Ch. 9 Electrodiagnostic medicine I: Fundamental Principles Flashcards
What is saltatory conduction?
Action potential jumps from one node of Ranvier to another
What kind of voltage-gated channels do myelinated nerves lack and contain?
Voltage-gated potassium channels
Only contain voltage-gated Na channels
What are the differences in nerve AP b/w men and women?
Women have inc antidromic sensory nerve amp in median and ulnar nerves
Women have great NCV for upper and lower limbs
How is SNAP affected by age?
CV declines 1-2m/s per decade
Duration 10-15% longer in 40-60 yo and 20% longer in 70-88 yo
Amp 1/2 in 40-60 yo and 1/3 in 70-88 yo
Newborn’s motor NCV are ___ that of adults
1/2
When do children’s NCV reach that of adults?
3-5 yo
What happens to NCV after 50 yo?
Fastest motor fibers decline by 1-2m/s per decade
Inc in distal motor latency
Dec in motor amp
How does height affect NCV?
Slower lower limb NCV in taller patients
What is the most influencing factors on NCV?
Temperature
How does temperature affect NCV?
As temp lowers the amount of current required to generate an AP inc and reduces NCV
How does temperature affect CMAP and SNAP?
Inc Amp, duration, rise time and area
With every __ drop in temperature there is a ___ decrease in conduction velocity
1 deg C temp dec 2.4 m/s dec in CV
What should the minimum temperature of limbs be when doing NCS?
32 deg C in upper
30 deg C in lower
What is an antidromic technique?
Induced neural impulse propagates along the nerve in a direction opposite to its physiologic direction
What is an orthodromic technique?
Impulses propagate in direction along the nerve in physiologic direction
What is a mixed nerve repsonse?
Component of both othrodromic and antidromic responses
What is the shape of a SNAP waveform?
Biphasic negative-positive potential
How can distance b/w active and recording electrodes change SNAP?
Distance <40 mm amp dec and peak latency shortens
Distance > 40 mm neg peak amp will not grow but terminal + phase will change configuration
What is normal insertional activity?
Inserting needle into muscle results in brief bursts of electrical potentials
What is decreased insertional activity?
Few or no electrical waveforms when needle inserted in fibrous or electrically inexcitable tissue
What is increased insertional activity?
Insertional activity persists after needle movememnet cessation
What is a miniature end-plate potential?
Waveform is short duration (0.5-2 ms), small (10-50uV), irregularly occuring (1/~5 sec per axon terminal) monophasic negative waveform
What do miniature end-plate potential represent?
Random release of Ach vesicles
What doe miniature end plate potentials (MEPPs) sound like?
Seashell murmur
What is an end-plate spike?
Short duration (3-4 ms) of moderate amp (100-200uV) irregularly firing and biphasic with initial negative deflection when needle placed near the end-plate region
How can one differentiate a PSW from an end-plate spike?
PSW and fibs have a regular firing rate and slowly trail off
End-plate spikes are irregular
What is a motor unit?
One anterior horn cell, its axon and the single musce fibers supplied by that nerve
What is a motor unit action potential (MUAP)?
Electrical activity from all muslce fibers summates together
What is the MUAP amplitude?
maximum peak to peak CRT trace displacement
What is the rise time of a MUAP?
Temporal aspect of a potential’s peak
What is the duration of a MUAP?
Depature from and return to baseline
What are phases of MUAP?
Number of baseline crossings plus one
What is peak-to-peak MUAP amp arised from?
<12 single muscle fibers located w/in 0.5 mm of needle electrode
What is the shape of a MUAP?
Triphasic: positive-negative-positive
What are polyphasic potentials?
MUAPs with 5 or more phases
What are satellite potentials?
Late waves linked to the rest of the waveform
What is collateral sprouting?
Denervated muscle fibers induce nearby terminal axons of intact nerves to send out neural projections to reinnervate orphaned muscle fibers
What do neurogenic diseased MUAPS look like?
Larger amplitude, longer-duration and highly polyphasic MUAPs
What do myopathic MUAP’s look like?
Shorter-duration, highly polyphasic, low-amplitude
Where is a muscle’s end-plate or motor point located?
Midway b/w muscles origin and insertion where active electrode is placed to record a CMAP
Where is the reference electrode placed to record a CMAP?
On or distal to the tendinous insertion of the muscle
What does a positive deflection preceding the negative phase of a CMAP mean?
The active electrode is off of the motor point
What are fibrillation potentials?
Regularly firing spontaneous depolarization of denervated a single muscle fiber
What do fibrillation potentials look like?
<5 ms duration
<1 mV amp
Fire b/w 1-15 Hz
What do fibrillation potentials sound like?
Rain on a tin roof
What are PSW?
Waveforms recorded from a single muscle fiber w/ unstable resting membrane potential secondary to denervation or intrinsic muscle disease
What does a PSW look like?
Large primary sharp deflection followed by a small neg potential
Duration of 100 ms or longer
What doe PSW sound like?
Regular firing rate (1-15Hz) and a dull thud sound
Where can transient runs of “PSW appearing potentials” be found?
Paraspinal, hand and foot intrinsic muscles
What is a complex repetitive discharge?
Spontaneously firing group of action potentials that stop abruptly
What do complex repetitive discharges look like?
Continuous run of simple or complex spike patterns that repeat at 0.3-150 Hz
What doe complex repetitive discharges sound like?
