Nerves conduction study

Nerves conduction study
Basic neurophysiology
DR SACHIN ADUKIA

Elements of the peripheral nervous system.

Uses
 paraesthesias (numbness, tingling, burning)
 weakness of the arms and legs
 Differentiation and Localization: ?? Nerve – muscle- NMJ
 d/b local or diffuse disease process (mononeuropathy or
polyneuropathy)
 Classify peripheral nerve conduction abnormalities due to axonal
degeneration, demyelination, and conduction block
 Early diagnosis eg. AIDP
 Extent of severity
 Get prognostic information on clinical course and treatment response

Disorders of the Peripheral Nervous System

Classification of nerve fibres

Limitations
 conduction velocity and latency measurements are from largest and fastest
fibers.
 Large-diameter fibers have the most myelin and the least electrical resistance
faster conduction velocities.
 NCS study only largest A-alpha fibers
 poses a problem where strength , & vibration and position senses are
unaffected
 but pain and temperature sensations are abnormal: "small fiber neuropathy”
 Thus, neuropathies affecting small fibers may not reveal any abnormalities
on NCSs

The procedure
Electrodes
• Skin will be cleaned
• electrodes will be attached to the skin along the
nerves being studied
Stimulus
• Small stimulus is applied (electric current) that
activates the nerves
• ?? Discomforting but not painful
Current
• Electrodes measure the current that travels along the
nerve pathway

Procedure for motor study
 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 recording electrode
 Stimulator / recording electrode is placed over the nerve that
supplies the muscle, cathode closest to the recording electrode.
 Current needed (our lab)
1. 15-35 mA for motor NCS
2. < 20 mA for sensory NCS
 Supramaximal stimulation is given in motor studies

Components of NCS
 Compound Motor Action Potential (CMAP)
 Sensory Nerve Action Potential (SNAP)
 F-wave study
 H-reflex study

CMAP
Compound action potential- motor or sensory

Motor conduction study
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
 Ground electrode- In between stimulating
and recording electrode

Technique
 As current is slowly increased by 5- 10 mA more nerve fibers are brought
to action potential and subsequently more muscle fiber action potentials
 Threshold stimulus
 Maximal stimulus
 When CMAP no longer increases in size, can presume that all nerve fibers
excited  supramaximal stimulation
 At our lab: 30% above maximal stimulation

Compound muscle action potential (CMAP)
 CMAP: summation of all
underlying individual muscle
fiber APs
 Biphasic potential with an
initial negativity, or upward
deflection from the baseline.
Comprises of :
•Latency
•Amplitude
•Duration
•Area
Stimulation

Motor Latency
•Represents the largest
conducting fibres
•nerve conduction time from
stimulus site to NMJ
•time delay across NMJ
•depolarization time across
muscle

 Amplitude of M wave
Amplitude:
commonly measured from baseline to
the negative peak
Represents no. of fibres that depolarize
Causes of reduced amplitude
(1) Axonal neuropathy
(2) Demyelation with conduction block
(3) Presynaptic NMJ disorder
(4) Advanced myopathy

 CMAP area: measured
between the baseline and the
negative peak
 Is determined by no. of fibres
that depolarise
 Reduced in Conduction block
from demyelination
Distal area
Proximal area

CMAP duration:
 measure of synchrony (extent to
which individual muscle fibers fire
simultaenously).
 measured from the initial
deflection from baseline to the first
baseline crossing (i.e., negative
peak duration)
 also measured from initial to
terminal deflection back to
baseline.
 increased in demyelinating disease.

Conduction velocity
 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)
 Reduced in demyelination due to
increased electrical resistance from
damaged myelin

 Motor nerve conduction:
 Latency on stimulation of peripheral n.- T1
 same nerve stimulated at a more distal point, this latency is (T2)
 Distance between 2 points of stimulation is measured in cm.
T
1
T
2

References values at our lab
Motor ms mA m/s
Latency Amplitude Velocity
Median <4.2 >5 >48
Ulnar <4 >5 >48
Deep peroneal <6 >2 >42
Posterior tibial <6 >5 >42
Radial <2.9 >2 >49
Musculocutaneous <5.7 Compare
Axillary <4.9 Compare
Suprascapular <3.7 Compare
Femoral >3
Sensory uV
Median <3.4 >15 >48
Ulnar <3.4 >15 >48
LABC <3 >15 >50
MABC <3.2 >10 >50
Superfical peroneal <3.4 >5 >42
Sural <3.4 >5 >42
Sapheneous <3.9 >2 >40

Sensory nerve conduction
 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
S = Stimulus point,
T = Takeoff point,
P = Peak
Time (latency) is from S to T
measured in milliseconds.
Amplitude = microvolts (μV).

Sensory
nerve action
potential
(SNAP)
 SNAP: summation of all individual sensory nerve fiber action potentials
 Onset Latency:
 is the time from the stimulus to the first deflection from baseline
 represents nerve conduction time 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
 Used when onset latency is unclear
 Increases in demyelination

Onset latency vs Peak latency
Peak
latency
Onset latency ???

