NEEDLE ELECTROMYOGRAPHY (EMG)
EMG is the study and recording of the intrinsic electrical properties of skeletal muscles. EMG is performed to diagnose, define, and follow diseases of the peripheral nervous system and muscle. Needle EMG is performed by inserting an electrode through the skin into appropriate muscles, one at a time. The needle translates the amount and intensity of the electrical activity into waveforms that are displayed on a computer screen. Generally, the electrodes are of two types: monopolar or concentric. In addition, needle EMG can be performed as a part of intraoperative neurophysiological monitoring (INM), which monitors the integrity of neural pathways during surgeries where there is a potential for damage to neural integrity.
After the needle EMG is performed on the muscle, the data (the waveforms, sounds, and feel of the muscle) are analyzed to determine if additional muscles need to be studied. The muscles studied will vary depending upon the differential diagnosis and the ongoing synthesis of new information that is obtained while the test is being performed. The electromyographer relies on ongoing real-time clinical diagnostic evaluation when deciding whether to continue, modify, or conclude a needle EMG. This requires a comprehensive knowledge base of anatomy, physiology, and neuromuscular diseases, as well as an awareness of the influence of age, temperature, and body height on the results.
Needle EMG results reflect the integrity of the functioning connection between a nerve and its innervated muscle, as well as the integrity of the muscle itself. The axon innervating a muscle is primarily responsible for the muscle's volitional contraction, survival, and trophic functions. Thus, interruption of the axon will alter the EMG.
Neurogenic disorders can be distinguished from myopathic disorders by a carefully performed needle EMG. For example, both polymyositis and amyotrophic lateral sclerosis (ALS) produce manifest weakness; however, polymyositis carries a very different prognosis and treatment than ALS. A needle EMG is essential in making a distinction between the two disorders because the needle electrode allows the muscle's electrical characteristics at rest and during activity to be interpreted by the electromyographer. This interpretation includes an analysis of oscilloscope tracings and the characteristic sounds that are produced by electrical potentials. The final interpretation of the study is a combination of the individual's history, a physical examination, and the muscles tested. Similarly, classification of nerve trauma into axonal versus demyelinating categories, with corresponding differences in prognoses, is possible with a needle EMG.
In single-fiber needle EMG, a specially designed needle electrode is used to record and identify action potentials (APs) from individual muscle fibers. These recordings are used to calculate the neuromuscular jitter and the muscle fiber density (FD). Jitter is defined as the variability in time between activation of the motor nerve and generation of the muscle fiber AP; it reflects the normality of nerve-to-muscle transmission. Normal jitter varies among muscles and muscle fibers within individual muscles. To determine if jitter is abnormally increased, statistical analysis is performed from the recordings of a population of muscle fibers within each treated muscle. When neuromuscular transmission is sufficiently abnormal and nerve activation produces no muscle AP, blocking is seen. Increased jitter, blocking, or both may occur in a variety of primary disorders of neuromuscular transmission (e.g., myasthenia gravis). Jitter and FD may be measured in one or more muscles depending on the condition being evaluated and the results of the testing.
NERVE CONDUCTION STUDIES (NCS) CONDUCTED WITH NEEDLE EMG
NCS is performed together with a needle EMG. NCS measures the electrical activity of muscles or the integrity of the functioning connection between a nerve and its innervated muscle, as well as the integrity of the muscle itself. The American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM) recommends that except under unique situations (e.g., presence of lymphedema, current use of anticoagulants), NCS and needle EMG should be performed together in a study for an accurate clinical diagnosis of peripheral nervous system disorders. The reporting of NCS and needle EMG study results should be integrated into a unifying diagnostic impression.
SURFACE EMG (SEMG)
A surface EMG (SEMG) is not the same as a conventional EMG. SEMG involves a noninvasive, computer-based technique that records the electrical impulses of the nerves and muscles through electrodes that are placed on or passed over the surface of the skin. SEMG differs from needle EMG with respect to technical requirements and electrical properties. SEMG electrodes measure from a wide area of muscle, have a relatively narrow frequency band, have low-signal resolution, and are highly susceptible to movement artifact. According to the AANEM, there are no clinical indications for the use of SEMG in the diagnosis and treatment of disorders of the nerves or muscles.