Positive sharp waves (a) and fibrillation potentials (b). Note the initial positive (downward) deflection of both types of potentials. Courtesy Hugh J. McMillan, MD
The analysis of motor unit morphology in the pediatric age group is dependent on the ability of the child to activate a muscle in a controlled fashion, or in the case of an infant, the ability of the neurophysiologist to capture and interpret motor unit action potentials with practiced eyes and ears when they may only be present fleetingly. In both cases, the number of distinct action potentials available for viewing and analysis may be less than would be expected in a typical adult needle examination. In most situations, a skilled neurophysiologist will still be able to assess patterns of amplitudes, durations, and phases of the motor units that present themselves (Fig. 9.2). As in adults, high amplitudes and long durations are suggestive of a neurogenic injury, which can be focal (Fig. 9.3) or generalized (Fig. 9.4) depending on the pattern of abnormalities seen. Low amplitudes and short durations are suggestive of myopathic disease (Fig. 9.5). However, the amplitudes and durations of normal motor unit action potentials may be smaller in younger children and especially in infants. Thus, there is a dual risk of missing neurogenic motor units and “overcalling” myopathic motor units in this age group particularly when studying facial muscles. It is also important to remember that the duration is often more informative than the amplitude with respect to sensitivity and specificity, though the measurement of duration may be confounded when the recorded motor units are distant from the recording electrode.
Normal voluntary motor units and recruitment pattern observed upon needle examination in a 16-year-old adolescent male. Courtesy Hugh J. McMillan, MD
Polyphasic, rapidly firing motor unit action potentials in a 14-year-old with radial neuropathy. Courtesy Hugh J. McMillan, MD
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