What Do Nerve Conduction Studies and Electromyography Diagnose?

Electromyography and nerve conduction studies use electrodes to test the function of nerves and muscles to diagnose various problems.

Nerve conduction studies and needle electromyography (EMG) are tests performed to assess the health of nerves and muscles. A neurophysiologist stimulates specific nerves and muscles and studies the resulting activity to evaluate if the nerves and muscles are functioning normally.

Electromyography

A thin needle electrode is inserted into the muscle. The muscle’s electrical activity displays on the EMG’s monitoring device, highlighting to doctors important functional information such as:

• Response to electrode insertion
• Spontaneous electrical activity during rest
• Electrical activity during voluntary muscle movement

Nerve conduction studies

Nerve conduction studies are performed with electrodes placed on the skin to stimulate specific nerves. A mild electric impulse stimulates the nerve, and the resulting neural activity demonstrates how well the nerves are conducting

• Signals of sensation from the tissues to the brain
• Signals from the brain to the muscles to function 

An EMG test is used to assess if a muscle functions normally when stimulated by its nerve, while nerve conduction studies are performed to test if specific nerves function normally.

EMG and nerve conduction studies are commonly performed together to diagnose whether a neuromuscular disorder is caused by muscle or nerve damage, in order to accurately diagnose and provide appropriate treatment. These tests are generally performed only when physical examination does not reveal the cause of the disease.

An EMG and nerve conduction studies may be performed when a person has unexplained neuromuscular symptoms such as:

• Muscle cramps and pains
• Tingling and numbness
• Muscle weakness
• Muscle paralysis

EMG and nerve conduction studies are not performed on people who have

• Temporary external pacing wires for cardiac procedures
• Guidewires for catheter placement

EMG and nerve conduction studies are performed with caution in people who have implanted electrical devices such as:

• Cardiac pacemaker
• Implantable cardioverter-defibrillator
• Spinal cord stimulator

EMG and nerve conduction studies are useful in diagnosing neuromuscular diseases such as:

• Muscular dystrophy: progressive weakness and loss of muscle mass
• Radiculopathy: pain and weakness caused by pinched nerves from herniated discs or degenerative disc disease
• Peripheral neuropathy: nerve pain from damage to peripheral nerves
• Myasthenia gravis: muscle weakness due to problems in the nerve and muscle junction
• Carpal tunnel syndrome: pain, numbness and tingling in the arm due to compressed nerve
• Guillain-Barre syndrome: a condition where the body’s immune system attacks the nerves in the arms and legs
• Charcot-Marie-Tooth disease: an inherited condition which affects the nerves and causes weakness of foot and lower leg muscles

EMG and nerve conduction studies are usually performed by a neurophysiologist, as an outpatient procedure. A technician operates the device while the neurophysiologist studies and interprets the results for a diagnosis. The tests may take one or two hours depending on how many muscles and nerves are tested.

Preparation

The patient may be required to

• Shower but must not use any lotions or oils on the skin
• Avoid caffeine and sugary beverages for two to three hours prior
• Avoid smoking
• Inform the doctor of any recent infections
• Inform the physician if they have any implanted electrical devices 

Procedure

Needle electromyography

• The neurologist inserts a thin needle electrode into the muscle that is to be tested.
• The muscle’s electrical activity is transmitted to the EMG machine.
• The neurologist will instruct the patient to flex or relax the muscle.
• The corresponding nerve may also be stimulated to record the muscle’s response.

Nerve conduction studies

The technician

• Applies a gel over the skin and places electrodes at different locations along the specific nerves.
• Delivers a small impulse of electric current through the stimulating electrode.
• The other electrodes record the nerve’s response and the time for conduction of the signal.
• The nerve response is recorded in the device and appears on a monitor.

The nerve conduction study may cause a certain amount of discomfort when the mild shock is delivered to the nerve. There usually is no pain after the procedure. The insertion of EMG needle can cause a pain like a muscle injection. The muscle may feel sore for a few days after the test.

EMG and nerve conduction studies are low-risk procedures. A very mild electrical impulse is used in the nerve conduction study that is unlikely to cause any harm. With the needle insertion during EMG procedure there may be a small risk of:

• Infection
• Bleeding
• Nerve injury