31

They affect onset latency but not peak latency

Question 32

What are the typical low and high frequency filter settings for motor nerve conduction studies?

Answer 32

10 Hz and 10 kHz

Question 33

What are the typical low and high frequency filter settings for sensory nerve conduction studies?

Answer 33

20 Hz and 2 kHz

Sensory high frequency filter is set lower than for motor studies to reduce high frequency noise since SNAPs contain more high-frequency components compared to CMAPs