Nerve conduction study

1. Nerves conduction study
Part 1 : Techniques of recording
For post basic neurophysiology course

2. Outlines
• Overview
• Sensory nerves conduction
study
• Motor nerves conduction study
• Repetitive nerves conduction
study
• Late response (F wave & H
reflexes)

3. Overview

4. Elements of the peripheral nervous system.

5. 5
Nerve Conduction Study (NCS)
• NCS is a test commonly used to evaluate
the function of the motor and sensory
nerves of the human body.
• mainly for peripheral nerves
Peripheral nerves are stimulated with an
controlled electrical stimulus
Responses recorded

6. Uses
• Nerve conduction studies are used mainly for
evaluation of paresthesias (numbness, tingling,
burning) and/or weakness of the arms and legs.
• The type of study required is dependent in part,
by the symptoms presented.
• Indications:
– Symptoms indicative of nerve damage as numbness, weakness.
– Differentiation between local or diffuse disease process
(mononeuropathy or polyneuropathy).
– Get prognostic information on the type and extent of nerve
injury.
6

7. Limitations:
• Routine motor and sensory conduction velocity and
latency measurements are from the largest and fastest
fibers.
• Large-diameter fibers have the most myelin and the least
electrical resistance, both of which result in faster
conduction velocities.
• Thus, neuropathies that preferentially affect only small
fibers may not reveal any abnormalities on NCSs.

8. Classification of nerves

9. Common disorders diagnosed by NCS
Peripheral neuropathy
• Mononeuropathy (ex: carpal tunnel syndrome)
• Mononeuritis multiplex (ex: vasculitides, rheumatoid arthritis, lupus
erythematosus [SLE], sarcoidosis, leprosy, Lyme disease, amyloidosis)
• Polyneuropathy (ex: diabetic neuropathy,)
Myopathy
• Muscular dystrophies (ex: Facioscapulohumeral muscular dystrophy)
• Myotonia
• Congenital myopathies
• Metabolic myopathies
Radiculopathy (problem in which one or morenerves are
affected with emphasis on the nerve root; Radix="root")
• Nerve damagefrom herniated discs
Diseases of neuromuscular junction
• Myasthenia gravis
9

10. Disorders of the Peripheral Nervous System

11. Hardware and Software

14. Sweep,
Sensitivity,
Current
Controller

15. Description of the procedure
Electrodes
• Skin will be cleaned
• electrodes will be taped to the skin along the nerves
that are beingstudied
Stimulus
• Small stimulus is applied (electric current)that
activate nerves
Current
• Theelectrodes will measure the current thattravels
down the nervepathway
15

16. Procedure
•
•
•
Active electrode placed on the center of the muscle
belly (over the motor endplate)
Reference electrode placed distally about 3-4 cm from
active electrode (over tendon or bone).
Ground electrode in between active and reference
electrode
•
•
Stimulator placed over the nerve that supplies the
muscle, cathode closest to the recording electrode.
– Current needed
1. 20-50 mA for motor NCS
2. 5-30 mA for sensory NCS
Supramaximal stimulation is given.

18. Stimulator
• Cathode (Negative pole ) –
depolarize underlying nerve
segment
• Anode (Positive pole )–
hyperpolarize underlying
nerve segment
•
• Placing the cathode closer
to the recording site avoids
anodal conduction block
Cathode & anode - 2-3
cm apart

19. CMAP
•
•
•
Latency – time interval between the onset of a stimulus
and the onset of a response
Amplitude – the maximal height of the action potential.
Conduction velocity – how fast the fastest part of the
impulse travels

20. latency
Latency:

21. DIRECTION OF CONDUCTION
• Orthodromic conduction
• Antidromic conduction
• Orthodromic – when the electrical impulse travels in the same
direction as normal physiologic conduction
• Antidromic – when the electrical impulse travels in the opposite
direction of normal physiologic conduction

22. Orthodromic
(physiological)
pain
AP

23. Antidromic
(non-
physiological)

24. Orthodromic sensory study
Stimulating away from the sensory
receptor

25. Antidromic sensory study
Stimulating toward the
sensory receptor.
The antidromic method has the advantage
of a higher-amplitude SNAP but is
followed by a large volume-conducted
motor potential.