Heavy machinery or idling motorcycle
What cannot effect CRDs?
Curare or nerve blocks
What do CRD’s indicate?
Chronic process of a group of muscle fibers becoming separated from their NMJ’s
What is myotonia?
Delayed muscle relaxation after muscle contraction
What is percussion myotonia?
Delayed muscle relaxation after activation with a reflex hammer
What does “warm up” do to myotonia?
Continued muscle contraction lessens myotonia
What is a fasciculation?
Visible contraction of a portion of a muscle
What is a fasciculation potential?
Electrically summated voltage of depolarzing muscle fibers belonging to all or part of one motor unit
Describe fasciculation potentials
Discharge 1Hz to many/min
Irregular and random
No under voluntary control
Influenced by mild contraction of agonist or antagonist muscles
Fasciculation potentials occur in ___
Normal foot intrisics or gastrosoleus muscles Motor neuron disorders Radiculopathies Entrapment neuropathies Cervical spondylotic myelopathy Metabolic disturbances
What is myokymia?
Rippling movement or “bag of worms” movement of the skin
What is a myokymic discharge?
Bursts of normal appearing group of motor units with interburst intervals of electrical silence
Describe myokymic discharges
Firing rate 0.1-10 Hz in semirhythmic pattern
What do myokymic discharges sound like?
Sputtering of a low-powered motorboat engine
What can facial myokimc potentials be seen in?
Multiple sclerosis
Brainstem neoplasm
What can segmental myokymic discharges be seen in?
Syringomyelia
Radiculopathies
What can generalized myokymic discharges be seen in?
Uremia
Thyrotoxicosis
Inflammatory polyradiculoneuropathy
Issac’s syndrome
What can limb myokymic discharges be seen in?
Radiation plexopathy
Chronic compressive neuropathies
What is seen on EMG in stiff man syndrome?
Normal MUAPs producing a sustained discharge pattern in agonist and antagonist muscles
What can induce cramps?
Hyponatremia
Hypocalcemia
Vitamin deficiency
Peripheral neuropathies
How can cramps be induced in the calf muscles of normal patients?
Exercises
Abnormal positioning
maintained fixed position for a prolonged period
What is seen on EMG in a cramped muscle?
Multiple motor units firing b/w 40-60 Hz up to 20-300 Hz
What is tetany?
Spontaneous muscle twitiching, cramps and carpopedal spasm
What is Chvostek’s sign?
Inducing tetany by tapping the facial nerve
What is the peroneal sign?
Inducing tetany by tapping the peroneal nerve at the fibular head
What is Trousseau’s sign?
Placing BP cuff around arm for 3 minutes inducing hand tetany
Describe a motor unit potential in tetany
Fires rapidly with inderdischarge interval of 2-20 ms
Doublet: fires twice
Triplet: fires 3 times
Multiplet: fires >3 times
How does Seddon classify nerve injury?
Combo of functional status and histologic appearance
How is neurapraxia?
Mild degree of neural insult that results in conduction block of impulse across the affected segment
Describe EMG in neurapraxia
No fibs should be seen as axon is intact and muscle innervation maintained
What is axontmesis?
Only axon is disrupted w/ preserved perineurium and epineurium causes wallerian degeneration
What is the prognosis of axontmesis?
Good with axonal regeneration
What is neurotmesis?
Complete disruption of axon and all supporting CT structures.
What is the prognosis of neurotmesis?
Poor prognosis for complete recovery and requires surgery
How does Sunderland classify nerve injury?
Trauma with respect to the axon and supporting CT
What does compression neuropathy cause?
Thinning of myelin and widening of nodes of Ranvier, causing slowing of latency and velocity on NCS
What are the two types of electrodes used in EMG/NS?
Surface and needle
Describe a monopolar needle
Solid, stainless steel shaft coated in Teflon except bare metal tip which acts as the recording surface
Describe a concentric needle
Hollow, stainless steel hypodermic needle with central platinum or Nichrome-silver wire surrounded by expoxy resin
Where are the ground and reference in the monopolar needle?
Separate from needle
Where are the ground and reference in the concentric needle?
Ground is separate and reference in cannula
Describe pros and cons of the monopolar needle
Wider recording territory
Distant reference makes recording “noisy”
Teflon coating dec patient discomfort
Describe pros and cons of the concentric needle
Active and reference electrodes close together making them quieter
More discomfort
Describe motor units found with concentric needles compared to monopolar
Smaller amp
Fewer phases
Comparable duration
Less distant motor unit activity
Describe a single-fiber electrode
Modified modified concentric needle with small 25 um recording port opposite of electrodes bevel
What is an amplifier?
Device w/ ability to magnify wanted signals and minimize unwanted signals or noise
Which amplifier is connected to the active electrode?
Noninverting amplifer
Which amplifier is connected to the reference electrode?
Inverting amplifier
What does a high-frequency filter do?
Eliminates frequencies higher than its numeric designation and permits lower frequencies to pass
What does a low-frequency filter do?
Eliminates frequencies lower than its numeric designation and permits higher frequencies to pass
Is the stimulating cathode negative or positive?
Negative
Is the stimulating anode negative or positive?
Positive
Describe the optimal condition of the patients skin during EMG/NCS
Dry without perspiration, lotion, make up or other surface conductors
Where should the ground electrode be placed?
Between active and reference electrodes
What is the most effective what to reduce stimulus artifact?
Rotate anode about cathode
