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Page 4 of 20 Kobylarz et al. Plast Aesthet Res 2023;10:2 https://dx.doi.org/10.20517/2347-9264.2022.38

morphological changes in the structure and recruitment of the motor unit action potential (MUAP) during
voluntary contraction. In this portion of electrodiagnostic testing, a needle electrode is placed into a muscle
of interest. Movement of the electrode through the muscle at rest results in a burst of muscle fiber potentials
that generally last no longer than 300 ms after needle movement. In denervated muscle, single muscle fibers
spontaneously twitch at rest due to denervation hypersensitivity; this is detected as fibrillation potentials
and positive sharp waves that reflect the activity of denervated muscle fibers due to nerve fiber axonal
loss [2,4,8,13,14] .

When the muscle is contracted, the morphology of individual MUAPs and the recruitment of those
potentials are studied [Figure 2]. Recruitment reflects the gradual addition of more MUAPs as increasing
force is generated in a muscle. Reduced recruitment occurs when an insufficient number of MUAPs are
recruited for a given muscle contraction; this occurs in the setting of a neurogenic lesion or with sufficient
demyelination to cause conduction block (drop out of the nerve action potential due to demyelination).
Changes in the duration, number of phases (polyphasia), or amplitude of a MUAP reflect chronic changes
resulting from reinnervation. These changes can reflect the reinnervation of muscle fibers as collateral nerve
fibers sprout from adjacent intact axons. The conduction speed of sprouting nerve fibers is slower
concerning mature branches due to their size and incomplete or absent myelination; therefore, the potential
from the muscle fibers innervated by new sprouts is slightly delayed increasing in phases, turns, and the
duration of the MUAP [Figure 2]. This reflects the asynchronous firing of potentials from a reinnervated
muscle [2,4,8,13-15] . Therefore, polyphasia can occur in the setting of early reinnervation with poorly myelinated
axonal sprouts that eventually become more established, allowing for the polyphasia to resolve (see “Time
course of MUAP changes”).

Time course of MUAP changes
A predictable pattern of MUAP changes is seen after nerve injury resulting in axonal loss. Within the first
week to 2 weeks, MUAP morphology is normal, but recruitment is reduced, reflecting drop out of MUAPs
innervated by injured axons. By 4 weeks after injury, distal sprouting to denervated muscle fibers occurs
from adjacent axons; this results in polyphasic MUAPs because of unstable neuromuscular transmission
and asynchronous conduction along immature, poorly myelinated/unmyelinated nerve fiber sprouts
branching from intact axons, as described above. These are referred to as “nascent units”. Because
reinnervation is still early and nascent units have reinnervated a few muscle fibers, motor units can appear
small. As more distant muscle fibers are reinnervated over the subsequent several months, the MUAP size
and duration grow while remaining polyphasic. After ~6 months, the degree of polyphasia will diminish as
nascent units mature and develop more stable neuromuscular transmission and conduction. However,
MUAPs will continue to appear long in duration and large amplitude, reflecting the more significant
number of muscle fibers taken over by the reinnervating motor unit [2,4,13-15] .

Intraoperative electrodiagnostic techniques
Electrodiagnostic techniques, including NCS, free-running, and stimulated EMG, have been used in the
surgical setting for over five decades. These techniques have been helpful for various surgeries, particularly
involving the spine, nerve roots, and peripheral and cranial nerves.

EMG can be monitored intraoperatively in any muscle accessible by paired surface or needle electrodes,
which provide a better signal-to-noise ratio. After the patient is anesthetized and sedated, multiple EMG
electrodes are placed into or over the limb or cranial muscles of interest based on which neural structures
are at risk during the surgery. EMGs are recorded continuously during the surgical procedure with low-
noise differential amplifiers. Due to significant electrical noise emanating from the many electronic devices