Electromyography (EMG) is a procedure used to diagnose disorders that affect muscle and nerve health, including Charcot-Marie-Tooth disease (CMT). It measures the electrical activity connecting muscles to specialized nerve cells called motor neurons.
What can EMG diagnose?
Physicians may perform an EMG if a patient feels tingling, numbness, pain, cramping, or weakness in the muscles. EMG helps to diagnose conditions that interfere with normal muscle contraction, such as muscle disorders, nerve disorders, and disorders of the neuromuscular junction (connection between a nerve and muscle cells).
Relevant to CMT, this procedure can determine damage in the peripheral nerves, those outside the brain and spinal cord.
EMG and Charcot-Marie-Tooth disease
The speed with which electrical signals are transmitted in people with CMT is slower than normal.
In those with CMT type 1, the disorder’s most common and characterized by damage to the myelin sheath (the insulating cover of nerve extensions), nerve conduction velocity is usually less than 38 meters per second (m/sec). Notably, normal values are greater than 50 m/s.
In CMT type 2, characterized by damage to nerve extensions (axons), conduction velocity is higher than 38 m/s, while in people with congenital or early childhood forms (CMT type 3), values are typically less than 20 m/sec. Velocity values in patients with dominant intermediate CMT are in the intermediate range (24 to 50 m/s).
How is an EMG performed?
Muscles contract in response to electrical signals transmitted by motor neurons. These signals can be translated into numbers, graphs, or sounds using EMG.
EMG is performed in two parts. In the first part, a nerve conduction study (NCS) is conducted to measure the ability of nerves to transmit electrical impulses. This involves applying mild electrical impulses, via electrode patches, to the skin. Conduction speed is calculated by measuring the distance and the time taken by electrical impulses to travel between the electrodes.
The test’s second part focuses on muscle health and function. This involves inserting a tiny needle into the muscle, so that it records the signal when the muscle is at rest or when it contracts. The procedure can cause a pinprick sensation when the needle is inserted.
An EMG is considered safe, but patients may experience bleeding, especially if they are using blood thinner medications. Bruising at the site where the needle was inserted and temporary muscle soreness are also possible.
Last updated: Oct. 21, 2021
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