Use of the EMG as an outcome measure

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The ability to predict a patient's eventual prognosis is an important part of the overall treatment plan. Unfortunately the current literature dealing with electrodiagnostic prognostication is meager. There are only a few areas that electrophysiologic testing provides important outcome data. One of these areas is the evaluation of traumatic peripheral nerve injury. Traumatic nerve injuries can result in different grades of injuries as outlined by Seddon; neuropraxic, axonotmesis, and neurotmesis. They are based on morphologic criteria and the presence of Wallerian degeneration. The electromyographer is frequently asked to participate in the clinical evaluation and therapeutic decision-making of patients with peripheral nerve injury. Careful EMG evaluation can identify a lesion that is a result of neuropraxia versus axonotmesis and neurotmesis. In the latter case there is varying degrees of Wallerian degeneration. The timing of the EMG study is important and is guided by experimental studies of sensory and motor nerve regeneration (Gilchrist and Sachs, 2004). Nerves have been shown to regenerate at an average rate of 1-2 mm/day (Seddon et al., 1943). Knowing when the lesion occurred allows the evaluator a time course in which one can predict the course of regeneration and likewise when a nerve is not regenerating as one would anticipate. For most nerve lesions in continuity, surgical exploration is generally recommended if there is no evidence of axonal regeneration by 3-6 months post injury. Prognostically, lesions with the least amount of Wallerian degeneration recover best (Cros, 2001).

NCs are useful in mapping the recovery from a compression neuropathy or a neuropraxic lesion. Neuropraxic lesions result from a disruption of myelin with intact axons and stroma. This type of lesion can also be seen in traumatic brachial plexus injuries. In this situation the large myelinated fibers are more susceptible. Thus motor fibers are usually more affected than sensory fibers, resulting in motor paralysis. Motor paralysis from, a neuropraxic lesion, is expected to last from 1 to 6 months, although most resolve by 3 months (Rudge, 1974). Nerve conductions serve to document and outline the recovery of this type of injury.

Electrodiagnostic testing is perhaps the most important predictor of outcome in patients with GBS (Katirji et al., 2002). Patients with severe axonal loss have poorer outcomes. The amplitude of the CMAP is the most reliable measure of axonal loss: low CMAP amplitude (less than 10-20% of normal) has been correlated with poor outcomes (McKhann et al., 1985).

NCs can be used in assessing the response of diabetic neuropathy to various therapeutic interventions. Early studies revealed improvement in conduction velocity following glycemic control in newly diagnosed diabetics (Fraser et al., 1977).

Lastly, electrophysiologic testing has played an important role in determining the extent of facial nerve injury when there is early, complete paralysis of the facial muscles. Testing is best performed on the 5th day after onset or within 2 weeks (Gutierrez and Sumner, 2003). The facial nerve conduction is the most useful. CMAP side-to-side comparisons seem to hold the best prognostic values. May demonstrated that a 25% or greater sparing of CMAP amplitude when compared to the unaffected side had a 98% chance of satisfactory recovery (May et al., 1983).

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