Electromyography (EMG) is the study of electrical functions of skeletal muscles, the activity of which can be used to assess muscle and nerve function. This method of examination is used whenever diseases of the peripheral nervous system are suspected, which includes muscles and nerves of the head, trunk, and limbs.
What is electromyography?
Electromyography determines the electrical activity of muscles. Depending on a muscle’s response to rest or tension, it can be determined if a particular condition is present. Electromyography determines the electrical activity of muscles. Depending on a muscle’s response to rest or tension, it can be determined whether a particular disease is present. While a muscle shows no electrical activity at rest, the corresponding muscle groups contract when the muscle cells are excited. This activity is measured by means of electrodes and then made visible and audible. Healthy muscles react differently than diseased muscles. Through the strength and type of current impulses acting during an electromyography, the physician can assess muscle functions as well as possible nerve and muscle diseases.
Function, effect, and goals
Before electromyography is used, a preliminary physical examination of the patient is necessary to make a tentative diagnosis. This is the only way to specifically examine the muscles. The skin area over the muscle to be examined is disinfected and then thin needle electrodes are inserted into the corresponding muscle. These measure the electrical voltage generated by the muscle at rest and when it is tensed. This voltage is displayed on a screen in the form of voltage curves and is also output through loudspeakers. Electromyography is divided into three steps. First, muscle activity is measured during insertion and at rest. Then, the activity is tested when the muscle is moderately tense. In a final step, muscle activity is determined at the greatest possible muscle tension. If the muscle or the associated nerve is damaged, deviating electrical activity is detected. The muscle action potentials can shorten or lengthen, for example, in their duration, and also decrease or increase in their potential curve. As a rule, between three and five muscles are examined during electromyography. By means of concentric needle electrodes, potential fluctuations of individual muscle groups can be derived. Special needles are used to record individual muscle fibers (single-fiber myography). Alternatively, surface electrodes can be applied to the respective muscle, but this method does not allow conclusions to be drawn about the activity of individual muscle fibers, since the action potential of entire muscles or several muscle groups is measured here. An electromyography takes about 15 to 30 minutes, during which the puncture site and puncture depth are changed several times. Electrical activity patterns of the examined muscle resulting from an electromyography make it possible to distinguish between muscular and nerve-related diseases. For this reason, this examination method is used, for example, to diagnose muscle weaknesses, muscle inflammations, nerve injuries and to differentiate between certain nerve diseases (polyneuropathies). EMG is also used in cases of suspected spinal cord diseases. Electromyography is often combined with electroneurography (ENG), which is used to measure nerve conduction velocity. In some diseases, electromyography can also be used to make prognostic statements regarding the course of healing, for example, in the case of nerve injuries resulting from an accident or pressure-induced nerve damage and also in certain types of muscle inflammation. In addition, various treatment methods of chronic or acute nerve or muscle inflammation sometimes require an exact electromyographic classification of the respective disease.
Risks, side effects, and hazards
Usually, no serious complications occur during electromyography. The insertions of the needle electrodes, which are much thinner than the hypodermic needles used to draw blood, can be compared to those used in acupuncture. The examined muscle or nerve fibers are not damaged by electromyography.However, the muscle may still hurt or feel numb for a few days after the examination. Electromyography should not be performed if blood coagulation is disturbed by a disease or if anticoagulant medication is being taken, due to the increased risk of bleeding. Since the needles used in electromyography can transmit skin germs to deeper tissue layers, infections are possible but occur extremely rarely. If the patient suffers from blood-borne diseases (AIDS, infectious hepatitis), it is imperative that the examiner be notified so that appropriate protective measures can be taken.
