Aminoff's Electrodiagnosis in Clinical Neurology E-Book

Michael J. Aminoff

• Publisher: Saunders
• Year: 2012
• ISBN:
  9781455726769
• Edition: 6
• Topic: Medicina
• Subtopic: Neurología

Appropriately select, implement, and interpret electrodiagnostic tests to identify a full range of central and peripheral nervous system disorders with Aminoff's Electrodiagnosis in Clinical Neurology! Covering everything from basic principles to the latest advances in electrodiagnosis, this medical reference book helps you make optimal use of this powerful but complex diagnostic modality in compliance with regulatory and professional standards, so you can diagnose patients accurately and initiate effective treatment and management strategies.

• Deepen your understanding of the principles, scope, limitations, diagnostic importance, prognostic relevance, and complications for each technique.
• Visually grasp the technical and practical aspects of electrodiagnostic tests with almost 800 charts, figures, and tables.
• Rely on the knowledge, experience, and perspective of renowned expert Dr. Michael J. Aminoff and an international team of contributors comprised of a virtual "who's who" of clinical neurophysiology.

• Keep up with developments in the field through significant updates, includingnew chapters on Artifacts and Normal Variants in the Electroencephalogram; Microneurography; Clinical Applications of Nerve Excitability Testing; Ultrasound of Muscle and Nerve; The Blink Reflex and Other Brainstem Reflexes; Visual Evoked Potentials, Electroretinography and Other Diagnostic Approaches to the Visual System; and Magnetic Stimulation in Clinical Practice and Research.
• Meet regulatory and professional standards and apply best practices with state-of-the-art guidance (for both non-specialists and specialists) emphasizing the clinical applications of each electrodiagnostic technique.
• Get easily actionable information and avoid mistakes with electrophysiologic findings integrated into the clinical context in which they are obtained.

Section III

Electromyography, Nerve Conduction Studies, and Related Techniques

Chapter 11 Clinical Electromyography

Michael J. Aminoff

Practical aspects

Procedure

Electrical activity of normal muscle

EMG Activity at Rest

EMG Findings During Activity

Motor Unit Action Potentials

Motor Unit Recruitment Pattern

EMG activity in pathologic states

EMG Activity at Rest

Insertion Activity

Fibrillation Potentials and Positive Waves

Fasciculation Potentials

Myotonic Discharges

Complex Repetitive Discharges

Motor Unit Action Potentials

EMG Findings During Activity

Motor Unit Action Potentials

Abnormalities of Recruitment Pattern

EMG findings in various clinical disorders

Myopathic Disorders

Muscular Dystrophies and Other Familial Myopathies

Inflammatory Disorders of Muscle

Endocrine and Metabolic Myopathies

Myopathies Caused by Drugs or Alcohol

Critical Illness Myopathy

Congenital Myopathies of Uncertain Etiology

Myotonic Disorders

Rippling Muscle Disease

Neuropathic Disorders

Spinal Cord Pathology

Root Lesions

Plexus Lesions

Peripheral Nerve Lesions

Disorders of Neuromuscular Transmission

Miscellaneous Disorders

Diaphragmatic electromyography

Sphincteric electromyography

Laryngeal electromyography

Surface electromyography

Quantitative aspects of electromyography

The term electromyography refers to methods of studying the electrical activity of muscle. Over the years, such methods have come to be recognized as an invaluable aid to the diagnosis of neuromuscular disorders. As is discussed in this chapter, electromyography (EMG) has been used to detect and characterize disease processes affecting the motor units and to provide a guide to prognosis. Electromyographic examination is often particularly helpful when clinical evaluation is difficult or equivocal. The findings commonly permit the underlying lesion to be localized to the neural, muscular, or junctional component of the motor units in question. Indeed, when the neural component is involved, the nature and distribution of EMG abnormalities may permit the lesion to be localized to the level of the cell bodies of the lower motor neurons or to their axons as they traverse a spinal root, nerve plexus, or peripheral nerve. The EMG findings per se are never pathognomonic of specific diseases and cannot provide a definitive diagnosis, although they may be used justifiably to support or refute a diagnosis advanced on clinical or other grounds.

