and neuropathy syndromes are autoimmune and easily and permanently
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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
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!)
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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