Background and Aims The brachial plexus is a complex anatomical structure and its variability is frequently observed.  Some rare arrangements of the plexus might represent a challenge even for experts in regional anesthesia.
Ultrasound scanning gave us the chance to identify and illustrate a very rare variant of the brachial plexus, never described so far (figure 1)
Methods A 29-year-old female resident was selected as a model for a live demonstration regarding ultrasound guided regional anesthesia.
Results Scanning on the right side of the neck:C5 – emerges between anterior and posterior tubercles of C5 transverse process (TP); courses anteriorly to the anterior scalene muscle (ASM), then becoming superficial, goes back, and dives into the interscalene groove alone without joining the C6 root.C6 and C7 – emerge from the corresponding TP, run inside the interscalene groove, joining each other below the transverse cervical artery (TCA), forming a sort of trunk.
C8 – becomes visible in the corner pocket and immediately joins the trunk.
T1 – never becomes visible
TCA level: C5 root and the trunk are clearly visible; a fascial sheet keeps them divided. (figure 2) (The course of the plexus is visible in a video linked to the qr code in the figure.)
The contralateral plexus was perfectly normal.
Conclusions This rare anatomical variation might represent a challenge in case of interscalene block. Since the upper trunk does not exist, a superior trunk block (STB)  would be impossible. An accurate ultrasound examination, aiming to recognize standardized structures  would be helpful.
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