AAEM minimonograph #25: Single-fiber electromyography

Single-fiber electromyography (SFEMG) is a selective recording technique in which a needle electrode with a small recording surface in the side is used to identify action potentials from individual muscle fibers. The SFEMG parameters of greatest clinical use are fiber density (FD) and neuromuscular jitter. FD reflects the local organization of muscle fibers within the motor unit; jitter reflects the safety factor of neuromuscular transmission at individual neuromuscular junctions. SFEMG can be of great value in demonstrating or excluding abnormalities in mild or questionable disease of nerve, muscle, or the neuromuscular junction. The neuromuscular jitter may be measured during nerve stimulation, which is particularly useful in uncooperative patients or when it is desirable to control the firing rate precisely, or during voluntary muscle activation, which is less subject to technical artifact. The SFEMG findings may not be specific to a particular diseases, but they frequently increase understanding of the disease process by demonstrating abnormal neuromuscular transmission or rearrangement of muscle fibers within the motor unit, which complements information from more conventional EMG examinations. © 1996 American Association of Electrodiagnostic Medicine. Published by John Wiley & Sons, Inc.     

AAEM minimonograph #46: Neurogenic muscle hypertrophy

Muscle hypertrophy occurs uncommonly in several neurogenic disorders including neuropathies, radiculopathies, spinal muscular atrophy, and post-polio syndrome. Its pathogenesis varies in different circumstances. In the presence of generalized myokymia and neuromyotonia (Isaacs' syndrome), symmetrical hypertrophy appears to be the result of continuous spontaneous electrical stimulation of myofibers and, in some cases, results in type 1 myofiber preponderance. Focal hypertrophy occurring with radiculopathies and mononeuropathies was associated with complex repetitive discharges (CRDs) in approximately half the cases. CRDs may play a role in the pathogenesis of myofiber hypertrophy by continuous myofiber stimulation, but in some cases, with and without CRDs, myofiber hypertrophy may be related to mechanical events. Muscle enlargement seen in old polio appears to involve a significant degree of pseudohypertrophy, although some myofiber hypertrophy occurs. The symmetrical occurrence of hypertrophy in genetically determined disorders, such as spinal muscular atrophy, and hereditary motor and sensory neuropathy types 1 and 2 may have both a genetic and a mechanical basis in addition to pseudohypertrophy in some cases. © 1996 American Association of Electrodiagnostic Medicine. Published by John Wiley & Sons, Inc.     

AAEM minimonograph #40: Clinical neurophysiology of the respiratory system

Disorders of nerve and muscle are frequent causes of respiratory insufficiency, but current methods often fail to adequately assess the problem. Phrenic nerve conduction and needle electromyography of the diaphragm are of great assistance in identifying the nature and site of the disorder: the various disturbances of central drive, axonal or demyelinating neuropathies of the phrenic nerves, and certain myopathies. These studies may be performed on adults, children, or infants, and in outpatient, general ward, or critical care settings. It is hoped these techniques will stimulate further developments in the field.     

AAEM minimonograph #3: motor unit recruitment.

Motor unit recruitment is the process by which different motor units are activated to produce a given level and type of muscle contraction. At minimal levels of muscle contraction (innervation), muscle force is graded by changes in firing rate (rate coding) of individual motoneurons (MNs). At higher levels of innervation, recruitment is accomplished by the addition of different motor units firing at or above physiologic tremor rate. During slowly graded and ballistic increases in force, motor units are recruited in rank order of their size. In addition to MN soma diameter, other factors contribute to the selectivity of MN activation. For la afferent MN activation in the cat, synaptic density and efficacy as well as specific membrane resistance are also rank ordered for slow, fatigue resistant, and fast fatigue motor units with slow motor units recruited first. The central drive for motor unit activation is distributed to all the MNs of the pool serving a given muscle. Size-structure organization of the MN pool determines the order of recruitment and how MNs interact with each other. Disorders of the motor unit affect recruitment. A method for the clinical electromyographic assessment of recruitment is suggested. Assessment is made at three levels of innervation: minimal contraction for onset and recruitment firing rates; moderate contraction required to maintain the limb against gravity for the maximum number of motor units, their firing rates, and motor unit spikes/s; maximal voluntary contraction (MVC) for detection of high threshold enlarged motor units characteristic of reinnervation and completeness of the interference pattern (IP). Loss of muscle fibers results in early and excessive recruitment at minimal and moderate levels of innervation. Loss of motor units can result in both an increased rate and range of single motor unit firing at all levels of innervation. With reinnervation and enlargement of motor units, firing rates increase significantly and the interference pattern during MVC is incomplete.     

