Sonographic convergence between a phrenic nerve branch and the upper trunk of the brachial plexus: a case report

The anatomical proximity of the phrenic nerve to the brachial plexus is a known cause of diaphragmatic paralysis following interscalene blocks. This case report aimed to describe a sonographic convergence between a phrenic nerve branch and the upper trunk of the brachial plexus, and its critical implications for nerve block strategy. A patient scheduled for ultrasound-guided interscalene brachial plexus block exhibited a rare anatomical variant. Preprocedural scanning at the C5-C6 level revealed the right phrenic nerve bifurcating; while one branch followed the typical course, the other branch (PN2, 0.8 mm diameter) coursed caudally and demonstrated sonographic convergence with the upper trunk of the brachial plexus. This variant indicated a high risk of inadvertent phrenic nerve involvement with any proximal approach. Consequently, the anesthetic plan was altered to an axillary brachial plexus block, which was performed successfully without any postoperative diaphragmatic dysfunction. This case illustrates a sonographic convergence between a phrenic nerve branch and the upper trunk of the brachial plexus, representing an anatomical variant that may increase the risk of phrenic involvement during proximal brachial plexus blocks, even with minimal local anesthetic volumes. It underscores the potential value of pre-block ultrasound screening to identify such high-risk variations and guide safer block selection.