The sheath of the brachial plexus - Fact or Fiction?

Background: The concept of the axillary "sheath" has been a central tenet of brachial plexus regional anesthesia for many years. Recent investigations have cast doubt on its nature and existence. This study further examines the issue. Methods: Computerized axial tomographic dye studies were performed using continuous catheter systems for the sciatic nerve and the brachial plexus. The resultant images were compared and contrasted. Results: The images of the two catheter systems were the same, with the exception that one was of the upper extremity and the other was of the lower extremity. Conclusions. The sciatic nerve is not surrounded or enveloped by a "sheath"-it lies in the tissue plane between rigid anatomical structures. Similarly, the brachial plexus lies in the tissue plane between the rigid anatomy of the chest wall, scapula, humerus, and pectoral fascia. This finding is inconsistent with the concept of the axillary sheath.