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                                  Electromyographic (EMG) examination 

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Electromyographic (EMG) examination ; Myopathy, Anterior horn cell disease, Neuropathies, Neuromuscular transmission disease

3) Electromyographic (EMG) examination  

This test consists of two parts: Nerve conduction studies and needle examination. 

a) Nerve conduction studies  

Since there are few pure motor nerves to study,  motor nerve conduction recording electrodes are placed over a distal muscle (i.e. thenar muscle group).  The appropriate nerve is then stimulated electrically and the evoked responses can be measured.  These evoked responses recorded from the surface of the muscle are called a compound muscle action potential (CMAP). The time it takes from stimulation to generation of the CMAP is the conduction speed. 

The CMAP represents the action potentials of all muscle fibers activated by the nerve stimulation and the measured response can be compared to a known standard for such stimulation.  Reduction in the strength of this response indicates a loss in overall muscle mass or the loss of motor fibers and must further be investigated as to its cause. 

For sensory nerve conduction studies, the recording electrodes are placed over superficial nerves (e.g. the sural nerve is a pure sensory nerve).  Stimulation of a sensory nerve leads to action potentials in all of the fibers of that nerve and an electrode on the surface of such a nerve records the sensory nerve action potential (SNAP).  Furthermore, by stimulating the same nerve over different segments the distances between stimulation sites can be measured and a conduction velocity for the nerve segment established. 

 

The conduction studies are followed by repetitive nerve stimulation studies. A routine motor nerve conduction study is performed but the nerve is stimulated supramaximally at 2 - 3 Hz and the amplitude of the first 4 CMAPs recorded. In neuromuscular transmission defects the CMAP amplitude decreases with successive stimuli as some muscle fibers are not depolarized due to the neuromuscular transmission defect (figure 12). This is called a decremental response.  (The exact mechanism of the decremental response is complex and beyond the scope of this course!!  Donít worry!)  

Figure 12  Repetitive nerve stimulation study.  Four CMAPs are shown in each tracing. Note that the amplitudes of the responses are the same in a normal muscle, but that a decremental response is recorded in neuromuscular transmission defects.  

 

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