If you have numbness, tingling, pain, weakness, or muscle cramps, your doctor might have ordered
a test called electromyography or EMG. If you looked up on the Internet,
you might be afraid of doing this test because it involves giving electrical shocks to your skin
and sticking needles on your muscles. Today, I will explain why we do these
tests and what you can expect when your doctor sends you to an EMG test.
At the end, I will also answer some common questions that patients ask me
when I tell them that I am sending them to an EMG with nerve conduction study.
So, let’s talk about EMG today.
The nerves are like wires that connect our brain to the rest of the body. The brain
receives information from the periphery, we call these “sensations”. And the brain
sends orders to the muscles and bones to move, we call these “motor function”.
The nerves are like roads and highways in a city, carrying information back and forth.
In roads with pavement, like concrete or asphalt,
the speed is much higher than rural roads covered by gravel.
There are also single lane roads or multiple lanes highways. The speed limit can be the same,
but the number of cars that travel is much more with multiple lanes than single lane roads.
We have a similar situation with nerves. This is a nerve,
the nerve contains the axons that are the raods carrying information back and forth to the brain.
Some axons are covered by myelin. The myelin speeds up the electrical transmission.
The more myelin around an axon the higher the speed, it is like having
a well paved road by concrete or asphalt. The nerves that do not have myelin or have
a little bit of myelin are similar to rural roads covered by gravel.
We also have differences in the amount of information the nerves can carry,
it is like roads with more lanes, in other words, more axons, and others that carry fewer
information because they don’t have many lanes, that is the same for nerves have fewer axons.
When we talk about EMG, we are actually talking about two tests:
The nerve conduction study is usually done first, and it will examine the speed of the nerves, and
the results will be measured in meters per second. The second part is the EMG test, and it will
basically examine how many lanes are opened in the road, but not the speed.
The nerve conduction study can be done in one arm, both arms, one leg, both legs, or all four limbs.
The EMG part can be done in any part of your body that has a muscle, including the face,
the tongue, neck, lower back, arms and legs. The nerve conduction study involves electrical
shocks to your skin, and the EMG part involves sticking needles in the muscles.
First, let me explain the nerve conduction study: In the arm, there are mainly three nerves that can
be examined for their speed: the radial nerve, median nerve and ulnar nerve. In the leg, there
are mainly three nerves that can be examined: the tibial nerve, the fibular and the sural nerve.
The speed in each nerve can be measured for the sensation fibers
or for the motor fibers. Usually speed of the motor fibers is higher than the sensory fibers,
because the motor fibers have more myelin. And here is how we measure the speed:
This device will give an electrical shock to the nerve,
here it is the median nerve above the wrist, and these are surface electrodes that will
detect the shock that was discharged by this device. The technician will then measure the
distance between the device emitting the electrical shock and the sensor.
The machine will tell the time it took for the electrical current go from point A to point B.
Then, the technician will calculate the speed in meters per second. Normal speeds
are usually around 50 to 60 meters per second. If the calculated speed is lower than expected,
then, it might be there is a lesion in the nerve here.
The speed of a nerve is determined by the amount of myelin around the nerve. If the
myelin is damaged, then the speed in this region will be decreased. The analogy is like an asphalt
road that is damaged and the cars can’t go fast. There are some diseases that cause a problem of
the myelin, for example multiple sclerosis or MS and diabetes.
Now, let me explain the electromyography part. In order for us to know how many axons are
getting to the muscle, or in our analogy, how many lanes of the highway are opened,
we must ask the muscle to contract and then we can see and hear the recruitment of the muscle fibers.
If all axons are connected to the muscle, or all lanes are opened, then we will see
and hear a typical noise that shows a lot of activity in the screen.
If we see a few fibers being recruited, then we know there is some blockage or loss of axons.
Some people get scared with the word “needle”. Here is an EMG needle. Well,
in fact this is not a needle, like one to give an injection. This is a monopolar electrode.
Here is an injection needle and
it is thicker than the EMG needle. But the EMG needle is thicker than an acupuncture needle.
So, the doctor will stick this electrode inside the muscle, then will look at the computer
screen and will ask the patient to contract the muscle slightly, then stronger, then very strong.
The doctor will also pay attention to the sound that is coming from the muscle.
The higher the sound it means there are more axons working, or more lanes opened.
There are diseases that affect the axons, like amyotrophic lateral sclerosis or ALS,
poliomyelitis, diabetes and alcoholism.
I would like to mention that we can also ask an EMG NCS to look at the recovery
after a nerve injury. For example, when a person has an accident that damages a nerve,
the muscles innervated by that nerve stop working and we call this paralysis.
The sensations from that part of the body are also lost and we call this numbness or anesthesia.
When we do EMG and NCS of this nerve we can see if there is nerve regrowth and how much
longer this is going to take. It is usually very good news to give to the patient that
their paralysis and numbness will likely recover. There is a disease called carpal tunnel syndrome,
that is a pinched nerve here at the level of the wrist.
Sometimes we can avoid surgery by doing a nerve conduction study, even without EMG. Because we
can demonstrate that the nerve is not so severely damaged, and we can postpone or avoid surgery.
Some common questions that I get from my patients when I send them to NCS/EMG exam:
1.What to expect when your doctor sends you to a EMG NCS?
First, you need to know how many parts of your body the doctor will examine.
One limb, two, three or all four limbs? In general it takes 30 minutes for each limb,
so, if your doctor asked for both arms and both legs, you
can expect 30 minutes times four which is 2 hours. First the technician will do the nerve conduction
studies, this involves attaching a lot of electrodes to your skin and then giving
electrical shocks to a distant part of the skin. The second part is done by a medical doctor,
they will insert the electrodes in your skin and look at a monitor for the shapes of the waves
and also hear some sounds coming from your muscles. They will ask you to move your muscles
while the needle is inside the muscle. 2. Do I need to prepare for the exam?
I usually tell patients not to apply any oil or cream to their skin
for a few days before the test. There is no need to stop eating
or drinking any liquids right before the exam. 3. Is there any contraindication to the test?
People with pacemakers, central lines, defibrillators and other implanted devices
that depend on electrical current should not take part on the nerve conduction studies.
Also, tell the doctor if you take any blood thinner, that is important to know for the
EMG part of the exam. 4. Is it painful?
The nerve conduction part is uncomfortable because the shocks are like tingling,
but not painful. The electromyography can be painful because of the needles.
For some people it can be very painful, and for others they don’t bother at all.
The majority of the people are in the middle. If during the exam you find that the pain is
too much, you may ask the doctor to modify the technique or stop completely the test.
5. Can I take some analgesics to feel less pain prior to the test?
Yes, you can take your usual painkillers prescribed by your doctor,
or an over-the-counter acetaminophen or anti-inflammatory.
6. Will the test show if I have a pinched nerve? Yes, when there is a pinched nerve like a
radiculopathy, carpal tunnel syndrome, or ulnar neuropathy,
the nerve conduction and the electromyography will show if myelin and/or axons are damaged.
7. I have tingling and numbness and my NCS/EMG study is normal. What does that mean?
The NCS/EMG is only capable to examining large nerve fibers. There are very tiny nerve fibers
that cannot be examined by NCS/EMG. This is called small fiber neuropathy, the best way to examine
these fibers are by doing a sensory exam using a cotton ball and a sharp object on the skin.
Please, don’t forget that this video is for educational purpose only.
If you have a condition that needs medical advice, please talk to your doctor.
And if there is an emergency, go to the nearest emergency department.
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