Fundamentals of nerve conduction

studyDr. Parag MoonSenior resident

Dept. of NeurologyGMC Kota

Peripheral nerves are stimulated with an controlled electrical stimulus

Responses recorded 1. Compound motor action potential (CMAP) 2. Sensory nerve action potential (SNAP) 3. F wave 4. H- reflex

Principle of NCS

Active recording electrode placed on the center of the muscle belly (over the motor endplate)

Reference electrode placed distally about 3-4 cm from active 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.

Procedure

CMAP- biphasic potential with an initial negativity (upward deflection).

M response For each stimulation site: latency, amplitude,

duration, and area of the CMAP are measured. A motor conduction velocity can be calculated

after two sites of stimulation, one distal and one proximal.

MOTOR CONDUCTION STUDIES

If an initial positive deflection exists, it may be due to:

1. Inappropriate placement of the active electrode from the motor point

2. Volume conduction from other muscles or nerves

3. Anomalous innervations

It is the time from the stimulus to the initial negative deflection from baseline

Made in milliseconds (ms). In CMAP latency represents three separate

processes: (1) the nerve conduction time . (2) the time delay across the NMJ (3) the depolarization time across the muscle.

Latency

Calculated by dividing the change in distance (between proximal stimulation site & distal stimulation site in mm) by the change in time (proximal latency in ms minus distal latency in ms)

Normal values are > 50 meters/sec in the upper limbs And > 40 meters/sec in the lower

limbs

Condution velocity

Most commonly measured from baseline to the negative peak (baseline-to-peak) and less commonly from the first negative peak to the next positive peak (peak-to-peak).

Reflects the number of muscle fibers that depolarize.

Low CMAP amplitudes most often result from loss of axons (as in a typical axonal neuropathy), conduction block.

Amplitude

Measured from the initial deflection from baseline to the final return

Also measured from the initial deflection from baseline to the first baseline crossing

2nd is preferred as the terminal CMAP returns to baseline very slowly and can be difficult to mark precisely.

Duration

This is a function of both the amplitude and duration of the waveform.

CMAP area is measured between the baseline and the negative peak.

Differences in CMAP area between distal and proximal stimulation sites for determination of conduction block from a demyelinating lesion(>50%)

Area

CMAP

A pair of recording electrodes (GI and G2) are placed in line over the nerve at an interelectrode distance of 3 to 4 cm, with the active electrode (G I) placed closest to the stimulator.

Recording ring electrodes are conventionally used to test the sensory nerves in the fingers

SENSORY CONDUCTION STUDIES

Onset latency is the time required for an electrical stimulus to initiate an evoked potential.

Onset latencies reflect conduction along the fastest nerve fibers

Peak latency in SNAP : it represents the latency along the majority of the axons and is measured at the peak of the waveform amplitude (first negative peak).

Both latencies are primarily dependent on the myelination of a nerve.

Latency

Peak latency can be ascertained in a straightforward manner.

Some potentials, especially small ones, it may be difficult to determine the precise point of deflection from baseline

Peak latency cannot be used to calculate a conduction velocity

SNAP amplitude -sum of all the individual sensory fibers that depolarize.

Low SNAP amplitudes indicate a definite disorder of peripheral nerve.

Conduction velocity-Only one stimulation site is required to calculate a sensory conduction velocity.

SNAP CMAP

Lesions proximal to it (injuries to the sensory nerve root or to the spinal cord) preserve the SNAP waveform despite clinical sensory abnormalities

This is because axonal transport from the DRG to the peripheral axon continues to remain intact.

Antidromic studies are performed by recording potentials directed toward the sensory receptors

Orthodromic studies are obtained by recording potentials directed away from these receptors.

Antidromic studies are easier to record a response than orthodromic studies.

May be more comfortable than orthodromic studies due to less stimulation required. May have larger amplitudes due to the nerve

being more superficial at the distal recording sites.

More chances of volume conducted motor potential.

Antidromic and Orthodromic Recording

1.Antidromic stim.2.orthdromic stim of same n

(SNAPs) and(CMAPs) both are compound potentials

They represent summation of individual sensory and muscle fiber action potentials, respectively.

With distal stimulation, fast and slow fiber potentials arrive at the recording site at approximately the same time

With proximal stimulation, the slower fibers lag behind the faster fibers.

Temporal dispersion and phase cancellation

Temporal dispersion & phase cancellation is more prominent withSNAP than CMAP for 2 reasons:– The CMAP duration is much longer than the SNAP– The range of fiber conduction velocity is less spread in motor thansensory fibers (12 m/sec vs. 25 m/sec).

SNAP

CMAP

For this reason a drop of 50% is considered normal when recording a proximal SNAP.