26. 26
Components of NCS
• The NCS consists of the following components:
– Compound Motor Action Potential (CMAP); also
called Motor nerve conduction study
– Sensory Nerve Action Potential (SNAP); also called
Sensory nerve conduction study
– F-wave study
– H-reflex study
– Repetitive nerve study
– A-(Axon) wave study
– Blink Reflex study
– Direct Facial Nerve Study
will not
be
discussed
…

27. Sensory nerves conduction
study

28. Sensory nerve conduction (SNC):
• S
C
Vm
e
a
su
r
em
en
t
sdifferf
r
o
mM
N
Cinthattheactionpotentialofthenerveitselfratherthanofa
m
uscleservesastheobservablee
n
dpoint.
• S
N
A
Pareofm
u
c
h
smalleramplitude.
• S
N
C
Sarem
o
r
esensitivethanM
N
C
Sindetectingearlya
n
dm
ilddisorders.

29. Sensory nerve conduction (SNC)
• Most sensory responses are very small (1 to 50 µV)
• Sensitivity: 10-20mcv/division
• Sweep: 20ms
• Electrical pulse: 100 or 200µs
• Stimulation: 5 to 30 mA
• sensory fibers usually have a lower threshold to
stimulation than do motor fibers.

30. Sensory nerve conduction (SNC)
•
•
•
•
Electrodes (GI and G2) are placed in line over the nerve
Interelectrode distance: 2.5 to 4 cm
Active electrode (GI) placed closest to the stimulator.
Recording ring electrodes used for sensory nerves in the fingers
S = Stimulus point, T= Takeoff
point, P =Peak
The time (latency) from S to T is
typically about 3milliseconds.
The amplitude would be
measured in microvolts (μV).

31. Sensory nerve conduction (SNC)
• a compound potential that
represents the summation of all the
individual sensory fiber action
potentials.
• Usually are biphasic or triphasic
potentials.
• For each stimulation site, the onset
latency, peak latency, duration,
and amplitude are measured
• A sensory CV can be calculated with
one stimulation alone

32. Sensory
nerve
action
potential
(SNAP)
• Onset Latency:
– is the time from the stimulus to the first deflection from baseline
– represents nerve conduction time from the stimulus site to the
recording electrodes for the largest cutaneous sensory fibers
– used to calculate conduction velocity.
• Peak Latency:
– is measured at the midpoint of the first negative peak.
– Inter examiner variation is less (marking).

33. Onset latency vs Peak latency
• Onset latency
– represents the fastest
conducting fibers
– can be used to calculate a
conduction velocity.
– difficult to mark precisely
• Peak latency
– the population of fibers
represented is unknown
– cannot be used to calmlate
a conduction velocity.
– easy to mark precisely
Peak
latency
Onset latency ???

34. Sensory nerve action potential (SNAP)
• Duration:
– measured from the onset of the potential to the
firstbaseline crossing (i.e., negative peak duration).
– The SNAP duration typically is much shorter than the
CMAP duration (typically 1.5 ms vs 5-6 ms).

35. Sensory nerve action potential (SNAP)
• Amplitude:
– commonly measured from baseline to negative peak.
– Low SNAP amplitudes indicate a definite disorder of
peripheral nerve.

36. Sensory nerve conduction velocity
• Sensory conduction velocity represents the
speed of the fastest, myelinated cutaneous
sensory fibers.
• Sensory conduction velocity can be determined
with one stimulation, by dividing the distance
traveled by the onset latency

37. Orthodromic method:
• Stimulating electrode
(supramax.) over distal
sensory branches of n.
• Recording electrode
over more proximal
point on n. trunk.
• The nerve will conduct
the impulse
orthodromically as
normal from distal to
proximal.

38. Antidromic method
•
•
• Stimulating electrode over
proximal point on n. trunk.
Recording electrode at distal
sensory branches of n.
The nerve will conduct the
impulse antidromically
opposite to normal from
proximal to distal.
Diabetic neuropathies
striking ↓↓ in SNCV.

39. 39

40. Sensory nerve conduction study – sites
Median nerves (R & L) at;
• index finger
• thumb
Ulnar nerves (R & L) at;
• little finger
• ring finger
Sural nerves (R & L) at;
• behind the Lateral Malleolus
Saphenous nerves(R & L) at;
• anterior to the MedialMalleolus
40

41. Normal value

42. Press
to
stimul
ate
Motor nerve conduction

43. Motor Conduction Studies
• Technically less demanding than sensory and mixed
nerve studies
– usually are performed first.
• Unit: millivolts (mV)
– Sensory and mixed nerve responses; microvolt (mcV)
• Less affected by electrical noise and other technical
factors.

