Page 84 - Read Online

Page 8 of 20 Kobylarz et al. Plast Aesthet Res 2023;10:2 https://dx.doi.org/10.20517/2347-9264.2022.38

irritation can pose a significant risk of injury, particularly with repetitive trauma.

Electrodes detect interference or ambient electrical noise from various sources within the operating room,
such as bed warmers, pumps, electrocautery devices, drills, or microscopes. It is essential to distinguish such
artifacts from EMG activity [Figure 5]. In addition, various neuromuscular conditions and treatments can
interfere with EMG recording, e.g., muscular dystrophy, myasthenia gravis, chronic neurogenic conditions,
and botulinum toxin. EMG activity can also change due to several non-surgical factors, including the level
of sedation and pharmacologic neuromuscular blockade. Motor unit potentials may saturate the recording
due to irritation of multiple axons or voluntary muscle activation as the patient begins to awaken from the
sedated state [Figure 6]. This EMG activity is usually bilateral and often involves the axial muscles. In
addition, it is an anesthetic requirement that no paralytic agents are administered when monitoring
intraoperative EMG. The absence of neuromuscular blockade can be verified with train-of-four (TOF)
testing. Four successive supramaximal electrical stimuli at 2 Hz are applied to a peripheral nerve (e.g., ulnar
nerve), and the resultant CMAPs are recorded from a corresponding muscle (e.g., abductor digiti minimi).
Four robust CMAPs of equal amplitude should be induced for the EMG recording to be of sufficient
sensitivity to detect intraoperative changes reliably. For short-acting agents, the neuromuscular blockade
effects can dissipate during surgical exposure. However, if TOF testing indicates that neuromuscular
blockade is still present, this can be resolved by administering reversal agents such as sugammadex [18,20-23] .

Stimulation-triggered electromyography
Polyphasic CMAPs in innervated muscles can be evoked with motor nerve stimulation, either directly or
through intervening tissue, e.g., tumor or bone [Figure 7]. To localize a nerve or nerve root intraoperatively,
a sterile handheld monopolar or bipolar stimulating probe is used within the operative field by the surgeon.
Typically, the surgeon will use the monopolar stimulator during dissection for stimulation through tissue,
such as bone or tumor, to determine if a neural structure, i.e., nerve root, plexus, or peripheral nerve, is
nearby. The bipolar stimulator is used when the neural structure is in clear view. An absent CMAP may
indicate the absence of motor nerve function, technical problems (poor nerve contact, current shunting,
subthreshold stimulation intensity, erroneous recording settings), or pharmacologic neuromuscular
blockade. Figure 8 illustrates how the surgeon can sweep the stimulator across the surgical field to locate the
peripheral nerve precisely; the stimulation location corresponding to the largest amplitude CMAP would be
closest to this neural structure. In addition to providing information regarding localization, the integrity of
the nerve or nerve root can be quantified by determining the stimulation threshold and how consistently
this is maintained during the procedure. Another way to quantify nerve function is to apply supramaximal
stimulation to the motor nerve: the size of the CMAP generated is correlated with the number of
functioning axons between the stimulation and recording locations. Stimulation can be performed
intermittently during surgery to monitor the integrity of the nerve. Preservation of the response and
consistency of the threshold with proximal stimulation provides objective evidence that no significant
neural injury has occurred. If a decrement of the CMAP amplitude or increase in the stimulation threshold
occurs, the surgeon should consider altering the surgical approach. A significant reduction or complete
disappearance of the response often correlates with the severity of the postoperative neural deficit in the
short-term and potentially long-term [18,19] .

Bipolar probe utilization results in localized current stimulation that minimizes undesired current spread to
adjacent nerves. This can be advantageous when multiple nerves are nearby, such as in the brachial or
lumbosacral plexus. However, current shunting between the two electrodes can occur within conductive
fluids, e.g., blood in the surgical field, resulting in a false negative motor nerve response. This can be
avoided by ensuring that the surgical field is sufficiently dry to avoid this aberrant local conduction. For