Electromyography (EMG):

Needle electromyography (EMG) is used to assess the health and integrity of motor nerve fibers of the spinal cord, the spinal nerve root, the plexus and the peripheral nerves.  Electromyography is used to measure tiny electrical discharges produced within a muscle.  A physician may recommend the EMG test when a patient reports muscle weakness and the physical examination confirms a reduction of muscle strength or the presence of unexplained muscle atrophy. The needle electromyographic study is a common procedure used to assess the health and integrity of muscle in the presence of muscle atrophy and/or weakness. The needle EMG study can be used to localize the site of nerve compromise and to assess the degree and duration of nerve injury and related muscle compromise.

Muscles receive a constant supply of electrical signals, which travel along nerve pathways. Muscles also produce their own electrical signals during contraction.  The EMG study of muscle requires the careful placement of a small sterile recording needle into the muscles.  There may be local discomfort associated with the testing and there may be occasional bruising at the site of needle placement/insertion.  A thin recording electrode is used to record the pattern of electrical activity within the muscle.  The electrode is connected to sophisticated testing equipment, which has software that records and analyzes the patterns of electrical activity when the muscle is at rest and during voluntary contraction of the muscle.  The size, duration and frequency of the muscle signals help determine whether there is compromise of the muscle or the nerves that innervate the muscle.

The needle EMG study can be particularly helpful in distinguishing peripheral nerve damage from compromise of a spinal nerve root. The results of EMG tests are often correlated with the results from nerve conduction studies performed during the same testing session.  Needle electromyography, and nerve conduction studies, are essential for the evaluation of suspected spinal cord radicular and peripheral nerve disorders.

Specialized forms of EMG include:

Quantitative Electromyography (QEMG): Quantitative electromyography is reserved for those patients with unusual or complicated neurological and/or muscular disease.  It may also be used to evaluate whether there is significant muscle reinnervation after a course of care.  It is a more time-consuming study than routine needle electromyography.  Select muscles are assessed in greater detail than would be done during a routine EMG study.  QEMG requires specialized training, sophisticated equipment and detailed protocols. 

There are a variety of tests, which fall under this heading. Categories of QEMG assessment include: triggered Single Fiber EMG, Stimulated Single Fiber EMG, Macro EMG, Template Matching, Parametric Matching, Manual Interference Pattern Analysis, On-line Interference Pattern Analysis, Myofrequency Assessment and Recruitment analysis.

The types of parameters, which are quantifiably measured, include assessment of the individual muscle fiber (Single Fiber EMG), which often reflect widespread involvement within a muscle, and the assessment of electrical activity arising from an entire motor unit that refers to all those muscle fibers attached to a single nerve fiber.  A Template Match Motor Unit analysis provides quantitative characterization of individual nerve and muscle fiber relationships.  QEMG provides detailed assessment of the pattern of muscle fiber recruitment during muscle contraction, the quantity of muscle fibers fired, the firing speed, waveform appearance and the collective pattern of muscle fiber firing in detail.