Nerve Conduction Studies AProf Candice Delcourt

In this podcast, Associate Professor Candice Delcour, a neurologist at MQ Health Neurology, presents a comprehensive overview of neurophysiology, focusing on nerve conduction studies (NCS) and electromyography (EMG). She begins by outlining the essential procedures carried out at her facility, highlighting the daily performance of nerve conduction studies, alongside electromyography and other diagnostic tests like electroencephalograms and evoked potentials. Delcour emits a sense of accessibility by mentioning the possibility of a practical demonstration for attendees during the lunch break, encouraging questions and engagement.

Delcour sets the stage by introducing foundational concepts essential for understanding the complex functions of the nervous system. She elaborates on the structure and physiology of the motor unit, which consists of the motor neuron, its axon, neuromuscular junction, and associated muscle fibers. An emphasis is placed on the recruitment of motor units, where smaller units are activated at lower force levels, leading to a gradual increase in muscle contraction as larger units are recruited in response to increased effort. This aspect is crucial in EMG analysis, as it illustrates how muscle activity is represented graphically, showing variations in amplitude and frequency that correlate with physical exertion.

Transitioning to nerve injuries, Delcour explains critical terminologies and concepts relevant to neurophysiology, emphasizing the importance of understanding various motor and sensory potentials. She skillfully navigates through the pathophysiological changes occurring after nerve damage, explaining how muscles and motor units respond to injury. For example, the spontaneous depolarization of muscle fibers leads to fibrillation, observable only through needle EMG after a nerve injury. Delcour discusses the process of axonal sprouting, detailing how surviving nerve fibers attempt to reconnect with denervated muscle fibers, which marks the initial stage of recovery post-injury.

Key to Delcour's lecture is the detailed discourse on carpal tunnel syndrome, a common condition in her practice. She describes how NCS and EMG can be utilized to assess this syndrome, illustrating the differences in nerve action potentials between healthy and affected individuals. She shares graphical data that illustrates typical latency and amplitude responses in both normal and abnormal studies, encouraging attendees to understand how to interpret these results, which is invaluable in clinical practice.

Lastly, Delcour summarizes the key points with clarity, reiterating the variability in motor units, the significance of fibrillations and fasciculations in diagnosing nerve pathologies, and the impact of injury severity on recovery outcomes. Her lecture not only offers foundational knowledge in neurophysiology but also equips practitioners with practical understanding relevant to their clinical assessments and patient management in neurology.

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So our next speaker is Associate Professor Candice Delcour. So Candice is a

neurologist here at MQ Health Neurology, where she's been since December in 2017.

Candice and I met 20 years ago when she came to do some training in Sydney,

but most of her training was actually in Northern Europe, so Belgium and France.

She has training both in clinical neurophysiology and stroke.

In her first of two talks, we've got her working hard today,

she's going to give us a refresher on nerve conduction studies and electromyography. So thank you, Candice.

Do you want this one? No, thank you. Be blessed, Jane.

Good morning, everyone. Thank you, James, for the introduction.

This is the last talk before lunch, so hopefully I'll keep you entertained.

I'm going to talk about neurophysiology.

At Macquarie Health Neurology, we do nerve conduction studies,

electromyography and electroencephalograms, and also evoked potentials.

So I have actually a machine that we use here in the room. So if you are interested

during lunchtime, we can have a trial.

So we do nerve conduction studies every day, so five days a week.

And usually the way it works is we have a technician who does the test.

And then the doctor, whether it's James, myself, or Dom, will come to the room

to review what the technician has done, discuss symptoms,

review the patient, and then we decide if we do further testing and electromyography.

We can provide you with a report as soon as the test is finished.

And as I said, we do it five days a week, so we can always accommodate urgent

requests if you have any.

It's a bit tricky to talk about this in 20 minutes, so I've been trying to make it short.

And first, we'll talk about some basic concepts. So I'll talk about motor units and recruitment.

I will talk about nerve injury, the terms we use in neurophysiology,

even if we try not to put too many acronyms there. I'm sure you have seen them.

And then I will just give an example of what we commonly see,

which is carpal tunnel syndrome.

So, first, the motor unit, so it's important to just refresh this concept because

it's really key when we talk about electromyography.

So the motor unit is composed by the motor neuron, the motor neuron axon,

the axon terminals, the neuromuscular junction,

and muscle fibers which are scattered through the muscle.

And there are multiple motor units of different size in each muscle.

And if we think about muscle contraction, the first motor unit which will contract

will be small motor units,

what we used to call type 1 or red.

They will activate first with small forces. So when you do an electromyogram,

you first see a small motor unit appearing which will,

beat, I guess, in a regular, quite low frequency.

And then when you increase the effort, larger motor units will be recruited,

so the trace of the electromyogram will become bigger and the frequency will increase.

So initially you just have a little trace which comes regularly and then it

becomes a much busier trace.

This is also important because smaller motor units generate small forces and

a larger motor unit are required for bigger forces.

And as this graph shows, like what I've already mentioned, is the small motor

units have a lower threshold of activation, so they will activate first.

And also, the accuracy of the motor unit is inversely proportional to the size,

and the accuracy of the movement is inversely proportional to the force.

So, if you think about extraocular muscles, the motor units will be very small,

between two and ten muscle fibers, because the movement is very accurate. it.

Now if you think about much bigger muscles, the motor units will be also much

bigger to exert more power.

So this is what it shows on electromyography.

So you see here, actually.

This is a motor unit action potential, which is typically polyphasic, so several phases.

The down phase is usually what we call a positive phase, and an up phase,

negative phase, and then another positive phase.

