Components of a needle EMG evaluation

What are the components of a needle EMG evaluation

Insertional/spontaneous activity: An EMG needle inserted into a normal muscle should evoke brief electrical discharges of muscle fibers. Increased or prolonged electrical activity may indicate abnormalities of the muscle fibers or the nerves supplying them. Fibrillations, positive sharp waves, and complex repetitive discharges are electrical signals that represent abnormal spontaneous firing of muscle fibers due to nerve or muscle damage. There should be no spontaneous activity in a healthy relaxed muscle.

Motor unit analysis: When a patient slightly contracts a muscle, MUAPs can be recorded. The parameters of interest include the amplitude, duration, number of phases, and firing pattern of the MUAPs. Assessment of these parameters occurs in real time and is generally subjective. The quality of the interpretation depends on the skill and experience of the electromyographer, technical recording conditions, and patient cooperation.

Recruitment: When a patient contracts a muscle more forcefully, a large number of MUAPs can be recorded. How “full” this pattern of MUAPs is reflects the underlying health of the motor units and the ability of the patient to “recruit” available motor units. In myopathic conditions, recruitment may be “early” because myopathic motor units generate less force than healthy ones. In neurogenic conditions, recruitment may be reduced as a result of axon or neuron loss.

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