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 per second. Voluntary firing caused by incomplete relaxation can often be silenced by passively changing the posture of the limb or by slight activation of the antagonist. Voluntary motor units never fire as single isolated discharges, a useful point in distinguishing them from fasciculations. |
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Abnormal Recruitment.In a normal muscle increasing voluntary effort causes an increase in the rate of firing of individual MUAPs and initiates the discharge of additional MUAPs. The relationship between the rate of firing of individual potentials to the number of potentials firing is constant for a particular muscle and is called the recruitment pattern. Normal and abnormal recruitment has been discussed previously. In disorders in which there is a loss of MUAPs, the rate of firing of the remaining individual potentials will be disproportionately high compared to the number of potentials firing; this is referred to as reduced recruitment. Reduced recruitment may be found in any disease process that destroys or blocks conduction in the axons innervating the muscle or destroys a sufficient proportion of the muscle so that muscle fibers of entire motor units are lost. This pattern occurs in association with all neurogenic disorders associated with axonal loss and may be the only finding in a neurapraxic lesion in which the sole abnormality is a focal conduction block. Reduced recruitment may be the earliest finding in an acute axonal lesion in which fibrillation potentials or other MUAP changes have not yet developed. Although a hallmark of neurogenic disorders, reduced recruitment may also be seen in severe or endstage myopathies, where entire motor units are lost due to primary muscle fiber degeneration, such as in muscular dystrophies. Rapid recruitment of MUAPs occurs in disorders in which the force that a single motor unit can generate is decreased due to loss of muscle fibers within the motor unit. As a result, more motor units are activated than would be expected for the force exerted by the patient. The recruitment frequency and rate of firing in relation to number are normal with rapid recruitment; however, the number of motor units that fire are increased relative to force. Rapid recruitment occurs primarily in myopathies. While in many cases abnormalities in MUAP configuration will occur along with abnormal recruitment, this is not always the case, and rapid recruitment may be the only abnormality identified on needle EMG, particularly in early or mild myopathies.
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     MUAP duration is measured as the time from the initial baseline deflection to the time of the return to baseline, and it reflects the density and area of fibers within a motor unit, as well as the synchrony of firing of those fibers. The size of MUAPs in a muscle is dependent on the level of activation and with larger MUAPs it becomes active at a stronger force. Normal values for MUAP duration have been published Individual MUAPs that are longer than the normal range for a particular muscle or groups of MUAPs that have a mean duration greater than the normal range for the same muscle in a patient of the same age are called long-duration MUAPs. Long-duration MUAPs occur in diseases in which there is increased fiber density in a motor unit, an increased number of fibers in a motor unit, or loss of synchronous firing of fibers in a motor unit, typically due to collateral sprouting and reinnervation of a motor unit. Long-duration MUAPs generally have high amplitude and show reduced recruitment, but since the spike amplitude reflects only the few muscle fibers closest to the needle recording tip, they may have normal or low amplitude. When assessing MUAP duration, those MUAPs recorded from damaged muscle fibers that are preponderantly positive with a long late negativity, which is a recording artifact, should not be measured or interpreted as long duration.
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