Electromyography (EMG) is a procedure performed to diagnose conditions that affect muscle function and strength. EMG measures the electrical activity of muscles and the associated nerves called motor neurons.
What can an EMG diagnose
Physicians may perform an EMG if a patient feels tingling, numbness, pain, spasms, or weakness in the muscles. EMG helps diagnose conditions that interfere with normal muscle contraction, such as muscle disorders, nerve disorders, and disorders of the neuromuscular junction (connection between a nerve fiber and a muscle). EMG can also determine damage in the peripheral nerves, or those outside the brain and spinal cord, by testing different muscles.
EMG and Charcot-Marie-Tooth disease
EMG is usually the first test that performed to diagnose Charcot-Marie-Tooth disease (CMT). It measures the strength and speed of electrical signals down the peripheral nerves and the electrical activity in the muscles. The motor neurons in patients with CMT transmit electrical signals at a very slow speed, about 20 to 25 meters per second.
How is an EMG performed
Muscles contract in response to electrical signals transmitted by motor neurons. This muscle contraction itself produces electrical signals that can be translated into numbers, graphs, or sounds using EMG.
EMG is performed in two parts. In the first part, the physician performs a nerve conduction study (NCS) to measure the ability of nerves to transmit electrical impulses to their respective muscles. NCS involves applying small, brief electrical shocks to the nerve and then recording the nerve’s response.
In the second part, the physician measures the electrical activity in the muscles. This involves inserting a needle electrode into the muscle that receives the electrical signal during muscle contraction. This signal is then transmitted from the needle electrode to a receiver or amplifier that is connected to a device that displays readouts in the form of graphs or numbers.
The procedure may cause mild discomfort that can be managed with an anesthetic or sedative cream.
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