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634: Electrotherapy Modalities > Components of a NCS > Flashcards

Components of a NCS Flashcards

1
Q

What are the 4 tests commonly performed under the label of NCS?

A

1) sensory nerve studies
2) motor nerve studies
3) reflex studies (F-wave and H-reflex)
4) repetitive stimulation testing

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2
Q

What are sensory nerve studies aka?

A

Sensory Nerve Action Potential (SNAP)

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3
Q

What does a sensory nerve conduction study represent?

A

the conduction of an impulse along the sensory nerve fibers

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4
Q

How is a sensory nerve conduction study performed?

A

by electrical stimulation of a peripheral nerve and recording from a purely sensory portion of the nerve, such as on a finger

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5
Q

In sensory nerve conduction studies are the electrodes placed orthodromically or antidromically?

A

The active electrode is placed proximal to the stimulating electrode, therefore they are arranged orthodromically

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6
Q

Describe the setup for a typical sensory NCS for the median nerve

A

1) The stimulating ring electrodes are placed on the index finger, with the cathode placed proximally and the anode placed distally
2) The active electrode is typically placed 14 cm proximal to the cathode on the skin of the wrist over the nerve
3) The reference electrode is placed several cm proximal to the ground electrode along the course of the nerve
4) The ground electrode is positioned on the same extremity, typically on the opposite side of the limb between the point of stimulation (cathode) and the point of pickup (active)

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7
Q

What are the 4 specific parameters measured in association with a SNAP?

A
  • amplitude of the size of the potential
  • shape of the AP
  • latency or the time that it takes from stimulus to the response over a predetermined distance
  • nerve conduction velocity (NCV) or the speed by which a nerve conducts an AP
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8
Q

What is AP amplitude measured in?

A

microvolts

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9
Q

What is the typical shape of the AP generated via sensory NCS?

A

biphasic with a phase on each side of the baseline

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10
Q

What is AP latency measured in?

A

milliseconds

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11
Q

What is NCV measured in?

A

m/s

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12
Q

How is NCV calculated?

A

the distance between the stimulating and recording electrode divided by the latency (proximal latency – distal latency)

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13
Q

Where should stimulation be applied in order to perform a sensory nerve conduction study of the median nerve?

A

Index finger or Thumb

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14
Q

Where should stimulation be applied in order to perform a sensory nerve conduction study of the ulnar nerve?

A

5th finger

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15
Q

Where should stimulation be applied in order to perform a sensory nerve conduction study of the superficial peroneal nerve?

A

Approx. 2cm medial to the Lateral Malleolus

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16
Q

Where should stimulation be applied in order to perform a sensory nerve conduction study of the sural nerve?

A

Posterior to the Lateral Malleolus

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17
Q

Do signals detected via SNAP produce a lot of “signal noise”? Explain why or why not

A

The signals detected are very small and require a greater level of amplification, therefore there is increased “signal noise”

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18
Q

Amplitude is measured from ____ to _____

A

peak to trough

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19
Q

Latency is measured from ____ to _____

A

stimulus onset to peak of the negative potential

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20
Q

In some cases an ______ generated AP will be larger or easier to elicit than one obtained _____.

A

antidromically

orthodromically

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21
Q

When are antidromic and orthodromic latencies equivalent?

A

When the distance between the active and directive electrodes is 4 cm apart

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22
Q

An antidromic generated AP typically produces a ____ amplitude SNAP

A

higher

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23
Q

An ortodromic generated AP typically produces a ____ amplitude SNAP

A

smaller

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24
Q

What is the NCV of the median, ulnar, and radial nerves?

A

> 35 m/s

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25
Q

What is the NCV of the sural, superficial peroneal, and saphenous nerves?

A

> 40 m/s

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26
Q

What is the amplitude for all nerves typically?

A

> 6 microvolts

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27
Q

What can SNAPs also be useful in localizing?

A

a lesion in relation to the dorsal root ganglion

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28
Q

What can be expected of lesions proximal to the DRG (in the root or spinal cord)?

A

The SNAP is preserved despite the clinical sensory abnormalities

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29
Q

Why is the SNAP preserved in a lesion proximal to the DRG?

A

because axonal transport from the cell body to the axon continues to remain intact

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30
Q

What are motor nerve studies aka?

A

Compound Motor Action Potential (CMAP)

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31
Q

Are SNAPs or CMAPs typically considered more sensitive in the detection of an incomplete peripheral nerve injury?