AAEM minimonograph #44: Diseases associated with motor unit activity

Stiff-man syndrome is due to hyperexcitability of anterior horn cells, possibly related to interference with the synthesis or action of gammaaminobutyric acid. Unexpected acoustic and exteroceptive stimuli produce exaggerated muscle responses. Needle electrode examination of involved muscles yields nonspecific findings and demonstrates involuntary motor unit activity. The appearance and firing pattern of motor units are normal except that agonist and antagonist muscles may contract concurrently. Continuous muscle fiber activity (Isaacs' syndrome) comprises a heterogeneous group of hereditary and acquired disorders that cause hyperexcitability of peripheral nerves. Some are associated with electrophysiologic evidence of peripheral neuropathy and some are not. Repetitive afterdischarges often follow the M-, H-, and F-waves. Needle electrode examination reveals an abnormal pattern of motor unit firing, consisting of myokymic discharges, doublets and multiplets, neuromyotonic discharges, and fasciculations. These abnormalities may occur alone or in combination.     

AAEM minimonograph #37: facial and limb myokymia.

Myokymia is a clinical phenomenon associated with characteristic electromyographic activity referred to as myokymic discharges. These are spontaneously generated bursts of individual motor unit potentials with each burst recurring rhythmically or semirhythmically, usually several times per second. It involves facial muscles more commonly than those of the extremities, and is most often seen in association with Guillain-Barré syndrome, multiple sclerosis, radiation plexopathy, pontine tumors, and timber rattlesnake envenomation. An alteration in the biochemical microenvironment of axon membranes at one of the various sites along the motor axon is the likely basis for the altered membrane excitability that underlies the myokymic discharges in most cases. The similarity of these discharges to those seen with hypocalcemic tetany, and the ability to manipulate myokymic discharges by altering serum-ionized Ca++, suggests that decrease in the ionized Ca++ in the microenvironment of the axon may play an important role.     

AAEM minimonograph #11: Needle examination in clinical electromyography

The physiologic and histologic principles underlying clinical electromyographic studies are briefly reviewed as an introduction to the normal and abnormal findings in human subjects. Technical aspects of recordings as well as the specific types of discharges and their significance are discussed.     

AAEM minimonograph #41: Neuromuscular diseases associated with HIV-1 infection

Neuromuscular diseases occur in as many as 50% of patients infected with human immunodeficiency virus type 1 (HIV-1). All forms of neuromuscular disease have been reported, including axonal neuropathy, demyelinating neuropathy, mononeuropathy multiplex, polyradiculitis, ALS-like syndromes, disorders of neuromuscular transmission, myopathy, and toxic neuropathies due to medication side effects. Neuromuscular disease is often the presenting manifestation of HIV-1 infection. Infection with cytomegalovirus (CMV) is associated with different types of neuropathy including mononeuritis multiplex and polyradiculopathy. There is effective treatment for many of the associated disorders including chronic inflammatory demyelinating neuropathy, CMV-mediated neuropathies, and myopathy. Treatment of CMV-mediated mononeuropathy multiplex may be life saving. The different neuromuscular syndromes associated with different stages of HIV-1 infection may be due, in part, to different levels of immunocompetence.     

AAEM minimonograph #47: Normative data in electrodiagnostic medicine

This article reviews, without mathematics, the important principles governing the acquisition and use of normative data in electrodiagnostic medicine. Common flaws in neurophysiological normative data include vague clinical criteria for establishing freedom from disease, samples that are too small and inadequately stratified, and application of Gaussian statistics to non-Gaussian variables. Other problematic issues concern the trade-off between permissible false-positivity and false-negativity in defining the limits of normative from sample data, test-retest variability, and the use of multiple independent test measurements in each electrodiagnostic examination. The following standards for normative data are proposed: (1) standardized objective determination of freedom from disease; (2) appropriately large sample of normal subjects; (3) proportional statification of normal subjects for known relevant variables; (4) test of Gaussian fit for application of Gaussian statistics; and (5) data presentation by percentiles when Gaussian fit is in doubt. Many existing normative studies in clinical neurophysiology do not meet these standards. High-quality normative data, readily accessible, is essential for the accurate electrodiagnosis of neuromuscular diseases. © 1997 American Association of Electrodiagnostic Medicine. Published by John Wiley & Sons, Inc.     

