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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
What is the NCV of the sural, superficial peroneal, and saphenous nerves?
> 40 m/s
26
What is the amplitude for all nerves typically?
> 6 microvolts
27
What can SNAPs also be useful in localizing?
a lesion in relation to the dorsal root ganglion
28
What can be expected of lesions proximal to the DRG (in the root or spinal cord)?
The SNAP is preserved despite the clinical sensory abnormalities
29
Why is the SNAP preserved in a lesion proximal to the DRG?
because axonal transport from the cell body to the axon continues to remain intact
30
What are motor nerve studies aka?
Compound Motor Action Potential (CMAP)
31
Are SNAPs or CMAPs typically considered more sensitive in the detection of an incomplete peripheral nerve injury?
SNAPs
32
A CMAP is recorded as what?
A compound evoked potential from a motor point within the muscle
33
What are the 6 specific parameters measured in association with a CMAP?
- latency - amplitude - rise time - duration - shape - NCV
34
The time it takes for electrical impulse to travel from the stimulation to the recording site is called what?
latency
35
Can NCV across different segments be measured? Explain...
Yes, by stimulating two or more different locations along the same nerve
36
A motor nerve conduction study corresponds to what?
the integrity of the motor unit
37
Can motor nerve conduction studies distinguish between pre- and post-ganglionic lesions?
No, because the cell body is located in the spinal cord
38
What occurs if the active and reference electrode are placed too close together?
Similar waveforms are recorded at both sites and rejected, dropping the amplitude of the waveform
39
Where should stimulation be applied in order to perform a motor nerve conduction study of the median nerve at the wrist and elbow?
Abductor Pollicis Brevis for both wrist and elbow
40
Where should stimulation be applied in order to perform a motor nerve conduction study of the ulnar nerve at the wrist and elbow?
Wrist: First Dorsal Interosseous Elbow: Abductor Digiti Minimi
41
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?
Ankle: Extensor Digitorum Brevis | Head of fibula: Tibialis Anterior
42
Where should stimulation be applied in order to perform a motor nerve conduction study of the tibial nerve at the ankle?
Along the Abductor Hallucis and/or Abductor Digiti Quinti Pedis
43
Why must two sites be stimulated for a motor NCS?
The data is more valid with 2 stimulation sites, because you are checking two different sites
44
Given the same distance are latencies for motor NCS longer or slower than those for a SNAP?
a little longer
45
What are 3 reasons why motor NCS latencies longer than sensory NCS latencies?
- 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
Is amplitude higher in motor NCS or sensory NCS?
motor, by as much as 1000 times larger
47
What is the typical shape of the AP generated via motor NCS?
biphasic
48
For a CMAP the duration is measured from what to what?
from the initial deflection away from the baseline to the first baseline crossing
49
How is distal latency calculated?
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
How is proximal latency calculated?
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
So how do you to calculate the time between distal and proximal stimulation sites?
Subtract distal latency from proximal latency
52
What is the range of distal latencies for the median, ulnar, radial, deep peroneal, tibial, femoral, and sciatic nerves?
< 3.5 - 6.0 milliseconds
53
What is the range of distal velocities for the median, ulnar, radial, deep peroneal, tibial, femoral, and sciatic nerves?
> 40 - 55 m/s
54
What is the range of amplitudes for the median, ulnar, radial, deep peroneal, tibial, femoral, and sciatic nerves?
> 2 - 5 mV
55
What happens to distal latency and conduction velocity following axonal loss?
Distal latency increases slightly | Conduction velocity decreases slightly
56
What happens to distal latency and conduction velocity following demyelination with and without conduction block?
Distal latency increases drastically | Conduction velocity decreases drastically
57
What are 2 other motor nerve conduction studies?
F-Wave, H-Reflex, and repetitive nerve stimulation
58
What does a central conduction study (F-wave) evoke?
a small late response from a short duration supramaximal stimulation
59
Describe the transmission of the AP during a F-wave study
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
Why is the secondary contraction of a F-wave considered secondary?
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
What is this initial activation of the muscle fibers called?
The M-wave
62
What does the F-wave represent?
the overall conduction status of the entire nerve under investigation
63
Describe the amplitude of the F-wave
It is small with significant variability
64
The configuration and latency of the F-wave changes with each stimulation, why is that?
Due to a polysynaptic response in the spinal cord, where Renshaw cells inhibit impulses from traveling the same path each time
65
F-wave latency <__ msec in the LE
30 58
66
The F-wave is an inconsistent response and must be calculated with at least __ successive trials
10
67
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?
proximal - Guillain-Barré syndrome - Thoracic outlet syndrome - Brachial plexus injuries - Radiculopathies with more than one nerve root involved
68
What is Hoffman's reflex (H-reflex) useful in diagnosing?
radiculopathy and peripheral neuropathy
69
What is the H-reflex most commonly used to test?
the integrity of the sensory and motor monosynaptic pathways of S1 nerve roots (sometimes C6 and C7)
70
The H-reflex is analogous to what?
a tendon tap reflex
71
How is an H-reflex deemed abnormal?
If there is greater than 1 msec difference between extremities
72
Describe the process for eliciting a H-wave
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
In what direction does the AP travel during the Hoffman's reflex?
Proximally along the afferent axon toward the spinal cord into the segmental level of the spinal cord
74
Once the H-wave reaches the anterior horn of the spinal cord what happens?
A CMAP results, which causes an impulse to travel distally via efferent axons along the motor nerve where it depolarizes muscle
75
The H-Wave has a _____ | amplitude, _____ latency, and _____ optimal stimulus intensity.
Smaller Longer Lower
76
What percentage of the motor neuron pool may participate in H-waves?
24-100%
77
What are the 2 advantages of the H-reflex?
- Stable reflex | - Indicates problems along the motor nerve
78
What are the 2 disadvantages of the H-reflex?
- Only used with mixed nerves | - Difficult to obtain
79
What is repetitive stimulation testing designed to test for?
a neuromuscular junction transmission disorder
80
What are 2 types of neuromuscular junction transmission disorders?
- Myasthenia Gravis | - Lambert-Eaton syndrome
81
Describe how repetitive stimulation testing can test for a neuromuscular junction transmission disorder
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
A decrease of __% from the 1st to the 5th stimuli is considered abnormal
10%
83
What technique should be used if the nerve to be tested is deeper in the tissues and more difficult to reach?
Near-Nerve Stimulation Technique via needle electrode
84
What technique can be used to identify the specific location of a peripheral nerve problem?
"Inching" technique in which the stimulating electrode is moved small distances the nerve
85
What are 3 types of sensory nerve studies to conduct when carpal tunnel is suspected?
- 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
What are 6 types of motor nerve studies to conduct when carpal tunnel is suspected?
- 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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