Electromyography (EMG) is a diagnostic tool that assesses muscle-nerve cell interaction. It can help determine the health of nerve cells and muscles, and can be used in diagnosing Lambert-Eaton myasthenic syndrome (LEMS).
LEMS is an autoimmune condition in which the immune system mistakenly attacks the nerve endings. This results in a shortage of neurotransmitter, the chemical agent released by nerve cells to communicate with the muscles, and instruct them to contract and relax. The shortage of neurotransmitter results in muscle weakness.
How EMG works
EMG measures the electrical activity in muscles and nerve cells to determine muscle response and strength. A neurologist or a trained technician can perform the test.
A traditional EMG has two parts. One involves measuring the electrical activity of muscles using thin needles called electrodes that are inserted into the muscles. These electrodes read the tiny electrical impulses sent by nerve cells to the muscles. The signals are amplified by the electrodes, and converted into graphs that are read by the specialist. The second part is called a nerve conduction study (NCS), which measures the ability and speed of nerve cells to relay signals. A very low-intensity electric current is applied using a small flat electrode placed on the skin, and the response of the nerve cell is recorded. The speed at which the electrical signal is relayed by the nerve cell determines the degree of damage.
The nerve response is usually very low in LEMS patients. One of the symptoms of LEMS is a sudden increase in muscle strength immediately after the start of exercise. Given that, the muscles may be exercised for about 10 seconds before the test. This stimulates the muscles, and an increase in nerve response — called an “increment” — may occur. An increment of more than 100% may be indicative of LEMS.
In some cases, a traditional EMG may be inconclusive. Such patients can benefit from a more sensitive EMG called single-fiber electromyography (SFEMG). This is a specific type of EMG that is used to study the neuromuscular junction, or the point where the nerve cell meets muscles. An increase in irregular transmission, known as a neuromuscular jitter, is indicative of LEMS.
There are two types of SFEMG: stimulated and voluntary. In stimulated SFEMG, a thin needle electrode is placed in the cheek muscle, and another is placed above the eyebrow. Clinicians measure the twitch in the eye and face muscles due to stimulation by the electrodes. In voluntary SFEMG, electrodes are placed on the face in a similar way to stimulated SFEMG. Patients are asked to make small movements, such as looking up and down, and the response of the muscles are recorded.
An EMG can take anywhere between 30 and 90 minutes.
Before an EMG
Neurologists must be informed in advance about any pre-existing medical condition, such as hemophilia, or the use of medications like blood thinners or herbal supplements. They also must be made aware of any internal electrical devices, such as pacemakers. Some medicines may interfere with the tests, and the doctors may ask those to be discontinued until after testing.
No lotions or creams should be used before the test. Bathing prior to testing is recommended to remove oils from the skin, which can interfere with the EMG.
Patients are recommended to wear loose clothing for the test, although most testing centers provide gowns to be worn during testing.
Slight pain and discomfort should be expected during EMG because small needles are inserted at multiple places in the body. However, patients should inform the neurologist or the technician if the pain is excessive.
When the nerves are stimulated for testing, it is normal for patients to experience slight tingling.
After an EMG
Overall, EMG is a safe procedure. No specific care is required after an EMG. There may be slight soreness and some bruising due to the needle insertion, but such effects heal within days of the test.
Last updated: July 22, 2019
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