Nerves conduction study

Nerves conduction studyPart 1 : Techniques of recording

For post basic neurophysiology course

Dr Ahmad Shahir MawardiNeurology DepartmertHospital Kuala Lumpur

19 October 2015

Outlines

• Overview• Sensory nerves conduction

study• Motor nerves conduction study• Repetitive nerves conduction

study• Late response (F wave & H

reflexes)

Overview

Elements of the peripheral nervous system.

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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

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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.

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.

Classification of nerves

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Common disorders diagnosed by NCSPeripheral 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 more nerves are affected with emphasis on the nerve root; Radix = "root")• Nerve damage from herniated discs

Diseases of neuromuscular junction• Myasthenia gravis

Disorders of the Peripheral Nervous System

Hardware and Software

Sweep, Sensitivity, Current Controller

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Description of the procedure

Electrodes• Skin will be cleaned• electrodes will be taped to the skin along the nerves

that are being studied

Stimulus• Small stimulus is applied (electric current) that

activate nerves

Current• The electrodes will measure the current that travels

down the nerve pathway

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 needed1. 20-50 mA for motor NCS2. 5-30 mA for sensory NCS

• Supramaximal stimulation is given.

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

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

Press to

stimulat

latency

Latency:

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

Orthodromic(physiological)

pain

AP

Antidromic(non-physiological)

Orthodromic sensory study

Stimulating away from the sensory receptor

Antidromic sensory study

Stimulating toward thesensory receptor.

The antidromic method has the advantage of a higher-amplitude SNAP but is followed by a large volume-conducted motor potential.

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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…

Sensory nerves conduction study

Sensory nerve conduction (SNC):

• SCV measurements differ from MNC in that the action potential of the nerve itself rather than of a muscle serves as the observable end point.

• SNAP are of much smaller amplitude.

• SNCS are more sensitive than MNCS in detecting early and mild disorders.

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.

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 3

milliseconds.The amplitude would be measured in microvolts

(μV).

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

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).

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

Peaklatency

Onset latency ???


• 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).


• Amplitude: – commonly measured from baseline to negative peak. – Low SNAP amplitudes indicate a definite disorder of

peripheral nerve.

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

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.

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.

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Sensory nerve conduction study – sitesMedian 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 Medial Malleolus

Normal value

Press to

stimulate

Motor nerve conduction

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.

Motor Conduction Studies

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

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

Compound motor action potential (CMAP):

Pres

s to

sti

mu

lat

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.

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 CMAPStimulation

Motor conduction velocity can be calculated after 2 sites (proximal & distal) stimulated

Pres

s to

sti

mula

t

Motor Latency


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

Pres

s to

stim

ulat Amplitude of M wave


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


• 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


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.


• 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)

Motor nerve conduction:

• The directly evoked muscle action potential recorded after stimulation at T1 of 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.

Press to stimulat

T1

T2

DL (m/s) CV (msec) Amp(mV)Wrist-APB 3.2 15.0Elbow-Wrist 55 14.8

Velocity of conduction = Distance (cm) x 100(Meter/sec) T1-T2 (millisecond)

T1

T2

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Motor nerve conduction study – sitesMedian nerves (R & L) at;• Wrist Abductor Pollicis Brevis• Elbow

Ulnar nerves (R & L) at;• Wrist First Dorsal Interosseous (FDI) • Elbow Abductor Digiti Minimi (ADM)

Peroneal nerves (R & L) at;• Ankle Extensor Digitorum Brevis• Head of fibula Tibialis Anterior

Tibial nerves(R & L) at;• Ankle Abductor Hallucis

Abductor Digiti Quinti Pedis

Normal Values

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


• 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


• Exercise testing1.Brief maximal voluntary exercise for tested

nerves -10 seconds (painless)2.Rapid RNS- 50-Hz supramaximal nerve

stimulation (painlful)


A repetitive nerve stimulation study demonstrating a 61 percent decrement in area and a 54 percent decrement in amplitude from the first to the fourth stimulation.

Increment during rapid repetitive nervestimulation. Recording the hypothenar muscles, stimulating the ulnar nerve at 50 Hz in a patient with Lambert-Eaton syndrome.


Technical Factors in RNS

1. Immobilization: Isometric Electrode Position Is Essential2. Stimuli Must Be Supramaximal3. Temperature Must Be Controlled4. 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.6. Stimulation Frequency

– The optimal frequency for slow RNS is 2 or 3 Hz.7. Number of Stimulations

– 5 to 10 pulses is preferable for slow RNS.

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.

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)

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)

Late response (F wave & H reflexes)

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.

Late response

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

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

F-response • Represent 1-5% of muscle fiber• Latency UL: 25-32 ms• Latency LL: 45-65 ms

• Normal persistence: 80-100% (always above 50%)• Normal chronodispersion: 4ms (UL) 6 ms (LL)

• Peroel nerve can be difficult to elicit in normal subject• Maybe absent in sleeping or sedated patient• best obtained with distal stimulation

Chrono dispersion

F WAVE

Press to stimulat

The nerve isstimulated supramaximally distally withthe cathode placed proximally to avoid thetheoretic possibility of anodal block

Uses:•Early GBS•C8-T1, LS-S1 radiculopathy•Polyneuropathy•Internal control (entrapment neuropathy)

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F-response

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.

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 threefingerbreadths distal to where the soleus meets the two bellies ofthe gastrocnemius.

Optimal location

H - REFLEX

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.

F Response vs H reflex

F Response vs H reflex

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

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

3. Height • Taller individual have slower conduction

velocity.• Adjustment no more than 2-4m/s below

lower limit of normal4.Proximal vs distal• Proximal nerve segment conduct slightly

faster than distal.

Non physiologic factors

1. Electrical impendance• 60 HZ noise made by different electrical

devices.• Identical noise at each electrode best

achieved by ensuring same electrical impendance at both electrodes.

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

2. Cathode position reversed

• Theoretical possibility of anodal block

• Distal latency prolonged by 0.3-0.4ms

• Slowing of sensory CV by 10m/s

4. Co-stimulation of adjacent nervesCan be reduced by1. Stimulator directly over nerve2. Watch for abrupt change in waveform3. Change in resultant muscle twitch4. Avoid excess current5. Co-record muscles simultaneously frm

adjacent nerve

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.

CARDINAL RULES OF NCS AND EMG

5. When in doubt, do not overcall a diagnosis.

6. Always think about the clinical-electrophysiologic correlation.

Health & Medicine

Nerves conduction study