The Lambert-Eaton myasthenic syndrome, sometimes associated with oat cell carcinoma of the lung, as discussed in Chap. 46, is characterized by a presynaptic blockage of acetylcholine release and, with rapid rates of stimulation, produces the opposite effect on neuromuscular transmission to the one recorded in myasthenia gravis. There is instead an increment in the amplitude of the CMAP with continued stimulation. During very rapid repetitive stimulation (20to 50-per-second), the muscle action potentials, which are small or practically absent with the first stimulus, increase in voltage with each successive response until a more nearly normal amplitude is attained (see Fig. 2-15B). Exercising the muscle for 10 s before stimulation will cause a posttetanic facilitation in patients with the Lambert-Eaton syndrome (200-fold increases are not uncommon). A less important decremental response to slow stimulation may occur, but it is difficult to discern because of the greatly diminished amplitude of the initial responses. The effects of botulinum toxin and of aminoglycoside antibiotics are similar, that is, being active at the presynaptic membrane, they produce an incremental response at high rates of stimulation.
The single-fiber EMG, discussed in a later section, is an even more sensitive method of detecting failure of the neuromuscular junction.
Needle Examination of Muscle (Electromyography)
In the usual EMG examination, a plan for the study is made based on detailed knowledge of muscular innervation and focusing on the regions affected by weakness. In some patients, as in those with motor neuron diseases or polymyositis, a wider sampling of muscles is required to detect changes in asymptomatic regions. This technique requires the use of monopolar or concentric bipolar needle electrodes, which are inserted into the body of the muscle to record the electrical activity generated by contraction. With concentric electrodes, the tip of the wire that runs in the hollow of the needle is in proximity to many muscle fibers belonging to several different overlapping motor units; this is the active recording electrode. The shaft of the needle, in contact over most of its length with intercellular fluid and many other muscle fibers, serves as the reference electrode. Monopolar electrodes use the uninsulated needle tip as the active electrode, while the reference electrode may be another monopolar needle electrode placed elsewhere in subcutaneous tissue or a surface electrode on the skin overlying the muscle. Patients almost invariably find this portion of the test uncomfortable and should be prepared by a description of the procedure. Rapid and brief needle insertion by the skilled examiner makes the test more tolerable.
As the electrical impulse travels along the surface of the muscle toward the recording electrode, a positive potential is recorded on the oscilloscope, that is, the recorded signal is deflected downward by convention (at A in Fig. 2-16). When the depolarized zone moves under the recording electrode, it becomes relatively negative and the recorded signal is deflected upward (at B). As the depolarized zone continues to move along the sarcolemma, away from the recording electrode, the current begins to flow outward through the membrane toward the distant depolarized region, and the recording electrode becomes relatively positive again (at C). There is then a return to the resting isopotential position. The net result is a triphasic action potential, as in Fig. 2-16. This configuration is typical of the firing of a single fiber.
The electrical activity of various muscles is recorded both at rest and during active contraction by the patient. Muscle fibers do not normally discharge until activated together in motor unit activity. This involves the almost simultaneous contraction of all the muscle fibers innervated by a single anterior horn cell. Although the typical configuration of a motor unit potential (MUP) is triphasic, up to 10 percent of normal MUPs consist of four or more phases (polyphasic potentials); however, an excess of polyphasic potentials beyond this is pathologic.