So there are several distinct phases.

That's just one motor unit potential.

And if we do a very, very small contraction, we can see these individual motor units.

But as soon as we exert a bigger force, many more motor units will recruit.

And if the muscle is normal, we will have this kind of trace here.

So very busy, what we call an interference pattern.

Now, what happens if a muscle fiber is disconnected from the motor neurone?

So what happens is there will be spontaneous depolarization of the muscle fiber,

and there are lots of different theories on how that exactly works,

but basically there will be an abnormal electrical activity at the level of

the membrane of the muscle fiber, which will create what we call fibrillation.

Since this is at the level of the muscle fiber, you're not going to see that

at the surface of the skin.

But you will see it if you put a needle in the muscle when the patient is at rest.

And we will see that about three weeks after the nerve injury.

So you can see the fibrillation on this graph here very short biphasic potential

with usually an initial negative deflection.

There is also another form of these what we call spontaneous activities where

you will only have the positive part of the potential that really depends where the needle is,

where you're recording this electrical activity from,

and that's what we call positive sharp waves.

So when the muscle fiber is disconnected from the motor neuron,

this fibrillation will actually trigger sprouting from nearby axons.

And this is the first process which will happen in recovery.

So surviving terminal nerve fibers will form new branches and grow towards the

other fibers which have lost their nerve.

And there will be new synapses forming and the motor unit will become bigger.

So this is when the nerve is partly damaged, this is the first thing that will happen.

It takes about three months, which is very much the time for forming new neuromuscular

junction, as you know, like from patients receiving botulinum toxin injections.

Now, if we have an irritated or dying motor neuron, what will happen is not

at the muscle fiber here, it's at the level of the motor unit.

So the abnormal activity will be much bigger than this fasciculation.

Bigger in size and broader, sorry, than the fibrillation. So,

fibrillation is from the muscle fiber, and here we're talking about fasciculation,

which is at the side of the motor unit.

And these fasciculations are visible through the skin.

Some fasciculations are benign when the same motor unit flies regularly,

and some fasciculations randomly fly.

This is what we see in motor neurone disease.

So, if we completely cut a nerve, it's important to know that the axons distilled

to the injury side remain intact.

So, if you look at here, cutting a median nerve in the forearm,

if we stimulate the median nerve at the wrist,

within the first 48 to 96 hours we will still have a recordable potential distally.

So even if you cut a nerve, that end distal part of the nerve remains stimulable for 48 to 96 hours,

so your nerve conduction study might not give you the true result.

Now, if you stimulate above the cut, then you will not get an answer if the

nerve is completely cut, so there will be no response, or there will be a small-sized response.

So this is a no response, or there will be a small-sized response is the nerve is just partly cut.

But after that time, the extremity of the nerve will degenerate.

That's what we call valerian degeneration.

So you will not be able to stimulate that distal part anymore.

So, this slide actually summarizes the nerve regeneration after an injury.

So, if there is an incomplete injury, the regeneration will be done by sprouting

from the nearby axons, and that will take about four months.

And that will happen if some of the nerve is still in continuity.

So, if there are 10 to 20% of fibers still in continuity, sprouting can lead

to good clinical recovery.

But now, if you completely cut the nerve, the recovery will depend on axonal regeneration,

which will be slow from the neuron down to one millimeter per day.

So coming to the terms used in neurophysiology, so we talk about sensory nerve

action potentials, so SNAP,

compound motor action potentials, CMAPs, fibrillations, so discharge it from

motor fibers, fasciculations, discharge it from motor units.

We talk about the amplitude of the response, the latency and conduction velocities,

and on electromyography,

you will hear us talk about more like spontaneous activity, so at rest,

which will be these fasciculations, fibrillations, and positive sharp waves,

and volitional activity, so following recruitment.

Coming to the example of carpal tunnel syndrome now,

on the right hand side here you see a typical way of doing a motor study to

assess the median nerve.

So you stimulate the median nerve at the wrist and record the response on the

abductor Policis brevis.

And this one is just a grant.

So that's a motor study. And for the sensory study,

again, the way we do this probably varies a little bit between centers and doctors,

but this is one of the typical ways to do it.

So we use rings to stimulate the median nerve into the index and we record the response at the wrist.

So hopefully that project's alright. So these are the kind of graphs you will

have if you do sensory studies.

So we have the sensory nerve action potential here, and the software we use

comes up with a table which will give you the amplitude,

and usually the onset-to-peak amplitude and the peak-to-peak amplitude,

the latency at the onset and the peak latency.

And if you put the distance there, you will have a conduction velocity.

So this is a typically normal study there on the left.

Trivialize this. And if you look on the right,

you will see, even if you're not familiar with this, that first the response

comes later and that the response is smaller.

And that's also what's shown in the table there.

So we have a decreased amplitude there and a slowing of conduction velocity.

The first things that will happen with carpal tunnel syndrome is a slowing of

conduction velocity, especially a slowing of the sensory velocity.

When things tend to progress, we have an increase in motor latency and decrease in amplitude.

So these are the motor studies now. Now, again, on the left-hand side is a normal

study, and on the right-hand side, an abnormal study.

When we look at motor studies, we look at latencies, amplitudes, and velocities.

And as I see, this is a normal range for the patient. And on the right hand

side you see that the motor potential comes later and is of smaller amplitude.

So, just some takeaways.

Motor units have variable size and the accuracy is inversely proportional to their size.

If a motor unit is disconnected from the motor neuron, it will generate fibrillations

when fasciculations are discharges from a motor unit.

And recovery after nerve injury depends on the severity of the injury.

Thank you for your attention.