Unintended Medial Cord Block Following Pectoral Nerve (PECS) I and II Blocks: A Case Report Highlighting Anatomical Considerations and Technical Optimization.

The pectoral nerve (PECS) block is a relatively newer regional anesthetic technique that provides analgesia to the upper anterior chest wall. It is commonly used for breast surgery and has also been applied to a range of other procedures, from pacemaker insertion to anterior shoulder surgeries. PECS blocks provide effective pain relief while avoiding more invasive techniques such as paravertebral blockade, making them an appealing option for perioperative analgesia. Although generally considered safe, rare complications may occur if the local anesthetic spreads beyond its intended fascial planes, potentially affecting the brachial plexus. We report the case of a 35-year-old woman who underwent bilateral augmentation mammoplasty under general anesthesia, with ultrasound-guided PECS I and II blocks administered postoperatively at the conclusion of the procedure (15 mL per plane of a mixture of liposomal and standard bupivacaine). Approximately one hour after surgery, she developed right upper extremity paresthesia, motor weakness, and sensory deficits consistent with medial cord (C8-T1) involvement. She was managed conservatively with protective measures and counseling, with complete resolution of symptoms by postoperative day 6. This case highlights the possibility that surgical disruption of pectoral fascial planes, together with injectate volume, may permit unintended proximal spread of local anesthetic and underscores the importance of meticulous anatomical technique.