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Electromyography (EMG), the recording of electrical activity in muscle, should be regarded as an extension of the clinical examination. It can distinguish myopathic from neurogenic muscle wasting and weakness. It can detect abnormalities such as chronic denervation or fasciculations in clinically normal muscle. It can, by determining the distribution of neurogenic abnormalities, differentiate focal nerve, plexus, or radicular pathology; and it can provide supportive evidence of the pathophysiology of peripheral neuropathy, either axonal degeneration or demyelination. EMG is an obligatory investigation in motor neurone disease to demonstrate the widespread denervation and fasciculation required for secure diagnosis.
Recordings are made with a disposable concentric needle electrode inserted into the muscle. A fine wire in the axis of the needle is insulated from the shaft, the end of the needle being cut at an acute angle. The area of the recording surface determines the volume of muscle that the needle can “see”. Conventional EMG needles record from a hemisphere of radius of about 1 mm. Within this volume there are some 100 muscle fibres. The many hundreds of muscle fibres belonging to one motor unit are distributed widely throughout the cross section of the muscle and, therefore, within the pick-up region of the needle there may be just 4–6 fibres of a single motor unit. Analysis of the waveforms and firing rates of single motor or multiple motor units can give diagnostic information.
Electromyographers are skilled at interpreting both the appearance of muscle activity and the sound of the activity transmitted through a loud speaker. Normal resting muscle is silent. Patients often have difficulty completely relaxing a muscle. The motor unit activity associated with incomplete relaxation is distinguished from abnormal spontaneous activity by its rhythmicity. Motor units when first recruited or on the point of being de-recruited fire regularly at 6–10 spikes …
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