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Background electrical noise
a. Larger in sensory potentials because of low amplitude of signal (low signal-to-noise ratio)
b. Background noise from voluntary muscle activity needsto be minimized (high amplitude relative to signal)
Stimulus artifact is reduced by
- Preventing electrode or sweat “bridging”
- Minimal duration and intensity of stimulus (just enough to obtain supramaximal response)
- Minimizing distance between stimulator and nerve to allow supramaximal depolarization with minimal possible current, reducing overstimulation and muscle artifact
Important factors in stimulation
1) Axon size: larger axons are stimulated more easily
2) Myelin: myelinated axons are stimulated more easilythan unmyelinated axons
Compound muscle action potential (CMAP) conductionaffected by
nerve conduction time, neuromuscular junction transmission, and the muscle.
So in sensory exam Only nerve axons are assessed (as compared with motorconduction studies, which measure conduction alongmotor nerve, neuromuscular junction, and muscle)
Disadvantages of antidromic conduction study inhand:
muscle artifact and volume conductionfrom lumbricals (with ring electrodes)
Placement of electrodes and recording
1) Two recording electrodes (G1 and G2) are placed over the nerve, 3.5 to 4 cm apart
2) G1, the active recording electrode, is placed closer to stimulator
3) Ground electrode: relatively large and placed between stimulating and recording electrodes
- Conduction velocity Can be determined with one stimulation site (ascompared with the motor conduction velocity thatrequires two stimulation sites)
- Compound potential that is summation of individual nerve action potentials, may be biphasic or triphasic.
- Usually more sensitive to both generalized and focalnerve disease than motor nerve conduction studies (e.g., entrapment mononeuropathies such as carpal tunnel syndrome)
- SNAP abnormalities may be first abnormalities of a neuropathic process such as generalized sensorimotor neuropathy (earlier than motor studies) and may be selectively involved in sensory neuropathies.
- Proximal sensory studies result in smaller amplitudepotentials and are difficult to perform
- - Temporal dispersion and phase cancellation withproximal stimulation normally occur with SNAPs
Antidromic technique (compared with orthodromic technique)
- Amplitude of the responses is higher inantidromically conducted potentials because therecording electrodes (e.g., ring electrodes) are closer to underlying sensory nerves
- Useful when recording small potentials in neuropathic conditions
- Because a mixed motor-sensory nerve is stimulated, SNAP is usually followed by, and may be mistaken for, a volume-conducted motor response
Reason for tempral dispersion and phase canellation in proximal SNAP
- Lag time between faster and slower conductingfibers is exaggerated with proximal stimulation,leading to increased duration and decreased amplitude.
- Phase cancellation occurs from overlap of negative phase of asingle-fiber sensory action potential and positivephase of another sensory action potential.
Note: Onset latency is not affected by temporal dispersion and phase cancellation and can be used to measure conduction velocity.
Why SNAPs potentially may be abnormal in lower lumbar or upper sacral radiculopathies
Because the DRG of lower lumbar and upper sacral segments may be inside the spinal canal and axonal injury related to a compressive radiculopathy in these segments may be at or distal to the DRG, SNAPs potentially may be abnormal in lower lumbar or upper sacral radiculopathies
- Lateral antebrachial cutaneous sensory --> upper trunk or lateral cord distribution
- Superficial radial sensory --> middle trunk or posterior cord distribution
- Medial antebrachial cutaneous --> lower trunk or medial cord distribution
CMAP in myopathy
CMAP may be normal in myopathy until disease isadvanced and there is loss of muscle tissue
Temporal dispersion and phase cancellation in CMAP study is more common in .......
acquired demyelination with orwithout conduction block (less common ininherited demyelination)
Axonal loss (motor and sensory conduction studies)
a. Reduced amplitude (but reduced amplitudes do not necessarily imply axonal loss)
b. Conduction velocity and distal latency may be normal, assuming largest and fastest conducting axons are intact (Large reduction of amplitude and some slowing of conduction velocity or mild prolongation of distal latencies may indicate axonal loss, with relative loss of large fast-conducting fibers and relative preservation of slowlyconducting fibers) --> Conduction velocities may be decreased, but neverless than 70% of lower limit of normal and Distal latencies expected to be normal or slightly prolonged,but no more than 130% of upper limit of normal.
