Nerve conduction studies

Question 1

What do nerve conduction studies check?

Answer 1

Diseases of peripheral nerves

Question 2

TRUE or FALSE? NCSs study all the nerves

Answer 2

FALSE

They measure the fastest nerves only

Question 3

What are the types of NCS stimulations?

Answer 3

Supramaximal stimulation

Submaximal stimulation

Question 4

What is supramaximal stimulation?

Answer 4

Completely non-physiological: use a supramaximal stimulus (enough to make all the axons in the bundle make an action potential) - never happens in real life

Question 5

What are F waves?

Answer 5

A late CMAP evoked from a muscle by a supramaximal electrical stimulus to the nerve, and it is due to antidromic activation (backfiring) of alpha motor neurons.

Question 6

What are supramaxial stiumulations used to check?

Answer 6

compound motor action potentials

sensory action potentials

mixed sensory and motor responses

Question 7

What are H waves?

Answer 7

Hoffman reflex:(stimulate muscle spindle fibres –> sensory fibres –> come back)

electical correlate of tendon reflex

Question 8

What is the NCS amplitude determined by?

Answer 8

the number of axons
impedance
distance from nerve

Question 9

TRUE or FALSE? NCS amplitudes vary a lot between people.

Answer 9

TRUE

Question 10

What does reduced NCS velocity suggest?

Answer 10

demyelination

Question 11

What does a reduced NCS amplitude suggest?

Answer 11

axonal loss/degeneration/lesion

Question 12

What are examples of when NCS amplitudes are decreased

Answer 12

Axonotmesis: injury to the peripheral nerve axons

Neurotmesis: cut of axon

Question 13

How do median sensory studies work (SAP)?

Answer 13

1. Use ring electrodes around finger to electrocute fingers
2. Find out how long it takes AP to get to wrist
3. Distance/time = velocity

Question 14

Define orthodromic

Answer 14

normal direction of AP

Question 15

Define antidromic

Answer 15

reverse direction of AP

Question 16

In a median motor study which wave is bigger? Muscle or nerve?

Answer 16

Muscle

Question 17

Why do we not study velocity in motor studies?

Answer 17

bc of NMJ being involved (distorts)

Question 18

What is temporal dispersion?

Answer 18

when there is myelin damage due to inflammation then certain areas are more affected than others thus velocities are spread out in a random way

Question 19

What are the conduction velocities in distal and proximal parts of upper and lower limbs?

Answer 19

Upper limb:
Distal >50m/s
Proximal >60m/s

Lower limbs:
Proximal >50m/s
Distal >40m/s

Question 20

Why are leg velocities slower than arms?

Answer 20

Because the axon diameter is smaller in the legs

Question 21

What are the types of neuropathy defined by neurophysiological findings + examples?

Answer 21

Length dependent (e.g. CMT)

Non-length dependent (e.g. CIDP) -patchy

Mono-neuropathy (carpal tunnel syndrome)

Question 22

What are the patterns of NCS abnormalities?

Answer 22

Gereneralised: length dependent

Patchy: acquired inflammatory conditions

Anatomically defined abnormalities - carpal tunnel

Question 23

What do we see in preganglionic lesions?

Answer 23

Sensory conduction looks normal bc has cell body but connection to NS is damage

SAP: normal
CMAP : abnormal

Question 24

What do we see in postganglonic lesions?

Answer 24

SAP: abnormal
CMAP: abnormal

Question 25

Give an example of preganglionic damage

Answer 25

Radiculopathies Motor neuron disease (e.g. anterior horn cell disease)

Question 26

Give examples of post ganglionic damage

Answer 26

Plexopathy Mononeuropathy peripheral neuropathy

Question 27

What does repetitive nerve stimulation do in myasthenia gravis?

Answer 27

If you stimulate a motor response repetitively you get a drop in amplitude due to failure of transmission across NMJ

Question 28

Which part of the synapse does MG affect?

Answer 28

post synaptic disorder

Question 29

What happens if you repetitively stimulate lambert-eaton myasthenic syndrome nerves?

Answer 29

it improves after repeated stimulation

Question 30

Which part of the synapse does lambert-eaton myasthenic syndrome affect?

Answer 30

pre-synaptic disorder

Question 31

What happens if you repetitively stimulate botulism nerves?

Answer 31

And you can force the Ach across the NJM so you get an increase in amplitude

Question 32

How does botulinum toxin work?

Answer 32

Prevents Ach Release

Question 33

How do you do an EMG??

Answer 33

Put needle in the muscle and look at the pattern of electical activity uncomfortable

Question 34

What happens if you denervate a muscle?

Answer 34

the muscle will start to fire off on its own - useful to see axon damage

Question 35

What are the limitation of NCSs?

Answer 35

Supra maximal stimulation is non physiological (Can show abnormalities which are not useful) Normal ranges are very wide Only accessible nerves can be examined The method only examines large demyelination (Most fibres are small) We can look at fibres that supply skeletal fibres and those for proprioception Some people can't tolerate the stimulus