AAEM minimonograph #45: The early development of electromyography

The use of electricity for therapeutic purposes began in the first century and became more refined as the properties of electricity became more understood. The works of Franklin, Galvani, Volta, and others contributed to this body of knowledge. Development of the string galvanometer, the advent of the vacuum tube, the introduction of concentric needle electrodes, and the development of the cathode-ray oscilloscope occurred during the first half of the 20th century. The science of electromyography and electrodiagnosis grew in its sophistication, leading to the formation of the American Association of Electromyography and Electrodiagnosis (now the American Association of Electrodiagnostic Medicine) with James Golseth, MD, as its first president in 1953. © 1995 John Wiley & Sons, Inc.     

AAEM minimonograph #2: Important anomalous innervations of the extremities

Anomalous innervations of the extremities are common and influence the interpretation of electrophysiologic studies in normal patients and those with peripheral nerve lesions. The following anomalous innervations are reviewed: median to ulnar nerve communication; ulnar to median nerve communication; variations in the innervation of intrinsic muscles of the hand; accessory deep peroneal nerve; and tibial to peroneal nerve communication. The electrophysiologic recognition of these anomalies and the manner in which they affect the interpretation of electrodiagnostic studies in various conditions is emphasized.     

AAEM minimonograph #38: neuropathies in connective tissue disease.

Neuropathies are common in patients with known or suspected connective tissue disease. A vasculitic mononeuropathy multiplex is often seen in patients initially presenting with polyarteritis nodosa or developing arteritis as a complication of rheumatoid arthritis. However, vasculitic neuropathy may become confluent and present as as distal symmetrical polyneuropathy or occur without systemic necrotizing vasculitis. Distal symmetrical polyneuropathies without associated vasculitis are also common in many connective tissue diseases. Compression neuropathies, especially carpal tunnel syndrome, occur with increased frequency in rheumatoid arthritis. Finally, certain neuropathies may be the major presenting feature of particular connective tissue diseases. For example, trigeminal neuropathy often heralds the onset of systemic sclerosis or mixed connective tissue disease, and sensory neuronopathy may be the initial presenting feature of Sj?gren's syndrome.     

AAEM minimonograph #43: Neuromuscular problems in the performing arts

Over the past 10–15 years, there has been increasing interest in the health problems of performing artists. In this review, I will discuss the major playing-related disorders seen in instrumental musicians. Among the 672 instrumentalists evaluated, the major diagnoses identified included musculoskeletal disorders in 64%, peripheral nerve problems in 22.5%. and focal dystonia in 7%. Sixty percent of instrumentalists were female although males predominate in the group with focal dystonia. The average age of those evaluated was 32 years. Among musculoskeletal disorders overuse syndrome is the most common. Frequent peripheral nerve disorders include thoracic outlet syndrome, carpal tunnel syndrome, and ulnar neuropathy. A characteristic distribution of symptoms and signs is identified for each instrument group. Electrodiagnostic studies are an important part of the evaluation of these disorders. With carefully designed treatment, the majority of instrumental musicians can be returned to full and pain-free playing activities. The success rate is highest in some entrapment neuropathies but remains low in focal dystonia. © 1994 John Wiley & Sons, Inc.     

AAEM minimonograph #26: The electrodiagnosis of carpal tunnel syndrome

The electrodiagnosis of carpal tunnel syndrome (CTS) is reviewed, including discussions of old and new techniques of motor and sensory nerve conduction, anomalous innervation, and needle electrode examination. A variety of sensitive nerve conduction studies (NCSs) are available for the evaluation of a patient with suspected CTS. For any particular patient, the NCS method chosen by the clinical neurophysiologist may vary for a numer of reasons, including the severity of the deficit and the presence of superimposed conditions. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1477–1486, 1997     

AAEM minimonograph #42: Intraoperative monitoring of peripheral and cranial nerves

Intraoperative studies of focal neuropathies have become increasingly common both as an aid to the localization of the neuropathy and a means of monitoring the integrity of the nerve in the course of surgery. Success very often depends on careful attention to the technical problems presented by working in the operating theater. Despite these difficulties, assessment of conduction directly from the exposed nerve offers opportunities for better understanding the pathophysiology of these neuropathies and correlating these findings with conventional electrodiagnostic studies. © 1994 John Wiley & Sons, Inc.     

