Nerve conduction studies Flashcards
What do nerve conduction studies check?
Diseases of peripheral nerves
TRUE or FALSE? NCSs study all the nerves
FALSE
They measure the fastest nerves only
What are the types of NCS stimulations?
Supramaximal stimulation
Submaximal stimulation
What is supramaximal stimulation?
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
What are F waves?
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.
What are supramaxial stiumulations used to check?
compound motor action potentials
sensory action potentials
mixed sensory and motor responses
What are H waves?
Hoffman reflex:(stimulate muscle spindle fibres –> sensory fibres –> come back)
electical correlate of tendon reflex
What is the NCS amplitude determined by?
the number of axons
impedance
distance from nerve
TRUE or FALSE? NCS amplitudes vary a lot between people.
TRUE
What does reduced NCS velocity suggest?
demyelination
What does a reduced NCS amplitude suggest?
axonal loss/degeneration/lesion
What are examples of when NCS amplitudes are decreased
Axonotmesis: injury to the peripheral nerve axons
Neurotmesis: cut of axon
How do median sensory studies work (SAP)?
- Use ring electrodes around finger to electrocute fingers
- Find out how long it takes AP to get to wrist
- Distance/time = velocity
Define orthodromic
normal direction of AP
Define antidromic
reverse direction of AP
In a median motor study which wave is bigger? Muscle or nerve?
Muscle
Why do we not study velocity in motor studies?
bc of NMJ being involved (distorts)
What is temporal dispersion?
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
What are the conduction velocities in distal and proximal parts of upper and lower limbs?
Upper limb:
Distal >50m/s
Proximal >60m/s
Lower limbs:
Proximal >50m/s
Distal >40m/s
Why are leg velocities slower than arms?
Because the axon diameter is smaller in the legs
What are the types of neuropathy defined by neurophysiological findings + examples?
Length dependent (e.g. CMT)
Non-length dependent (e.g. CIDP) -patchy
Mono-neuropathy (carpal tunnel syndrome)
What are the patterns of NCS abnormalities?
Gereneralised: length dependent
Patchy: acquired inflammatory conditions
Anatomically defined abnormalities - carpal tunnel
What do we see in preganglionic lesions?
Sensory conduction looks normal bc has cell body but connection to NS is damage
SAP: normal
CMAP : abnormal
What do we see in postganglonic lesions?
SAP: abnormal
CMAP: abnormal
Give an example of preganglionic damage
Radiculopathies
Motor neuron disease (e.g. anterior horn cell disease)
Give examples of post ganglionic damage
Plexopathy
Mononeuropathy
peripheral neuropathy
What does repetitive nerve stimulation do in myasthenia gravis?
If you stimulate a motor response repetitively you get a drop in amplitude due to failure of transmission across NMJ
Which part of the synapse does MG affect?
post synaptic disorder
What happens if you repetitively stimulate lambert-eaton myasthenic syndrome nerves?
it improves after repeated stimulation
Which part of the synapse does lambert-eaton myasthenic syndrome affect?
pre-synaptic disorder
What happens if you repetitively stimulate botulism nerves?
And you can force the Ach across the NJM so you get an increase in amplitude
How does botulinum toxin work?
Prevents Ach Release
How do you do an EMG??
Put needle in the muscle and look at the pattern of electical activity
uncomfortable
What happens if you denervate a muscle?
the muscle will start to fire off on its own - useful to see axon damage
What are the limitation of NCSs?
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
