Needle EMG, although providing valuable insights into a patient's peripheral pathology, has a number of limitations that are important to identify. Similarly, these limitations go hand-in-hand with several often misunderstood aspects of the procedure.
Whereas we would like to think of an individual MUP as the electrical signal of a single motor unit, in fact, a single MUP is actually the electrical signature of a single motor unit as recorded by a needle electrode in a specific position relative to that motor unit. In other words, the orientation and distance of the needle to an individual motor unit determines the exact configuration of the MUP. Small movements of the needle may turn a polyphasic motor unit into one with normal phases; amplitude will also fluctuate. At least one study has demonstrated that MUPs will appear smaller when evaluated with the needle inserted toward the periphery of a muscle then when inserted more deeply into it. Duration of a given MUP usually is the most stable characteristic, is less dependent on needle position, and should be considered the parameter of choice to evaluate.
9.2. Sampling Error
When a needle is inserted in a muscle, only a small area of the muscle is evaluated. Hence, for mild lesions, needle examination in one area of the muscle might show results different from another area. In some situations, in which the findings are very subtle, examining several different parts of a muscle can be helpful in establishing the overall condition of that muscle.
As noted, MUPs will gradually enlarge as a person ages, and most people who undergo EMG who are in their 80s will demonstrate MUPs that are relatively large compared with those of younger people. Whether this represents true disease or whether it is merely a consequence of aging, caused by normal motor neuron loss, is never clear in an individual patient. This situation is analogous to finding degenerative changes on imaging of the spine of asymptomatic elderly individuals. Such "abnormalities" may not be meaningful. Ultimately, how to interpret mild degrees of MUP enlargement in a given elderly individual is really up to the discretion of the electromyographer.
It must be recalled that although EMG is very useful test, it does not evaluate all aspects of the muscle. It only evaluates the electrical activity of the muscle fiber—that produced by the depolarization of the muscle fiber membrane. Although strength depends on this being normal, it also depends on normal release of Ca2+ into the sarcoplasm and normal function of actin and myosin, as well as other associated proteins, to produce a muscle contraction. Strength also depends on having an intact connection between the muscle and bone (i.e., the presence of an intact tendon). Another cause of milder degrees of weakness in the setting of relatively normal EMG is mildly reduced activation or recruitment that is simply difficult to observe on the oscilloscope screen. Patients with subtle weakness, in the 4+ to 5-range, may have relatively normal-appearing EMG studies. This is especially true in mild myopathies, in which subtle changes in MUP morphology may not be readily observed and in which computer-assisted techniques at quantifying the results may be helpful.
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