| Electromyography (EMG) is a neurophysiological test of nerve and muscle function. Technically speaking, EMG is often referred to as a 2-part test that includes nerve conduction velocity (NCV) studies and EMG. EMG, however, specifically refers to the aspect of this test in which a needle records the electrical activity of muscle groups during rest and contraction (tightening). Since muscle contraction ordinarily follows stimulation of a motor nerve, EMG reflects motor (but not sensory) nerve function. The goal of the testing is to assess the muscle activity at rest and upon contraction. By recording this muscle activity, a skilled physician may assess both nerve and muscle function.
A motor nerve consists of a bundle of axons (single fiber projections of neurons) whose cell bodies are located in the spinal cord. Each axon branches as it enters the muscle to end on a group of muscle cells (motor end plate). A motor neuron and the muscle cells that it stimulates are referred to as a motor unit. A muscle is made up of several units. When motor neurons are stimulated, electrical impulses travel along the axons. Muscle contraction occurs when electrical impulses are received by the muscle cells. A sensory nerve consists of axons traveling from receptors in the periphery through cell bodies just outside the spinal canal and then into the dorsal horn of the spinal cord. Sensory nerves convey incoming impulses from the sensory receptors that are processed by the central nervous system so as to become part of the individual's perception of self and his environment.
During electromyography, testing needle electrodes are inserted into muscles to record their electrical activity. Electrical impulses display as waves on a device called an oscilloscope and can also be heard through a loudspeaker. NCV studies stimulate the axons, motor and sensory together and measure a response, either motor or sensory, at a distance along the nerve path. It is then assessed.
After a peripheral nerve injury, regeneration of the nerve may occur. Before there is clinical improvement in muscle strength, signs of re-innervation (growth of living nerve back into the paralyzed muscle) can be detected on EMG. Repeat EMGs may be done to follow an individual's progress. Although an abnormal EMG is an indicator of disease, a normal EMG does not necessarily rule out disease. The EMG reflects motor function; it cannot detect damage to a sensory nerve directly.
EMGs are often avoided or done with great caution on individuals who are receiving anticoagulants. EMGs are generally not problematic for persons with pacemakers, but nerve conduction studies requiring stimulation near pacemakers should be avoided. Before the EMG, anticholinergics and muscle relaxants must be discontinued for 72 to 144 hours. A minimum of 3 hours prior to the EMG, individuals must stop smoking cigarettes. |
Source: Medical Disability Advisor
| Electromyography is a useful diagnostic procedure for evaluating nerve and muscle disorders. It is typically performed on individuals who may have nerve root impingement within the vertebrae due to disc herniation or bone spurs, or peripheral nerve injuries. Muscle recruitment patterns, specifically during maximal contraction, may be helpful in distinguishing between different kinds of muscle and nerve disorders. EMG (usually in conjunction with nerve conduction studies) is helpful in determining the location of a nerve lesion. |
Source: Medical Disability Advisor
| There are four phases to an EMG test. In the first phase, a group of skeletal muscles are chosen, based on the nerve root and peripheral nerves that are suspected of dysfunction. The epidermis over the muscle is rubbed with isopropyl alcohol and a needle electrode is inserted into the muscle. The needle electrode records any electric discharges and displays them on a screen that shows the wave formations of electrical current (oscilloscope). The oscilloscope is observed when the needle is first inserted into the muscle. A short burst of potentials will be seen when the needle is first inserted; however, injured muscle fibers produce increased insertional or injury potentials when the needle is moved around.
The second phase is the observation of the action potentials when a muscle is voluntarily contracted. The individual is asked to try to cause gentle contraction of the muscle. When the needle electrode is near the motor end plate, where there is the highest concentration of motor neurons, a high frequency noise will be heard. The recorded action potential is studied to compare its duration (in milliseconds), its amplitude (in microvolts), and its waveform (or number of phases) against normal values for the particular muscle group being studied.
The third phase examines the sequence of action potentials generated when a muscle is contracted at a gradually increasing intensity. In a normally innervated muscle, the amplitude of the wave progressively increases, as does the firing rate.
The fourth phase examines spontaneous activity when a muscle is at rest. Healthy muscle yields no action potentials in the absence of contraction. However, in a denervated muscle, action potentials are spontaneously discharged (fibrillation).
The data are collected and examined to ascertain the nature of the injury. |
Source: Medical Disability Advisor
| EMG, when correlated with clinical findings, is usually helpful in determining the diagnosis. It is generally a definitive procedure.
Abnormal electrical activity on EMG confirms the presence of neural involvement in muscular dysfunction. However, additional testing such as an MRI scan is often necessary to determine the location of the nerve injury. With positive test results, individuals are referred for rehabilitation specific to the nerve injury detected.
Individuals who have a negative EMG do not have the presence of nerve damage due to compression or lesion. These individuals may require other tests such as an MRI to rule out muscular dysfunction such as tendon rupture. These individuals then undergo rehabilitation appropriate for the diagnosis that is given once all diagnostic tests are performed. |
Source: Medical Disability Advisor
| Pain at the site of needle electrode insertion may occur during and after the EMG. The muscles may also experience pain and spasm after the EMG is completed. Bruising is rare, and with sterile, single use (disposable) needles, infection is also unlikely. |
Source: Medical Disability Advisor
| No restrictions are necessary as a result of the test. If the EMG reveals nerve injury, appropriate accommodations may be necessary. |
Source: Medical Disability Advisor
| Barkhaus, Paul E., and Sanjeev D. Nandedkar. "Electrophysiologic Biopsy." eMedicine. Eds. Dianna Quan, et al. 3 Mar. 2005. Medscape. 20 May 2005 <http://emedicine.com/neuro/topic610.htm>. "Electromyography (EMG) and Nerve Conduction Studies." WebMD.com. 5 Oct. 2004. WebMD Inc. 20 May 2005 <http://my.webmd.com/hw/health_guide_atoz/hw213852.asp>. |
Source: Medical Disability Advisor