A sound knowledge of anatomy is a vital prerequisite for achieving successful nerve blocks. In simple terms, the course of the brachial plexus, for example, can be compared to an hourglass. Near the spine, its roots extend out broadly and then narrow around the middle of the clavicle, where the nerve bundles form thick clusters that are packed tightly together. This, to continue our metaphor, would be the waist of our hourglass. Towards the armpit, the nerves fan out again. In regional anaesthesia of the arm, this hourglass form has the following consequences:

More or less complete anaesthesia of the arm (excepting the shoulder) is best achieved at the hourglass‘s waist, namely infra- and medioclavicular. Proximal to this (interscalene nerve blocks), as the technique also includes the sensory parts of the superficial cervical plexus (supraclavicular nerve), anaesthesia will reach the shoulder, but not always the ulnar parts of the lower forearm and the hand. These types of block will rarely reach the caudad segments of the brachial plexus. Distad to this (e.g. as with axillary nerve blocks) anatomical gaps can be expected in the region of the radial and musculocutaneous nerves. Thus, in order to choose the most suitable procedure for the patient it is especially important to be properly informed about the localisation and the extent of the planned surgical intervention.