A

SNAPs

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32
Q

A CMAP is recorded as what?

A

A compound evoked potential from a motor point within the muscle

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33
Q

What are the 6 specific parameters measured in association with a CMAP?

A
  • latency
  • amplitude
  • rise time
  • duration
  • shape
  • NCV
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34
Q

The time it takes for electrical impulse to travel from the stimulation to the recording site is called what?

A

latency

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35
Q

Can NCV across different segments be measured? Explain…

A

Yes, by stimulating two or more different locations along the same nerve

36
Q

A motor nerve conduction study corresponds to what?

A

the integrity of the motor unit

37
Q

Can motor nerve conduction studies distinguish between pre- and post-ganglionic lesions?

A

No, because the cell body is located in the spinal cord

38
Q

What occurs if the active and reference electrode are placed too close together?

A

Similar waveforms are recorded at both sites and rejected, dropping the amplitude of the waveform

39
Q

Where should stimulation be applied in order to perform a motor nerve conduction study of the median nerve at the wrist and elbow?

A

Abductor Pollicis Brevis for both wrist and elbow

40
Q

Where should stimulation be applied in order to perform a motor nerve conduction study of the ulnar nerve at the wrist and elbow?

A

Wrist: First Dorsal Interosseous

Elbow: Abductor Digiti Minimi

41
Q

Where should stimulation be applied in order to perform a motor nerve conduction study of the peroneal nerve at the ankle and head of fibula?

A

Ankle: Extensor Digitorum Brevis

Head of fibula: Tibialis Anterior

42
Q

Where should stimulation be applied in order to perform a motor nerve conduction study of the tibial nerve at the ankle?

A

Along the Abductor Hallucis and/or Abductor Digiti Quinti Pedis

43
Q

Why must two sites be stimulated for a motor NCS?

A

The data is more valid with 2 stimulation sites, because you are checking two different sites

44
Q

Given the same distance are latencies for motor NCS longer or slower than those for a SNAP?

A

a little longer

45
Q

What are 3 reasons why motor NCS latencies longer than sensory NCS latencies?

A
  • the recorded AP has to cross the neuromuscular junction, which takes time
  • the AP has to spread across the muscle fiber, which is relatively slow
  • even the largest motor axons cannot conduct as fast as the largest sensory axons
46
Q

Is amplitude higher in motor NCS or sensory NCS?

A

motor, by as much as 1000 times larger

47
Q

What is the typical shape of the AP generated via motor NCS?

A

biphasic

48
Q

For a CMAP the duration is measured from what to what?

A

from the initial deflection away from the baseline to the first baseline crossing

49
Q

How is distal latency calculated?

A

The time it takes for the AP to travel from the distal stimulation site to the neuromuscular junction, to cross the neuromuscular junction, to depolarize the muscle

50
Q

How is proximal latency calculated?

A

The time it takes for the AP to travel from the proximal stimulation site to the distal stimulation site, to the neuromuscular junction, to cross the neuromuscular junction, and to depolarize the muscle

51
Q

So how do you to calculate the time between distal and proximal stimulation sites?

A

Subtract distal latency from proximal latency

52
Q

What is the range of distal latencies for the median, ulnar, radial, deep peroneal, tibial, femoral, and sciatic nerves?

A

< 3.5 - 6.0 milliseconds

53
Q

What is the range of distal velocities for the median, ulnar, radial, deep peroneal, tibial, femoral, and sciatic nerves?

A

> 40 - 55 m/s

54
Q

What is the range of amplitudes for the median, ulnar, radial, deep peroneal, tibial, femoral, and sciatic nerves?

A

> 2 - 5 mV

55
Q

What happens to distal latency and conduction velocity following axonal loss?

A

Distal latency increases slightly

Conduction velocity decreases slightly

56
Q

What happens to distal latency and conduction velocity following demyelination with and without conduction block?

A

Distal latency increases drastically

Conduction velocity decreases drastically

57
Q

What are 2 other motor nerve conduction studies?

A

F-Wave, H-Reflex, and repetitive nerve stimulation

58
Q

What does a central conduction study (F-wave) evoke?

A

a small late response from a short duration supramaximal stimulation

59
Q

Describe the transmission of the AP during a F-wave study

A

The AP is transmitted proximally (antdromically) via efferent motor axons to the level of the anterior horn cell, which then “bounces back” (in an orthodromic direction) along the same efferent axons to result in a secondary contraction of the innervated muscle fibers

60
Q

Why is the secondary contraction of a F-wave considered secondary?

