The action of nerves and muscle is essentially electrical. Information is transmitted along nerves as a series of electrical discharges carrying information in a pulse repetition frequency. This may be in the range of 1 to 100 pulses per second.
Contraction of muscle fibres is also associated with an electrical discharge which can be detected by measuring electrodes or brought about by electrical stimulation. EMG is most often used when people have symptoms of weakness, and examination shows impaired muscle strength. It can help to differentiate primary muscle conditions from muscle weakness caused by neurological disorders. EMG can be used to differentiate between true weakness and reduced use because of pain or lack of motivation.
EMG equipment consists of recording electrodes, preamplifiers (which are normally placed very close to the patient to avoid pick-up of electrical interference), amplifiers to provide the correct gain, calibration and frequency characteristics, a display system (usually a CRT), a range of integrators and averagers - partly to achieve some data compression (chart records may be very long and difficult to read), and a recording medium, which is often a photographic (fibre-optic) system.
The most typical method for testing uses a needle electrode inserted through the skin into the muscle. The electrical activity detected by this electrode is displayed on a CRT (and may be displayed audibly through a speaker). Because skeletal muscles are isolated and often large units, each electrode gives only an average picture of the activity of the selected muscle. Several electrodes may need to be placed at various locations to obtain an accurate study. After placement of the electrode(s), the patient may be asked to contract the muscle (for example, by bending the arm). The presence, size, and shape of the wave form produced on the oscilloscope (the action potential) provide information about the ability of the muscle to respond to nervous stimulation. Each muscle fibre that contracts will produce an action potential, and the size of the muscle fibre affects the rate (how frequently an action potential occurs) and size (amplitude) of the action potential(s). A nerve conduction velocity test is often done at the same time as an EMG.
There may be some discomfort with insertion of the electrodes (similar to an intramuscular injection). Afterward, the examined muscle may feel tender or bruised for a few days. Muscle tissue is normally electrically silent at rest. Once the insertion activity (caused by the trauma of needle insertion) quiets down, there should be no action potential on the oscilloscope. When the muscle is voluntarily contracted, action potentials begin to appear. As contraction is increased, more and more muscle fibres produce action potentials until a disorderly group of action potentials of varying rates and amplitudes (complete recruitment and interference pattern) appears with full contraction.
Many EMG tests involve the use of stimulators to induce discharges in a nerve trunk, and detect the response by surface electrodes over a muscle served by that nerve. In this case the signals may be as large as 2 mV, and may be presented audibly or for recording on a high-speed chart recorder. Such evoked response tests might be for determining the nerve conduction time, or for assessing the performance of the neuromuscular control system. There are many different disorders of the nervous system and EMG examination has to be tailored to the particular requirements of the individual patient. Thus, these tests are normally carried out by a specialist in electromyography within the neurology department.
A test to measure muscle response to nervous stimulation (electrical activity within muscle fibres).
When a muscle fibre loses its nerve supply, it exhibits a characteristic irritability manifested as spontaneous discharges at rest. Single muscle discharges, called FIBRILLATIONS have a short duration (.5 to 1.5 msec), low amplitude (50-300 microvolts) and a REGULAR rhythm. They are usually positive (downward) in their initial deflection.
SEPs can provide a unique insight into nervous function since they result from direct stimulation of sensory nerves. Particular aspects of the response can be investigated by choice of stimulation and recording sites. Clinical applications of SEPs are the investigation of Multiple Sclerosis, Brachial Plexus and Spinal Cord injuries, Coma, Brain Death and other neuropathies.
The H Reflex results from stimulation of sensory fibres with the resulting afferent discharge causing an excitatory potential in the motor neurone pool, following a synaptic delay. Exceeding the potential threshold for a particular motor neurone generates an action potential and the resulting afferent discharge will cause the muscle fibres innervated by that neurone to be activated.
Single Fibre EMG can be used to estimate the density of fibres belonging to a single motor unit. These fibre density measurements are valuable in identifying reorganisation of the motor unit in neurogenic conditions and in certain primary myopathies.
Single Fibre Jitter measurement is a technique used for assessing transmission at the motor end plate. Measurements can be made manually or with an automated analysis package providing a sensitive technique for detecting abnormalities of neuromuscular transmission at an early stage.
Measurements of various parameters of MUAPs can provide valuable data for differential diagnosis. MUAP measurements can be made manually or with an automated analysis package for recognising and averaging potentials from particular motor units. In general, myopathies lead to a decrease in duration and amplitude of MUAPs, whereas neuropathies cause an increase in duration and amplitude.
The F Response results from a centrifugal volley in an alpha motor neurone, following antidromic excitation of the nerve cell body in the ventral horn of the spinal cord. Clinical applications of F Response are conditions such as entrapment neuropathies and root compression syndrome and estimation of motor neurone excitation.
Disorders or conditions that cause abnormal EMG results include:
- Denervation (reduced nervous stimulation)
- Carpal tunnel syndrome
- Myopathy (muscle degeneration, may be caused by a number of disorders, including muscular dystrophy)
- Myasthenia gravis
- Alcoholic neuropathy
- Axillary nerve dysfunction
- Becker's muscular dystrophy
- Brachial plexopathy
- Cervical spondylosis
- Common peroneal nerve dysfunction
- Distal median nerve dysfunction
- Duchenne's muscular dystrophy
- Facioscapulohumeral muscular dystrophy (Landouzy-Dejerine)
- Familial periodic paralysis
- Femoral nerve dysfunction
- Friedreich's ataxia
- Lambert-Eaton Syndrome
- Mononeuritis multiplex
- Peripheral neuropathy
- Radial nerve dysfunction
- Sciatic nerve dysfunction
- Sensorimotor polyneuropathy
- Shy-Drager syndrome
- Thyrotoxic periodic paralysis
- Tibial nerve dysfunction
- Ulnar nerve dysfunction