A recording of the electrical activity in muscles. An EMG can record electrical activity through surface electrodes applied on the skin or through fine needle electrodes inserted into muscle groups to measure the electrical impulses within them. Some people experience slight discomfort with the needle electrodes, but they provide much more information and are the standard of care for diagnostic examinations. Skin electrodes provide only very gross information about muscle activity and are largely relegated to interoperative monitoring, sleep monitoring, EEG monitoring, or other situations in which only gross information about whether a muscle is active is all that is required. The electrodes convey electrical signals to an amplification unit that converts the signals to visual patterns, which then display on a screen called an oscilloscope, or to audio signals, which transmit as sounds. Some amplification units present both visual and audio signals and can generate a print copy of the readings.
During an EMG the technician tells the person to perform movements to activate the muscle group and records electrical activity at reset. completing a typical EMG takes about 30 minutes to an hour, depending on the muscle groups tested. The doctor may order an EMG for muscle groups where there are spasms or tremors. Sometimes EMG is incorporated as an element of more comprehensive musculoskeletal function testing such as gait analysis or before and after surgery such as pallidotomy or thalamotomy to assess the surgery’s effectiveness objectively.
Many neuromuscular disorders have uniquely characteristic EMG patterns, which help to rule them out as the cause of symptoms. Differences can exist in the amplitude, frequency, and rhythm of electrical activity. In Parkinson’s disease, the EMG is normal except for the rhythmic alternating pattern of contraction and relaxation in opposing muscles affected by tremor. Hence, EMGs are rarely done unless to rule out other disorders. other neu-romuscular disorders show different patterns that may include combinations of increased or decreased activity during rest, extension, or contraction. For Parkinson’s disease, EMG, like other diagnostic tests, may provide one piece of the overall picture and not a definitive diagnosis.