C. With hyperacute lesions, nerve conduction studies performedwithin first 4 to 6 days may be normal
Demyelination (motor and sensory conduction studies)
a. Slowing of conduction velocities: slower than 70% of lower limit of normal if amplitudes are preserved, 50% if amplitudes are decreased
b. Prolongation of distal latency, Longer than 130% of upper limit of normal
c. Focal slowing --> More than 10 m/s slowing over 10-cm segment and More than 0.4-millisecond change in latency over1-cm segment
d. Temporal dispersion and phase cancellation
f. complete block --> loss of CMAP
Inherited demyelinating polyneuropathies
Usually all myelin is affected equally and demyelination is symmetric --> Slowing is uniform --> Dispersion and block are uncommon
Normal late responses (F waves and H waves)
- F waves:
- Named “F” because originally recorded from the foot
- Both afferent and efferent arms are motor; no synapse involved
- Not a true reflex
- May be spared in conditions selectively involving sensorypathways or a relatively small number of motor axons
- More commonly abnormal in demyelinating than axonal lesions
Evoked F-wave latencies
F-wave estimate = (2 × distance/conduction velocity)+ distal latency --> Should be within 3-5 milliseconds
- If shorter than the estimated range of F-wave estimates: distal conduction is slower than proximal conduction (e.g., peripheral neuropathy)
- If longer than the estimated range of F-wave estimates: proximal conduction is slower than distal conduction (e.g., radiculopathy, polyradiculopathy)
F wave clinical application
- Useful in identifying demyelination that selectively affects proximal segments (e.g., inflammatory demyelinating polyradiculo-neuropathy or inflammatory plexopathy)
- Limited practical usefulness in processes predominantly involving axonal loss (e.g., motor neuron disease or radiculopathies)
- Obtained by stimulating tibial nerve in the poplitealfossa and recording the gastrocnemius-soleus muscle
- True monosynaptic reflex with Ia muscle spindle (sensory) afferents and alpha motor neuron efferents
- May be abnormal with S1 radiculopathy or plexopathy
- Not a true reflex
- Produced by impulse traveling antidromically (like F waves) to a branch point in the nerve and then orthodromically along the second branch (unlike F waves )toward the recording electrode to create an A-wave response
- May be seen in peripheral neuropathy, polyradiculopathy, or plexopathy
- Usually do not occur in normal subjects
- Initial antidromic impulse: latency decreases when stimulation site is moved proximally (as with F waves)
Posttetanic potentiation or facilitation vs. Decrement
In summary: potentiation oraugmentation of the amount of acetylcholine released after brief repetitive exercise or rapid rate of stimulation (>10 Hz)
Decrement: reduction in amount of acetylcholine released with slow rates of repetitive stimulation (2-3Hz), caused by depletion of acetylcholine stores in activezones of nerve terminal
note : Normally, safety margin of neuromuscular transmissionis large, and no decrement of CMAP occursdespite the decrease in EPP amplitude (see the pic)
Lambert-Eaton myasthenic syndrome pathophysiology
AB to voltage-gated calcium channels reduce acetylcholine release --> Reduced acetylcholine release causes reduced postsynaptic EPP --> Result is low-amplitude CMAPs at rest and marked facilitation of amplitudes after brief exerciseor.
a) Safety margin of EPP is reduced by acetylcholine receptor deficiency
b) Repetitive stimulation at slow rates may cause lower subsequent EPPs that may not reach the threshold, causing transmission failure across neuromuscular junction and CMAP decrement
c) CMAP decrement is greatest between first and second stimuli in a train of four stimuli
d) Immediately after exercise, there is postactivation facilitation of CMAP amplitude and decrease inthe decremental response
e) There is a greater decremental response 4 minutesafter exercise
Repetitive nerve stimulation: Normal vs. MG
Note: Brief period of exercise has same effect as rapid stimulation at 20 to 50 Hz, and is more tolerable.
a. In normal subjects --> no decrement should occur with 2-Hz stimulation and Immediately after exercise, normal subjects may show small increments in responses caused by synchronizedfiring of motor units.
b. In MG --> Decrement with repetitive stimulation at 2 Hz --> Greatest relative change between the first and second response in the train of four stimuli.
and Repair of decrement with exercise or rapid stimulation at 20 to 50 Hz
Larger decrement noted 1 to 3 minutes after exerciseg.