AAEM minimonograph #14: The influence of temperature in clinical neurophysiology.

AAEM MINIMONOGRAPH # 14 Temperature affects biologic and neurophysiologic processes and is, therefore, always well controlled in in vitro experiments. Its role is equally important in the clinical laboratory but has often been neglected. Lower temperature cause slower nerve conduction velocities (NCVs), and increased amplitudes of muscle and nerve potentials. Fibrillations may disappear, and muscle contraction will be slower and weaker. Neuromuscular transmission improves. Somatosensory evoked potentials (SEPs) are similarly vulnerable in the peripheral segments, or with changes in central temperature. As a result, abnormalities are artificially created or existing defects are not detected, resulting in false or missed diagnoses. Control of temperature, albeit somewhat time consuming, will result in greater diagnostic accuracy.     

AAEM minimonograph #34: polyneuropathy: classification by nerve conduction studies and electromyography

Electrodiagnostic evaluation of patients with suspected polyneuropathy is useful for detecting and documenting peripheral abnormalities, identifying the predominant pathophysiology, and determining the prognosis for certain disorders. The electrodiagnostic classification of polyneuropathy is associated with morphologic correlates and is based upon determining involvement of sensory and motor fibers and distinguishing between predominantly axon loss and demyelinating lesions. Accurate electrodiagnostic classification leads to a more focused and expedient identification of the etiology of polyneuropathy in clinical situations.     

AAEM minimonograph #39: Digital filtering: Basic concepts and application to evoked potentials

Filtering of evoked potentials has been performed in clinical laboratories using both analog and digital methods. Analog methods introduce distortion caused by nonlinear phase shift which may be quite severe. Digital methods, while avoiding distortion caused by phase shift, reveal evoked potential components which may or may not correspond to distinct singular neuranatomic generators or homogeneous neuroanatomic systems. Thus, components identified with zero phase shift digital filters at restricted bandpass must be compared with components seen in open bandpass recordings. In some specific circumstances, high-pass filtering of short-latency somatosensory-evoked potentials may distinguish slow asynchronous synaptic activity from fast and synchronous synaptic, lemniscal, or axonal activity. © 1992 John Wiley & Sons, Inc.     

AAEM minimonograph #16: Instrumentation and measurement in electrodiagnostic medicine–part II

A review of instrumentation and measurement in electrodiagnostic medicine is continued in this Part II which focuses on digital instrumentation principles, gain and sweep effects, noise, nerve stimulation, and conduction measurement limitations. With the adoption of microprocessor-based equipment, the neurophysiologic signal must undergo analog-to-digital conversion (ADC) before analysis and display on a video monitor. ADC resolution and sampling rates affect accuracy and measurement precision. Following waveform display, the visual assessment of latency and duration may be influenced by sweep and gain settings, often overlooked sources of error. Undesired signal or noise typically originates from power-line interference, electronic amplifier noise, background muscle activity, or nerve stimulation artifact. Noise often interferes with clinical studies but techniques exist to reduce noise to acceptable levels in virtually all situations. An awareness and understanding of these technical issues will lead to an appreciation of the limitations of electrodiagnostic testing and improve interpretation and clinical decision-making. © 1995 John Wiley & Sons, Inc.     

AAEM minimonograph #16: Instrumentation and measurement in electrodiagnostic medicine–part I

Technical and instrumentation factors play an important role in obtaining reliable information during electrodiagnostic studies. With contemporary electrodiagnostic equipment, neurophysiologic potentials are detected using a variety of electrodes and undergo differential amplification, filtering, conversion to digital form, and finally, analysis and display. Understanding the signal processing principles, limitations, and sources of errors that can occur during this multistep process can improve the technical quality of studies, minimize preventable errors, and improve clinical interpretation. Part I of this minimonograph reviews the basic principles of action potential generation and overviews electrodiagnostic instrumentation. The concept of waveform frequency content is related to the role of filters in suppressing noise while preserving waveform latency, amplitude, and morphology. The electrical characteristics of various surface and needle electrodes influence instrument design and the nature of the potentials recorded. This is especially important in understanding the differences in motor unit characteristics obtained from monopolar and concentric needle electrodes. © 1995 John Wiley & Sons, Inc.