• Discussion:

    Question 1 Explanation

    Answer: Postganglionic injury to C6.

    The lateral antebrachial cutaneous sensory distribution is the C6 distribution.  There is loss of motor function in the C6 distribution and loss of sensation.  The loss of sensory nerve action potential suggests a postganglionic injury.  When a preganglionic injury occurs, there is no disconnection between the distal sensory axons and the cell bodies in the dorsal root ganglion, so no Wallerian degeneration happens in the axons and the sensory nerve action potential is preserved.  There is a disconnection between the distal motor axons and the cell bodies in the ventral horn of the spinal cord, so Wallerian degeneration does occur.  Thus, a preganglionic injury is suggested a preserved sensory action potential in an area of anesthesia.  In this case, the loss of the lateral antebrachial cutaneous nerve sensory action potential is more consistent with a post-ganglionic injury to C6.

    Question 2 Explanation

    Answer: Phrenic nerve.

    The nerve labelled *** is the phrenic nerve.  The phrenic nerve receives contributions from C3, C4, and C5.  The nerve runs longitudinally and is the only nerve running longitudinally and the only nerve running from lateral to medial in a typical supraclavicular brachial plexus exposure.  The nerve runs on the anterior surface of the anterior scalene muscle, while the brachial plexus emerges from the interval between the anterior scalene and middle scalene muscles.

    Question 3 Explanation

    Answer: The nerve labeled $$$, which is the anterior division of the upper trunk

    The upper trunk of the brachial plexus trifurcates into the suprascapular nerve, posterior division of the upper trunk, and anterior division of the upper trunk.  The suprascapular nerve supplies the supraspinatus and infraspinatus muscles.  The anterior division of the upper trunk supplies the biceps via the musculocutaneous nerve.  The posterior division of the upper trunk supplies the deltoid via the axillary nerve.  Hanna has described the SPA arrangement of the trifurcation: The lateral-most branch is the suprascapular nerve, with the posterior division of the upper trunk next, and the anterior division of the upper trunk medial-most.  This is not appropriately depicted in many textbooks.  It is important to recognize the anatomic arrangement in order to be able to appropriately target the branches for repair.  The nerve labeled $$$, which is the anterior division of the upper trunk, should be the recipient in order to target the biceps for reinnervation.

    Question 4 Explanation

    Answer: Triggered SSEP, stimulating proximal to the nerve injury

    A variety of intraoperative electrophysiological testing modalities can be useful in operative brachial plexus explorations, including triggered EMG, nerve action potential testing, and triggered somatosensory evoked potentials.  Nerve action potential testing can be very useful in testing any identified neuroma-in-continuity to see whether axonal regeneration has traversed the neuroma.  In order to test for a functional connection to the central nervous system, an injured nerve root may be tested by generating a stimulated somatosensory evoked potential proximal to the area of injury.

    References:

    • Hanna A. The SPA arrangement of the branches of the upper trunk of the brachial plexus: a correction of a longstanding misconception and a new diagram of the brachial plexus. J Neurosurg. 2016;125(2):350-354.
    • Lukas Rasulić, Andrija Savić, Milan Lepić, Vojin Kovačević, Filip Vitošević, Nenad Novaković, Stefan Mandić-Rajčević, Miroslav Samardžić, Viable C5 and C6 Proximal Stump Use in Reconstructive Surgery of the Adult Brachial Plexus Traction Injuries, Neurosurgery, nyz179, https://doi.org/10.1093/neuros/nyz179

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