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Cuccurullo Board Pearls > Electrodiagnostics > Flashcards

Electrodiagnostics Flashcards

1
Q

What is the innervation ratio (IR) in relation to an axon?

A

The amount of muscle fibers belonging to an axon = the innervation ratio, which varies depending on the function of the motor unit
- Muscles of gross movement (i.e. leg muscles) have a LARGER amount of fibers innervation by one axon (high ratio) - the higher the IR, the greater force generated by that motor unit
- Muscle of fine movement have a SMALLER amount of their fibers innervated by one axon (low ratio)

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2
Q

What does a motor unit consist of?

A

Anterior horn cell (alpha motor neuron) and all of the muscle fibers that it innervates (including peripheral nerve, NMJ)

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3
Q

What is the order of recruitment for motor units?

A

Smaller motor units are recruiting first by sequential activation for a smooth increase of contractile force (Henneman Size Principle = a smaller alpha motor neuron has a lower threshold of excitation, causing it to be recruited first during voluntary contraction; a larger motor neuron has a higher threshold of excitation and is recruited when more motor units are needed to generate a greater contractile force)

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4
Q

What are the three types of motor neurons?

A

Alpha - innervates extrafusal fibers (skeletal muscle)
Gamma - innervated intramural fibers (muscle spindle)
Beta - innervates intrafusal and extrafusal fibers

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5
Q

What are the innervation characteristics of Type I fibers?

A

Small cell body
Thinner diameter axon
Lower innervation ratio
“Slow twitch” muscle fibers

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6
Q

What are the innervation characteristics of Type II muscle fibers?

A

Larger cell body
Thicker diameter axon
Higher innervation ratio
“Fast twitch” muscle fibers

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7
Q

EDx studies evaluate which type of muscle fiber?

A

Type Ia (large, myelinated) fibers

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8
Q

How does decreased temperature affect Na channels along the axon?

A

A decrease in temperature affects the protein configuration and causes a delay in opening and closing of the Na+ gates

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9
Q

What are the expected waveform changes due to a decrease in temperature (30-32 degrees C)?

A

Latency - prolonged (approx 1 msec)
Amplitude - increased (approx 20%)
Duration - increased
Conduction velocity - decreased (approx by 10 m/sec)
Phases - increased
* amplitude can drop due to increased temporal dispersion or phase cancellation

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10
Q

What is the mechanism of propagation of depolarization?

A

Na+ flows into the cell during depolarization, it moves away from the cell membrane and spreads the current down the path of least resistance along the length of the axon - the myelin sheath covering prevents Na flow back out through the membrane and the potential “jumps” to the next group of Na+ channels at the nodes of Ranvier (saltatory conduction)

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11
Q

Orthodontic recording occurs in which direction?

A

The action potential is traveling in the direction of its typical physiologic conduction (normal conduction along motor fibers travels AWAY from the spinal cord, whereas conduction along sensory fibers travel TOWARDS the spinal cord)

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12
Q

Antidromic recording occurs in which direction?

A

Opposite direction of the typical physiologic conduction
- antidromic motor study: action potential travels towards the spinal cord
- antidromic sensory study: action potential travels away from the spinal cord

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13
Q

What is the motor endplate?

A

Distal portion of the motor axon (terminal branches) that innervate individual muscle fibers, contains NMJ

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14
Q

What is the physiology behind the development of a MUAP?

A

Nerve depolarization opens voltage-gated Ca++ channels that cause a release of multiple quanta (containing ACh) into the synaptic cleft, which increases the amount of miniature endplate potential (MEPP), these MEPPs summate to form an endplate potential, which generates a motor unit action potential

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15
Q

What is the basic unit of muscle myofibril?

A

The sarcomere (which runs Z-line to Z-line)

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16
Q

What is the basic physiology of a skeletal muscle contraction?

A

Muscle fiber depolarization -> T-tubule system, causes Ca++ to be released from the sacroplasmic reticulum, which binds to the troponin-tropomyosin complex and exposes actin’s active sites.

