Electromyography

Electromyography (EMG) is a medical technique for evaluating and recording physiologic properties of muscles at rest and while contracting. EMG is performed using a instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these cells contract, and also when the cells are at rest.

Electrical Characteristics

The electrical source is the muscle membrane potential, about -70mV. Due to the applied method the resulting measured potentials range between smaller than 50μV and 20 to 30mV.

Typical repetition rate of muscle unit firing is about 7-20Hz, depending of the size of the muscle (eye muscles versus seat (gluteal) muscles), previous axonal damage and other facts.

Uses

EMG can help to distinguish primary muscle conditions from muscle weakness caused by neurologic disorders. It can be used to differentiate between true weakness and reduced use due to pain or lack of motivation. It is used to find causes of weakness, paralysis, involuntary twitching, and abnormal levels of muscle enzymes. It can help diagnose neuromuscular disorders such as Motor Neurone Disease, neuropathy, nerve damage and muscle damage. See #Abnormal results.

Electromyography is also used in biofeedback studies and training. However, this type of electromyography uses surface electrodes to record the electrical activity, rather than needle electrodes inserted directly into the muscle. Surface electromyography training is a kind of biofeedback in which patients learn to control muscle tension in the face, neck, and shoulders. For example, such training is sometimes given to migraine patients.

Procedure

To perform EMG, needle electrodes are inserted through the skin into the muscle tissue. A medical doctor (most often a physiatrist or neurologist) observes the electrical activity while inserting the electrode. The insertional activity provides valuable information about the state of the muscle and its innervating nerve. Normal muscles at rest make certain, normal electrical sounds when the needle is inserted into them. Then the electrical activity when the muscle is at rest is studied. Abnormal spontaneous activity might indicate some nerve and/or muscle damage. Then the patient is asked to contract the muscle smoothly. The shape, size and frequency of the resulting motor unit potentials is judged. Then the electode is retracted a few millimeters, and again the activity is analyzed until at least 10-20 units have been collected. Each electrode track gives only a very local picture of the activity of the whole muscle. Because skeletal muscles differ in the inner structure, the electrode has to be placed at various locations to obtain an accurate study.

A motor unit is defined as one motor neuron and all of the muscle fibers it innervates. When a motor unit fires, the impulse (called an action potential) is carried down the motor neuron to the muscle. The area where the nerve contacts the muscle is called the neuromuscular junction, or the motor end plate After the action potential is transmitted across the neuromuscular junction, an action potential is elicited in all of the innervated muscle fibres of that particular motor unit. The sum of all this electrical activity is recorded as a motor unit potential. This electrophysiologic activity is avaluated during an EMG. The composition of the motor unit, the number of muscle fibres per motor unit, the metabolic type of muscle fibres and many other factors affect the shape of the motor unit potentials in the myogram.

Nerve conduction testing is also often done at the same time as an EMG.

Because of the needle electrodes, EMG may be somewhat painful to the patient, and the muscle may feel tender for a few days. However, so called "needleless EMG"—an EMG performed using surface electrodes—. will not give the information required for neurological diagnosis.

Normal results

Muscle tissue at rest is normally electrically inactive. After the electrical activity caused by the irritation of needle insertion subsides, the electromyograph should detect no abnormal spontaneous activity (i.e. a muscle at rest should be electrically silent, with the exception of the area of the neuromuscular junction, which is normally electrically very spontaneously active). When the muscle is voluntarily contracted, action potentials begin to appear. As the strength of the muscle contraction is increased, more and more muscle fibers produce action potentials. When the muscle is fully contracted, there should appear a disorderly group of action potentials of varying rates and amplitudes (a complete recruitment and interference pattern).

Abnormal results

Abnormal results may be caused by the following medical conditions (please note this is nowehere near an exhaustive list of conditions that can result in abnormal EMG studies):

Alcoholic neuropathy
Axillary nerve dysfunction
Becker's muscular dystrophy
Brachial plexopathy
Carpal tunnel syndrome
Centronuclear myopathy
Cervical spondylosis
Charcot_marie_tooth
Common peroneal nerve dysfunction
Denervation (reduced nervous stimulation)
Dermatomyositis
Distal median nerve dysfunction
Duchenne muscular dystrophy
Facioscapulohumeral muscular dystrophy (Landouzy-Dejerine)
Familial periodic paralysis
Femoral nerve dysfunction
Fields condition ^*
Friedreich's ataxia
Guillain-Barre
Lambert-Eaton Syndrome
Mononeuritis multiplex
Mononeuropathy
Motor neurone disease
Myasthenia gravis
Myopathy (muscle degeneration, which may be caused by a number of disorders, including muscular dystrophy)
Myotubular myopathy
Peripheral neuropathy
Poliomyelitis
Polymyositis
Radial nerve dysfunction
Sciatic nerve dysfunction
Sensorimotor polyneuropathy
Shy-Drager syndrome
Thyrotoxic periodic paralysis
Tibial nerve dysfunction
Ulnar nerve dysfunction

References