Electromyography is also used in conjunction with nerve conduction studies to obtain information of prognostic significance in the management of patients with peripheral nerve lesions. For example, EMG evidence of denervation implies a less favorable prognosis than otherwise in patients with a compressive or entrapment neuropathy. Again, evidence that some motor units remain under voluntary control after a traumatic peripheral nerve lesion implies a more favorable outlook than otherwise for ultimate recovery, indicating as it does that the nerve remains in functional continuity, at least in part.

The clinical utility of electrodiagnostic testing in patients presenting with a chief complaint of weakness has been examined.¹ The referring diagnosis was compared with the diagnosis immediately after electrophysiologic evaluation, and then with the final diagnosis as recorded 9 months later. This revealed that the testing had resulted in a single correct diagnosis in 73 percent of patients; where it resulted in more than one possible diagnosis, one of them was ultimately confirmed as correct in another 18 percent of patients, to yield an overall diagnostic accuracy of 91 percent. The electrophysiologic diagnosis was unsuspected by the referring physician, regardless of his or her specialty, in approximately one-third of cases.¹

Over the years, the activity of individual muscles in the maintenance of posture and during normal or abnormal movement has also been studied by EMG. Such studies are of considerable academic interest, and their clinical relevance is considered further in Chapter 20. The interested reader is also referred to a glossary of terms commonly used in electromyography.²

Practical aspects

The electrical activity of muscle is studied for diagnostic purposes by inserting a recording electrode directly into the muscle to be examined. The bioelectric potentials that are picked up by this electrode are amplified, then displayed on a cathode ray oscilloscope for visual analysis and fed through a loudspeaker system so that they can be monitored acoustically. A permanent photographic record of the oscilloscope trace can be made, if desired, or the amplified bioelectric signals can be recorded for retrieval at a later date. Modern, commercially available equipment includes analog-to-digital converters that permit the easy storage of data, advanced signal processing and analysis, and alteration in the display characteristics (e.g., time base and sensitivity) of stored potentials. Signal averaging is possible, and voltage trigger and delay lines facilitate the viewing of potentials.

The concentric needle electrode is a convenient recording electrode for clinical purposes. It consists of a pointed steel cannula within which runs a fine silver, steel, or platinum wire that is insulated except at its tip. The potential difference between the outer cannula and the inner wire is recorded, and the patient is grounded by a separate surface electrode. Alternatively, a monopolar needle electrode can be used. This consists of a solid needle (usually of stainless steel) that is insulated, except at its tip. The potential difference is measured between the tip of the needle, which is inserted into the muscle to be studied, and a reference electrode (e.g., a conductive plate attached to the skin or a needle inserted subcutaneously). The pick-up area of the concentric needle electrode is smaller than that of the monopolar electrode, and it is asymmetric as opposed to the more circular pick-up area of the monopolar electrode. Both concentric and monopolar electrodes are available in disposable or reusable forms. There is no evidence that one type of electrode is more painful to patients than the other.³

An electrode exhibits some opposition or impedance to the flow of an electric current, and it is therefore important that it is connected to an amplifier having a relatively high input impedance to prevent loss of the signal. The amplifier, and the recording system to which it is connected, should have a frequency response of 2 or 20 to 10,000 Hz so that signals within this frequency spectrum are amplified uniformly without distortion. When necessary, however, the frequency response of the amplifier can be altered by the use of filters. This allows the attenuation of noise or interference signals that have a frequency different from that of the potentials under study.

Noise, which appears as a random fluctuation of the baseline, is generated within the amplifier and by movement of the recording electrodes or their leads. It can obscure the bioelectric signals to be studied, as can any unwanted interference signals that are picked up by the recording apparatus. Interference signals usually are generated by the alternating-current (AC) power line, by appliances such as radios, or by paging systems; occasionally, however, they are biologic in origin. Technical and safety factors are important when the electrical activity of muscle is to be recorded. They are considered in detail in Chapter 2 and the review by Git...