Repetitive nerve stimulation: Lambert-Eaton myasthenic syndrome
1) Low-amplitude resting CMAP
2) Decrement with repetitive stimulation at 2 Hz,usually less prominent than in myasthenia gravis
3) Marked increment (more than 100%) or facilitation (more than 200%) with rapid rates of stimulation orbrief exercise
Single fiber electromyography
a. Selective technique of recording potentials from single muscle fibers within a motor unit
b. Allows measurements of jitter and blocking, which reflect efficiency of neuromuscular transmission
Highly sensitive: normal SFEMG finding in clinically weak muscle confirms stability of the neuromuscular junction and rules out a clinically important disorder of neuromuscular transmission (myasthenia gravis, Lambert-Eaton myasthenic syndrome, botulism, congenital myasthenic syndromes)
Single fiber electromyography: jitter vs. blocking
Jitter --> Disease of neuromuscular junction can causevariability in rise time of the EPP and, thus, timeinterval between two single fiber potentials in samemotor unit
Very sensitive measure of neuromuscular junction disease: findings are abnormal even in very mild disease, Findings may be abnormal when repetitive stimulation studies are normal
Blocking --> EPP may fail to reach threshold and may not causean action potential, Occurs in more moderate to severe disorders of neuromuscular junction transmission
Motor unit potential (MUP)
aggregate potentialrecorded from a group of muscle fibers innervated by asingle lower motor neuron (anterior horn cell)
Cross-sectional diameter of the area containing musclefibers of a single motor neuron is normally 5 to 10 mm
MUP Characteristics - Duration
- Short-duration MUPs:
- - Myopathies: loss of muscle fibers or ability of muscle fibers to generate action potentials
- - Disorders of nerve terminals: failure of neuromuscular transmission (e.g., myasthenia gravis)
- - Severe axonal loss with limited or very early reinnervation of target muscles
- Long-duration MUPs: may be seen in conditions that increase spatial and temporal distribution of muscle fiber action potentials in a motor unit
- - Axonal or motor neuron loss, with collateral or robust proximal-to-distal reinnervation
- - Chronic myopathies (e.g., inclusion body myositis)
Increased with collateral reinnervation, and decreased temperature
in a myopathic disease or reinnervation after axonal loss
change in configuration and characteristics ofMUPs during the same recording -->
Usually an indicator of abnormal neuromuscular junction
Recruitment frequency: rate of firing of the first MUP atthe time a second MUP begins to fire (normal, 9-11 Hz)
“Rule of Five”: reduced recruitment if first MUP firesfaster than 15 Hz before the second MUP appears or 20Hz before third MUP appears
Reduced recruitment --> Axonal loss, NM junction dis, severe destruction of muscle fibers
Rapid recruitment: recruitment of many MUPs (near interference pattern, with minimal muscle contraction, always associated with reduction in size of MUPs) --> Usually seen in myopathies due to loss of individual muscle fiber
Poor activation signifies poor voluntary effort or a central lesion
- Increased insertional activity
- a. Normal variants
- b. Neuropathic conditions
- c. Myopathic conditions
Reduced insertional activity: seen in long-standing neuropathic or myopathic conditions in which muscle is replaced by connective tissue
Seen in denervation as well as myopathic or severe NM junction blocking.
Regular firing rate (0.5-20.0 Hz), which may graduallyslow down (rarely irregular). Smaller amplitudes may occur with advanced muscle atrophy.
- A-D, Fibrillation potentials of different amplitudes and severity. Smaller amplitudes may occur with advanced muscleatrophy. Note regular firing of each waveform. Note also fasciculation potential at beginning of recording in C. This was recordedfrom anterior tibialis muscle in patient with amyotrophic lateral sclerosis.
Are fibrillation potentials and positive sharp waves the same?
No! Although in most cases these two potentials have the same clinical significance, there are at least five different situations in which they do not have an identical meaning:
(1) positive sharp waves can be recorded earlier after a peripheral nerve injury than can fibrillation potentials;
(2) occasionally, nonclinically significant diffuse positive sharp wave activity may be seen in the absence of fibrillation activity (i.e., "EMG disease");
(3) positive sharp waves may be seen in distal muscles of "normal" subjects without the presence of fibrillation activity or clinical significance;
(4) positive sharp waves without fibrillation potentials may be seen following local muscle trauma; and
(5) positive sharp waves may be seen alone in some demyelinating polyneuropathies.
long duration and biphasic with briefinitial positivity followed by long negativity
Causes: same as fib and sometimes in normal subject
Complex repetitive discharges
Ephaptic transmission of action potentials to and between adjacent muscle fibers
Nonspecific: may be seen in myopathic or neurogenic disorders or otherwise normal proximal muscles inelderly subjects
- Complex repetitive discharge recurring at 30-35 persecond.