Myosin heads, powered by ATP, bind with the active sites and slide over the actin filaments to shorten the muscle

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17
Q

What is a conduction block?

A

> 50% drop in CMAP amplitude between proximal and distal stimulation sites across a site of demyelination (failure of an action potential to propagate past an area of demyelination along axons that are otherwise structurally intact)

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18
Q

What are common etiologies of conduction blocks?

A

Focal compression causing transient ischemic episode, edema, or myelin invaginations w/ paranormal intussusceptions
Chronic diseases causing degradation of myelin leading to peripheral neuropathies

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19
Q

What are the primary NCS findings in demyelinating conditions?

A

Prolonged latency, slowed conduction velocity, increased temporal dispersion

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20
Q

What are the two primary mechanisms of axonal injury?

A

Axonal degeneration: a nerve injury that begins in a “dying back” fashion and affects the nerve in a length-dependent manner (degeneration starts dismally and ascends proximally)

Wallerian degeneration: the axon degenerates distally from the site of a nerve lesion (the nerve segment proximal to the injury site is essentially intact with some minor dying back at the lesion by 1-2cm
- motor axons: degeneration is usually complete in 7 days
- sensory axons: degeneration is usually complete in 11 days
*motor degeneration occurs faster than sensory degeneration

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21
Q

Primary EDx findings in axonal injury?

A

Decreased amplitude (representation of axonal loss)

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22
Q

How does collateral sprouting during axonal recovery affect EMG waveforms?

A

Remodeling results in motor units w/ poor firing synchronicity - resulting in polyphasic waveforms with increased amplitudes

(Collateral sprouting = part of axonal recovery, a neuritis sprouts off the axon of an intact motor unit and innervates denervated muscle fibers of an injured motor unit. The sprouts connect w/ smaller terminal branches, thinner myelin, and weaker NMJs compared to an uninjured axon)

23
Q

If the supporting soft tissue remains intact after axonal injury, how fast will an axon re-grow?

A

Approx 1mm/day or 1 inch/month (35mm/month)

These axons will have decreased diameter, thinner myelin, and shorter intermodal distance. With reinnervation, low-amplitude, long duration and polyphasic potentials (nascent potentials) are formed.

24
Q

Who will dominate if an axon regrows to innervate its original muscle fibers but collateral sprouting to these fibers has already occurred?

A

The nerves possessing the largest axon, the thickest myelin, the strongest NMJ will prevail and keep the muscle fibers

25
Q

What is Ohm’s Law as it relates to an electric current passing through a wire?

A

Current = voltage/Resistance (I = V/R or voltage = current x resistance)

An electric current passes through a wire at an intensity of the current (I) measured in amperes, equal to the voltage (V) from an electromotor source measured in volts divided by the resistance (R) in ohms

26
Q

What is a mono polar EMG needle?

A

Teflon coated needle w/ exposed tip (A)

Advantages:
- Inexpensive
- conical tip: Omni-directional recording
- less painful (teflon decreases friction)
- larger recording area (2x concentric)
- Records more positive sharp waves and more abnormal activity in general

Disadvantages
- Requires a separate needle or surface reference
- Nonstandarized tip area
- May have more interference if the reference is not near the recording electrode

27
Q

What is a standard concentric EMG needle?

A

24-26 gauge needle (reference) with bare inner wire (active) - B in figure

Advantages:
- Standardized exposed area
- Fixed location from reference
- Less interference
- No separate reference
- Used for quantitative EMG

Disadvantages:
- Beveled tip - unidirectional recording
- Smaller recording area
- MUAPs have smaller amplitudes
- More painful

28
Q

What is a bipolar concentric EMG needle?

A

Needle with the active and reference wires within its lumen (C in diagram)

Advantages:
- best for isolating MUAPs
- less artifact

Disadvantages:
- Expensive
- More painful

29
Q

What is a single fiber EMG needle?