 Duration:
 measured from onset of potential to the first baseline 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)
 Amplitude:
 measured from baseline to negative peak
 Low SNAP amplitudes indicates axonal loss

Orthodromic method
 Stimulating electrode 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.

Antidromic method
 Stimulating electrode over
proximal point on n. trunk.
 Recording electrode at distal
sensory branches of n.
 Nerve will conduct impulse
antidromically from proximal
to distal

References values at our lab
Motor ms mA m/s
Latency Amplitude Velocity
Median <4.2 >5 >48
Ulnar <4 >5 >48
Deep peroneal <6 >2 >42
Posterior tibial <6 >5 >42
Radial <2.9 >2 >49
Musculocutaneous <5.7 Compare
Axillary <4.9 Compare
Suprascapular <3.7 Compare
Femoral >3
Sensory uV
Median <3.4 >15 >48
Ulnar <3.4 >15 >48
Superfical peroneal <3.4 >5 >42
Sural <3.4 >5 >42
LABC <3 >15 >50
MABC <3.2 >10 >50
Superficial radial <2.9 >15 >48
Sapheneous <3.9 >2 >40

Limitation of Sensory conduction
 Lesions Proximal To DRG Normal Sensory Potentials.
 Eg. Sensory Roots, Spinal Cord or Brain since Cell bodies are in DRG
 If pt. has Sensory Symptoms or Sensory LOSS with Normal Sensory Study
consider a Proximal Lesion
 In case of Proximal MOTOR Root lesion or AHC Lesion 
 Degeneration of Motor Fibers throughout the Nerve – thus NCS is
abnormal

DISORDER LATENCY VELOCITY AMPLITUDE DURATION
AXONAL NORMAL
(<130%)
NORMAL
(>75%)
DECREASED NORMAL
DEMYELINATING PROLONGED
(>130%)
DECREASED
(<75%)
<35 in UL
<30 in LL
NORMAL
OR
Reduced if
Conduction
Block
PROLONGED
(Temporal
Dispersion)
NMJ
DISORDER
NORMAL NORMAL NORMAL
(REDUCED in
Presynaptic)
NORMAL
MYOPATHIC NORMAL NORMAL NORMAL
(REDUCED in
Severe Distal)
NORMAL

F-response
 First described by Magaladery and McDougal.
 derives its name from Foot because it was first recorded from the intrinsic foot
muscles.
 This NCS evokes a small late response from a short duration supramaximal
stimulation.
 It initiates an antidromic motor response to the spinal cord followed by an
orthodromic motor response to the recording electrode.

F-wave study
 It is approximately 5% of CMAP amplitude
 The configuration and latency change with each stimulation.
 This is due to a polysynaptic response in the spinal cord, where
Renshaw cells (R) inhibit impulses from traveling the same path each
time.

Supramaximal distal stimulation with cathode placed
proximally to avoid the theoretic possibility of anodal block

 This is not a reflex, as only motor pathway is involved
 F- waves latency can be used to derive CV of nerves between limb and spinal cord
 Uses:
• Early GBS
• C8-T1, LS-S1 radiculopathy
• Polyneuropathy
 Limitation:
 only accesses motor fibers, not useful in sensory radiculopathies
 Mild Proximal Lesions: F-wave Normal
 Peroneal nerve can be difficult to elicit in normal subject
 Maybe absent in sleeping or sedated patient
 best obtained with distal stimulation,
 conduction across long segments will dilute its utility.

H - REFLEX
 named after Hoffmann, who first evoked the response in 1918
 true reflex with a sensory afferent, synapse, and motor efferent segment
 stimulating the tibial nerve in popliteal fossa, recording the gastroc-soleus muscle
 prolonged H reflex
 in polyneuropathy,
 proximal tibial and sciatic neuropathy,
 lumbosacral plexopathy,
 lesions of S1 nerve root

H- reflex
 This NCS creates a late response that is an electrically evoked analogue to a
monosynaptic reflex.
 It is initiated with a submaximal stimulus at a long duration (0.5–1.0 milliseconds).
 This preferentially activates the IA afferent nerve fibers, causing an orthodromic
sensory response to the spinal cord, and then an orthodromic motor response
back to the recording electrode.
 The morphology of wave pattern and latency remains constant with each stimulation at
the appropriate intensity.

Protocols at our lab- upper limb
Median Ulnar Radial Axillary Musculo
cutaneous
Supra-
spinatus
MABC LABC
Brachial
Plexus
M + S M + S M + S M M M S S
F wave F wave
CTS M + S M + S Plus comaprison studies at 3 sites for Median and ulnar
muscles
F wave F wave

Protocols at our lab- lower limb
Tibial Peroneal Femoral Sup.
Peroneal
Sural Sapheneous LCN
LS
Plexopathy
M M M S S S S
Meralgia M M - S S - S
F wave F wave

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