Doublets and multiplets
a. Spontaneous depolarization of two to five “time-locked”MUPs
b. Same clinical significance as fasciculation potential
Spontaneous bursts (“marching soldiers”) of 2 to 10 grouped potentials firing at 40 to 80 Hz in semi-regular rhythm
Essentially grouped fasciculations, but clinical significance different from fasciculation potentials
Myokymic discharge recorded from patient withhistory of radiation-induced brachial plexopathy
Clinical significance of Myokimia
If recorded from facial muscles: multiple sclerosis, facialpalsy due to brainstem neoplasm or idiopathic,
If recorded from extremity muscles: radiation plexopathy, compression neuropathy (e.g., carpal tunnel syndrome), diffuse demyelinating disorders such as CIDP or disorder of peripheral nerve hyperexcitability (e.g.,cramp-fasciculation syndrome, Isaacs’ syndrome)
fasciculations (a), myokymic discharges (b), doublets (c), and multiplets (d)
Abnormal spontaneous activity.
(A) Fibrillations (*) and positive sharp waves (**) in an acutely denervated hand muscle.
(B) Single, doublet, triplet, and multiplet motor unit neuromyotonic discharges.
(C) Fasciculations in the tongue in a patient with ALS. The single discharges are irregular and occur on a background of ongoing EMG activity caused by poor relaxation.
(D) Myotonic discharges in a patient with dystrophia myotonica. There is a characteristic waxing and waning in frequency.
Recruitment patterns during maximal voluntary contraction of the deltoid muscle in (A) a healthy subject, (B) a patient with spinal muscular atrophy, and (C) a patient with polymyositis.
Note the different amplitude calibrations
Fib vs. Fascic
Fasciculations arise from the discharge of part or the whole of a single motor unit.
They are larger and more complex than fibrillation potentials.
Spontaneous, prolonged discharge of a group of muscle fibers induced by activation of sodium channels
on muscle fiber membrane
triggers: mechanical irritation of muscle fiber,changes in body temperature, voluntary activation,ischemia, changes in acid-base balance of microenvironment.
- conditions with clinical myotonia (e.g., myotonic dystrophy)
- acquired disorders without clinical myotonia (e.g., inflammatory muscle disease or acid maltase deficiency)
- May be seen in:
- - inherited channelopathies (myopathic conditions associated with clinical myotonia, e.g., paramyotonia congenita or periodic paralysis).
- Myotonic discharge recorded from patient with myotonic dystrophy. In addition, there were slow, tall fibrillation potentials (arrows) in the recording, two of which are shown here. The myotonic discharge ends with the second fibrillation potential represented in this segment.
High-frequency (100-300 Hz) regular repetitive discharges of a single MUP characteristically wanes in amplitude and frequency
b. Originates from terminal portion of motor axon
c. Observed in disorders of peripheral nerve hyperexcitability, usually secondary to altered conductance of voltage-gated potassium channels (e.g., Isaacs’ syndrome,cramp-fasciculation syndrome, some neuropathies)
Axonal Loss or Injury
- Hyperacute (<4-6 days old)
- a. Before wallerian degeneration occurs
- b. Stimulation distal to the lesion: normal conductions
- c. Stimulation proximal to the lesion: reduced or absent conductions (cannot differentiate between axonal loss and conduction block at this stage)
- d. Reduced recruitment (will persist)
- e. Normal MUP morphology and spontaneous activity
- Acute (>4-6 days but <10 days)
- a. Nerve conduction studies: reduced amplitudes (proximal and distal to lesion) with relatively preserved conduction velocities and distal latencies
- b. Reduced recruitment but no fibrillation potentials (inadequate time)
- Early subacute (10-15 days)
- a. Increased spontaneous activity (increased insertional activity and possibly some fibrillation potentials)
- b. Reduced recruitment with normal MUP morphology
- Late subacute (16-60 days)
- a. fibrillation
- b. Reduced MUP recruitment
- c. By 4 weeks, MUPs will likely have increased duration and be polyphasic.
- e. Reinnervation eventually produces long-duration, high amplitude neurogenic potentials, which may become less polyphasic over time in absence of ongoing denervation
- f. By this stage, amplitudes of CMAPs (if preganglionic or postganglionic localization of lesion) and SNAPs (if postganglionic) may be reduced, with relative preservation of conduction velocities and distal latencies
- Chronic axonal loss (after 180 days): in absence of ongoing denervation
- a. Nerve conduction studies are expected to be normal or show reduced amplitude, depending on severity of axonal injury and success of reinnervation (conduction velocities may be mildly reduced because re innervated axons have smaller diameter and less myelin)
- b. “Nascent units” may be present with proximal-to-distal reinnervation
- c. Both collateral and proximal-to-distal reinnervation eventually result in long-duration, high-amplitude, stable MUPs with reduced recruitment, which may or may not be polyphasic.
- d. Insertional activity is often normal, in absence of ongoing denervation
True axonal regeneration leads to the formation of nascent potentials, which are usually low in amplitude, polyphasic in configuration, and can have a short or normal and sometimes even long duration.
