This is a special technique for the recording of single-muscle-fiber action potentials that has found utility in measuring muscle fiber density and in detecting the so-called jitter in disease of the neuromuscular junction, particularly myasthenia gravis. Jitter is the variability of the interpotential interval of successive discharges of two single muscle fibers belonging to the same motor unit. This phenomenon is largely a result of the very slight variability of delay at the branch points in the distal axon and by synaptic delay at the neuromuscular junction, especially in myasthenia gravis where it has found its main clinical use. Fiber density and jitter may, however, also be increased in neuropathic disorders that cause denervation with reinnervation. Both are usually normal or only slightly increased in myopathic disorders.
Testing for jitter is carried out by having the patient voluntarily contract a muscle to the slightest degree possible so as to activate only one motor unit (requiring a great deal of cooperation by the patient) or by stimulating an intramuscular nerve twig (requiring great patience on the part of the examiner). The EMG needle is advanced until two muscle fibers from the same motor unit are recorded. If the oscilloscope sweep is triggered by the firing of the first fiber, a fluctuating latency of the second fiber potential can be seen on the screen as a movement (jitter) of the second peak. The degree of jitter can be quantitated by measuring the interval between the activation of the two muscle fibers (the result of slightly differing lengths of the terminal axons) from which a mean interpeak interval is determined. Approximately 20 fiber pairs are sampled, and an average of the mean intervals can be derived. In a muscle such as the extensor digitorum communis, the average variation should be no more than 34 ms. The acceptable average is lower for large proximal muscles. Also, in disease of the neuromuscular junction, one muscle fiber in a pair may fail to fire intermittently as a result of a blocking of conduction. Further details of this technique and its clinical applications are discussed by Stålberg and Trontelj.
Imaging of Muscle and Nerve
Imaging techniques—CT, MRI, and ultrasonography—enable one to measure muscle volume and to recognize qualitative changes in muscle structure (see review of Filler and colleagues). Such methods are finding some clinical and research use in the diagnosis of disorders of muscle and in gauging the effects of treatment. CT scans of dystrophic muscle show foci of decreased attenuation, representing masses of fat cells. The fatty masses spread gradually from multiple foci and eventually replace muscle fibers. The original shape of the muscle is retained; indeed, an enlarged weak muscle containing mostly fat confirms the clinical impression of pseudohypertrophy. In denervative atrophy, the muscles are obviously small and contain multiple punctate areas of decreased attenuation, which represent interstitial fat. Eventually, large portions of chronically denervated muscle may be replaced by fat. Blood, blood products, and calcium deposits are expressed by increased attenuation in CT. This may be helpful in the diagnosis of muscle trauma, myositis ossificans, and dermatoand polymyositis.