Electromyography (EMG)

Electromyography is a minimally invasive diagnostic study that measures electrical activity produced in a muscle in response to stimulation by a nerve. The electrical activity is analyzed while the muscle is at rest, slightly contracted, and fully contracted. The procedure typically involves inserting a fine needle into the target muscle to capture the electrical signals, which are displayed on a screen in the form of waves and may also be heard through an audio amplifier.

In back and neck pain conditions, EMG may be used to analyze the severity of radiculopathy, muscle weakness caused by spinal nerve compression, and the number of spinal nerves involved.1 It can also help determine if weakness is due to a degenerative muscular disorder, such as spinal muscular atrophy, among others.

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When EMG May Be Useful for Back or Neck Pain

EMG is generally useful when motor deficits, such as muscle weakness, are present in radiculopathy. Nerve root impingement or compression that causes purely sensory symptoms, such as pain, are not quantified by EMG.1,

An EMG referral is typically considered when the symptoms and signs of radiculopathy need to be correlated with the results of imaging tests. The presence of an imaging lesion in addition to EMG evidence of altered nerve function may suggest significant nerve root compromise.1,

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EMG Procedure for Back or Neck Pain

An EMG test is done on an outpatient basis in a clinic or a hospital. Neurologists and physiatrists trained in performing the procedure conduct the test.

  • Patients usually change into a hospital gown after removing personal clothing, jewelry, and metal objects that may interfere with the test.
  • The study may be performed in a sitting or lying-down position.
  • Once the target muscle is identified, the skin over the area is cleaned with an antiseptic and a fine needle is inserted into the muscle. A ground electrode is positioned under the patient’s arm or leg.
  • A few more muscles are always tested in a similar fashion to help isolate the cause of weakness.
  • The physician guides the patient to perform basic muscle contractions, such as bending or extending the leg, with little or full force, with periods of relaxation in between.
  • The electric signals from the muscle are displayed on the computer screen. An audio amplifier may be used to evaluate the sound associated with the signals, each of which has a unique sound further helping to determine which muscles are compromised, how severely, and for what duration.

Slight pain and bleeding may be experienced at the site of needle insertion. This discomfort is usually temporary and resolves within a few hours after the test.

Nerve Conduction Study and EMG

A related test, the nerve conduction study (NCS), is often performed alongside an EMG. NCS measures the speed of conduction and electrical signals of a particular nerve supplying a specific set of muscles (a myotome). By analyzing the waveform generated from the electrical pulse that passes a single or set of nerves, the extent of nerve damage can be assessed.

Purpose of an NCS test

In diagnosing radiculopathy, NCS is useful in ruling out conditions that mimic spinal nerve compression. For example, L5 radiculopathy (compression of the L5 spinal nerve at the L5-S1 spinal segment) and peroneal neuropathy (compression of the peroneal nerve near the knee) can both cause foot drop. NCS is typically normal in radiculopathies and shows abnormal neural activity in other nerve conditions, helping to differentiate between the two.1

NCS test procedure

NCS is an independent test and may be performed before or after the EMG. The NCS procedure involves taping an electrode over the skin and passing small, mild bursts of electric current to a specific nerve. The response of the target nerve is displayed on the computer screen.

Slight discomfort may be felt during the process, but the test is typically not considered painful. Caffeinated drinks are usually avoided a few hours before this test to prevent caffeine-induced alterations to the study results.

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The findings of EMG and NCS when performed by trained physicians can help narrow down the cause of nerve damage and pain. The results of imaging tests combined with the findings of the physical electrodiagnostic studies can improve the accuracy of a diagnosis and help physicians formulate a targeted treatment plan.

When performed and interpreted accurately, EMG and NCS may be valuable in ruling out, diagnosing, and/or treating radiculopathy stemming from the spinal nerves in the neck or back.

Read more about Getting an Accurate Back Pain Diagnosis

References

  • 1. Sandoval AEG. Electrodiagnostics for low back pain. Physical Medicine and Rehabilitation Clinics of North America. 2010;21(4):767-776. doi: 10.1016/j.pmr.2010.06.007 Wright R, Inbody SB. Radiculopathy and degenerative spine disease. In: Neurology Secrets. Elsevier; 2010:121-130.
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