Terminal collateral sprouting always leads to the formation of long-duration polyphasics. Although both are polyphasic, their configuration is distinctly different and it is important to remember that only the nascent potentials represent true axonal regeneration.
- “Nascent units.” Early nascent units have shortduration and low amplitude, but with ongoing reinnervationthey become more polyphasic and duration increases, as seenhere. Note the severely reduced recruitment that distinguishesthese potentials from myopathic motor unit potentials. Also,there is variability of motor unit potentials due to neuromuscularjunction instability.
- Focal proximal demyelination
- a. Normal distal sensory and motor conduction studies
- b. Stimulation across site of demyelination (if possible) demonstrates focal slowing or conduction block
- c. Abnormal late responses
- d. Normal MUP morphology (in pure demyelination)
- e. Reduced recruitment if conduction block; normal recruitment with focal proximal slowing
- f. If less than 4 days old, difficult to distinguish from hyperacute axonal loss.
- Transient loss of axonal conduction due to altered nodal function caused by
- a. Ischemia
- b. Metabolic dysfunction or
- c. Alteration of nodal architecture (demyelination, mechanical distortion of node, alteration of axonal membranein nodal region)
2. Conduction returns with repair of nodal function
- a. Upper trunk
- - Abnormal SNAPs: lateral antebrachial cutaneous (± median and radial sensory responses)
- -Neurogenic MUPs: Supraspinatus and infraspinatus (direct branches of upper trunk through Suprascapular) AND musculocutaneous-innervated muscles and C5, 6 innervated muscles,including: Deltoid, Biceps , Brachioradialis, Pronator teres,
- - Sparing of rhomboid muscles (C5 root/dorsal scapular nerve)
- - Sparing of muscles innervated primarily by C7 root
- b. Middle trunk
- - Abnormal SNAPs: median and radial nerves
- - Neurogenic MUPs: C7-innervated muscles (pronator teres, triceps brachii, etc.)
- c. Lower trunk
- Abnormal SNAPs: ulnar nerve
- - Neurogenic MUPs: ulnar-innervated muscles, median and radial-innervated muscles (C8/T1 distribution)
- d. Lateral cord:
- - Abnormal SNAPs: lateral antebrachial cutaneous an dmedian nerves
- - Neurogenic MUPs: musculocutaneous-innervated muscles and C5-7/median-innervated muscles (if radial-innervated muscles affected, think upper trunk)
- e. Posterior cord:
- - Abnormal SNAPs: radial sensory, with normal median and ulnar
- - Neurogenic MUPs: radial and axillary innervated muscles, as well as latissimus dorsi and teres major, directly innervated by the posterior cord
- f. Medial cord:
- - Abnormal SNAPs: ulnar and/or medial antebrachial cutaneous nerve (ulnar nerve may be spared)
- - Neurogenic MUPs: ulnar-innervated muscles
Motor Neuron Disease
- Nerve conduction studies:
- - NL SNAPs
- - NL motor responce or Reduced CMAP amplitude with relative preservation of distal latency and conduction velocity
- - Some slowing may be expected if larger, faster axons are affected
- - Presence of large unstable MUPs with reduced recruitment (reinnervation) and poor activation if upper motor neuron involved
- - Fibs, Fascic
- SNAPs are normal (except if DRG is inside spinal canal and axonal injury is distal to DRG, as at L5-S1 root level)
- CMAP amplitudes are normal with mild axonal loss
- With severe axonal loss, CMAP amplitudes are reduced— if fastest axons are affected, conduction velocities may be slowed and distal latencies prolonged
- H reflex may be absent with S1 radiculopathy (may also be absent in plexopathy or sciatic neuropathy)
- 1. Nerve conduction studies may be normal or CMAP amplitudes may be reduced when advanced
- 2. Normal or abnormal spontaneous activity
- a. Fibrillation potentials due to muscle fiber necrosis, splitting,or vacuolation, may also be positive sharp waves
- b. Complex repetitive discharges common with chronic myopathies, but non-specific
- c. Myotonic discharges are more specific, may be seen in certain myopathies
3. Insertional activity: normal, increased, or decreased, depending on the myopathy, severity, and stage of progression
- 4. Rapid (early) recruitment: fewer muscle fibers permotor unit, thus activation of more motor units to generate the same force
- 5. Classic myopathic morphology
- a. Short-duration, low-amplitude (sometimes polyphasic) MUPs
- b. Chronic severe or end-stage myopathies may appear “neurogenic” (i.e., long duration, large amplitude, polyphasic), probably because of excessive motor unit remodeling that occurs with advanced disease of the muscle fibers.
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