Electrodiagnostic examination

Question 1

electrodiagnostic examination

Answer 1

comprises:

1. Nerve conduction studies
2. EMG

Question 2

Nerve conduction studies

Answer 2

tests conduction velocity and AP amplitude in motor, sensory and/or mixed nerves

uses electrodes to apply a small electrical stimulation to test nerve and muscle responses.
conduction velocity = distance/time
the rate at which nerves conduct electrical impulses can be calculated

Question 3

EMG

Answer 3

examines spontaneous and voluntary activity in skeletal muscles

small needle electrode is inserted into the muscle and measures voluntary and spontaneous activity in the muscle

voluntary contraction as measured by recorded signals are known as motor unit potentials. These examine the amplitude, duration, phases and recruitment patterns and may show evidence of myogenic or neurogenic disease.

Question 4

Nerve conduction studies of the upper limb

Answer 4

median, ulnar and radial nerve in the arms

Question 5

Nerve conduction studies of the lower limb

Answer 5

sural nerve and superficial perineal nerves in the legs

Question 6

EMG types

Answer 6

1. Surface EMG
2. Needle EMG
3. Single fibre EMG

Question 7

Needle EMG

Answer 7

axis of needle (recording electrode portion) is insulated from the shaft of the needle. the shaft of the needles acts as the reference electrode.

conventional EMG records from hemisphere radius of 1 mm.

recorded muscle activity can be targeted both visually (oscilloscope) and by ear (loudspeaker translates visuals into sound)

Question 8

EMG considerations

Answer 8

muscle characteristics, age of the patient, patient cooperation, electrical specifications

Question 9

EMG analysis

Answer 9

AP frequency, duration, amplitude, area and variability

Question 10

motor unit diseases

Answer 10

associated with muscle wasting or weakness

Question 11

7 sites of motor unit disease involvment

Answer 11

1. pre upper motor neurons:
   cortical neurons
   corticospinal tracts
2. lower motor neurons
   anterior horn neurons
   peripheral nerve axons e.g. myelin sheath breakdown as with multiple sclerosis
3. NMJ e.g. myasthenia gravis
4. Muscle membrane (sarcolemma) e.g. myotonia
5. contractile elements e.g. muscular dystrophy

Question 12

denervation

Answer 12

severing of the nerve

Question 13

fasciculation

Answer 13

muscle twitch that is spontaneous, involuntary muscle contract and relaxation

Question 14

EMG for diagnosis

Answer 14

distinguish between myopathic and neuropathic muscle wasting and weakness.

detect abnormalities such as chronic denervation or fasciculations in the muscle.

differentiate between focal and diffuse pathologies (in conjunction with nerve conduction studies)

evidence for pathophysiology of peripheral neuropathy

Question 15

principle of selective vulnerability

Answer 15

motor neurone diseases affect only motor systems and not sensory or autonomic systems.

Question 16

4 steps of EMG

Answer 16

1. Insertional activity
2. Spontaneous activity
3. Motor unit potentials
4. Recruitment and inference pattern

Question 17

inference

Answer 17

greater contraction results in distortion and overlap of individual shape tracings to where they cannot be identified

Question 18

spontaneous activity

Answer 18

activity at rest. in normal individual there should be a slight initial spiking activity where needle insertion has damaged small part of the muscle releasing K+ causing depolarisation. the muscle becomes quiescent very rapidly.

if reduced then could be linked to fibrotic conditions e.g. muscular dystrophy (replaced with collagenous material so signal is almost non existent).

if increases then could be linked to denervation and inflammatory processes.

Question 19

voluntary activity

Answer 19

mild voluntary contraction induces isolated discharges of one or few motor units at a rate of 5 - 10 spikes per second.

force increases with two mechanisms of greater muscle force activity both occurring simultaneously

1. recruitment of inactive units by size principle of motor unit recruitment
2. more rapid firing of already active units

ultimately results in inference pattern.

Question 20

normal voluntary EMG activity

Answer 20

appropriate motor discharge for effort input.

effort input is the examiner knowing the normally number of motor units that should be recruited for a given force exerted.

Question 21

reduced EMG activity = ALS

amplitude increased, freq decreased

Answer 21

recruitment pattern is reduced in ALS = amyotrophic lateral sclerosis.

full inference is not detected. the amplitude of AP is much higher but frequency is drastically decreased.

Question 22

increased EMG activity = limb girdle dystrophy

amplitude decreased, freq increased

Answer 22

exaggerated voluntary recruitment activity as in limb girdle dystrophy. minimal activity results in activation of 2/3 motor units in comparison to 1.

full recruitment pattern is established at maximal contraction but amplitude is smaller than in normal muscle.

Question 23

muscle weakness (babinski sign)

Answer 23

diseases of premotor neurons and/or their axons characterised by spasticity, overactive tendon reflexes, abnormal plantar flexion (babinski sign)

distal weakness = neuropathy
proximal weakness = myopathy

Question 24

more reliable diagnosis of muscle weakenss

Answer 24

fibrillation or fasciculation

Question 25

fibrillation (denervation) (can be detected with voluntary movement)

Answer 25

only in lower motor neuron disorders + some myopathies + muscle inflammation generated by the muscle due to circulating ACh due to denervation supersensitivity.

fibrillation is invisible through skin but detected with needle EMG. can appear in healthy muscles.

confirmed by reproducible changes in at least 2 different areas of muscle is highly suggestive of lower motor neuron disorders and myopathic diseases.

denervation examples:

1. acute injury
2. chronic pressure and nerve entrapment
3. death of alpha motor neurons in the anterior grey horn
4. polyneuropathy e.g. diabetes

Question 26

polyneuropathy

Answer 26

damage or disease affecting peripheral neurons in roughly the same areas on both sides of the body e.g. diabetes or guillain barre syndrome

Question 27

denervation supersensitivity

Answer 27

@ site of lesion, the muscle fibres innervated remain viable and function BUT following a lesion (7 - 10 days) nAChR spread over the surface of muscle fibres known as denervation sensitivity and become so excitable that they discharge spontaneously causing fibrillation.

Question 28

muscle with neurogenic disease
ALS and poliomyelitis
amplitude increased
frequency decreased

Answer 28

the number of motor units under voluntary control is reduced.

fibrillations present at rest as muscle undergoes denervation sensitivity.

number of MUAPs (frequency reduced) but MUAPS larger (amplitude increased) as denervated nerve fibres become re innervated by sprouts from viable neuron.
decrease in fineness of muscle control but allows for some strength.

Question 29

muscle with myopathic disease
Muscular dystrophies
amplitude decreased
frequency increased

Answer 29

the number of muscle fibres is reduced as fibres shrink and become non functional.

fibrillation may or may not be present at rest.

number of MUAPs the same (frequency the same) but MUAP strength is reduced (amplitude decreased) and results in much weaker voluntary contractions.

Question 30

fasciculations (involuntary)

slowly progressive diseases

Answer 30

involuntary discharge of motor neurons induing contraction of part or whole single motor unit occurring at irregular intervals

larger and more complex than fibrillation and visible at skin if occurring near surface of muscle.

characteristic of slowly progressive diseases of the motor neuron itself

Question 31

myokymic discharges

e.g. multiple sclerosis or guillain barre syndrome

Answer 31

more complex bursts of repetitive discharges seen in muscles innervated by cervical nerves