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Kobylarz et al. Plast Aesthet Res 2023;10:2 https://dx.doi.org/10.20517/2347-9264.2022.38 Page 15 of 20

sensory axon degenerates. This allows the electromyographer to test several different sensory nerves,
allowing for the localization of a plexus lesion. For instance, alteration of sensation over the lateral aspect of
the hand and forearm prompts evaluation of the median sensory response to the thumb, radial sensory
response, and lateral antebrachial cutaneous nerves, which localizes to the upper trunk of the plexus. The
differential of this distribution of sensory alteration would include mononeuropathies involving any of these
single nerves; if more than one is affected, however, a plexopathy is supported. Side-to-side comparison can
help detect more subtle axonal injury and support plexopathy versus mononeuropathy. For example,
sensory abnormalities over the medial hand and forearm with reduced amplitudes in the ulnar sensory
response to the fifth digit, the dorsal ulnar cutaneous response, and the medial antebrachial cutaneous
response localize to the lower trunk and medial cord. These are conduction studies that are generally not
performed but can be of value in the assessment of plexopathy.

To establish the correct pattern, the subsequent needle EMG study in plexopathy is more extensive than it is
for radiculopathy or mononeuropathy. Muscles tested must represent trunks, cords, and individual nerves
of the upper extremity. Muscles innervated proximal to the plexus, including cervical paraspinal muscles,
serratus anterior (long thoracic nerve), and rhomboids (dorsal scapular nerve), are essential to rule out
radiculopathy. Of particular importance in possible surgical management is loss of axonal continuity; this is
evidenced by the absence of recruitment in weak muscles (that is, no motor units produced on a voluntary
contraction of a muscle). Even in the acute period before the appearance of fibrillation potentials and
positive sharp waves, the absence of MUAP recruitment in the setting of good effort should prompt surgical
exploration or nerve grafts, considering tendon transfer in the chronic setting.

Errors in the electrodiagnosis of plexopathy
An advantage of the brachial plexus study over radiculopathy is the availability of sensory responses as part
of the diagnostic arsenal. This not only helps to distinguish a lesion as localized distal to the dorsal root
ganglion (DRG) but also increases the number of responses that can be obtained to detect an abnormality.
Similar considerations regarding limitations in plexus studies apply, as have been discussed for
radiculopathy studies. As in radiculopathy studies, there exist similar timing considerations, with poor
sensitivity during the hyperacute period (< 3 weeks). The injury must either result in axonal loss or
conduction block to be detected. An adequate sample of muscles representing trunks, cords, and nerves
must be tested to be confident of localization. Brachial plexus lesions can often be patchy to complicate
these challenges further, leading to incomplete lesions of specific anatomic branches. There can also be
variation in the spinal levels supplying the plexus, leading to an incomplete denervation picture or
denervation in unexpected muscles. As discussed above, differentiation of plexus injury from radicular or
polyradicular involvement depends on the sensory NCS, the pattern of denervation on needle EMG, and,
essentially, denervation in paraspinal muscles. However, errors can occur in these metrics: (i) sensory nerve
action potentials can be affected if the DRG is intraspinal or very proximal; (ii) needle study can be
insensitive in the absence of active denervation; (iii) fascicular involvement can mislead localization or
result in incomplete data; and (iv) paraspinal muscle study may be contraindicated due to anticoagulation
or limited by incomplete relaxation. Therefore, plexus studies are often more extensive and difficult to
localize.

Intraoperative monitoring of plexopathy
Electrodiagnostic assessment of the brachial plexus can be quite challenging due to the number of neural
elements and the intricacies and variability of the neuroanatomy involved, i.e., roots, trunks, divisions, and
cords. Given the inherent complexities, pre-operative assessment of the nerves that comprise the brachial
plexus can be limited. A thorough understanding of the functional anatomy and neuropathologic changes is
crucial, so IONM could be of greater importance for surgery involving this portion of the peripheral