44. Motor Conduction Studies
•
•
•
•
Sensitivity- 2 to 5 mV per division
Sweep -30ms
Current-20 to 50 mA
Duration of current: 200ms

45. MCS-belly-tendon montage
• Active recording electrode (GI) is
placed on the center of the muscle
belly (over the motor endplate),
• Reference electrode (G2) is placed
distally, over the tendon to the
muscle
• Stimulator- placed over the nerve
that supplies the muscle
– cathode placed closest to the
recording electrode.(Black to black)
• Ground electrode- In between
stimulating and recording electrode

46. Compound motor action
potential (CMAP):

47. Technique
• As current is slowly increased by 5- to 10-mA
– more of the underlying nerve fibers are brought to action potential
and subsequently more muscle fiber action potentials
• When CMAP no longer increases in size, one presumes
that all nerve fibers have been excited --> supramaximal
stimulation achieved.
• The "+" another 20% to ensure supramaximal
stimulation.

48. Compound muscle action potential (CMAP)
• Compound term
represents the
summation of all
underlying individual
muscle fiber action
potentials.
• Biphasic potential with
an initial negativity, or
upward deflection from
the baseline.
Comprises of :
•Latency,
•Amplitude,
•Duration, and
•Area of the CMAP
Stimulation
Motor conduction velocity can be calculated
after 2 sites (proximal & distal) stimulated

49. Motor Latency

50. Compound muscle action potential (CMAP)
Latency (ms):
•the time from the stimulus to the
initial CMAP deflection from
baseline.
•Latency represents three separate
processes:
(1)the nerve conduction time from
the stimulus site to the NMJ,
(2) the time delay across the NMJ
(3)the depolarization time across
the muscle.
Stimulation

51. Amplitude of M wave

52. Compound muscle action potential (CMAP)
Amplitude:
•commonly measured from
baseline to the negative peak
•less commonly from the first
negative peak to the next
positive peak.
•Causes of low CMAP-
(1) Axonal neuropathy
(2)Demyelation with
conduction block
(3) Presynaptic NMJ disorder
(4) Advanced myopathy

53. Compound muscle action potential (CMAP)
• CMAP area: also is
conventionally measured
between the baseline and
the negative peak.
• Differences in CMAP
area between distal and
proximal stimulation sites
take on special
significance in the
determination of
conduction block from a
demyelinating lesion.
Distal area
Proximal area

54. Compound muscle action potential (CMAP)
CMAP duration:
•
• measured from the initial
deflection from baseline to the
first baseline crossing (i.e.,
negative peak duration)
also can be measured from the
initial to the terminal deflection
back to baseline.
•
• Duration is primarily a
measure of synchrony (i.e., the
extent to which each of the
individual muscle fibers fire
atthe same time).
Duration increased in
demyelinating disease.

55. Compound muscle action potential (CMAP)
• Measure of the speed
of the fastest
conducting motor
axons.
• Conduction velocity
(m/s) calculated as:
distance between 2
stimulus sites (m)
difference between 2
latency (s)

56. Motor nerve
conduction:
•
•
• The directly evoked muscle action
potential recorded after
stimulation at T1of peripheral n.
this AP called M response.
The same nerve is stimulated
similarly at a more distal point and
this latency (T2) is also recorded.
The distance between 2 point of
stimulation is measured in cm. T
1
T
2

57. CV (msec)
DL(m/s)
3.2
Wrist-APB
Elbow-Wrist 55
Amp(mV)
15.0
14.8
Velocity of conduction = Distance (cm) x 100
(Meter/sec) T1-T2 (millisecond)
T
1
2

58. Motor nerve conduction study – sites
Median nerves (R & L) at;
• Wrist Abductor Pollicis Brevis
• Elbow
Ulnar nerves (R & L) at;
• Wrist
• Elbow
First Dorsal Interosseous (FDI)
Abductor Digiti Minimi (ADM)
Peroneal nerves (R & L) at;
• Ankle
• Headof fibula
Extensor Digitorum Brevis
Tibialis Anterior
Tibial nerves(R & L) at;
• Ankle Abductor Hallucis
Abductor Digiti Quinti Pedis
58

59. Normal Values

60. Repetitive nerves conduction
study

61. Repetitive nerves conduction study
• Easy to learn, easy to perform, and
requires no special equipment.
• However, it is poorly tolerated in some
patients and is prone to technical
problems
• Repeated stimulation given
• Response measure at different time frame
– Baseline, Exercise, 30 secs, 1 min, 2 min, 3
minit, 5 min

