Basic Electromyography: Analysis Of Spontaneous Activity

Question 1

What information can be yield from the presence of abnormal spontaneous activity?

Answer 1

1) the neuroanatomic localisation of the lesion
2) specific diagnostic information / specific disorders (e.g. myotonic discharges - myopathies and hyperkalemic periodic paralysis)
3) the severity of the lesion
4) the time course of the lesion

Question 2

Which attributes must be analysed in a waveform?

Answer 2

1) morphology
2) stability
3) firing characteristics

Question 3

Which aspects of waveform morphology can help identify the source of a spontaneous discharge?

Answer 3

Amplitude
Duration
Number of phases
Initial deflection

Question 4

What can be the source of a spontaneous discharge?

Answer 4

1) the neuromuscular junction
2) a single muscle fiber
3) the terminal axon twig
4) a motor neuron / axon
5) multiple muscle fibers linked together

Question 5

Miniature endplate potentials (endplate noise): how are they produced and how is their morphology

Answer 5

They result from the normal spontaneous exocytosis of individual quanta of Ach traveling across the NMJ
They have a small amplitude and monophasic negative morphology
They fire irregularly at 20 to 40 Hz and have a characteristic seashell sound

• Normal findings
• • They are not action potentials!

Question 6

What kinds of waveforms are produced from the depolarisation of a muscle fiber?

Answer 6

Endplate spikes
Fibrillation potentials
Positive sharp waves
Complex repetitive discharges
Myotonic discharges

Question 7

What kinds of waveforms are produced by the motor neurons or their axons (MUAPs)?

Answer 7

Fasciculation potentials
Doublets, triplets, multiplets
Myokymic discharges
Cramp potentials
Neuromyotonic discharges
Rest tremor

Question 8

How is the stability of a waveform assessed?

Answer 8

A waveform can be:

A) Stable
B) Unstable: 
Waxing and waning
Waning
Stops/ starts/ changes abruptly

Question 9

What firing characteristics can a waveform have?

Answer 9

A) Firing rate
Very slow (<2Hz)
Slow (2-10 Hz)
Fast (10-100 Hz)
Very fast (100-250 Hz)

B) Firing pattern
Regular
Irregular (sputtering, bursting, waxing and waning, waning, stops/ starts/ changes abruptly)

Question 10

What is the minimum firing rate of a voluntary potential?

Answer 10

4-5 Hz

Question 11

What is insertional activity and how is it assessed?

Answer 11

Insertional activity is produced from the depolarisation in a brief burst of muscle fibers when a needle is quickly moved through muscle. For the evaluation of insertional activity at least four to six brief needle movements are made in four quadrants of each muscle

Question 12

How much is the normal duration of insertional activity and when is it considered abnormal?

Answer 12

The normal duration of insertional activity is 300ms
The duration is increased in both neuropathic and myopathic conditions
The duration is decreased when muscle is replaced by fat and fibrous connective tissue

Question 13

What are endplate spikes?

Answer 13

They are muscle fiber action potentials produced by needle-induced irritation of a terminal nerve twig and the subsequent activation of a nerve action potential
They are biphasic with an initial negative deflection and fire irregularly up to a frequency of 50Hz
They only occur when a needle is in the muscle and close to the endplate zone

Question 14

How to distinguish endplate spikes from fibrillation potentials?

Answer 14

Endplate spikes have an initial negative phase and have a highly irregular rate

Question 15

What are fibrillation potentials?

Answer 15

They are the electrophysiologic markers of active denervation
Their morphology consists of a brief spike with an initial positive deflection, 1 to 5 ms in duration, and low in amplitude (10-100μV)
The firing pattern is very regular with a rate usually 0,5 to 10 Hz occasionally up to 30 Hz
In very chronic conditions they may become very tiny
They sound like ‘‘rain on the roof’’

Question 16

When are the fibrillation potentials found?

Answer 16

Neuropathic disorders (neuropathies, radiculopathies, motor neuron disease)
Muscle disorders (inflammatory myopathies and dystrophies)
Severe diseases of the NMJ (botulism)

Question 17

Which are the kinds of spontaneous activity?

Answer 17

Endplate noise (MEPPs)
Endplate spike
Fibrillation potential
Positive sharp wave
Myotonic discharge
Complex repetitive discharge
Fasciculation potential
Doublets, triplets, multiplets
Myokymic discharge
Cramp potential
Neuromyotonic discharge
Rest tremor

Question 18

How to distinguish positive sharp waves from voluntary motor units potentials at a distance (which occasionally have a positive wave morphology)?

Answer 18

Positive sharp waves - like fibrillation potentials - usually have a regular firing pattern

Question 19

What do fibrillation potentials and positive sharp waves represent?

Answer 19

They both represent the spontaneous firing of a single muscle fiber with an unstable resting membrane.
Their difference is that in the case of positive sharp waves there is needle induced deformation of the muscle membrane.

Question 20

How are fibrillation potentials and positive waves graded?

Answer 20

0 None present
+1 Persistent single trains of potentials (>2-3 seconds) in at least two areas
+2 Moderate number of potentials in three or more areas
+3 Many potentials in all areas
+4 Full interference pattern of potentials

Question 21

Describe +4 grade fibrillation potentials

Answer 21

The screen is completely filled and individual potentials cannot be seen
It can be confused with voluntary activity
Sound of ‘‘rain on the roof’’ but heavy downpour
Uncommon and seen in situations where all muscle fibers are denervated simultaneously as in trauma and infarction (vasculitis)