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Page 14 of 20 Kobylarz et al. Plast Aesthet Res 2023;10:2 https://dx.doi.org/10.20517/2347-9264.2022.38

during spine surgeries, they are less sensitive than EMG for monitoring nerve root function and
irritation [18,20] .

Intraoperative EMG monitoring nerve roots is greatly useful during spine surgery for several reasons. Free-
running, continuous EMG provides real-time feedback regarding the integrity and irritation of nerve roots,
which are at significant risk of injury during surgery. The surgical procedure can be altered, limited, or
discontinued if excessive bursts or trains of neurotonic discharges in the corresponding monitored muscles
occur . Gunnarsson et al. reported that the sensitivity of intraoperative EMG activation to identify patients
[26]
with new neurologic deficits after thoracolumbar spine surgery was 100%, although the specificity was only
24% . Relatively few patients in their series (6.6%) had postoperative deficits despite the large percentage
[26]
(78%) with significant neurotonic discharges: therefore, it is likely that the surgeons could avoid permanent
neural injury by modifying the surgical strategy. Other IONM modalities can be employed to provide
complementary information regarding intraoperative neurophysiologic integrity. Triggered EMG, e.g.,
pedicle screw stimulation, may also inform the surgeon regarding the proximity of instrumentation to nerve
roots [21-23,26,27] .

Pedicle screw testing
Intraoperative pedicle screw stimulation is a highly sensitive and specific method for detecting pedicle wall
breach into the spinal canal during screw placement. A malpositioned pedicle screw poses a significant risk
to the nerve root. Although the secure placement of pedicle screws can be determined by fluoroscopy, the
local neuroanatomy cannot be visualized, so triggered EMG provides an essential complementary tool for
this purpose. The holes and screws correctly positioned within the pedicle wall are separated from the
adjacent nerve roots by a high-impedance cortical bony layer. When a pedicle screw is placed correctly, the
surrounding bone impedes electrical conduction to adjacent nerve roots. Therefore, a higher electrical
current will be needed to stimulate the local nerve root. During testing, a monopolar electrical stimulator is
used to apply a series of increasing currents to the pedicle screws. Electrodes placed into muscles innervated
by the respective spinal nerve root levels will record CMAPs synchronized with the stimulation. If a pedicle
wall breach occurs, the stimulation threshold is significantly reduced. A stimulation threshold greater than
15-20 mA reliably indicates adequate screw position. A stimulation threshold of less than 10 mA is often
associated with significant pedicle cortical perforation by the screw. When less bone separates the screw
from the nerve root, the stimulation threshold will be reduced due to lower electrical impedance. This
should prompt the surgeon to remove or redirect the screw at that site, particularly if there is concurrent
neurotonic EMG activity in a corresponding muscle. Stimulation thresholds between these two current
levels usually indicate adequate screw position, although surgical exploration to determine the integrity of
the pedicle wall should be considered. False negatives can occur due to various factors, such as the use of
muscle relaxants or pre-existing nerve damage, which can result in a higher stimulation threshold or no
CMAP response at all. In addition, the region around the pedicle screws should be sufficiently dried to
avoid current shunting, which can also result in an erroneously high stimulation threshold. In such cases, if
possible, one should first determine the stimulation threshold to evoke a CMAP by directly stimulating the
exposed chronically compressed spinal nerve root and then performing pedicle screw testing [18,20,27,28] .

Outpatient electrodiagnostic studies in brachial plexopathy
As with the other entities, the electrodiagnosis of plexopathies is based on inference from clinical and
electrophysiologic data. Nerve conduction data are an essential aspect in evaluating plexopathies,
particularly in distinguishing plexopathy from radiculopathy or polyradiculopathy. This is partly because of
the informative nature of sensory nerve studies in plexopathy. As noted above, sensory nerve studies are
normal in radiculopathy because the lesion is usually proximal to the dorsal root ganglion, leaving the
sensory axon intact. In plexopathies, however, the lesion is distal to the dorsal root ganglion, and the