A

Needle (reference) consisting of an exposed 25um diameter wire (active) - E in figure

Advantages:
- Looks at individual muscle fibers
- Used to assess fiber density
- Used to assess jitter
- Used assess fiber blocking
- Helpful in assessing NMJ disorders and motor neuron disorders

Disadvantages:
- Not used for traditional EMG
- Expensive

30
Q

What is volume conduction?

A

With stimulus intensity set too high, volume conduction spreads through tissue surrounding the nerve; skin, extra cellular fluid, muscles, and other nerves may be stimulated, which can lead to:
- Decreased conduction times and shortened latencies
- Altered waveforms
- Amplitudes remain unchanged

31
Q

What is the signal-to-noise ratio w/ relation to EDx?

A

The process of averaging improves the S:N by a factor that is the square root of the number of averages performed - the number of average must be increased by a factor of 4 to double the S:N

S:N = signal amplitude x (# of averages performed)-1/2/noise amplitude

32
Q

What is the Common Mode Rejection Ratio (CMRR) in relation to EDx amplifiers?

A

Selectively amplifying different signals and rejecting common ones - usually expressed as dB and should be > 90dB

The larger the CMRR, the more efficient the amplifier

33
Q

With respect to EDx - what is the role of the high frequency (low pass) filter?

A

The high-frequency (low pass) filter removes signals with frequencies higher than its cutoff setting (signals with frequencies lower than the cutoff setting are not affected) - this affects the faster portions of the summated waveform

34
Q

With respect to EDx, what is the role of the low-frequency (high-pass) filter?

A

The low-frequency (high pass) removes signals with frequencies lower than its cutoff setting (signals higher than the cutoff setting are not affected)

35
Q

EDx - what happens when the low-frequency filter is elevated/raised?

A

Reduces the peak latency
Reduces the amplitude
Changes potential from bi-to tri-phasic
Does not changes the onset latency

36
Q

EDx - what changes when the high-frequency filter is reduced/lowered?

A

Prolongs the peak latency
Reduces amplitude
Creates a longer negative spike
Prolongs the onset latency

37
Q

What is sweep speed?

A

Time allocated for each x-axis division and is measured in milliseconds
- Sensory - usually 5msec
- Motor - usually 2 msec
- EMG - usually 10 msec

38
Q

Contraindications to EDx?

A

External cardiac pacemakers (external pacing wires can be electrically sensitive to NCS)

*not an absolute contraindications - platelet < 50K, INR > 3.0, central line catheters may pose a risk of generating a stimulus in the heart (peripheral lines are not considered to be problematic)

39
Q

How are conduction velocities different in infants/young children compared to adults?

A

Infants: 50% of adult conduction velocities
1 year old: 80% of adult CV
3-5 years old: equal CV to adults

40
Q

What is the difference in sensory involvement in pre-ganglionic and post-ganglionic lesions?

A

The DRG is located in the intervertebral foramen
- Pre-ganglionic lesions are proximal to the DRG and preserve the SNAP waveform despite clinical sensory abnormalities
- Post-ganglionic lesions are distal to the DRG, results in Wallerian degeneration of both motor and sensory axons

41
Q

What technical considerations should be reviewed if there are initial negative deflections on CMAPs/NCS?

A
  • inappropriate placement of the active electrode from the motor point
  • volume conduction from other muscles or nerves
  • anomalous innervations
42
Q

What can occur if the active and reference electrodes are placed too close together during NCS?

A

Falsely decreased amplitude and inaccurate latency (similar waveforms are recorded at both sites and rejected, decreasing the amplitude of the waveform)

43
Q

H reflex is typically used to monitor for which radiculopathy?

A

An S1 radiculopathy (or C7 radiculopathy in the upper extremity)

H-reflex is an electrically evolved analogue to monosynaptic reflex (equivalent to the Achilles reflex in the lower extremity)
- initiated w/ a submaximal stimulus at a long duration (1.0 msec). This preferentially activates the 1a 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 and latency remain constant with each stimulation at the appropriate intensity (a mean of 10 F-waves can be used as a surrogate for one H-reflex)
- > 0.5 to 1.0 msec side-to-side difference is significant
- Location
— soleus muscle: tibial nerve (S1 pathway)
— flexor carpi radialis (FCR): median nerve (C7 pathway)

44
Q

What is a the F-Wave?

