EMG

Electromyography (EMG)

Electromyography (EMG) is a diagnostic procedure used to assess the health of muscles and the nerve cells that control them (motor neurons). EMG results can reveal nerve dysfunction, muscle dysfunction, or problems with nerve-to-muscle signal transmission.

History

The development of electromyography began in the early 20th century. The first EMG recordings were made by Duchenne de Boulogne in the 19th century, but it was not until the 1920s that Joseph Erlanger and Herbert Gasser developed the cathode ray oscilloscope, which allowed for more precise recordings of electrical activity in muscles.

Principles of EMG

EMG measures the electrical activity of muscle tissues. When a muscle contracts, it produces electrical signals. These signals are detected by electrodes and are displayed as waveforms on a monitor. The two main types of electrodes used in EMG are:

• Surface electrodes: These are placed on the skin over the muscle being tested. They are non-invasive and are used for general assessments.
• Intramuscular electrodes: These are needle electrodes inserted directly into the muscle. They provide more detailed information about muscle activity.

Procedure

The EMG procedure involves several steps:

1. Preparation: The skin is cleaned to remove oils and dirt, which can interfere with the signal.
2. Electrode placement: Depending on the type of EMG, either surface or needle electrodes are placed.
3. Recording: The patient is asked to contract the muscle, and the electrical activity is recorded.
4. Analysis: The waveforms are analyzed to assess the muscle's electrical activity.

Clinical Applications

EMG is used in the diagnosis of several conditions, including:

• Carpal Tunnel Syndrome
• Amyotrophic Lateral Sclerosis (ALS)
• Myasthenia Gravis
• Peripheral Neuropathy

Interpretation of Results

EMG results are interpreted by a neurologist or a specialist in physical medicine and rehabilitation. The key aspects analyzed include:

• Amplitude: The size of the electrical signal, which can indicate the number of muscle fibers activated.
• Frequency: The rate of signal firing, which can indicate the speed of nerve conduction.
• Waveform shape: Abnormal shapes can indicate nerve or muscle damage.

Risks and Considerations

EMG is generally safe, but there are some risks and considerations:

• Discomfort: Needle electrodes can cause discomfort or pain.
• Bleeding or bruising: At the site of needle insertion.
• Infection: Rare, but possible at the needle insertion site.

See Also

• Nerve Conduction Study
• Electroencephalography
• Neurophysiology