62. Repetitive nerves conduction study
• Repeated electrical stimulus applied to the
motor neuron at a rate of 3-5 / second , the
amplitude of the muscle recorded
• Decrement of more than 10% is abnormal
• Supramaximal stimulus
• Used to find NMJ abnormalities
– Eg: myaesthenia gravis
Lambert Eaton syndrome

63. Repetitive nerves conduction study
• Exercise testing
1.Brief maximal voluntary exercise for tested
nerves -10 seconds (painless)
2.Rapid RNS- 50-Hz supramaximal nerve
stimulation (painlful)

64. Repetitive nerves conduction study
Arepetitive nerve stimulation study
demonstrating a 61percent decrement in
area and a 54 percent decrement in
amplitude from the first to the fourth
stimulation.
Increment during rapid
repetitive nerve
stimulation. Recording the
hypothenar muscles, stimulating
the ulnar nerve at 50 Hz in a
patient with Lambert-Eaton
syndrome.

65. Repetitive nerves conduction study

66. Technical Factors in RNS
1. Immobilization: Isometric Electrode Position Is Essential
2. Stimuli Must Be Supramaximal
3. Temperature Must Be Controlled
4. Acetylcholinesterase Inhibitors Should Be Withheld Prior
to the Study (3-4 hours)
5. Nerve Selection
– any motor nerve.
– The nerves most commonly used are the ulnar, median,
musculocutaneous,axillary, spinal accessory, and facial.
1. Stimulation Frequency
– The optimal frequency for slow RNS is 2 or 3 Hz.
1. Number of Stimulations
– 5 to 10 pulses is preferable for slow RNS.

67. Protocol for Evaluating Disorders of the
Neuromuscular Junction (I)
1. Warm the extremity (33°C).
2. Immobilize the muscle as best as possible.
3. Perform routine motor nerve conduction studies first to ensure that
the nerve is normal.
4. Perform RNS at rest. After making sure that the stimulus is
supramaximal, perform 3-Hz RNS at rest for 5-IO impulses,
repeated three times, 1 minute apart. Normally, there is < 10%
decrement between the first and fourth responses.

68. 5. If> I0% decrement occurs and is consistently
reproducible:
– Have the patient perform maximal voluntary exercise for I0
seconds.
– Immediately repeat 3-Hz RNS postexercise to demonstrate
postexercise facilitation and repair of the decrement.
6. If <10% decrement or no decrement occurs:
– Have the patient perform maximal voluntary exercise for 1
minute, then perform 3-Hz RNS immediately and 1, 2, 3 and 4
minutes after exercise to demonstrate postexercise exhaustion.
– If a significant decrement occurs, have the patient perform
maximal voluntary exercise again for IO seconds and
immediately repeat 3-Hz RNS to demonstrate repair of the
decrement.
Protocol for Evaluating Disorders of the
Neuromuscular Junction (II)

69. 7. Perform RNS on one distal and one proximal motor nerve. Alwaystry
to study weak muscles.
8. If the compound muscle action potential amphtude is low at
basehne, have the patient perform I0 seconds of maximal voluntary
exercise, then stimulate the nerve supramaximally immediately
postexercise, looking for an abnormal increment (>40% above the
baseline).
If the patient exercisesfor> I0seconds or the nerve is not stimulated
immediately postexercise,a potential increment may be missed.
9. Alwaysperform concentric needle EMG of proximal and distal
muscles, especially of clinically weak muscles. Any muscle with
denervation or myotonia on needle EMG may demonstrate a
decrement on RNS. In these situations, a decrement on RNS does
not signify a primary disorder of the neuromuscular junction.
Protocol for Evaluating Disorders of the
Neuromuscular Junction (III)

70. Late response (F wave & H
reflexes)

71. Late response
• When a nerve stimulus
applied it travels in to 2
direction, peripheral
stimulation (orthodromic)
result in M-response
while towards anterior
horn cell
stimulation(antidromic)
result in to late
response.
• In routine only one late
response is measured
i.e.F response.

72. Late response
• For eliciting the late
response, some author
change the direction of
stimulator (cathode end
proximally) so that
maximum number of
nerve stimulated.