Drop of 15% is considered normal when recording a proximal CMAP

Conduction block DEFINITE > 50% drop in CMAP

amplitude with <15% prolongation of CMAP duration, or

> 50% drop in CMAP amplitude and area, or

> 20% drop in CMAP amplitude and area over a short nerve segment (10 cm)

PROBABLE 20‐50% drop in CMAP

amplitude with < 15% prolongation of CMAP duration, or

20‐50% drop in CMAP amplitude and area

Conduction block Most common with acute

nerve lesions – Peroneal at fibular neck – Radial at spiral groove – Ulnar at elbow Is due to segmental

internodal demyelination Is the electrophysiological

correlate of neurapraxia (first degree nerve injury)

Differential (desynchronized) slowing Is due to conduction

slowing along a variable number of the medium or small nerve fibers (average or slower conducting axons)

Often it is associated with focal slowing

1. F reflex2. H reflex3. A reflex

Late responses

Stimulation is followed by depolarization which travels in both directions: first directly to the muscle fiber producing the M response, and retrograde up to the motor axon and to anterior horn, where it is re propogated back through the axons to produce the delayed F response.

Small late motor response occurring after the CMAP.

Late response Approximately 1–5% of the CMAP amplitude. Supramaximal stimulation Pure motor response Not represent a true reflex Usually polyphasic& varies with each stimulation

F wave

Amplitude 1%-5% CMAP Measurements: Minimal, maximal latency

Chronodispersion and Persistence Minimal latency= less than 32 in UL and <56 in

LL Chronodispersion: it’s the time delay bet.

Minimal& maximal latencies (<4ms in UL and <6ms in LL)

Persistence >50% F estimate=2D/CVx10+1ms+DL

F wave

Normally peroneal F waves may be absent or nonpersistent

F responses may be absent in sleeping or sedated patients

F responses may be absent with low-amplitude distal CMAPs

F wave

1. Early AIDP2. C8-T1, L5-S1 radiculopathy3. Polyneuropathy

Significance of F wave

H reflex

Submaximal stimulation of the afferent sensory fiber(1A) ->orthodromic conduction to the spinal cord->synaptic stimulation of the alpha motor neuron->evoked H response in the muscle.A rudimentary M response is produced when a few motor axons are directly stimulated

LatencyNormal: 28–30 millisecondsSide to side difference: greater than 0.5–1.0 ms is

significantAbove 60 years: adds 1.8 milliseconds H/M ratio <50% Location Soleus muscle: tibial nerve: S1 pathway Flexor carpi radialis: median nerve: C7 pathway Vastus medialis : femoral n : L4 pathway

H reflex

1. Early polyneuropathy2. S1 radiculopathy3. Early GBS4. Tibial and sciatic neuropathy, sacral

plexopathy5. Electrical correlate of ankle reflex

Significance of H reflex

Not a true reflex It is another late potential that often is

recognized during the recording of F responses. Typically occurs between the F response and the

direct motor (M) response An axon reflex is identified as a small motor

potential that is identical in latency and configuration with each successive stimulation.

A-(AXON) WAVE

Axon reflexes typically are seen in reinnervated nerves, especially when a submaximal stimulus is given

Function1. This waveform represents collateral sprouting

following nerve damage.2. Also shows that stimulus is submaximal.

Normal. Submax. Supramax.

Axonal neuropathy Amplitude decreased May manifest with

conduction block early (before Wallerian degeneration)

CV is normal or slightly slowed(<75%)

DL is normal or slightly prolonged(<130%)

Morphology does not change between proximal and distal sites.

Demyelination associated with inherited conditions (uniform)•CV is markedly slowed < 75% lower limit of normal)• DL is markedly prolonged (>130% upper limit of normal). •Usually no change in configuration between proximal and distal stimulation

Demyelination with conduction block usually acquired as GBS•Marked slowing of conduction velocity and distal latency

•Change in potential morphology (conduction block/temporal dispersion) between distal and proximal stimulation sites

Occurs in approximately 15‐20% Fibers cross from the median to the ulnar

nerve in the forearm. Communicating branch(es) usually consists

of motor axons that supply the ulnar‐innervated intrinsic hand muscles,

1. first dorsal interosseous muscle2. hypothenar muscles3. ulnar thenar muscles4. A combination of these muscles

Anomalies.Martin‐Gruber anastomosis

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

Artifacts and technical error

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.

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.

Non physiologic factors

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

Temperature effect and cold limb Sloppy measurement of distances Anatomic abnormalities of patient Technical factors: edema, large limbs, long limbs Too few nerve conduction studies, lack of

comparisons Too many nerve conduction studies:

Interpretation of non-existing abnormality

Pitfalls of nerve conduction studies

Thank u

Electromyography and neuromuscular disorders; D. Preston, B.Shapiro: 3rd edition: pg 19-90

Nerve condution studies: essentials and pitfalls: A Malik et al: J Neurol Neurosurg Psychaitry 2005:76:pg 23-31

AANEM guidelines for nerve conduction studies

References