A

Because there is initially a direct activation of the muscle fibers from the distal segment of the efferent axons that directly activate the muscle fibers, creating a CMAP

61
Q

What is this initial activation of the muscle fibers called?

A

The M-wave

62
Q

What does the F-wave represent?

A

the overall conduction status of the entire nerve under investigation

63
Q

Describe the amplitude of the F-wave

A

It is small with significant variability

64
Q

The configuration and latency of the F-wave changes with each stimulation, why is that?

A

Due to a polysynaptic response in the spinal cord, where Renshaw cells inhibit impulses from traveling the same path each time

65
Q

F-wave latency <__ msec in the LE

A

30

58

66
Q

The F-wave is an inconsistent response and must be calculated with at least __ successive trials

A

10

67
Q

The F-wave is most helpful in diagnosis of conditions where the most ____ portion of the axon is involved. What are 4 examples of this?

A

proximal

  • Guillain-Barré syndrome
  • Thoracic outlet syndrome
  • Brachial plexus injuries
  • Radiculopathies with more than one nerve root involved
68
Q

What is Hoffman’s reflex (H-reflex) useful in diagnosing?

A

radiculopathy and peripheral neuropathy

69
Q

What is the H-reflex most commonly used to test?

A

the integrity of the sensory and motor monosynaptic pathways of S1 nerve roots (sometimes C6 and C7)

70
Q

The H-reflex is analogous to what?

A

a tendon tap reflex

71
Q

How is an H-reflex deemed abnormal?

A

If there is greater than 1 msec difference between extremities

72
Q

Describe the process for eliciting a H-wave

A

A submaximal stimulus is applied to the tibial nerve at the popliteal fossa and a motor response is recorded from the median portion of the soleus muscle

73
Q

In what direction does the AP travel during the Hoffman’s reflex?

A

Proximally along the afferent axon toward the spinal cord into the segmental level of the spinal cord

74
Q

Once the H-wave reaches the anterior horn of the spinal cord what happens?

A

A CMAP results, which causes an impulse to travel distally via efferent axons along the motor nerve where it depolarizes muscle

75
Q

The H-Wave has a _____

amplitude, _____ latency, and _____ optimal stimulus intensity.

A

Smaller
Longer
Lower

76
Q

What percentage of the motor neuron pool may participate in H-waves?

A

24-100%

77
Q

What are the 2 advantages of the H-reflex?

A
  • Stable reflex

- Indicates problems along the motor nerve

78
Q

What are the 2 disadvantages of the H-reflex?

A
  • Only used with mixed nerves

- Difficult to obtain

79
Q

What is repetitive stimulation testing designed to test for?

A

a neuromuscular junction transmission disorder

80
Q

What are 2 types of neuromuscular junction transmission disorders?

A
  • Myasthenia Gravis

- Lambert-Eaton syndrome

81
Q

Describe how repetitive stimulation testing can test for a neuromuscular junction transmission disorder

A

A muscle is repetitively stimulated via its nerve at a frequency of at least 1/sec in order to deplete the ACh in the neuromuscular junction. If a transmission disorder is present, the amplitude of successive stimuli will decrease.

82
Q

A decrease of __% from the 1st to the 5th stimuli is considered abnormal

A

10%

83
Q

What technique should be used if the nerve to be tested is deeper in the tissues and more difficult to reach?

A

Near-Nerve Stimulation Technique via needle electrode

84
Q

What technique can be used to identify the specific location of a peripheral nerve problem?

A

“Inching” technique in which the stimulating electrode is moved small distances the nerve

85
Q

What are 3 types of sensory nerve studies to conduct when carpal tunnel is suspected?

A
  • DSL of the median nerve (both palm-wrist and 2nd digit-wrist)
  • DSL of the ulnar nerve (both palm-wrist and 5th digit-wrist)
  • DSL of the superficial radial nerve (forearm-wrist)
86
Q

What are 6 types of motor nerve studies to conduct when carpal tunnel is suspected?

A
  • DML of the median nerve (wrist-APB)
  • NCV of the median nerve (elbow-wrist segment)
  • F-wave of the median nerve
  • DML of the ulnar nerve
  • NCV of the ulnar nerve (both below and above elbow-wrist; forearm and across the elbow)
  • F-wave of the ulnar nerve

634: Electrotherapy Modalities (11 decks)

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