Electromyography (EMG)

General Description

Electromyography (EMG) refers to a set of electrophysiological techniques—methods that study and record the electrical activity of the body—used to evaluate the functioning of the peripheral nervous system and skeletal muscles (those that move voluntarily). These tests allow the detection of deficiencies in nerve function, muscle function, or the transmission of signals between nerves and muscles.

During the electromyography process, two procedures are performed:

• Electroneurography (ENG): a noninvasive study of nerve conduction and the physiological status of peripheral nerves. The speed and strength with which electrical impulses are transmitted from the skin surface until a muscular response is obtained are measured.
• Needle electromyography: a minimally invasive technique in which the electrical activity of muscles is recorded both at rest and during movement.

Electromyography is adapted to the symptoms presented by the patient, as well as to the pathological findings detected in previous tests. Therefore, only the nerves and muscles that may be damaged are examined.

When is it indicated?

Electromyography is indicated in patients who present with any of the following symptoms:

• Muscle weakness.
• Loss of strength.
• Loss of muscle mass.
• Numbness.
• Tingling.
• Cramps.
• Pain.

It is a highly effective technique for diagnosing and monitoring the progression of the following diseases:

• Muscular dystrophy: progressive hereditary diseases that cause muscle weakness.
• Polymyositis: an autoimmune inflammatory disease that causes pain, weakness, and inflammation of skeletal muscles.
• Amyotrophic lateral sclerosis (ALS): a degenerative neuromuscular disease that accelerates the degeneration and death of neurons.
• Myasthenia gravis: a chronic autoimmune disease that causes muscle weakness and fatigue.
• Poliomyelitis: an infectious disease in which neurons in the spinal cord and brainstem become inflamed.
• Myopathies: a group of diseases that cause dysfunction of muscle fibers.
• Polyneuropathies: disorders that simultaneously affect several peripheral nerves, which do not function properly.
• Radiculopathies: commonly referred to as a "pinched nerve," because they reduce the sensory or motor function of the nerve root.
• Peripheral neuropathy: dysfunction of the nerves, which do not properly transmit information from the brain or spinal cord, resulting in loss of sensation and pain.
• Carpal tunnel syndrome: the median nerve is compressed within the bony canal of the wrist.
• Guillain-Barré syndrome: an autoimmune disorder in which the immune system attacks the peripheral nervous system.

How is it performed?

The procedure for performing electromyography is as follows:

• Electroneurography: electrodes are placed on the skin and secured with adhesive along the path of the nerve to be studied. These electrodes are connected by a cable to a device that emits electricity to stimulate the nerve and record its response at the most distal point. Conduction velocity and response amplitude are measured, that is, the number of nerve fibers functioning properly.
• Needle electromyography: an electrode is inserted directly into the muscle to evaluate its response to an electrical stimulus. The procedure records the response at rest (to detect abnormal activity), during minimal contraction (to identify whether the damage has a nerve or muscle origin), and during maximal contraction (to help determine the severity of the lesion).

In both cases, electrical activity is recorded in comparative charts in which one line represents the response of the nerve under study and another waveform shows the reference values. The recorded parameters are the following:

• Latency (L): time, expressed in milliseconds (ms), between the stimulus and the onset of the response.
• Conduction velocity (CV): the distance divided by the propagation time of the electrical signal. It is expressed in meters per second (m/s).
• Amplitude (A): the maximum height of the recorded wave. It may indicate:
  • Compound nerve action potential: sensory capacity, measured in microvolts (µV).
  • Compound muscle action potential: motor capacity, measured in millivolts (mV).
• Morphology: shape, size, and characteristics of the electrical waves recorded during the procedure. Some of the features that may appear include:
  • Fibrillation potential: a brief abnormal signal representing a spontaneous discharge of muscle fibers. It usually indicates denervation or muscle inflammation.
  • Polyphasic wave: presents more cycles than the two typical phases (positive and negative) seen in healthy muscles. It therefore indicates neuromuscular injury or reinnervation.
  • Long-duration wave: the response lasts longer than expected. It may be indicative of ALS or neuropathy.

Risks

Neurography does not pose a health risk to patients, although some may develop a rash in the area where the electrode was attached. However, although rare, some side effects of needle electromyography include:

• Mild external bleeding at the site where the needle electrode is inserted.
• Bruising.
• Muscle infection caused by bacteria entering from the skin surface.

What to expect from an electromyography

On the day of the procedure, patients can attend the appointment without prior preparation, although it is advisable to avoid creams and gels. Once in the examination room, the patient lies on their back on the examination table with the arm uncovered. Sensations vary depending on the specific test performed:

• Electroneurography: a mild electric shock or tingling sensation may be felt when the electrical impulse is delivered, but it is not a painful procedure.
• Needle electromyography: when the electrode is inserted, some patients experience mild discomfort while others may feel pain. Both sensations disappear once the needle is removed. When electrical currents are applied, a transient tremor or cramp may be felt, but it does not usually cause intense pain.

After the procedure, bruising is common and usually resolves within several days.

Results are received several days later during the specialist consultation, as the recorded information must be carefully studied and interpreted.

Specialties in which electromyography is requested

Electromyography is performed by specialists in Clinical Neurophysiology, usually at the request of Neurology.