Phrenic-sparing approaches in the interscalene block for shoulder surgery: a narrative review

The interscalene brachial plexus block is widely regarded as the gold standard for perioperative analgesia in shoulder surgery. However, its near-universal association with phrenic nerve block and subsequent hemidiaphragmatic paralysis has raised concerns, particularly in patients with compromised pulmonary reserve. This narrative review examines the mechanisms underlying phrenic nerve involvement, including cephalad and anterior spread of local anesthetic, and summarizes current evidence on preventive strategies. We investigate approaches such as low-volume injection, reduced local anesthetic concentration, superior trunk block, extrafascial deposition, and slow injection under ultrasound guidance to minimize phrenic nerve blockade while maintaining effective analgesia. Each technique presents potential advantages and limitations, including technical challenges, reduced block consistency, and diminished analgesic efficacy. In addition, we discuss the clinical implications of these strategies, highlight gaps in the literature, and propose future research to optimize analgesia while preserving diaphragmatic function. The present review aims to provide clinicians with a comprehensive overview of phrenic-sparing modifications to the interscalene block, thereby enhancing patient safety and promoting individualized patient care.