A

F-wave is a small late motor response occurring after the CMAP. It represents a late response from approximately 1% to 5% of the CMAP amplitude.
- produced using short duration, supramaximal stimulation, which initiates an antidromic motor response to the anterior horn
- pure motor response (not a true reflex)
- The configuration and latency change with each stimulation due to activation of different groups of anterior horn cells with each stimulation
- May be helpful in polyneuropathies and plexopathies but not overly useful in radiculopathies

45
Q

What is an A-wave (axon wave)?

A

Waveform that represents collateral sprouting following nerve damage
- typically occurs between the CMAP and F-wave at a constant latency
- the stimulus travels antidromically along the motor nerve and becomes diverted along a neural branch formed by collateral sprouting due to previous denervation and reinnervation

46
Q

What are the afferent and efferent pathways of the blink reflex?

A

Afferent = sensory branches of CN V (Trigeminal)
Efferent = motor branches of CN VII (Facial)

  • Analogue to the corneal reflux
  • initiated by stimulating the Supra-orbital branches of the trigeminal nerve
  • response propagates into the pons, branches to the lateral medulla - it then branches to innervate the ipsilateral and contra lateral orbicularis oculi via the facial nerve
47
Q

What does the Blink reflex set-up measure?

A

Ipsilateral R1 - early response through the pons
— Affected by lesions of the trigeminal nerve, pons, facial nerve
Bilateral R2 (blink is associated w/ R2 response) - late response through the pons and lateral medulla
— affected by consciousness level, Parkinson’s disease, lateral medullary syndrome, Valium, habituation

48
Q

What is synkinesis?

A

Aberrant regeneration of axons that can occur w/ facial nerve injuries leading to reinnervation of inappropriate muscles - may present as lip twitching when closing eyes or tears when chewing, etc

Conservative treatment options for lesions include prednisone, massage, or electrical stimulation

49
Q

Which nerves are commonly used for testing somatosensory evoked potentials?

A

Upper limb = median nerve
Lower limb = tibial nerve

Somatosensory evoked potentials (SSEP) monitor for problems such as peripheral nerve injuries, CNS lesions (such as MS), or intraoperative monitoring of spinal surgery

*During spinal cord surgery, loss of tibial nerve potentials with preservation of median nerve potentials can indicate nerve injury at the level of intervention
**anesthesia will affect SSEP potentials in both upper and lower limbs

50
Q

What does insertional activity represent on needle EMG?

A

Discharge potentials that are mechanically provoked by physically disrupting the muscle cell membrane w/ a needle

51
Q

Abnormal spontaneous EMG activity generated from muscle fibers includes what?

A

Fibrillations
Positive sharp waves
Complex repetitive discharges (CRD)
Myotonic discharges

52
Q

Abnormal spontaneous activity from a motor unit includes what?

A

Fasciculation potentials
Myokymic discharges
Neuromyotonic discharges

53
Q

What are satellite potentials on needle EMG?

A

Small potentials that are seen in early reinnervation - trail the main MUAP, can be due to the newly formed neural sprouting that often is small, unmyelinated or thinly myelinated, thus very slowly conducting

54
Q

What is the rule of 5s regarding MUAP recruitment on EMG?

A

Onset frequency of the first MUAP begins are approximately 5Hz
When the firing rate reaches approximately 10 Hz, a second MUAP begins at approx 5 Hz
When the first MUAP reaches a rate of 15Hz, the second should be at 10 Hz and a third will begin at 5Hz
As more force is needed, the firing rate of the first MUAP may reach 20 Hz, 2nd 15Hz, the 3rd at 10 Hz and the fourth approximately will begin at 5Hz

Cuccurullo Board Pearls (5 decks)

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