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Page 16 of 20 Kobylarz et al. Plast Aesthet Res 2023;10:2 https://dx.doi.org/10.20517/2347-9264.2022.38

neuroaxis, e.g., precisely determining the location(s) and severity of nerve injury, as well as the potential for
recovery.

The anatomic focus of the surgery will determine the selection of nerves and muscles to monitor. However,
one should be prepared for the possibility that this could change during the procedure, e.g., given the
intraoperative findings, the location of dissection can shift from the upper trunk to the lower trunk,
necessitating monitoring the median and ulnar nerves, as well as the respective muscles they each
innervate [21-23,29] .

As with radiculopathy and mononeuropathy, intraoperative neurophysiologic monitoring enables the
surgical team to identify components and determine the baseline neurophysiologic status of the brachial
plexus, e.g., the severity of the injury and the changes that may occur during surgery. Neurophysiologic
monitoring, i.e., EMG and NCS, also help the surgeon to avoid accidental injury to the brachial plexus
neural structures, including the spinal nerve roots, trunks, cords, and nerves, during dissection, neurolysis,
and tumor resection. Multimodal intraoperative testing, including NAPs and somatosensory and motor-
evoked potentials, can be quite helpful for determining whether ventral or dorsal root avulsions and
preganglionic injuries have occurred. However, the latter two neurophysiologic modalities are beyond the
scope of this review. IONM can also provide an assessment of the potential for spontaneous nerve
regeneration. If a NAP or CMAP can be recorded across scarring or a neuroma involving the brachial
plexus, local neurolysis might be a more successful surgical intervention rather than resection and nerve
grafting. However, if such responses are absent, resection would be necessary. Intraoperative electrical
stimulation could identify which nerve fascicles can be utilized for grafts and transfers; the resultant NAP
and CMAP responses enable the surgeon to determine which portion of the nerves and branches may be
expendable for this purpose. The specific noncritical muscles and potential overlapping innervation can be
confirmed using intraoperative stimulation testing with NAP and CMAP recordings.

Given the complex anatomy, IONM for brachial plexus procedures can be tedious and time-consuming.
However, it is worth the investment since this technology can provide essential information for achieving
favorable postoperative outcomes following tumor resection and post-traumatic nerve repairs and
reconstruction.

Outpatient electrodiagnostic studies of mononeuropathy
Electrodiagnostic studies are most useful for localizing the site of injury along the nerve in a
mononeuropathy and excluding alternative localizations, such as plexus or nerve root. Ulnar neuropathy at
the elbow (UNE) is an illustrative example of the second most common entrapment neuropathy in the
upper extremity, after median neuropathy at the wrist. Potential mimics of UNE include ulnar entrapment
at the wrist or alternative site, lower brachial plexopathy, or C8-T1 radiculopathy. Electrodiagnostic testing
can distinguish between these possibilities. EMG demonstrates denervation and reinnervation in the ulnar-
innervated muscles of the hand and forearm without the involvement of median or radial-innervated C8
muscles. The most proximal ulnar-innervated muscle is the flexor carpi ulnaris, often spared due to
differential fascicular sparing in compression at the elbow. On NCS, slowed conduction velocity or
conduction block at the elbow can be seen in a demyelinating UNE providing localization within the ulnar
nerve.

In contrast, for an axonal UNE, it may be more difficult to localize the lesion to the elbow site on NCS.
Additional studies could be considered in that setting, such as dorsal ulnar cutaneous sensory NCS, short
segment incremental inching studies, mixed and sensory NCS across the elbow, or motor NCS recording