Electromyography (EMG) is used to test the electrical activity of a skeletal muscle.
Who is a candidate for the test?
An EMG is used to detect disorders that mainly affect the muscles. It is also used to diagnose muscle problems caused by other diseases, such as nerve dysfunction. An EMG can show the difference between true weakness and the reduced use of a muscle due to pain or lack of motivation. Peripheral nerve damage, known as peripheral neuropathy, can also be detected on an EMG.
How is the test performed?
A needle electrode is placed into a skeletal muscle. Skeletal muscles are attached to bones, and are the muscles that control posture and movement. The electrical activity of the muscle is displayed on an oscilloscope as an electrical waveform. An amplifier can be used so that the sound of the electrical activity can also be heard. During the test, the person may be asked to contract the muscle slowly. The test takes about 30 to 60 minutes. There may be some discomfort when the needle electrode is inserted.
What is involved in preparation for the test?
There is usually is no special preparation for an EMG. The doctor can provide any specific instructions.
What do the test results mean?
Disorders or conditions that cause abnormal results include:
- polymyositis, or inflammation of the skeletal muscle tissue
- muscular dystrophy, a group of inherited muscle disorders that cause muscle weakness
- myopathy, which means any disease or abnormal condition of muscles
- traumatic injury
- hyperadrenalism, or increased secretion from the adrenal gland
- hypothyroidism, or decreased secretion from the thyroid gland
- carpal tunnel syndrome, which is pain or numbness that affects the hand
- sarcoidosis, a disease in which abnormal collections of inflammatory cells form in many organs of the body
- Guillain-Barre syndrome, an autoimmune disease in which the myelin sheathes of nerves are destroyed, causing a loss of motor function
- myasthenia gravis, an autoimmune disease that causes muscle weakness
- spinal cord injury
- multiple sclerosis, an autoimmune disorder of the central nervous system
- diabetic neuropathy, which causes a painful tingling or burning sensation in the hands and feet
- amyotrophic lateral sclerosis, a progressive disease marked by muscle weakness
Mosby's Manual of Diagnostic and Laboratory Tests, Pagana and Pagana, 1998.