73. F-response
•
•
•
Late motor response
First described by Magaladery
and McDougal.
F response derives its name
from foot because it was first
recorded from the intrinsic foot
muscles.
•
•
•
Sensitivity-500mcv/div
Sweep-100ms
Stimulation: Supramaximum

74. F-response
•
•
•
Represent 1-5% of muscl
Latency UL: 25-32 ms
Latency LL: 45-65 ms
•
•
Normal persistence: 80-1
Normal chronodispersion
•
•
•
Peroel nerve can be diffic
Maybe absent in sleeping
best obtained with distal s
e fiber
00% (always above 50%)
: 4ms (UL) 6 ms (LL)
ult to elicit in normal subject
or sedated patient
timulation
Chrono dispersion

75. F WAVE
The nerve is
stimulated supramaximally
distally with
the cathode placed proximally
to avoid the
theoretic possibility of anodal
block
Uses:
•Early GBS
•C8-T1, LS-S1 radiculopathy
•Polyneuropathy
•Internal control (entrapment
neuropathy)

76. 76
F-response

77. H - REFLEX
• The H reflex derives its
name from Hoffmann,
who first evoked the
response in 1918.
• It is a true reflex with a
sensory afferent, a
synapse, and a motor
efferent segment.
• stimulating the tibial
nerve in the popliteal
fossa, recording the
gastroc-soleus muscle.

78. H - REFLEX
• Start at very low stimulus
intensities.
• latency: 25 to 34 ms
• Enhancement:
– Jendrassik maneuver
– Plantaflexion (ankle)
• Do not stimulate faster than 2
seconds (avoid effect of
previous stimulation)
This location over
the soleus is
approximately two
to three
fingerbreadths
distal to where the
soleus meets the
two bellies of
the
Optimal location

79. H - REFLEX

80. H - REFLEX
•
•
•
• H reflex with the shortest latency is measured and
compared with a set of normal controls for height
Unilateral lesion: Comparison with the contralateral side
– significant if > 1.5 ms
H reflex should always be present if ankle reflex present
may still be present, If the ankle reflex is absent
prolonged H reflex
– in polyneuropathy,
– proximal tibial and sciatic neuropathy,
– lumbosacral plexopathy,
– and lesions of the S1 nerve root.

81. F Response vs H reflex

82. F Response vs H reflex

83. Artifacts and technical error
1. Temperature
•
•
•
• Cooler temperature prolong time of
depolarisation
Conduction velocity slows between 1.5-
2.5m/s, distal latency prolong by 0.2 ms
for every degree drop in temperature
Higher amplitude and longer duration
Temperature to be maintained between
32-34 degree

84. 2. Age
•
•
•
•
Conduction decrease with age
More prominent after 60 yrs
Correction factor of 0.5-4m/s for older
pts. can be used.
Sural nerve may not be ellicitable in
some

85. 3. Height
• Taller individual have slower conduction
velocity.
• Adjustment no more than 2-4m/s below
lower limit of normal
4.Proximal vs distal
• Proximal nerve segment conduct slightly
faster than distal.

86. Non physiologic factors
1. Electricalimpendance
• 60 HZ noise made by different electrical
devices.
• Identical noise at each electrode best
achieved by ensuring same electrical
impendance at both electrodes.

87. 2. Stimulus artifact
•
•
•
•
•
•
• Reduced by placement of
ground between recording and
stimulator
Decrease electrical
impendance
Coaxial electrodes
Stimulator directly over nerve
Lower stimulus
Rotate anode while
maintaining cathode
Stimulator and recording
cables do not overlap

88. 2. Cathode position
reversed
• Theoretical possibility
of anodal block
• Distal latency
prolonged by 0.3-
0.4ms
• Slowing of sensory
CV by 10m/s

89. 4. Co-stimulation of adjacent nerves
Can be reduced by
1. Stimulator directly over nerve
2. Watch for abrupt change in waveform
3. Change in resultant muscle twitch
4. Avoid excess current
5. Co-record muscles simultaneously frm
adjacent nerve

91. CARDINAL RULES OF NCS AND EMG
1.NCSs and EMG are an extension of the clinical
examination. NCSs and EMG cannot be
performed without a good clinical examination.
2.When in doubt, always think about technical
factors.
3.When in doubt, reexamine the patient.
4.EDX findings should be reported in the contextof
the clinical symptoms and the referring
diagnosis.

92. CARDINAL RULES OF NCS AND EMG
5.When in doubt, do not overcall a
diagnosis.
6.Always think about the clinical-
electrophysiologic correlation.