What is an EMG and NCS (Electromyography and Nerve Conduction Study)?

What is so special about an EMG/NCS?

Electromyography (EMG) and Nerve Conduction Studies (NCS) are two types of tests that take advantage of the fact that your brain, nerves, and muscles use electricity to function. Although sometimes the term “EMG” is used to also include a NCS, they are really two separate and distinct tests. 

Electromyography (EMG) looks at how well you can turn on a muscle.  To activate a muscle, you first have to think about using it;  the signal from your brain gets transmitted along your nerves to the muscle; the nerve electrically excites or turns on the muscle.  The muscle then electrically “fires” or contracts moving your arm or leg.  An EMG uses a sterile wire as an antenna to look at individual muscles and see how well they turn on.

A Nerve Conduction Study (NCS) focuses more on the nerves themselves.  We know how fast an average nerve conducts an electrical signal.  They conduct at an amazing speed of 60 meters per second.  Put in miles per hour, that is an amazing 134 m.p.h.!  If the nerve conducts slowly or the size of the electric impulse is small, we know that the nerve is injured in some way. In a NCS, a small electrical impulse is applied to the nerve and we measure how long it takes to go from point A to point B along a nerve..

To perform an EMG and/or a NCS, we use a specialized computer that is very sensitive and can measure signal speeds down to the millisecond (ms) and electrical impulses down to microvolts (uV).  That’s a very small amount of electricity! To see how these tests are performed, view this AANEM website, the professional medical society for EMG/NCS.


What are we looking for on an EMG/NCS?

We are looking for an injury to nerves or muscles when using these tests.  What separates these tests from others is that they are functional tests.  Oftentimes we obtain radiologic studies, i.e. xrays, MRIs, or ultrasounds, to look at the structure of the body.  Those are anatomic tests.  I make the comparison of bringing your car into the auto shop.  The mechanic can lift the hood and look at the engine and its various parts.  Or, he can hook it up to a diagnostic computer and see how well the engine is running.  An EMG/NCS is like hooking your body up to a diagnostic computer to see how well it is running. 


An EMG/NCS is useful in diagnosing what types of injuries or illnesses?

These studies are useful in evaluating complaints of numbness, tingling, or weakness in your hand, arms, or legs.  Examples would be Carpal Tunnel Syndrome, Ulnar Neuropathy, or a nerve injury due to a traumatic event. People with a peripheral neuropathy, where the ends of the nerves die off or are unhealthy, may have similar symptoms.

Some people have symptoms that can be traced to an inherited or acquired muscle disease, such as  Duchenne’s Muscular Dystrophy, Polymyalgia Rheumatica, or dermatomyositis.   Adverse drug reactions can also cause muscle injury as well.  Other times, people may develop an infection, such as polio, which affects the nervous system.  An EMG/NCS is useful in these cases to help make the diagnosis.


When to get an EMG/NCS?

When a nerve or muscle injury or illness is suspected, it is useful to obtain an EMG/NCS.  It can confirm a diagnosis and also often pinpoint the location of the injury.  However, it is just one tool that a physiatrist, a specialist in bone, joint, muscle, and nerve, may use in the care of patients with suspected nerve or muscle problems.  If you have similar issues, make a consultation appointment at the office of Frank Y. Wei, M.D., PLLC to have your issues evaluated, diagnosed, and treated.

Frank Y. Wei, M.D.

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