An Orthopedic Surgeon’s Guide to Interpreting Electromyography
Abstract & Commentary
Synopsis: Orthopedists ordering electrodiagnostic studies should be proficient in interpreting the results of this evaluation.
Source: Hogan C, Degnan G. Am J Orthop. 2001;30(10):745-750.
The article, "An Orthopaedic Surgeon’s Guide to Interpreting Electromyography," expertly accomplishes its mission as explained in the title. In a brief 6 pages, Hogan and Degnan provide an overview of neuromuscular anatomy, define essential terms common to electrodiagnostic testing, explain the significance of key findings in nerve conduction velocity studies, and elaborate on the value of each of the 4 phases of electromyography. The 4 phases of electromyography assess nerves and muscles at 4 different periods of activity: with needle insertion, with the muscle at rest, during a minimal muscle contraction, and finally with a maximal muscle contraction. The waveform as reviewed on the oscilloscope represents the sampled muscle activity with the amplitude representing the magnitude of muscle contraction and the duration representing the length of the response.
EMGs are a valuable source of information regarding the peripheral nerve, the neuromuscular junction, and the muscle itself. For example, a brief burst of electrical activity followed by silence on insertion of the needle electrode is the earliest sign of denervation. Fibrillations and sharp positive waveforms noted in phase 2 are also characteristics of denervation; whereas, polyphasic potentials in phase 3 may suggest a primary myopathic disorder or early re-innervation of an injured or repaired nerve. The interpretation of normal and abnormal results through each of the 4 phases of electromyography is presented by Hogan and Degnan in table form for ease of viewing by the reader. The article also has a helpful table that lists pathologic EMG findings for various disease states.
Comment by Letha Y. Griffin, MD, PhD
Electrodiagnostic studies are a valuable aid in making many diagnoses including myasthenia gravis, Guillain Barre Syndrome, Charcot-Marie-Tooth Disease, spinal muscular atrophy, and the various muscular dystrophies. These studies also aid in the identification of nerves injured by transection or compression as well as help define the extent of nerve recovery following injury. They can be used to assess the motor unit at a particular point in time, or serial EMGs can be obtained to monitor healing of the neuromuscular unit. Hogan and Degan suggest that just as physicians should not rely solely on the written x-ray report but rather view films themselves, so should they also review and interpret electrodiagnostic studies to confirm the accompanying written report. Hogan and Degnan provide us with a road map to be able to do just that.
Dr. Griffin, Adjunct and Clinical Faculty, Department of Kinesiology and Health, Georgia State University, Atlanta, GA, is Associate Editor of Sports Medicine Reports.
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