Unexpected Anatomical Variation While Performing an Ultrasound-Guided Interscalene Block for Shoulder Surgery

Anatomical variations of the brachial plexus are very common. Knowledge of the possible anatomical variations encountered in ultrasound imaging is crucial for the safe and effective practice of regional anesthesia. The interscalene block (ISB) targets the brachial plexus roots in the interscalene groove, between the anterior and middle scalene muscles (MSM), at the level of the sixth cervical vertebra. Blockade of the brachial plexus roots anesthetizes the shoulder region, making the ISB one of the preferred regional anesthesia options in shoulder surgeries. Abnormalities of the muscular structures surrounding the brachial plexus roots can pose a challenge while performing an ultrasound-guided ISB. We present a case of an unanticipated anatomical variation of the anterior scalene muscle (ASM) encountered on ultrasound imaging when performing an ISB. Our patient was found to have a small redundant ASM, which necessitated an alternative scanning approach and the use of a nerve stimulator to properly identify the brachial plexus roots. Based on our findings, we recommend placing the ultrasound probe parallel to the clavicle in the supraclavicular area and scanning in a cranial direction, tracing the brachial plexus back to the roots, and then confirming the needle placement by using a traditional nerve stimulator before local anesthetic deposition.