The first use of ultrasound to guide regional block was for brachial plexus block above the clavicle.¹ These pioneers used an offline Doppler technique to mark the position of the subclavian artery as a surrogate landmark of the brachial plexus. Although today there may be many criticisms of this technique, their results were impressive: a 98% success rate with no complications.

Ultrasound imaging blurs the distinction between interscalene and supraclavicular blocks. If the brachial plexus is seen stacked between the anterior and middle scalene muscles, the block is generally referred to as an interscalene block. If the brachial plexus is seen as a compact group of nerves lying superior and lateral to the subclavian artery, the approach is generally referred to as a supraclavicular block. Because this distinction can be subtle, both blocks will be treated together.

Variations in brachial plexus anatomy with respect to the scalene muscles are common. The cephalad components of the plexus (in particular, the C5 and C6 ventral rami) often pass over or through the anterior scalene muscle. This may pose a problem for nerve stimulation–based approaches to brachial plexus blocks above the clavicle. The incidence of scalene muscle anomalies is similar for sonography of volunteers and in cadaveric dissections, suggesting that ultrasound can accurately detect these anomalies.²

Cervical ribs are relatively uncommon, occurring in about 0.5% of the population.^3,4 Most cervical ribs are partial (incomplete) and therefore do not pose a problem. However, if the cervical rib is sufficiently large, transducer manipulation can be difficult, and acoustic shadowing by the bone obscures imaging of the brachial plexus.