How does EMG assess decreased motor unit recruitment in neurogenic conditions?

Updated: Oct 16, 2019
  • Author: Friedhelm Sandbrink, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Damage may occur to the neural portion of a motor unit, anterior horn cell, or corresponding axon. Such injury may result in wallerian degeneration of the motor axon, and all the muscle fibers previously innervated by this axon will be denervated. As a result of such motor unit loss, fewer motor units are available for muscle activation.

Normally, when the first recruited motor unit reaches a firing frequency of 10 Hz, a second unit should begin firing with increasing muscular effort. In a neurogenic condition, this second unit is missing and an increase in force can be achieved only by increasing the firing rate of the first unit.

Decreased recruitment in neurogenic conditions. Th Decreased recruitment in neurogenic conditions. This single motor unit is firing at 15 Hz. The firing rate is calculated from the presence of 3 MUAPs on a screen of 200 milliseconds. This rapid firing unit indicates a neurogenic pathology; the underlying condition in this patient is amyotrophic lateral sclerosis.

Successful activation of a second motor unit occurs only at a higher level of muscular effort than in the normal condition. The recruitment frequency, defined above as the firing rate of the first motor unit at the point when the second motor unit is activated, is therefore increased in a neurogenic lesion. Such an abnormally fast firing motor unit is called "rapid firing unit" (RFU). Because in such cases fewer MUAPs are active than expected, given the first motor unit firing rate, this pattern is called "decreased recruitment" or "reduced recruitment."

This pattern of decreased recruitment may occur whenever a lesion results in a reduced number of functionally intact motor neurons and axons, whether it is the result of actual motor unit loss or temporary conduction block as in neurapraxia. It is an early finding after acute nerve injury (eg, radiculopathy from disk herniation or nerve trauma) and may precede other evidence of denervation in the EMG study.

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