What is electromyography (EMG)?
Electromyography is a neurological diagnostic procedure used to record electrical signals from muscle cells. The electrical properties (electrophysiology) of healthy and diseased muscle cells may differ, and these differences can be detected by measuring the electrical currents and voltages of the muscle cells.
To record the electrical signals from muscle tissue, a fine needle (needle EMG) or surface electrodes (surface EMG) must be inserted. Needle EMG is frequently used in the diagnosis of ALS. Surface EMG is currently used primarily in research settings.
During a needle EMG examination, a physician experienced in electrophysiology inserts a special EMG needle (electrode) into the muscle tissue. The EMG needle is connected via a thin cable to an amplifier and an analysis unit (EMG device). The EMG machine provides the neurologist with a visual representation of the pattern of electrical potentials in the muscle tissue. In addition, the electrical signals are converted into an acoustic signal, which provides the neurologist with further important information about the electrical properties and changes typical of ALS.
A typical EMG examination involves the neurologist inserting a fine EMG needle into various locations within one or more muscles while observing the monitor (a graphical representation of the electrical potentials) and simultaneously listens for the acoustic signal audible through the EMG machine’s speakers. During the EMG examination, the neurologist asks the patient to relax the muscles or contract them in a specific way. In particular, contracting muscles while the needle is in place can be experienced as uncomfortable or painful. However, patients’ reactions to this sensation vary greatly: The majority of patients undergo the examination without discomfort, while others find the procedure stressful or can tolerate it only to a limited extent, despite the electrode’s thinness. Overall, the EMG examination is a common diagnostic procedure used for supplementary and differential diagnosis in ALS.