• The Brachial Plexus is a bundle of nerve roots arising from C5, C6, C7, C8, and T1.
• Brachial plexus injuries (BPI) result from severe traction forces on the limb.
• The most common sources of BPI are motorcycle accidents and birth palsy (i.e. Erb or Duchenne Palsy) affecting the upper part of the plexus (C5, C6) and causing shoulder and biceps muscle weakness.
• Injury to C7 >>> wrist weakness.
• Injury to C8 and T1 >>>  forearm and intrinsic hand muscle weakness.
• Injury to stellate ganglion or cervical sympathetic trunk >>> Horner's Syndrome.
• MRI of the upper extremity is the standard imaging modality used to make the diagnosis.

References

• Yoshikawa, et al. "Brachial Plexus Injury:  Clinical Manifestations, Conventional Imaging Findings, and the Latest Imaging Techniques," Radiographics, Volume 26. S133-44. Oct 2006.