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In a number of disorders of the endocrine glands, muscle weakness may be a prominent feature, and occasionally it may be the chief complaint Although these diseases are discussed in detail in Chap 51, it should be noted that such metabolic muscle weakness, acute or chronic, may occur in the absence of changes in serum electrolytes or enzymes Speci c hormone assays are then necessary for diagnosis This is particularly true of patients with thyrotoxicosis or Cushing disease and those receiving prolonged corticosteroid therapy In thyrotoxicosis, muscle paresis may appear without the classic signs of Graves disease
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ELECTRODIAGNOSIS OF NEUROMUSCULAR DISEASE (ELECTROMYOGRAPHY; EMG)
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It was long ago discovered that muscle would contract when a pulse of electric current was applied to the skin, near the point of entrance of the muscular nerve (motor point) The electrical pulse required is brief, less than a millisecond, and is most effectively induced by a rapidly alternating (faradic) current After denervation, an electrical pulse of several milliseconds, induced by a constant electrical (galvanic) stimulus, is required to produce the same response This change, in which the galvanic stimulus remains effective after the faradic one has failed, was the basis of Erb s reaction of degeneration, and varying degrees of this change were later plotted in the form of strength-duration curves For decades, this was the standard electrical method for evaluating denervation of muscle Though still valid, it was replaced long ago by nerve conduction studies and by the needle electrode examination The latter test, based on the sherringtonian concept of the motor unit (page 39), is accomplished by recording the ring characteristics of evoked motor unit potentials (CMAPs) and by the insertion into muscle of needle electrodes to measure spontaneous and voluntar-
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The red pigment myoglobin, responsible for much of the color of muscle, is an iron-protein compound present in the sarcoplasm of striated skeletal and cardiac bers Of the total body hematin com-
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DISEASES OF SPINAL CORD, PERIPHERAL NERVE, AND MUSCLE
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ily evoked muscle ber activity The terms electromyography and electromyogram (EMG) were used originally to describe the needle electrode examination but are now a common shorthand designation for the entire electrodiagnostic evaluation, including the nerve conduction studies, described below
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Studies of Nerve Conduction
The main laboratory technique for the study of peripheral nerve function involves the transcutaneous stimulation of motor or sensory nerves and recording of the elicited action potentials in the muscle (CMAP) and the sensory nerve action potential (SNAP) The results of these motor and sensory nerve conduction studies, expressed as amplitudes, conduction velocities, and distal latencies, yield certain quantitative information and additional qualitative observations regarding the waveform and dispersion of electrical impulses Hodes and coworkers in 1948 were the rst to describe nerve conduction studies in patients and the techniques used currently are not much changed An accessible nerve is stimulated through the skin by surface electrodes, using a stimulus that is large enough to recruit all the available nerve bers The resulting action potential is recorded by electrodes on the skin (1) over the muscle distally in the case of motor bers stimulated in a mixed or motor nerve (CMAP), (2) over the nerve proximally, using orthodromic techniques for sensory bers stimulated in the digital nerves, (3) over the nerve more distally, using antidromic techniques for sensory nerve conduction studies (this has technical advantages over orthodromic techniques), and (4) over the nerve more proximally for mixed nerve conduction studies (Fig 45-3) These techniques are the ones used most often in clinical work An alternative but much more demanding technique uses near nerve needle electrodes to record action potentials as they course through the nerve The main characteristics of the conventional nerve conduction studies are described below Distal (Terminal) Latencies, Conduction Times, and Conduction Velocities The conduction times from the most distal stimulating electrode to the recording site, in milliseconds, as determined by the latency from the stimulus artifact to the onset and to the peak of the CMAP, are termed the distal (or terminal) and peak motor latencies, respectively (see Fig 45-3) The former is the one used more often as a re ection of conduction time in routine work A stimulus may then be applied to the nerve at a second site more proximally (or if recording electrodes can be placed more proximally in the case of sensory bers), and a conduction time can be measured over a longer segment of nerve When the distance (in millimeters) between the two sites of stimulation is divided by the difference in conduction times (in milliseconds), one obtains a conduction velocity (in meters per second), which describes the maximal velocity of propagation of the action potentials in the largest diameter and fastest nerve bers These velocities in normal subjects vary from a minimum of 40 or 45 m/s to a maximum of 65 to 75 m/s, depending upon which nerve is studied (eg, slower in the legs than in the arms; Table 45-1) Values are lower in infants, reaching the adult range by the age of 2 to 4 years and decline again slightly with advancing age They are routinely diminished also with exposure to cold a potentially important artifact if these recordings are taken when the patient s skin is cool; measurement of skin temperature therefore is routinely made prior to performing the conduction tests