Background and objectivesThe paraneural sheath is a multilayered network of collagen fibers that surround the brachial plexus. Currently, there are no sonographic data on the paraneural sheath of the brachial plexus, which this study aimed to evaluate.MethodsUltrasound imaging datasets of 100 patients who received a costoclavicular brachial plexus block, using high-definition ultrasound imaging, were retrospectively reviewed. Video files, representing sonograms before and after the local anesthetic injection, from the costoclavicular space and lateral infraclavicular fossa were collated and reviewed by three experienced anesthesiologists. Frequency (yes/no) of ultrasound visualization of the paraneural sheath, septum, and the anterior and posterior compartments was assessed. Representative sonograms from the costoclavicular space and lateral infraclavicular fossa were visually correlated with archived cadaver microanatomic sections from the same location.ResultsDatasets of the 98 patients who achieved surgical anesthesia were evaluated. The paraneural sheath, septum, and the anterior and posterior compartments were visualized in 17.3%, 7.1%, 5.1% and 5.1%, respectively, at the costoclavicular space before the brachial plexus block; this contrasts (p<0.001) with their visibility post-block (94.9%, 75.5%, 75.5% and 75.5%, respectively). At the lateral infraclavicular fossa, the corresponding visibility of these structures post-block were 67.7%, 81.5%, 81.5% and 81.5%, respectively. Ultrasound images of the paraneural sheath and septum correlated well with that in the cadaver microanatomic sections.ConclusionWe have demonstrated the paraneural sheath and fascial compartments surrounding the cords of the brachial plexus at the costoclavicular space and lateral infraclavicular fossa using high-definition ultrasound imaging.Trial registration numberClinicalTrials.gov Registry (NCT04370184), (https://www.clinicaltrials.gov/).