Ultrasound-guided subomohyoid suprascapular nerve block and phrenic nerve involvement: a cadaveric dye study

The anterior approach to the subomohyoid suprascapular (SOS) nerve is a new, technically easy and reliable regional anesthesia technique for postoperative shoulder analgesia. However, due to its proximity, the injectate may spread to the brachial plexus and phrenic nerve. The goal of this anatomic study with dye injection in the subomohyoid space and subsequent cadaver dissection was to establish the likely spread of local anesthesia and the extent of brachial plexus and phrenic nerve involvement resulting from ultrasound-guided SOS nerve block.

The suprascapular nerve (SSN) under the inferior belly of omohyoid muscle in the posterior triangle of the neck was identified. Using a contrast dye, 10 ultrasound-guided SOS nerve injections of 5 mL were done bilaterally, in five fresh cadavers. The area was then dissected to evaluate the spread of the contrast dye in the immediate proximity of the brachial plexus, phrenic and SSN.

The SSN and omohyoid muscle were easily identified on each cadaver. SOS nerve staining with contrast dye was seen in 90% of dissections. The superior trunk was stained in 90% and the middle trunk was stained in 80% of dissections. The inferior trunk was stained in 20% of dissections. A spread of dye around the SSN was observed in 90% and the phrenic nerve was mildly stained in 20% of the dissections.

In-plane ultrasound-guided needle injection with a 5 mL volume for SOS block was sufficient to stain the SSN. This conservative volume involved other parts of the brachial plexus and may potentially spread to the phrenic nerve. Further clinical studies are required for confirmation.