Shear Wave Elastography of the Brachial Plexus and Target Muscle in Interscalene Block: A Narrative Review of Quantitative Tools for Patient-Centered Regional Anesthesia.

Ultrasound-guided interscalene block is a widely used analgesic technique for shoulder surgery, but its optimization is constrained by trade-offs between analgesic efficacy, motor impairment, and hemidiaphragmatic paresis. Shear wave elastography (SWE) quantifies tissue stiffness and has been proposed as a complementary, objective measurement that may help characterize the mechanical environment of nerves and muscles before and after local anesthetic injection. This narrative review synthesizes current evidence on SWE of the brachial plexus and deltoid muscle, with emphasis on technical considerations, reported reference values, and potential relevance to local anesthetic optimization in interscalene block. Available data show that SWE can quantify nerve and muscle mechanical properties, but direct clinical evidence in interscalene block remains limited; reproducibility at the cervical root level is variable; and, to date, no study has validated either brachial plexus SWE as a marker of intraoperative sensory blockade or deltoid SWE as a marker of regional anesthesia-induced motor block. We therefore propose a hypothesis-generating dual-target framework in which paired nerve and muscle SWE is investigated, alongside sensory testing, diaphragmatic assessment, and patient-reported recovery outcomes, as an exploratory surrogate of the local anatomical and mechanical changes that accompany the block, rather than as a direct readout of sensory or motor blockade itself. Standardized acquisition protocols, attention to floor effects in muscle measurements, and prospective validation studies are required before SWE can be considered a clinically actionable tool for patient-centered interscalene block.